8 Synthetic Methods By N. J. LAWRENCE Department of Chemistry UMIST PO Box 88 Manchester M60 lQD UK 1 Introduction The predominant theme of new synthetic methods this year has been the development of cleaner reactions and transformations. Methods for asymmetric catalysis and selective processes have as in recent years been prominent in 1995. The number of reports describing deliberately unselective synthesis in the guise of combinatorial chemistry has increased this year and has included some very useful reviews.ly2 Many impressive total syntheses of complex and important natural products have been reported this year including the construction of avermectin Bla,3tax01,~ curacin A,5 rapamycin6 and (+)-pancratistatin.' A candid report of the total synthesis of taxol disclosed last year by Nicolaou and Guy' is also well worth reading.Specialist reference works that have been published in 1995 include volume 49' of Organic Reactions which this year reviews aromatic lithiation reactions promoted by heteroatomic substituents,'" and the intramolecular Michael reactiongb A new major reference work The Encyclopaedia ofReayentsfor Organic Synthesis was published this year. This excellent work which comprehensively reviews over 3000 commonly encountered reagents will undoubtedly soon be indispensable to all those with an interest in organic synthesis. 2 Carbon-Carbon Formation The asymmetric catalytic addition of organozinc reagents-whose stereoselective reactions have been reviewed recently"-to aldehydes has proved as popular as ever this year (Figure 1).Some of the catalysts used include the diselenide (1)12 and the titanate complex (2).' The versatility of this type of reaction is being advanced by the development of new methods for the preparation of diorganozinc reagents. For example Knochel and co-workers report a useful nickel-catalysed conversion of alkenes to functionalized dialkylzinc reagents [e.y. (3) +(4)] (Scheme l).I4 An attractive feature of this reaction is that a large number of organozinc reagents can be made from diethylzinc (one of the few commercially available dialkylzinc reagents). The organozinc reagent can be trapped with various electrophiles or be used efficiently for the catalytic asymmetric addition to aldehydes. Two reports describing the direct oxidation of organozinc reagents with oxygen or dry air have been reported by the 22 1 N.J. Lawrence CI (1) (S)1 mol% 91% 98% ee Cl (2) (S)5 mol% 79% 91% ee Fig. 1 Selectivity in the addition of ZnEt to PhCHO neat 50 "C 2 Ft-+ Et2Zn (R+2zn + 2 CH2CH2 Ni(acac)2(1 mot%) (3) COD(1 mot%) (4) Scheme 1 groups of Kno~hel'~*'~ and Normant" respectively. Secondary and tertiary alkylzinc bromides have been found to add conjugatively to a$-unsaturated ketones in the presence of trimethylsilyl chloride and BF,*OEt, without a copper catalyst." Examples of the asymmetric catalytic addition of organolithium reagents to carbonyl compounds are rare. However Thompson et a!. have shown that the lithium acetylide (7) adds enantioselectively to the trifluoromethyl ketone (5)in the presence of the ephedrine alkoxide (8) to give the alcohol (6),a key intermediate in the synthesis of the reverse transcriptase inhibitor L-743,726 [Scheme 2(a)].I9 Normant and co-workers have developed a different strategy for the use of organolithium reagents in asymmetric carbon-carbon bond forming reactions.They have developed a potentially useful and intriguing process by finding that the asymmetric carbolithiation of cinnamyl derivatives can be performed using sparteine as a chiral promoter [e.g. cinnamyl alcohol -+(9) [Scheme 2(b)].,' The allylation of aldehydes by organometallic reagents provides useful homoallylic alcohols as products. Several new methods for achieving this transformation have been reported in 1995.The asymmetric allylation of aldehydes with diallyltin dibromide is promoted by chiral diamines,2 'for example the proline-derived diamine (10)[Scheme 3(a)]. Charette et af. have shown that the addition of allyltributylstannane to ketones of the type (1 1) [Scheme 3(b)] possessing the 2-benzyloxytetrahydropyranylgroup is highly selective in the presence of MgBr,.OEt,., New reagents for the allylation of aldehydes and ketones include allyl br~mide-tin-Me,SiCl-MeOH,~~allyl bromide and commercial zinc and allyl bromide and CUC~,.~H,O-M~.~~ Isaac and Chan describe a useful and related protocol for the coupling of aldehydes with prop-2-ynyl bromides in aqueous media mediated by indium to give allenyl alochols Synthetic Methods 0 CI CIecF.NHR NHR (5) (6) 82% 97% ee i BuLi-hexane (-)-sparteine 0 "C 1 h PhyOH Ph&OH ii H30+ B" (9)8l% 78% ee Scheme 2 f\\PhCHO (10) (1.1 equiv.) - Br2Sn \ CHzCIz -78 "C Phu 93% 79% ee MgBr2.0Et2 SnBY - Me OHa? \ (b) 90% 92% de = Br Me c J H In H,O R R =C8H17 99% Scheme 3 [Scheme 3(c)]. As with other processes that can be carried out in water this procedure offers the benefits of a cheap non-toxic non-flammable solvent; there is no need for hydroxy protecting groups; substrates that show minimal solubility in organic solvents can be used; and selectivity changes often occur.26 Aldehydes can be coupled with ally1 N.J. Lawrence bromides in a similar fashion.27 3-Bromo-2-bromomethylprop-1-ene and indium provide a water compatible trimethylenemethane dianion equivalent.28 A general useful review of synthetic organoindium chemistry29 has appeared this year.The addition of an organometallic reagent to carbonyl derivatives is promoted by ultrasound. Luche and co-workers3' report the use of the sonochemical Barbier reaction to make ketones. A mixture of alkyl chloride lithium carboxylate and lithium metal in tetrahydrofuran under sonication gives the required ketone in high yield. The rare earth triflates (trifluoromethanesulfonates)have been used to promote a variety of carbon-carbon bond forming reactions this year; their general utility has been reviewed by Mar~hrnan.~' Many reports have detailed further applications for this remarkable class of catalysts.For example hafnium triflate-catalysed Friedel-Crafts acylation reactions have been performed in lithium perchlor-ate-nitr~methane.~~ Lanthanide triflate-catalysed imino Diels-Alder reactions pro- vide a convenient synthesis of pyridine and quinoline derivatives. 33 Fries reactions of phenol and 1-naphthol derivatives with acyl chlorides proceed smoothly in the presence of a small amount of Sc(OTf) (5-20mol%) to afford the corresponding ketones in high yields.34 Lanthanide triflates are also effective for the allylation of imines with allyltrib~tylstannane.~~novel Mannich-type reaction between an A aldehyde amine and a vinyl ether is catalysed by lanthanide triflates in aqueous media.36 0 (12) Ph The design of new chiral auxiliaries and the improved syntheses of old ones for the asymmetric construction of carbon-carbon bonds has featured prominently this year.For example the oxa~olidinone~~ (12) derived from diphenylalinol (itself made from ~-serine~') is an excellent auxiliary for aldol alkylation and Diels-Alder reactions. McKillop Taylor and co-workers report a novel simple and efficient procedure for the synthesis of Evans-type oxazolidinones that avoids the use of borane and the intermediacy of water soluble 1,2-amino alcohols.39 The protected amino acid (13) is reduced with sodium borohydride in the presence of calcium chloride to give an alcohol (14) that is not water soluble; simple heating in toluene with potassium carbonate then gives the oxazolidinone (1 5) in high yield (Scheme 4).The process is very efficient and is well suited to the large scale preparation of this type of chiral auxiliary. Myers et al. have extended their elegant use of p~eudoephedrine~'as a chiral auxiliary to include the synthesis of r-amino acids.41 Treatment of the gly~inamide~~ (16) with lithium diisopropylamine followed by alkylation and hy- drolysis generates the amino acid (17) with high enantioselectivity (Scheme 5). Replacement of the hydrolysis step by treatment with a Grignard reagent generates r-amino ketones.43 Kise et al. report a very selective method for the construction of 2,3-disubstituted succinic acids by oxidative homocoupling of chiral en~lates.~~ The lithium enolate of the Evans oxazolidinone (18) is treated with the oxidant titanium Synthetic Methods (15) 91% Scheme 4 Me 0 i 2 x LDA LiCl Php+/NH2 4b HOLNH* I ii OH Me I iii H20 (16) (17) 63% > 97% de Scheme 5 X i,ii LDA .GX0 TiCI Et 0 x=ov (a) (18) (19) 95% 90% de 0 i LDA Y X ii I2 70% > 98% de Scheme 6 tetrachloride to give the coupled diester (19) with high diastereoselectivity [Scheme 6(a)].Helmchen and co-workers have also reported the synthesis of similar diesters (21) by coupling the enolate of the chiral imidazolidone derivative (20) with iodine as the oxidant [Scheme 6(b)]-45 Masamune et a!. have introduced benzopyranoisoxazolidines as a new class of chiral auxiliary.46 The auxiliary is very efficient in controlling the stereochemistry of alkylation of the acyl derivatives (22) in the manner (22) -,(23) (Scheme 7).In addition the new auxiliary offers many advantages over other auxiliaries such as oxazolidinones for example the acyl derivatives are simply made by combination of the isoxazolidine and the appropriate acid chloride in the presence of triethylamine. The alkylation with P-branched electrophiles can be performed by using the appropriate triflates. The N. J. Lawrence 1 > 90% de R' Scheme 7 auxiliary is removed (and recovered) in a variety of ways to release alcohols aldehydes or ketones. 1 lr-Binaphthyl-2,2'-diol a commonly used but expensive auxiliary has been resolved by reaction of its phosphoryl chloride with a variety of homochiral arnine~.~' De Lucchi and co-workers also report a convenient procedure for the resolution of racemic binaphthalene derivatives via fractional crystallization of the dia-stereoisomeric menthyl carbonate^.^^ 171'-Binaphthyl-2,2'-diolhas also been resolved by separation of its neomenthyl thi~acetates.~' Charette and Brochu have developed a new strategy for the Lewis-acid catalysed cyclopropanation of allylic alcohols.50 They found that treatment of an allylic alcohol with an (iodomethy1)zinc reagent-a class of reagent which has recently been reviewed' '-followed by addition of a Lewis acid triggered the cyclopropanation reaction.Enantiomerically enriched cyclopropylmethanol derivatives are obtained in the presence of homochiral Lewis acids.For example the titanium catalyst (24)effects the efficient enantioselective cyclopropanation of cinnamyl alcohol +(25) (Scheme 8). Radical-based Methods Majetich and Wheless have reviewed recent examples of remote intramolecular free radical functionalizations including those that involve carbon-carbon bond forma- ti~n.~~ Clive and Yang have introduced the polar stannane (26) for use in radical chemistry in much the same way as tributyl- and triphenyl- stannanes are It has been used to effect radical promoted dehalogenation Barton-type deoxygenation and deselenation reactions. The stannane being non-polar and relatively immobile on silica gel allows for the easy chromatographic separation of products from tin- containing by-products.Alkene Synthesis A practical synthesis of (2)-a,P-unsaturated esters using the new Horner-Emmons reagent (27) ethyl diphenylphosphonoacetate has been reported by and^.'^ The Synthetic Methods Zn(CH,I) (1 equiv.) (24) (0.25 equiv.) Ph*OH CHpCIp 0 "C 1.5 h * Ph (25)80% 90% ee Ph Ph Scheme 8 00 i KHMDS. 18-Crown-6 ~ Ph? (4 (pho)2b'doEt ii PhCHO -78 "C 1 h C02Et (27) (28) 98% Z:E 99:l 0 PhCHO CsF DMSO mC02Et do,, room temp. 35 min then * Ph \ (b) 100 "C 1 h (29) (30) 93% E:Z > 98:2 Scheme 9 reagent (27Fprepared from triethyl phosphonoacetate PCI and phenol-when deprotonated by a variety of bases at low temperature reacts with aldehydes with exceptionally high (Z)-selectivity (27) +(28) [Scheme 9(a)].Bellassoued and Ozanne have introduced a modification of the Peterson olefination reaction in which the coupling of the silyl reagent (29) and aldehyde and subsequent elimination of 'Me,SiOH' are both catalysed by fluoride ion in dimethyl sulfoxide (DMSO) in a one-pot operation (29) +(30) [Scheme 9(b)].55 N.J. Lawrence (31) (R) 1 mol% 8O% 93% ee (32)(R) 10mol% 90% 94% ee (33) (R) 10 mol% 89% 95% ee Fig. 2 Selectivity in the borane reduction of PhCOMe 3 Reduction Much effort has been spent on the development of new reagents for the selective reduction of carbonyl compounds and their derivatives especially ketones.56 Estab- lished asymmetric protocols using oxazaborolidine catalysts have seen widespread use for the borane reduction of ketones.57 Among the many new oxazaborolidine catalysts introduced this year are the p-amino alcohol (3 1),58 the serine-derived hydroxy aziridine (32)59 and cis-2-amino-1-acenaphthenol(33) (Figure 2).60 Two other organoboron reducing agents diisocamphenylchloroborane6' and diisobutyl-chloroalane62 have been used to chemoselectively reduce aldehydes in the presence of ketones.Wandrey and co-workers report the use of chiral titanium alkoxides as catalysts for the enantioselective reduction of ketones with boranes (Scheme For example the (r,sc,r',a'-tetraaryl- 1,3-dioxolane-4,5-dimethanol) TADDOL-like ligand (34) catalyses the catecholborane reduction of p-bromoacetophenone (35) with high selectivity to give the alcohol (36). The development of modified borohydride reagents has also been a particularly active area of research this year.For example titanocene borohydride [prepared in siru from Cp,TiCl and NaBH in 172-dimethoxyethane(DME)]has been shown to reduce ketones efficiently to the corresponding alcohols. The reduction of 4-terr-butyl- cyclohexanone is highly trans selective (trans cis = 97 3).64 Mukaiyama and co- workers have shown that the (p-oxoaldiminato)cobalt(rr)complex (37) is a highly efficient catalyst for the reduction of prochiral ketones with sodium borohydride e.g. (38) -P (39) (Scheme 1l).65Zinc borohydride (from ZnC1 and NaBH,) reduces carboxylic acids in refluxing tetrahydrofuran (THF) easily66 whilst calcium borohydr- ide reduces both aliphatic and aromatic esters to alcohols completely in the presence of alkene catalysts.67 A mixture of NaBH and BiCI has been used for the selective Synthetic Methods loo% 82% ee Ph (34) Scheme 10 (37) (5 mol%) NaBH [EtOH (3% in CHCI,)] (39)94% 92% ee Scheme 11 reduction of the carbon-carbon double bond of a,&unsaturated esters with high selectivity.68 The selective reduction of the carbon-carbon double bond of other x,P-unsaturated acid derivatives is also achieved using borohydride exchange resin-copper sulfate in methanol.69 The chemoselective 1,4-reduction of cx,p-un-saturated aldehydes or ketones is achieved with sodium dithionite in H,O-dioxane at 50°C in the presence of unsaturated (non conjugated) and saturated aldehydes or ketones.70 Diisobutylchloroalane effects the same change.71 Several useful modifications of the Meerwein-Ponndorf-Verley (MPV) reduction of ketones have been reported this year.Trifluoroacetic acid (TFA) has been found to greatly accelerate the MPV reaction of ketones with aluminium isopropoxide at room temperature in toluene.72 The aluminium reagent can be used in sub-stoichiometric N. J. Lawrence (411 A 99% 97% ee B:92% 93% ee [gives ent -(41)] A [BINAP-Ru"-(42)-KOH (1:1:2 mole ratio)] (0.2 mol%) H2 (4 atm) Pr'OH B:[RuC12(mesitylene)]2-(43)-KOH (1:2:5 mole ratio)] (0.5 mol%) Pr'OH Scheme 12 quantities in the presence of 1 mol equivalent of isopropyl alcohol (IPA) (i.e. IPA 100 mol%; Al(OPr'), 8 mol%; TFA 0.3 m~l%).~~ In the presence of the co-catalyst NaOH [NiCl,(PPh,),] acts as an efficient Meerwein-Ponndorf-Verley-like catalyst for the transfer of hydrogen from isopropyl alcohol to ketones and aldehydes.74 Noyori and co-workers report a useful related transfer hydrogenation reaction in which isopropyl alcohol is the ultimate source of hydrogen.They use a ruthenium(r1) catalyst in the presence of the sulfonamide (42) TsDPEN for the highly selective process A (40) +(41) (Scheme 12).75 A related process B using a catalyst system with a higher substrate catalyst ratio has been disclosed by the same group. The ruthenium catalyzed hydrogenation is highly enantioselective with the 2,2'-bis(dipheny1phos- phino)-1,l'-binaphthol (BINAP-Ru" complex is used in combination with the chiral diamine (43) and potassium hydroxide.76 Cutler and co-workers have shown that the manganese carbonyl complex [Mn(CO)(PPh,)Ac] (44) and related materials effect the catalytic hydrosilylation of carboxylic esters to the corresponding ether (45) -+ (46) [Scheme 13(a)].77 The modestly enantioselective radical-mediated reduction of the dihydrocoumarin (47) +(48)is achieved by combining Bu,SnH the chiral diamine (49) and magnesium iodide [Scheme 13(b)].'* The asymmetric induction is thought to derive from the close association of the enol radical with the Lewis acid and chiral ligand.Several new methods for the synthesis of amines by reduction have been developed this year including many involving the reduction of azides. For example lithium N,N-dimethylaminoborohydride reduces aliphatic and benzylic azides to the corre- sponding amines in excellent yield.79 Dichloroborane-dimethyl sulfide also reduces a variety of organyl azides with higher selectivity (halides esters nitriles and nitro groups are compatible with this process).*' Azides are transformed to N-boc-amines by treatment with tributylphosphine in the presence of di-tert-butyl dicarbonate.'' Organic azides are also efficiently reduced to primary amines with samarium Synthetic Methods 23 1 EtOhoEt (44) (3 mol%) PhSiH3 CsH6 24 "c (4 (45) 0 (46) 68% I Bu3SnH Mglo (1 equiv.) aoMe (49) (1 equiv.) (b) (47) (-)-(48) 88% 62% ee Scheme 13 dii~dide.'~,'~ Samarium diiodide also effects the reductive cleavage of N-0 bonds in hydroxylamine and hydroxamic acid derivatives.' Several new methods for the reductive amination of aldehydes and ketones have been reported in 1995.A general preparatively efficient simple method for the preparation of N,N-dimethylalkylamines via reductive alkylation of aldehydes and ketones with dimethylamine using titanium(1v) isopropoxide and NaBH has been reported by Bhatta~haryya.'~ Other amines can be used in this reaction.86 Dimare and co-workers report a simple and mild reductive amination protocol using methanolic pyridine-borane and 48 molecular ~ieves.'~ Amines are reductively methylated by the action of ZnCI, NaBH and formaldehyde in dichloromethane.88 Nitro compounds often function as precursors to amines in synthesis; new selective methods for achieving this transformation are desirable.For example anilines are prepared by the reduction of nitroarenes using catalytic FeCl,.H,O and N,N-dimethylhydra~ine.'~ The procedure is mild and compatible with a wide assortment of functional groups. Nitroarenes are also reduced to anilines by the binary combinations NaBH,-SbCl and NaBH,-BiC1,.90 Nitroarenes and nitroalkanes are reduced to anilines and alkylhydroxylamines using Na,S,O and octylviologen as an electron transfer reagent.g1 Aromatic aldehydes are reduced to the corresponding hydrocarbon (ArCHO -f ArMe) with borohydride exchange resin and nickel acetate in methan01.~' The same system reduces aromatic oximes to amine~.~, Aromatic ketones are reductively deoxygenated by the action of NaCNBH,-BF,*OEt, providing a practical alternative to the Clemmensen reduction and related processes.94 The reduction of sulfoxides and active halides is achieved with a mixture of Cp,TiCI or TiCl and samari~m.~~.~~ N.J.Lawrence (50) (DHQD)2DP-PHAL R = Ph (51) (DHQD)2-PHAL R = H ?H (DHQD),-PHAL oso, (01 (53) 63% ee with (50) 35% ee with (51) Scheme 14 Oxidation One of the most successful oxidative synthetic methods of recent years the Sharpless asymmetric dihydroxylation (AD) reaction has been highlighted as part of a review of ligand-accelerated catalysis.97 The method has been refined and exploited extensively this year. Sharpless and co-workers have extended this powerful reaction by making new ligands that possess a diphenylphthalazine spacer group (Scheme 14).98For example the ligand (50)(DHQD),DP-PHAL generally gives greater enantioselectivi- ties than those obtained from the PHAL series (51).cis-Olefins give improved selectivities [e.g. (52)-+ (5311 with (50). The diphenylphthalazine ligands are the most general for the asymmetric dihydroxylation reaction described to date. Among the many olefinic substrates studied this year are polyene~,~~,'~~ cyclopentene deriva- tives,"' polycyclic aromatic hydrocarbons,'02 vinylpho~phonates'~~ and cyclo- propylidene derivative^."^ The AD protocol has been used to make (20s)-cam- ptothecin,"' rhodinose derivatives,lo6 a bicyclic acetal apple aroma c~nstituent,"~ spiroacetals,' O8 bis(hydroxymethy1)piperidine derivative^,'^^ goniobutenolides A and B,' 'O tetrahydroxybutylimidazoles' '' and oxazolidinediones.' l2 Torii et a/.have described a procedure for the asymmetric dihydroxylation of olefins using a Sharpless method that incorporates a catalytic amount of potassium ferricyanide K,Fe(CN) which is recycled by electrooxidation.' ' New protocols for highly enantioselective dihydroxylation reactions using polymer supported cinchona ligands have been described this Warren and co-workers report a Sharpless-like racemic dihydroxylation protocol for the reaction of various alkenes (stilbenes sulfides and phosphine oxides).' '' Dihydroxylation with solid OSCI to provide the catalytic oxidant K,Fe(CN) as stoichiometric oxidant quinuclidine as the accelerating ligand with added K,CO and Synthetic Methods methanesulfonamide in a two-phase system (water and tert-butyl alcohol) gives excellent yields of racemic syn diols.An asymmetric version of the protocol incorporating the DHQD,-PHAL ligand has been used in the dihydroxylation of allylic phosphine oxides. l6 Many new catalysts and oxidants have been developed for the clean and efficient transformation of alcohols to their corresponding carbonyl compounds. For example benzylic and allylic alcohols are oxidized to ketones with Bu'OOH and catalytic [COC~,(PP~,),]."~The oxidation of alcohols is also effected by the catalytic use of palladium chloride and Adogen 464; the stoichiometric oxidant is 1,2-dich-loroethane.' l8 Rajendran and Trivedi have used ruthenium tetroxide and a phase- transfer catalyst in a biphasic system [CCl,-NaCl (as.)] for the oxidation of aromatic primary alcohols and aldehydes.The spent ruthenium oxidant is regenerated at a platinized titanium anode.' '' Secondary alcohols are rapidly oxidized to the corresponding ketones by the action of RuCl (2mol%) and peracetic acid in ethyl acetate.120 1,2-Diols are efficiently oxidized to the corresponding 1,2-diketones with aqueous hydrogen peroxide in the presence of catalytic peroxotungstophosphate. '" Alcohols are also oxidized by the 1 1 complex of N-bromosuccinimide and tet- rabutylammonium iodide,', by K,FeO and K10 Montmorillonite clay.123 18- Crown-6 complexes of N-butylammonium and pyridinium chlorochromates (PCCs) have been prepared and used as mild and selective oxidizing-agents for alcohols.'24 Unlike PCC these complexes oxidize benzylic alcohols more rapidly than primary alkyl alcohols.Dimethyldioxirane (DMDO) has been used to selectively mono-oxidize 1,2- and 1,3-diols to the corresponding hydroxy ketones in high yield.'25 The protocol exploits the inhibiting effect of carbonyl groups on DMDO promoted alcohol oxidation. A cheap efficient method for the oxidation of thiols to disulfides uses sodium chlorite.' 26 Transformation of sulfides to sulfoxides is effected by mercury(I1) oxide and iodine'27 and nitric acid catalysed by FeBr,.'28 Page et a/. report a highly enantioselective protocol for the asymmetric catalytic oxidation of sulfides to sulfoxideswith hydrogen peroxide.12' They used a series of acetals of oxocamphorsul-fonylimines as oxygen atom transfer mediators (Scheme 15). The sulfonylimine (54) generally gives the best selectivity [e.g. (55)+(56)]. The oxidation of electron-deficient sulfides to sulfones is achieved using excess HOF*CH,CN complex.'30 The related transformation of selenides to selenones is effected mildly and efficiently by Oxone (potassium peroxymonosulfate). ' ' The use of dioxiranes for the epoxidation of alkenes has seen much activity this year. Yang et a/. report a useful protocol for the epoxidation of olefins by methyl(trifluoromethy1)dioxirane which is generated in situ in a mixture of MeCN-H,O from trifluoromethylacetone and 0x0ne.l~~ Denmark et a/.have used a variety of ketones as catalysts for the epoxidation of alkenes with Oxone (Scheme 16).The best catalyst under biphasic reaction conditions was found to be the N-dodecyl-4- oxopiperidinium salt (57) in CH,CI2 at pH 7.5-8.0 [cinnamyl alcohol gave (5S)J.'33 Oxone in acetone is also a mild oxidant for the conversion of various substituted aniline derivatives to the corresponding nitroarenel, and the oxidation of the C-B bond of boronic acids and boronic esters.'35 New protocols for the epoxidation of alkenes using salen [H,salen = bis(salicy1idene)eth ylenediamine] complexes have been developed this 234 N. 3. Lawrence (55) (56) 100% >96% de Scheme 15 (57)(10 rnol%) Oxone CHzCIz-H,O Ph-*H pH 7.5,O"C,24 h * Ph&OH (58)83% 0 Scheme 16 year. Jacobsen and co-workers report the low temperature asymmetric epoxidation of unfunctionalized olefins by [Mn"'(salen)J complexes such as (59) [Scheme 17(a)].The combination of rn-chloroperbenzoic acid (MCPBA) and 4-methylmorpholine N-oxide (NMO) proved to be the best primary oxidant system for efficient asymmetric epoxidation of (60)to give (61).136The low temperature reaction allows epoxidation of a variety of alkenes with a significant increase in enantioselectivity relative to reaction using aqueous bleach. Katsuki and co-workers have also reported a low temperature epoxidation reaction [Scheme 17(b)]; using a solution of NaOCl saturated with NaC1 a reaction temperature of -18°C is possible. At this temperature and with the manganese complex (62) the epoxidation of cyclopentadiene is highly enantioselec- tive.' 37 Epoxidation of olefins can also be achieved with formamide-hydrogen peroxide in an aqueous medium,13* a synthetic hydrotalcite clay and hydrogen peroxide' 39 and silica treated with Ti(OPr') in combination with tert-butyl hydr~peroxide.'~' or,/?-Unsaturated ketones are rapidly converted to epoxy ketones by sodium perborate in the presence of a phase transfer reagent.',' The general use of sodium perborate in organic synthesis has been reviewed by Mu~art'~' and McKillop and Sander~0n.l~~ Synthetic Methods 235 (59) (2-8mol%) MCPBA / NMOGH&I, -78 "C (a) (60) (61)89% 96% ee (88% ee with NaOCI) PhHph TIPSO$-:;:$:boTl PS Bur But (59) NaOCl (aq.NaCI) 4-phenylpyridine* QO Koxide (20mol%) 82% 93% ee Scheme 17 Sanchez and Roberts have used poly-L-leucine and poly-D-leucine as catalysts for asymmetric epoxidation of a variety of a$-unsaturated ketones and dienones Ce.9.(63) -+ (64)] as an extension of the work of Julia (Scheme 18).'44 It has been known for some time that a phenyldimethylsilyl group can act as a latent hydroxy group. Taber et al. report a useful modification of the silyl-to-hydroxy conversion that is compatible with alkenes [Scheme 19(a)].145 The phenyl group in (65) is first reduced to the cyclohexa- 1,4-diene (66)with lithium in ammonia the cyclohexadiene group replaced by fluoride by reaction with tetrabutylammonium fluoride then the silyl fluoride is oxidized by hydrogen peroxide to the alcohol (67).This process has been used in the synthesis of a-dictyopterol. 146 Disilanyl groups are also readily converted to a hydroxy group by a simple two-step one-pot operation involving successive treatment with tetrabutylammonium fluoride (TBAF) and alkaline hydroperoxide [(68)-+ (69)] N. J. Lawrence ply-L-leucine Ph& *A Ph H2O2-NaOH4H2CI2 Ph Ph (63) (64)ee > 98% Scheme 18 i TBAF ii. H20 OH $30. (67)66%' OH OH Ph&SiMe2SiMe3 i TBAF THF room temp. ii H202 KHC03 MeOH. 40 "C Ph (68) (69)90% Scheme 19 [Scheme 19(b)].'47 Dirnethyl(5-methylthienyl)~ilyl'~~ and dimethylcl-(phenyl-thio)cyclopropyl)]silyl'4g groups have also been used as masked hydroxy groups. Resek and Meyers report the synthesis of x,P-unsaturated ketones nitriles and lactams from the saturated carbonyl compound using methyl phenyls~lfinate.'~~ The sulfinate is prepared by the treatment of diphenyl disulfide with bromine in the presence of methanol.a-Sulfinyl carbonyl compounds (71) are formed upon treatment 237 Synthetic Methods X= 0or NR Me3Si0 $J3) :::::2 -.;" OJ OJ (74) 86% Scheme 20 of a mixture of the methyl phenylsulfinate and the carbonyl compound (70) with potassium hydride. Thermal elimination of the sulfinate (71) generates the a$-unsaturated carbonyl compound (72) [Scheme 20(a)]. Larock et al. report a simple new effective palladium catalysed oxidation of silyl enol ethers to enones. For example the enol ether (73) is converted to the a$-unsaturated ketone (74) by treatment with Pd(OAc) (10mol%) in the presence of one atmosphere of oxygen in DMSO as the solvent [Scheme 20(b)].Pfaltz and co-workers have found that the copper(1) complex prepared in situ from the chiral bisoxazoline (75) and CuOTf effects the asymmetric allylic oxidation of alkenes. For example the bisoxazoline CuOTf and tert-butyl perbenzoate gives the allylic benzoate (76) from cyclopentene with good enantioselectivity (Scheme 2l).ls2 Chen and co-workers describe the potentially useful enzyme-mediated oxidation of substituted toluene derivatives to the corresponding aldehydes (77) -+(78).lS3 The enzyme used laccase (applications of which have recently been reviewedlS4) requires an artificial cofactor diammonium 2,2'-azinobis(3-ethylbenzothiazoline-6-sulfonate) [ABTS(NH,),] (79) [Scheme 22(a)].Titanium silicates (TS-1 and TS-2) efficiently catalyze the oxidation of cyclic ethers into the corresponding lactone (e.g. THF -+ 7-butyrolactone) in combination with H,O,. Ishii et a/.report the aerobic oxidation of a variety of organic substrates using N-hydroxyphthalimide as a novel catalyst [e.g. (80) +(81)] [Scheme 22(b)].155 This process is potentially very useful since no metal is involved in the oxidation reactions. Other advances in aerobic oxidation have been detailed by Mukaiyama and Yamada.lS6 Finally Hamelin and co-workers report a protocol for the rapid promotion of Beckmann reactions. Microwave-promoted reaction of cyclic ketones with hy- droxylamine-0-sulfonic acid over silica gives the corresponding lactams quickly and in high yield.15' Other microwave-assisted reactions have been extensively reviewed this year. 158,159 Two reviews of other specialized but important areas of synthetic chemistry serve as good introductions to synthesis in supercritical fluids,160 and the use of ultrasound.161 N. J. Lawrence (75) (6 mol%) CuOTf (5 mol%) PhCO,Bu' -20 "C 22 d MeCN * Q=ocoph (76)61Yo,84% ee But BU' (75) Scheme 21 "Y" laccase-ABTS( NH&02 87-1 00% (77) (78) Et SOSNH~ 02 PhCN 100 'C (b) (80) (81) 83% Scheme 22 Synthetic Methods 5 Protection and Functional Group Interconversion The use of protecting groups is essential in most organic syntheses.New developments in the use of protecting groups have recently been reviewed by Jarowicki and Kocienski.'62 New protecting groups and new methods for the manipulation of old ones described this year are outlined below. Alcohols and thiols Many new methods involving silicon based protecting groups have been detailed. Diphenyl-tert-butoxysilyl derivatives of alcohols are cleaved by sodium sulfide in ethanol [tert-butyldimethylsilyl (TBDMS) and tert-butyldiphenylsilyl (TBDPS)] groups remain ~naffected).'~~ Deprotection of diphenylmethylsilyl ethers of allylic or benzylic alcohols is achieved by UV irradiation in the presence of ~henanthrene.'~~ Lee et a!. report the chemoselective deprotection of tert-butyldimethylsilyl benzyl ethers in the presence of silyl ethers of secondary and tertiary alcohol^,'^^ by subjecting a 0.25 mol dm- solution of the silyl ether in CH,OH-CCl (1 1) to ultrasound.The use of various benzyl and ally1 protecting groups has seen much activity including the report of Srikrishna et ul. that sodium cyanoborohydride and boron trifluoride-diethyl ether provides a new convenient reductive procedure for the conversion of 4-methoxybenzyl (MPM) ethers to alcohols.'66 The combined use of tert-butyldimethylsilyl trifluoromethanesulfonate and triethylamine cleaves p-methoxybenzyl ethers to give the corresponding tert-butyldimethylsilyl ether direc- tl~.'~~ Benzyl ethers of o-substituted phenols are deprotected using hydrobromic acid in presence of a phase-transfer catalyst.16* Ammonia pyridine and ammonium acetate were found to be extremely effective as inhibitors of Pd/C catalysed benzyl ether hydrogenolysis while olefin benzyloxycarbonyl benzyl ester and azide groups are smoothly hydr~genated.'~' The use of iodine in methanol is described as a simple non-vigorous selective method for the cleavage of p-methoxybenzyl ethers in the presence of benzyl ethers.' 70 A 4,4'-dimethoxytrityl derivative of the levulinyl group has been developed for protection of nucleophilic functionalities such as hydroxy groups by reaction of its symmetrical anhydride.I7' It is rapidly removed under mild conditions using a hydrazine-pyridinium acetate buffer at near neutral pH.This protectinggroupcan bedetected withhighsensitivityat 513nm(~ = 78 600dm3mol-' cm-').Ally1 ethers are cleaved electrochemically using a nickel(I1) bipyridine com- plex.' New chemistry of acetal protecting groups include that from Srikrishna et ul. who report the simple reductive deprotection of tetrahydropyran-2-yl (THP) ethers using a combination of NaCNBH and BF,.OEt in THF.'73 x o-Diols are selectively mono-protected as a THP ether using a strongly acidic ion-exchange resin in a mixture of 3,4-dihydro-2H-pyran and toluene.' 74 Other catalysts used to promote the tetrahydropyranylation of alcohols include the Envirocat EPZG resin'75 and dicyanoethylene acetal. 76 THP ethers are readily hydrolysed to their corresponding alcohols in wet acetonitrile in the presence of a catalytic 2,3-dichloro-5,6-dicyano- 1,4- benzoquinone (DDQ).'" Related THF ethers are made by treating an alcohol with toluene-p-sulfonyl chloride and sodium hydride in THF.'78 Ley's new bisdihyd- 240 N. J. Lawrence ropyrans which can be used to protect alcohols as dispiroketals have been reviewed this year.'79*'80 The year has seen the introduction of some interesting new methods for the synthesis of esters. For example alcohols (as part of a polyol array) can be rapidly converted to esters using an acid chloride in the presence of sub-stoichiometric catalytic dibutyltin oxide under microwave heating.18' Yamamoto et al. report the use of scandium triflate as a remarkable acylation catalyst.'82 Hindered alcohols including tertiary ones are conveniently acetylated using acetic anhydride in acetonitrile at room temperature in the presence of scandium triflate (0.1mol%) (82 -+ 83) [Scheme 23(a)].The process is reported to be superior to the commonly used basic catalysts 4-dimethylaminopyridine (DMAP) and tributylphosphine. Scandium triflate was also used by the same group to effect the esterification of alchohols with carboxylic acids in the presence of p-nitrobenzoic anhydride. A similar protocol for the direct combination of carboxylic acids and alcohols using octamethylcyclotetrasiloxane and Ti"Cl(OTf) (cat. 10mol%) has been described by Mukaiyama and co-~orkers.'~~ Barrett et a!. report a useful Mitsunobu-like procedure for the acylation of alcohols. '84 They found that secondary alcohols are converted into benzoate esters with inversion of configuration via sequential reaction with (chloromethylene)dimethylammoniumchloride and po- tassium benzoate.An efficient procedure for the acylation and perfluoroacylation of activated aromatic substrates under mild conditions uses (RCO),O-Me,S-BF,.' 85 N-Formylbenzotriazole prepared by the reaction of benzotriazole and formic acid in the presence of dicyclohexylcarbodiimide is a superior N-and 0-formylating agent. '86 Alcohols are regenerated from their corresponding sulfonyl (mesyl) ester by simple treatment with methylmagnesium bromide in THF.18' Phenols are often protected as ethers Lee et al. have described the use of alkyl halide-caesium carbonate in acetonitrile for the efficient alkylation of phenols.' 88 Dodge et a[.have observed that methyl ethers of phenols para to electron withdrawing groups are selectively removed by alkaline thiolate in the presence of other methyl phenol ethers.189 Lemaire and co-workers report an alternative catalytic method for the synthesis of ethers (84) -+ (86) [Scheme 23(b)],190 made reductively from alcohols and ketones by the hydrogenolysis of the intermediate hemiketal(85) using Pd/C as the catalyst. Yamashita and co-workers have developed the optically active diazacyclophos- phamide (87) as a reagent for the determination of absolute configuration of optically active alcohols and amines by 'H and 31P NMR.lgl Finally thiols may be protected as the corresponding methoxymethyl (MOM) derivative by treatment with bromochloromethane and methanol under phase-transfer catalysis with benzyltriethylammonium ch10ride.l~~ Ketones and Aldehydes By far the most common way of protecting a ketone or aldehyde is formation of an acetal or ketal.Acetals can be made using microwave irradiation and ethylene glycol in the presence of toluene-p-sulfonic acid ferric chloride or acidic alumina. '93 Tartaric acid has been used as the acid catalyst for the efficient acetalization of acid-sensitive a$-unsaturated aldehydes.194Deprotection of dimethyl acetals of r-halo aldehydes is achieved with a mixture of acetic anhydride-acetyl chloride-sodium acetate trihydrate in refluxing chloroform. 19' The dithioacetalization of ketones and aldehydes is Synthetic Methods 241 I I Ac20 (1.5 equiv.) Sc(OTf) * CH,CN room temp.1 h 'OAc (4 9. 2- (82) (83) O=P-N Ph C6H13 CI (87) (86)92% Scheme 23 catalysed by BiX (X = C1 Br I) or Bi,(S0,),.'96 The cleavage of acetals is catalysed by a variety of reagents including copper sulfate supported on silica gel,lg7 [MoO,(a~ac),]'~~ and NO in CCl and silica Dilute methanolic HC1 in anhydrous THF promotes the cleavage of acetals and ketals without affecting tert-butyldimethylsilyl ethers.," A variety of other derivatives of ketones and aldehydes are converted to the parent carbonyl compound by new protocols. For example enol ethers are converted to aldehydes by Bu,NF-BF,.OEt catalysed hydrolysis.20' Ketones are oxidatively regenerated under neutral conditions from tosylhydrazones by treatment with tetrabutylammonium peroxydisulfate,202 and from oximes by treatment with activated manganese dioxide.*' Although not strictly a functional group interconversion the deracemization of chiral ketones is nevertheless an important process.Fuji et al. have used a series of binaphthalene derivatives as novel chiral proton sources for the enantioselective protonation of en~lates.~' The carbamate (90)is a particularly good proton source for the protonation of magnesium enolates (88) -,(89) [Scheme 24(a)]. The Kemp's acid-derived imide (93) also acts as a chiral proton source for the asymmetric protonation of enolates (91) -+ (92) [Scheme 24(b)].,05 In this case the imide is used catalytically. The imide is regenerated by the slow addition of the bulky phenol (94) to the reaction mixture.Carboxylic Acids and Derivatives The dicyclopropylmethyl (DCPM)group has been used to protect carboxylic acids and N. J. Lawrence (89) 72-85% 90% de (90) Ph OSiMe3 ii (93) (0.1 ~uIv.) ' -78 "C --u-'' iii (94) (1 equiv.) -78 OC 2 h (911 (92) 72-85% 90% ee ' (93) OH Scheme 24 carboxamides. DCPM esters are hydrolysed exploiting the exceptional stability of the cyclopropylmethyl cation with trifluoroacetic acid (1 YOin CH2C1,).206 Sibi et a!. describe a convenient synthesis of N-methoxy-N-methylamides from carboxylic acids using inexpensive 2-chloro- l-methylpyridinium iodide as the coupling agent.207 Nitriles are selectively converted into amides on unactivated alumina at 60 0C.208 Chloro iminium salts derived in situ from amides react with the new sulfur transfer reagent benzyl triet h ylammonium tetra thiomol ybda te to give the corresponding thioamide in very high yield.209 The overall process is a good method for converting an amide to a thioamide.Kende and Liu report that trifluoroacetyl groups attached to a carbon atom devoid of hydrogen undergo facile high-yield conversion to nitriles by reaction with MeAlClNH followed by KOBu' (95) +(96)+(97) [Scheme 25(a)]."' Seebach et al. have developed a very efficient procedure for the enantioselective opening of meso anhydrides of cyclic dicarboxylic acids [Scheme 25(b)]. The product hemiesters provide very useful building blocks for further elaboration. For example Synthetic Methods 243 Ph PcF3 phkcF3 MeAICINH2 BubK ph&c'N (a) I I I Me Me (95) (96) (97) Me 91% (100) Ar = p-CIOHI Scheme 25 Scheme 26 the anhydride (98)is converted to the isopropyl ester (99)upon reaction with the chiral Lewis acid r-Diazo esters (103),valuable synthetic intermediates can be prepared simply from an ester (101) by benzoylation to give an a-benzoyl ester (102) followed by diazo transfer (Scheme 26).2'2 A similar process transforms 2-benzoyl ketones via the same debenzoylation-diazo transfer strategy.213 Amines and phosphines The tetrachlorophthaloyl group has been used by Fraser-Reid and co-workers to protect amines.It is removed more easily than a phthaloyl group by simply heating with excess ethylenediamine in ethanol at 50 0C.214Davis and Gallagher introduce the tetraethyldisilaisoindoline (TEDI) group (104) (Figure 3) as a chromatographically (silica) stable group for the protection of primary amine~.~" Low-valent titanium can be used to facilitate the cleavage of N-allyl- and N-benzyl-amines.216 N,N-Di- allylamines are selectively deprotected using a Pdo catalyst and 2-thiobenzoic acid as the n~cleophile.~~~ The first ally1 group is cleaved at room temperature whilst the N.J.Lawrence ,Si Et \Et Fig. 3 second is cleaved at 60 "C. The pyridine-2-sulfonyl group is used to protect amines. The N-S bond is cleaved under mild conditions (SmI, room temp. THF) unlike that of the corresponding N-phenylsulfonylamines which requires much harsher conditions.21 * Gage and Wagner have found that treatment of a variety of aromatic amines with isobutylene in 1,4-dioxane in a pressure tube (90-140 "C) in the presence of HBr (as.48%) conveniently gives the N-tert-butyl aromatic amine.219 Trichloroethoxycar- bony1 (TROC) protected amines are efficiently reductively cleaved under neutral conditions with a cadmium-lead couple.220 The diethoxymethyl group is a useful nitrogen protecting group for lactams and amides. Treatment of lactams and amides with triethyl orthoformate leads to the N-diethoxymethyl substituted derivatives.221 The diethoxymethyl group is easily removed by subsequent treatment with trif- luoroacetic acid and NaOH. Amines are made from alcohols by a new protocol developed by Knight and co-workers where N-benzyltriflamide is used as a novel Mitsunobu nucleophile.222 Various primary and secondary alkyl azides potential precursors to amines have been synthesized in high yields by the fluoride anion induced S,2 substitution reactions of the corresponding alkyl halides phosphates or toluene-p-sulfonates (tosylates) and trimethylsilyl a~ide.~~~ Phosphines can be protected by formation of a tungsten pentacarbonyl complex [RPh,P.W(CO),] which is stable to alkylati~n.~~~ Aliphatic isocyanates react success- ively with mercury acetate and sodium borohydride to give aliphatic primary amine~.~~~ The selective mono-deprotection of phosphate phosphite phosphonate and phosphoramide benzyl esters is effected by stoichiometric amounts of 1,4-diazabicyclo[2.2.2]octane (DABCO) or quinuclidine in refluxing toluene.226 Ahman and Somfai report a simple procedure for the efficient preparation of the widely used base potassium bis(trimethylsily1)amide (KHMDS).227When a mixture of bis(trimethylsily1)amine and potassium in THF is sonicated for several hours a good yield of KHMDS is reliably obtained.KHMDS reacts with alkyl bromides iodides tosylates benzylic chlorides and allylic chlorides to give the corresponding N,N-bis(trimethylsily1) amines in high yields. Subsequent deprotection of the trimethylsilyl groups is performed under mildly acidic conditions to afford primary amines.228 A related procedure generates arylamines by the palladium catalysed coupling of KHMDS and an aryl halide.229 6 Organo Halides The year has seen many new methods for the synthesis of halogen-containing organic compounds many of which are included in a recent review.230 Synthetic Methods 245 Fluoro Compounds General organofluorine ~hemistry~~',~~~ and the asymmetric synthesis of fluoro- organic compounds233 has been reviewed this year as has the use of xenon difluor- ide234 and N-fluoropyridinium salts235 in synthesis.Perfluorohexane has been used as a replacement of carbon tetrachloride due to be phased-out as part of the Montreal Protocol as the solvent for bromination reactions. Perfluorohexane offers the advantages of being non-toxic non-ozone-depleting and readily a~ailable.~ 36 Fluorine is generally perceived to be too reactive to exhibit useful selectivity.However Chambers' group has demonstrated that elemental fluorine has great potential as a selective reagent for organic synthesis. For example elemental fluorine has been used as an enabler for the generation of powerful electrophiles; a combination offluorine diluted in nitrogen and iodine creates a system that will iodinate unactivated aromatic compounds in sulfuric acid.2 1,3-Dicarbonyl compounds react directly with elemental fluorine at room temperature to give the corresponding 2-fluoro derivatives.238 The electrophilic fluorination of aromatic compounds is achieved by the use of elemental fluorine in formic acid.239 In addition diaryl- 1,3-dithiolanes are converted into gem-difluoromethylene compounds by a combination of elemental fluorine and iodine.240 Aryltrifluoromethyl ketones are prepared conveniently by the palladium(0) catalyzed cross-coupling of aryltrialkyltin reagents-derived from aryl halides-with trifluoroacetic anhydride.241 Several uses of electrophilic fluorinating reagents have been described this year.Davis et al. report the synthesis of N-fluoro-o-benzenedisulfonimide (1054 and describe its use as a selective electrophilic fluorinating agent.242 It has been used to fluorinate enolates silyl enol ethers (106)-+(107) P-dicarbonyl compounds and aromatic compounds. Umemoto and Tomizawa have developed a series of N-fluoropyridinium- 2-sulfonates including (108)and illustrated their potential as highly selective fluorina- tion agents by their reaction with a variety of nucleophilic substrates such as activated aromatics enol trialkylsilyl and alkyl ethers [e.g.(109) -+ (1lo)] [Scheme 27(a)] activated olefins and sulfides.243 Poss and Shia have described the ./-fluorination of a$-unsaturated ketones [similar to the transformation (109) + (1 lo)] using N-fluorobenzenesulfonimide (1 11) [Scheme 27(b)].244 In their protocol the r,B-un- saturated ketone is first converted to a conjugated boron enolate. Many reports have detailed uses of the new electrophilic fluorinating agent Selectfluor F-TEDA-BF,245 (1 12) including ones describing the fluoro-decarboxylation of x-pyrrolecarb~xylic~~~ and x-furoic the fluorination of P-dicarbonyl compounds 248 nucleo~ides~~~ and substituted pyrimidine^.^^' The reagent has also been used to effect the conversion of 1-phenyl-substituted acetylenes to r,r-difluoro ketones.251 The related compound Accufluor NFTh (1 13) is reported to be highly effective for the fluoro-methoxylation and fluoro-acetylation of phenyl substituted alkene~.~~~ The novel fluoride source tetrabutylammonium (triphenylsily1)difluorosilicate (TBAT) is an excellent reagent for the nucleophilic substitution of halides mesylates and triflate~.~~~ Chloro Compounds The combination of trimethylsilyl chloride (TMSCl) and DMSO provides a new reagent system for the quick convenient and inexpensive conversion of alcohols to N.J. Lawrence TMSO 0 Me (105) CH&I, room temp. 2.5 h (106) (107) 86% room temp. 90 h * TIPSO 0 F (109) (110) 93% 3.8:l) I F (112)X=CH2CI (113)X=OH Scheme 27 Synthetic Methods chlorides.254 The efficient reverse conversion of an alkyl halide to an alcohol is achieved using triethylammonium formate to give first an alkylformate followed by acid or base ~atalysed-hydrolysis.~~~ Bromo Compounds Carrefio et a/.have shown that the use of N-bromosuccinimide (NBS) in acetonitrile provides an efficient method for the bromination of methoxybenzenes and naphtha- lenes with absence of side-chain reactions.256 x-Bromination of unsaturated ketones and dibromination of alkenes is effected by the polymeric bromine source poly(4- methyl-5-vinylthiazolium) hydrotribromide (114).257 Iodo Compounds Alcohols are converted directly to alkyl iodides (with inversion of stereochemistry) by reaction with iodine in refluxing light petroleum (60-80 0C).258 The method is mild and efficient and most importantly does not require an expensive iodine precursor.Iodination of phenols is achieved efficiently by the use of bis(syrn-collidine)iodine(I) hexafluor~phosphate.~~~ The same reagent also serves as an excellent source of ‘I” in the synthesis of oxepanes260 from hept-6-en-1-01s and the iodoacetylenes261 from terminal acetylenes. Orito et a/. describe the use of mercury(I1) oxide-iodine for the efficient electrophilic aromatic iodination of alkyl aryl ethers.262 Vicinal iodochlorides were synthesized in excellent yields by the electrophilic iodination of double bonds with potassium dichloroiodate(~).~~~ References 1 N.K. Terrett M. Gardner D. W. Gordon R. J. Kobylecki and J. Steele Tetrahedron 1995 51 81 35. 2 G. Lowe Chem. SOC. Rev. 1995 24 309. 3 J. D. White G. L. Bolton A. P. Dantanarayana C. M.J. Fox R. N. Hiner R. W. Jackson K. Sakuma and U. S. Warrier J. Am. Chem. SOC.,1995 117 1908. 4 J. J. Masters J. T. Link L. B. Snyder W. B. Young and S. J. Danishefsky Angew. Chem. In?. Ed. Engl. 1995 34 1723. 5 J.D. White T.-S. Kim and M. Nambu J. Am. Chem. SOC. 1995 117,5612. 6 A.B. Smith 111 S. M. Condon J.A. McCauley J. L. Leazer Jr. J. W. Leahy and R. E. Maleczka Jr. J. Am. Chem. Soc. 1995 117 5407. 7 B. M. Trost and S. R. Pulley,J. Am. Chem.SOC.,1995,117,lO 143; X. Tian,T. Hudlicky and K. Konigsberger J. Am. Chem. SOC.,1995 117 3643.8 K.C. Nicolaou and R. K. Guy Angew. Chem. In?. Ed. Engl. 1995,34 2079. 9 (a) R. D. Clark and A. Jahangir Org. React. (N.Y.),1995 47 1; (b) R. D. Little M. R. Masjedizadeh 0. Wallquist and J.I. McLoughlin Org. React. (N.Y.) 1995 47 315. 10 Encyclopaedia of Reagents for Organic Synthesis ed. L. A. Paquette Wiley Chichester 1995. N. J. Lawrence 11 P. Knochel Synlett 1995 393. 12 T. Wurth Tetrahedron Lett. 1995 36 7849. 13 X. M. Zhang and C. Guo Tetrahedron Lett. 1995 36 4947. 14 S. Vettel A. Vaupel and P. Knochel Tetrahedron Lett. 1995 36 1023. 15 I. Klement H. Lutjens and P. Knochel Tetrahedron Lett. 1995 36 3161. 16 I. Klement and P. Knochel Synlett 1995 1113. 17 F. Chemla and J. Normant Tetrahedron Lett. 1995,36 3157. 18 M.V. Hanson and R.D.Rieke J. Am. Chem. Soc. 1995 117 10775. 19 A.S. Thompson E. G. Corley M. F. Huntington and E. J. J. Grabowski Tetrahedron Lett. 1995,36,8937. 20 S. Klein I. Marek J.-F. Poisson and J. F. Normant J. Am. Chem. Soc. 1995 117 8853. 21 S. Kobayashi and N. Nishio Tetrahedron Lett. 1995 36 6729. 22 A. B. Charette A. F. Benslimane and C. Mellon Tetrahedron Lett. 1995 36 8557. 23 J.-Y. Zhou X.-B. Yao Z.-G. Chen and S.-H. Wu Synth. Commun. 1995 25 3081. 24 B.C. Ranu A. Majee and A. R. Das Tetrahedron Lett. 1995 36,4885. 25 C. Sarangi A. Nayak B. Nanda N. B. Das and R. P. Sharma Tetrahedron Lett. 1995 36 7119. 26 M. B. Isaac and T.-H. Chan J. Chem. Soc. Chem. Commun. 1995 1003. 27 M. B. Isaac and T.-H. Chan Tetrahedron Lett. 1995 36 8957. 28 C.-J. Li Tetrahedron Lett.1995 36 517. 29 P. Cintas Synlett 1995 1087. 30 M. J. Aurell Y. Danhui J. Einhorn C. Einhorn and J. L. Luche Synlett 1995,459. 31 R. W. Marshman Aldrichim. Acta 1995 28 77. 32 I. Hachiya M. Moriwaki and S. Kobayashi Tetrahedron Lett. 1995 36 409. 33 S. Kobayashi H. Ishitani and S. Nagayama Synthesis 1995 1195. 34 S. Kobayashi M. Moriwaki and I. Hachiya Synlett 1995 1153. 35 C. Bellucci P. G. Cozzi and A. Umani-Ronchi Tetrahedron Lett. 1995 36 7289. 36 S. Kobayashi and H. Ishitani J. Chem. Soc. Chem. Commun. 1995 1379. 37 M. P. Sibi P. K. Deshpande and J. Ji Tetrahedron Lett. 1995 36 8965. 38 M. P. Sibi P. K. Deshpande A. J. La Loggia and J. W. Christensen Tetrahedron Lett. 1995 36 8961. 39 N. Lewis A. McKillop R.J.K. Taylor and R.J. Watson Synth.Commun. 1995 25 561. 40 K. Ruck Angew. Chem. Int. Ed. Engl. 1995 34 433. 41 A.G. Myers J. L. Gleason and T. Yoon J. Am. Chem. Soc. 1995 117 8488. 42 A.G. Myers J. L. Gleason and T. Yoon Tetrahedron Lett. 1995 36 4555. 43 A.G. Myers and T. Yoon Tetrahedron Lett. 1995,36 9429. 44 N. Kise K. Tokioka Y. Aoyama and Y. Matsumura J. Organomet. Chem. 1995,60 1100. 45 T. Langer M. Illich and G. Helmchen Tetrahedron Lett. 1995 36 4409. 46 A. Abiko 0.Moriya S.A. Filla and S. Masamune Anyew. Chem. lnt. Ed. EngI. 1995 34 793. 47 M. Wang S.Z. Liu J. Liu and B. F. Hu J. Org. Chem. 1995,60,7364. 48 D. Fabbri G. Delogu and 0.De Lucchi J. Org. Chem. 1995,60 6599. 49 Z. Pakulski and A. Zamojski Tetrahedron A.,ymmetry 1995 6 11 1. 50 A.B. Charette and C. Brochu J.Am. Chem. SOC. 1995 117 11 367. 51 A. B. Charette and J. F. Marcoux Synlett 1995 1197. 52 G. Majetich and K. Wheless Tetrahedron 1995 51 7095. 53 D.L. J. Clive and W. Yang J. Org. Chem. 1995,60 2609. 54 K. Ando Tetrahedron Lett. 1995 36 4105. 55 M. Bellassoued and N. Ozanne J. Org. Chem. 1995,60 6582. 56 M. Wills and J. R. Studley Chem. Znd. 1994 552. 57 J. C. Caille M. Bulliard and B. Laboue Chim. Oggi 1995 13 17. 58 Y.-W. Zhang Z.-X. Shen C.-L. Liu and W.-Y. Chen Synrh. Commun. 1995 25 3407. 59 J.G.H. Willems F.J. Dommerholt J.B. Hammink A.M. Vaarhorst L. Thijs and B. Zwaneburg Tetrahedron Lett. 1995 36 603. 60 A. Sudo M. Matsumoto Y. Hashimoto and K. Saigo Tetrahedron Asymmetry 1995,6 1853. 61 J. S. Cha E. J. Kim 0.0.Kwon and J. M. Kim Synlett 1995 331.62 J.S. Cha 0.0.Kwon S. Y. Kwon J.M. Kim W. W. Seo and S.W. Chang Synlett 1995 1055. 63 G. Giffels C. Driesbach U. Kragl M. Weigerding H. Waldmann and C. Wandrey Angew. Chem. Int. Ed. Engl. 1995 34 2005. 64 M.C. Barden and J. Schwartz J. Org. Chem. 1995 60 5963. 65 T. Nagata K. Yorozu T. Yamada and T. Mukaiyama Angew. Chem. Znt. Ed. Engl. 1995,34 2145. 66 S. Narasimhan S. Madhavan and K.G. Prasad J. Org. Chem. 1995,60 5314. 67 S. Narasimhan K.G. Prasad and S. Madhavan Synrh. Commun. 1995 25 1689. 68 P.D. Ren S. F. Pan T. W. Dong and S.H. Wu Sph. Commun. 1995 25 3395. 69 T. B. Sim and N. M. Yoon Synlett 1995 726. 70 R.S. Dhillon R. P. Singh and D. Kaur Tetrahedron Lett. 1995 36 1107. 71 J.S. Cha O.D. Kwon and S.Y. Kwon Bull. Korean Chem.Soc. 1995 16 1009. 72 K.G. Akamanchi and V. R. Noorani Tetrahedron Lett. 1995 36 3571. Synthetic Methods 73 K. G. Akamanchi and V. R. Noorani Tetrahedron Lett. 1995,36 5085. 74 S. Iyer and J. P. Varghese J. Chem. SOC.,Chem. Commun. 1995 465. 75 S. Hashiguchi A. Fujii J. Takehara T. Ikariya and R. Noyori J. Am. Chem. SOC.,1995 117 7562. 76 T. Ohkuma H. Oaka S. Hashiguchi T. Ikariya and R. Noyori J. Am. Chem. SOC. 1995 117 2675. 77 Z. Mao B.T. Gregg and A. R. Cutler J. Am. Chem. Soc. 1995 117 10 139. 78 M. Murakata H. Tsutsui and 0.Hoshino J. Chem. SOC.,Chem. Commun. 1995,481. 79 S. G. Alvarez G. B. Fisher and B. Singaram Tetrahedron Lett. 1995 36 2567. 80 A.M. Salunkhe and H.C. Brown Tetrahedron Lett. 199536 7987. 81 C.A. M. Afonso Tetrahedron Lett.1995 36 8857. 82 C. Goulaouic-Dubois and M. Hesse Tetrahedron Lett. 1995 36 7427. 83 L. Benati P.C. Montevecchi D. Nanni P. Spagnolo and M. Volta Tetrahedron Lett. 199536 7313. 84 G. E. Keck S. F. McHardy and T.T. Wager Tetrahedron Lett. 1995 36 7419. 85 S. Bhattacharyya J. Org. Chem. 1995 60,4928. 86 S. Bhattacharyya A. Chatterjee and J. S. Williamson Synlett 1995 1079. 87 M.D. Bomann 1.C. Guch and M. DiMare J. Org. Chem. 1995,60 5995. 88 S. Bhattacharyya Synth. Commun. 1995 25 2061. 89 S. R. Boothroyd and M. A. Kerr Tetrahedron Lett. 1995 36 241 1. 90 P.-D. Ren S.-F. Pan T.-W. Dong and S.-H. Wu Synth. Commun. 1995 25 3799. 91 K. K. Park C. H. Oh and W.-J. Sim J. Org. Chem. 1995 60 6202. 92 B. P. Bandgar S.N. Kshirsagar and P.P. Wadgaonkar Synth.Commun. 1995 25 941. 93 B. P. Bandgar S.M. Nikat and P. P. Wadgaonkar Synth. Commun. 1995,25,863. 94 A. Srikrishna J. A. Sattigeri R. Viswajanani and C. V. Yelamaggad Synlett 1995 93. 95 Y. M. Zhang Y. P. Yu and W. L. Bao Synth. Commun. 1995,25 1825. 96 J.Q. Wang and Y.M. Zhang Synth. Commun. 1995,25 3545. 97 D. J. Berrisford C. Bolm and K. B. Sharpless Angew. Chem. Int. Ed. Engl. 1995 34 1059. 98 H. Becker S. B. King M. Taniguchi K. P. M. Vanhesscheand K. B. Sharpless,J. Org.Chem. 1995,60,3940. 99 H. Becker M.A. Soler and K. B. Sharpless Tetrahedron 1995,51 1345. 100 U. Hugger and K. B. Sharpless Tetrahedron Lett. 1995,36 6603. 101 C. W. Jefford and G. Timara J. Chem. Soc. Chem. Commun. 1995 1501. 102 X.-Q. Tang and R.G. Harvey Tetrahedron Lett.1995,36,6037. 103 T. Yokoatsu Y. Yoshida K. Suemune,T. Yamagishi and S.Shibuya Tetrahedron Asymmetry 1995,6,365. 104 H. Nemoto J. Miyata H. Hakamata and K. Fukumoto Tetrahedron Lett. 1995 36 1055. 105 S. S. Jew K. D. Ok H. J. Kim M. G. Kim J. M. Kim J. M. Hah and Y. S. Cho Tetrahedron Asymmetry 1995 6 1245. 106 A. Sobti and G.A. Sulikowski Tetrahedron Lett. 1995,36 4193. 107 M. A. Brimble D. D. Rowan and J. A. Spicer Synthesis 1995 1263. 108 S. Sauret A. Cuer J.G. Gourcy and G. Jeminet Tetrahedron Asymmetry 1995 6 1995. 109 H. Takahata S. Kouno and T. Momose Tetrahedron Asymmetry 1995,6 1085. 110 S.Y. KO and J. Lerpiniere Tetrahedron Lett. 1995 36 2101. 11 1 M. D. Cliff and S.G. Pyne Tetrahedron Lett. 1995,36 5969. 112 C.E. Song E. J. Roh S.G.Lee and 1.0. Kim Tetrahedron Asymmetry 1995 6 871. 113 S. Torii P. Liu and H. Tanaka Chem. Lett. 1995 319. 114 A. Petri D. Pini and P. Salvadori Tetrahedron Lett. 1995 36 1549. 1 15 J. Eames H. J. Mitchell A. Nelson P. O’Brien S. Warren and P. Wyatt Tetrahedron Lett. 1995,36 1719. 116 A. Nelson P. O’Brien and S. Warren Tetrahedron Lett. 1995 36 2685. 117 S. Iyer and J. P. Vargehese Synth. Commun. 1995 25 2261. 118 S. Ait-Mohand F. Henin and J. Muzart Tetrahedron Lett. 1995 36 2473. 119 S. Rajendran and D. C. Trivedi Synthesis 1995 153. 120 %-I. Murahashi T. Naota Y. Oda and N. Hirai Synlett 1995 733. 121 T. Iwahama S. Sakaguchi Y. Nishiyama and Y. Ishii Tetrahedron Lett. 1995 36 1523. 122 T. R. Beebe L. Boyd S. B. Fonkeng J. Horn T. M. Mooney M. J. Sadereholm and M.Skidmore J. Org. Chem. 1995 60 6602. 123 L. Delaude P. Laszlo and P. Lehance Tetrahedron Lett. 1995 36 8505. 124 H.S. Kasmai S.G. Mischke and T.J. Blake J. Org. Chem. 1995 60,2267. 125 P. Bovicelli P. Lupattelli A. Sanetti and E. Mincione Tetrahedron Lett. 1995 36 3031. 126 K. Ramadas and N. Srinivasan Synth. Commun. 1995 25 227. 127 K. Orito T. Hatakeyama M. Takeo and H. Suginome Synthesis 1995 1357. 128 A. R. Suarez L. I. Rossi and S. E. Martin Tetrahedron Lett. 1995 36 1201. 129 P.C. B. Page J. P. Heer D. Bethell E. W. Collington and D. M. Andrews Synlett 1995 773. 130 R. Beckerbauer B. E. Smart Y. Bareket and S. Rozen J. Org. Chem. 1995 60 6186. 131 P. Ceccherelli M. Curini F. Epifano M.C. Marcotullio and 0. Rosati J. Org. Chem. 1995 60 8412.132 D. Yang M. K. Young and Y. C. Yip J. Org. Chem. 1995 60 3887. 133 S. E. Denmark D. C. Forbes D. S. Hays J. S. Depue and R. G. S. Wilde J. Org. Chem. 1995 60 1391. 134 K.S. Webb and V. Seneviratne Tetrahedron Lett. 1995 36 2377. 250 N. J. Lawrence 135 K.S. Webb and D. Levy Tetrahedron Lett. 1995,36 5177. 136 M. Palucki G. J. McCormick and E.N. Jacobsen Tetrahedron Lett. 1995,36 5457. 137 D. Mikame T. Hamada R. Irie and T. Katsuki Synlett 1995 827. 138 Y. W. Chen and J.L. Reymond Tetrahedron Lett. 1995,36,4015. 139 C. Cativiela F. Figueras J. M. Fraile J. I. Garcia and J. A. Mayoral Tetrahedron Lett. 1995,36 4125. 140 J. M. Fraile J. I. Garcia J. A. Mayoral L. C. de Menoral and F. Rachdi J. Chem. Soc. Chem. Commun. 1995 19. 141 T.S. Straub Tetrahedron Lett. 1995 36 663. 142 J. Muzart Synthesis 1995 1325. 143 A. McKillop and W. R. Sanderson Tetrahedron 1995 51 10403. 144 M.E.L. Sanchez and S.M. Roberts J. Chem. Soc. Perkin Trans. 1 1995 1467. 145 D. F. Taber L. Yet and R.S. Bhamidipati Tetrahedron Lett. 1995,36 351. 146 D.F. Taber R.S. Bhamidipati and L. Yet J. Org. Chem. 1995 60,5537. 147 M. Suginome S.-I. Matsunaga and Y. Ito Synlett 1995 941. 148 Y. Landais D. Planchenault and V. Weber Tetrahedron Lett. 1995,36,2987. 149 R. Angelaud and Y. Landais Tetrahedron Lett. 1995 36 3861. 150 J. E. Resek and A. I. Meyers Tetrahedron Lett. 1995 36 7051. 151 R.C. Larock T. R. Hightower G.A. Kraus P. Hahn and D. Zheng Tetrahedron Lett. 1995,36 2423. 152 A. S. Gokhale A. B. E. Minidis and A.Pfaltz Tetrahedron Lett. 1995 36 1831. 153 A. Potthast T. Rosneau C.-L. Chen and J.S. Gratzl J. Org. Chem. 1995,60,4320. 154 A. I. Yaropolov 0.V. Skorobogatko S.S. Vartanov and S. D. Varfolomeyev Appl. Biochem. Biotechnol. 1994 49 257. 155 Y. Ishii K. Nakayama M. Takeno S. Sakaguchi T. Iwahama and Y. Nishiyama J. Org. Chem. 1995,60 3934. 156 T. Mukaiyama and T. Yamada Bull. Chem. Soc. Jpn. 1995 68 17. 157 A. Laurent P. Jacqualt J.-L. Di Martino and J. Hamelin J. Chem. Soc. Chem. Commun. 1995 1101. 158 S. Caddick Tetrahedron 1995 51 10403. 159 C. R. Strauss and R. W. Trainer Aus. J. Chem. 1995,48 1665. 160 T. Ikariya P.G. Jessop and R. Noyori J. Synth. Org. Chem. Jpn. 1995,53 358. 161 D.H. Bremner Ultrason. Sonochem. 1994 1 S 119. 162 K.Jarowicki and P. Kocienski Contemp. Org. Synth. 1995 2 315. 163 T. Schmittberger and D. Uguen Tetrahedron Letr. 1995 36 7445. 164 0. Piva A. Amougay and J.-P. Pete Synth. Commun. 1995 25 219. 165 A.S.-Y. Lee H.-C. Yeh and M.-H. Tsai Tetrahedron Lett. 1995 36 6891. 166 A. Srikrishna R. Viswajanani J.A. Sattigeri and D. Vijaykumar J. Org. Chem. 1995,60 5961. 167 T. Oriyama K. Yatabe Y. Kawada and G. Koga Synlett 1995 45. 168 U. T. Bhalerao B. C. Raju and P. Neelakantan Synth. Commun. 1995 25 1433. 169 H. Sajiki Tetrahedron Lett. 1995 36 3465. 170 A. R. Vaino and W.A. Szarek Synlett 1995 1157. 171 E. Leikauf and H. Koster Tetrahedron 1995 51 5557. 172 S. Oliver0 and E. Duiiach J. Chem. Soc. Chem. Commun. 1995 2497. 173 A. Srikrishna J.A. Sattigeri R. Viswajanani and C.V.Yelamaggad J. Org. Chem. 1995 60 2260. 174 T. Nishiguchi M. Kuroda M. Saitoh A. Nishida and S. Fujisaki J. Chem. Soc. Chem. Commun. 1995 2491. 175 B.P. Bandgar S. R. Jagtap B. B. Aghade and P. P. Wadgaonkar Synth. Commun. 1995,25 2211. 176 T. Miura and Y. Masaki Synth. Commun. 1995,25 1981. 177 S. Raina and V. K. Singh Synth. Commun. 1995 25 2395. 178 B. Yu and Y. Hui Synth. Commun. 1995,25 2037. 179 S.V. Ley R. Downham P. J. Edwards J. E. Innes and M. Woods Contemp. Org. Synth. 1995 393. 180 R. Downham P.J. Edwards D.A. Entwistle A.B. Hughes K.S. Kim and S.V. Ley Tetrahedron Asymmetry 1995 6 2403. 181 B. Herradon A. Morcuende and S. Valverde Synlett 1995 455. 182 K. Ishihara M. Kubota H. Kurihara and H. Yamamoto J. Am. Chem. Soc.1995,117,4413. 183 J. Izumi I. Shiina and T. Mukaiyama Chem. Lett. 1995 141. 184 A.G. M. Barrett N. Koike and P.A. Procopiou J. Chem. Soc. Chem. Commun. 1995 1403. 185 A.S. Kiselyov and R.G. Harvey Tetrahedron Lett. 1995 36 4005. 186 A. R. Katritzky H. X. Chang and B.Z. Yang Synthesis 1995,503. 187 J. Cossy J.-L. Ranaivosata V. Bellosta and R. Wietzke Synth. Commun. 1995 25 3109. 188 J.C. Lee J. Y. Yuk and S.H. Cho Synth. Commun. 1995 25 1367. 189 J. A. Dodge M.G. Stocksdale K.J. Fahey and C. D. Jones J. Org. Chem. 1995,60,739. 190 V. Bethmont F. Fache and M. Lemaire Tetrahedron Lett. 1995,36,4235. 191 T. Oshikawa M. Yamashita S. Kumagai K. Seo and J. Kobayashi J. Chem. Soc. Chem. Commun. 1995 435. 192 F. D. Toste and I. W.J. Still Synlett 1995 159.Synthetic Methods 251 193 F.M. Moghaddam and A. Sharifi Synth. Commun. 1995,25,2457. 194 T.J. Lu J. F. Yang and L.J. Sheu J. Org. Chem. 1995,60 2931. 195 M. Benincasa M. Boni F. Ghelfi and U.M. Pagoni Synth. Commun. 1995,25 1843. 196 N. Komatsu M. Uda and H. Suzuki Synlett 1995 984. 197 G.M. Caballero and E.G. Gros Synth. Commun. 1995,25 395. 198 M.L. Kantam V. Swapna and P.L. Santhi Synth. Commun. 1995 25 2529. 199 T. Nishiguchi T. Ohosima A. Nishida and S. Fujisaki J. Chem. Soc. Chem. Commun. 1995 1121. 200 K. Zimmermann Synth. Commun. 1995,25,2959. 201 V. Gevorgyan and Y. Yamamoto Tetrahedron Lett. 1995,36 7765. 202 F. Chen J. Yang H. Zhang C. Guan and J. Wan Synth. Commun. 1995,25 3163. 203 T. Shinada and K. Yoshihara Tetrahedron Lett.1995 36,6701. 204 K. Fuji T. Kawabata A. Kuroda and T. Taga J. Org. Chem. 1995,60 1914. 205 A. Yanagisawa T. Kikuchi T. Wanabe T. Kuribayashi and H. Yamamoto Synlett 1995 372. 206 L.A. Carpino H.-G. Chao S. Ghassemi E. M. E. Mansour C. Riemer R. Warrass D. Sadat-Aalaee G. Truran,H. Imazumi A. El-Faham,D. Ionescu M. Ishmael,T. L. Kowaleski C.H. Han H. Wenschuh M. Beyermann M. Bienert H. Shroff F. Albericio S.A. Triolo N. A. Sole and S.A. Kates,J.Org. Chem. 1995 60 7718. 207 M. P. Sibi C. C. Stessman J. A. Schultz J. W. Christensen J. Lu and M. Marvin Synth. Commun. 1995,25 1255. 208 C. P. Wilgus S. Downing E. Molitor S. Bains R. M. Pagni and G. W. Kabalka Tetrahedron Lett. 1995,36 3469. 209 P. Ilankumaran A. R. Ramesha and S. Chandrasekaran Terrahedron Lett.1995 36 831 1. 210 A.S. Kende and K. Liu Tetrahedron Lett. 1995 36 4035. 21 1 D. Seebach G. Jaeschke and Y. M. Wang Angew. Chem. Int. Ed. Engl. 1995 34 2395. 212 D.F. Taber K. You and Y. Song J. Org. Chem. 1995 60 1093. 213 D.F. Taber D.M. Cleave R. J. Herr K. Moody and M. J. Hennessy J. Org. Chem. 1995,60 2283. 214 J.S. Debenham R. Madsen C. Roberts and B. Fraser-Reid J. Am. Chem. Soc. 1995 117 3302. 215 A. P. Davis and P. J. Gallagher Tetrahedron Lett. 1995 36 3269. 216 S. Talukdar and A. Banerji Tetrahedron Lett. 1995 36 813. 217 S. Lemaire-Audoire M. Savignac and J.P. Genet Tetrahedron Lett. 1995 36 1267. 218 C. Goulaouicdubois A. Guggisberg and M. Hesse J. Org. Chem. 199.560 5969. 219 J. R. Gage and J. M. Wagner J. Org. Chem. 1995,60 2613.220 Q. Dong C.E. Anderson and M.A. Ciufolini Tetrahedron Lett. 1995 36 5681. 221 P. Gmeiner and B. Bollinger Synthesis 1995 168. 222 K.E. Bell D. W. Knight and M. B. Gravestock Tetrahedron Lett. 1995 36 8681. 223 M. Ito K. Koyakumaru T. Ohta and H. Takaya Synthesis 1995 376. 224 I. V. Komarov and M. Y. Kornilov Tetrahedron 1995 51 11 271. 225 C. Malanga A. Urso and L. Lardicci Tetrahedron Lett. 1995 36 8859. 226 M. Saady L. Lebeau and C. Mioskowski J. Org. Chem. 1995 60,2946. 227 J. Ahman and P. Somfai Synth. Commun. 1995 25 2301. 228 S. Itsuno T. Koizumi C. Okumura and K. Ito Synthesis 1995 150. 229 J. Louie and J. F. Hartwig Tetrahedron Lett. 1995 36 3609. 230 P. L. Spargo Contemp. Org. Synth. 1995 85. 231 J. M. Percy Contemp. Org. Synth. 1995 2 251.232 M. Hudlicky and A. E. Pavlath Chemistry of organic fluorine compounds 11 A critical review ACS Monograph Series 187 American Chemical Society Washington DC 1995. 233 K. lseki and Y. Kobayashi Rev. Heteroatom Chem. 1995 12 21 1. 234 M. A. Tius Tetrahedron 1995 51 6605. 235 L. Strekowski and A. S. Kiselyov Adc. Heterocycl. Chem. 1995 62 1. 236 S.M. Pereira G. P. Savage and G. W. Simpson Synth. Commun. 1995 25 1023. 237 R.D. Chambers C.J. Skinner M. Atherton and J.S. Moiliet J. Chem. SOC. Chem. Commun. 1995 19. 238 R.D. Chambers M. P. Greenhall and J. Hutchinson J. Chem. SOC. Chem. Commun. 1995 21. 239 R.D. Chambers C. J. Skinner J. Thomson and J. Hutchinson J. Chem. Soc. Chem. Commun. 1995 17. 240 R.D. Chambers G. Sandford and M. Atherton J.Chem. Soc. Chem. Commun. 1995 177. 241 J. W. Guiles Synlett 1995 165. 242 F.A. Davis W. Han and C. K. Murphy J. Org. Chem. 1995 60 4730. 243 T. Umemoto and G. Tomizawa J. Org. Chem. 1995 60 6563. 244 A. J. Poss and G. A. Shia Tetrahedron Lett. 1995 36 4721. 245 M. Abdul-Ghani R. E. Banks M. K. Besheesh I. Sharif and R. G. Syvert J. Fluorine Chem. 1995,73,255. 246 J. Wang and A. 1. Scott J. Chem. SOC. Chem. Commun. 1995 2399. 247 A. K. Forrest and P. J. O’Hanlon Tetrahedron Lett. 1995 36 2141. 248 D. S. Brown B. A. Marples P. Smith and L. Watson Tetruhedron 1995 51 3587. 249 G.S. Lal Synth. Commun. 1995 25 725. 250 G.S. Lal W. Pastore and R. Pesaresi J. Org. Chem. 1995 60 7340. N. J. Lawrence 251 M. Zupan J. Iskra and S. Stavber J. Org.Chem. 1995 60,259. 252 S. Stavber M. Zupan A. J. Poss and G.A. Shia Tetrahedron Lett. 1995 36 6769. 253 A.S. Pilcher H. L. Ammon and P. DeShong J. Am. Chem. Soc. 1995,117,5166. 254 D.C. Snyder J. Org. Chem. 1995,60 2638. 255 J. Alexander M. L. Renyer and H. Veerapanane Synth. Commun. 1995,25 3875. 256 M.C. Carreiio J. L.G. Ruano G. Sanz M.A. Toledo and A. Urbano J. Org. Chem. 1995,60 5328. 257 A. Babadjamian and A. Kessat Synth. Commun. 1995,25 2203. 258 R. Joseph P. S. Pallan A. Sudalai and T. Ravindranathan Tetrahedron Lett. 1995,36 609. 259 Y. Brunel and G. Rousseau Tetrahedron Lett. 1995,36 8217. 260 Y. Brunel and G. Rousseau Synlett 1995 323. 261 Y. Brunel and G. Rousseau Tetrahedron Lett. 1995,36 2619. 262 K. Oirto T. Hatakeyama M. Takeo and H.Suginome Synthesis 1995 1273. 263 N.S. Zefirov G.A. Sereda S. E. Sosonuk N. V. Zyk and T.I. Likhomanova Synthesis 1995 1359.