Saturated and partially unsaturated carbocycles CHRISTOPHER D. J. BODEN and GERALD PATTENDEN Department of Chemistry, The University, Nottingham NG7 2RD, UK Reviewing the literature published between August 1992 and January 1994 1 1.1 1.2 1.3 2 2.1 2.2 2.3 3 3.1 3.2 3.3 3.4 4 4.1 4.2 4.3 4.4 4.5 5 6 7 8 9 T hree-membered rings Simmons-Smith cyclopropanations Diazoester cyclopropanations Other routes to three-membered carbocycles Four-membered rings Transition metal based methods Photochemical and free radical methods Other routes to four-membered carbocycles Five-membered rings Free radical cyclizations Transition metal mediated cyclizations Other routes to five-membered carbocycles Pol yquinanes Six-membered rings Diels-Alder reactions Transition metal mediated cyclizations Free radical cyclizations Other routes to six-membered rings Fused six-membered rings Seven-membered rings Eight-membered rings Ten-membered and larger rings General carbocyclic ring synthesis References 1 Three-membered rings 1.1 Simmons-Smith cyclopropanations The Simmons-Smith reaction remains one of the most useful procedures for the synthesis of cyclopropane derivatives, and a wide range of chiral auxiliaries for asymmetric Simmons-Smith reactions have been described.In extensions of earlier work, Ukaji et al. have now shown that optically active silicon-substituted cyclopropylmethanols can be synthesized using the Simmons-Smith procedure utilizing diethyl tartrate as a chiral auxiliary, e.g. 1 -+ 2 in 92% e.e. Similarly, Charette et al. have developed a new and simple auxiliary derived from 1,2-trans cyclohexanediol for the Simmons-Smith cyclo- propanation of substituted alcohols, viz.3 -+ 4 ( > 20 : 1 d.s.); related work by the same research group using 2-hydroxy-~-~-glucopyranose as the chiral auxiliary has also been developed this year.3 1 2 02% (92% e.e.) wowpr Etgn, CH$ woT Pr OH CsH&de;-20"Cr 3h OH 3 4 By analogy with the Simmons-Smith reaction, Motherwell and Roberts have extended their earlier work on the deoxygenation of carbonyl compounds and shown that organozinc carbenoid species derived from zinc and chloroalkyl silanes can be trapped with alkenes leading to cyclopropanes in good to excellent yields." The reaction works particularly well using aryl and, in some cases, a,B-unsaturated carbonyl compounds as precursors of the organozinc carbenoid species (Scheme 1).IPh- 99% (cis : trans 25 : 1) Scheme 1 1.2 Diazoester cyclopropanations A very wide range of metal complexes are known to catalyse the cyclopropanations of alkenes by diazoesters. Rhodium( 111) porphyrin complexes are especially interesting in view of the scope that chiral porphyrins offer in asymmetric cyclopropanations. A transition state model has now been developed for these rhodium-catalysed reactions which provides some level of stereochemical predictive power.5 In a similar vein, the copper complex of the optically active bipyridine 56 and the chiral cobalt( 11) complex 6 from 1( R )-3-hydroxymethylenebornane-2-thione' have been used as chiral catalysts for the asymmetric cyclopropanations of styrenes and oct- 1-ene with ethyl diazoacetate.Boden and Pattenden: Saturated and partially unsaturated carbocycles 433n . .4 SiMe, Me38 O Y R MevMe 0 5 6 Scheme 3 Two novel cyclopropane ring-forming sequences have been employed in the synthesis of the natural products 13 and 14. Thus the cyclization step 11 -. 12 was used by White and Jensen14 in their biogenetically patterned synthesis of the cyclopropane-containing eicosanoid 13 produced by the coral Hexaura homomalla. Furthermore, the cyclopropane 17 has The homoallylic diazoacetates 9 have been shown to undergo enantioselective intramolecular cyclopropanation with the rhodium catalyst 7 leading to the bicyclic cyclopropane 10 in 7 1-90% e.e. and in 5 5-80°/0 chemical yield.8 Other related studies have been described, with ally1 diazomalonates, using the chiral phenylalaninol-derived copper catalyst 8.9 n U 7 coj2 Et - 9 8 10 80% (90% e.e.) been produced in one step from the mesylate 15, presumably via the intermediate 16, during Danishefsky 's synthesis of the unusual pentacyclic diterpene mycorin 14.lS 11 12 14 13 1.3 Other routes to three-membered carbocycles Meyers and his colleagues'" have described full details of their elegant studies on the synthesis of enantiomerically pure 172,3-trisubstituted cyclopropanes based on cycloaddition reactions of sulfur ylides to chiral unsaturated lactams (Scheme 2).Ho 'OH Reagents: (i) Ph,S=CHMe; (ii) Red-AI; (iii) BuCH=PPh3; (iv) H30+ 15 16 17 Scheme 2 In related work, Krief and Lecomte" have outlined further extensions of their studies of the stereoselective synthesis of cyclopropane derivatives from y-alkoxy- a, B-unsaturated carbonyl compounds derived from D-glyceraldehyde and sulfur ylides (Scheme 3).Developments of earlier strategies towards the synthesis of cyclopropanes, based on a 1,3-elimination protocol, have also been described.'*? l 3 A novel method for the synthesis of cyclopropanols has been described whereby solutions of a$-unsaturated aldehydes in DMF are simply treated with 2.2 mol of chromium chloride in the presence of 2 mol O/O nickel(n) chloride at room temperature, viz. 18 - 19.16 1,2-Cyclopropanediols are produced when acylsilanes are reacted with ketone enolates; the reaction proceeds via Brook rearrangement of the initial 1,2-adduct and subsequent aldol reaction (Scheme 4).17 434 Contemporary Organic Synthesiscrc12 R2 R'&o NiCIz, DMF * 16 \OH 19 OSiMe, HO ,OH Me,SiO 0- .. . , (ii) . , P h r n P i - PhmPi 0 Reagents: (i)+Pi, -80 O to -30 OC; (ii), MeOH Scheme 4 Substituted cyclopropanes of constitution 2 1 are produced via a novel rearrangement when the hydroxylated cyclobutane derivatives 20 are treated with F,B.OEt,-POCl, in the presence of pyridine or Raney nickel.' Another unusual atom-reorganization sequence has been used by Dowd et al. in their synthesis of the p, y-cyclopropyl ketones 23 by treatment of substituted cyclobutanones of constitution 22 with Bu,SnH-AIBN (Scheme 5).ly 20 21 22 23 Scheme 5 2 Four-membered rings 2.1 Transition metal based methods Transition metal based methods form the bulk of new work published in the area of cyclobutane ring synthesis during the period under review.Stille has reported the intramolecular addition of alkyl titanocene complexes to alkynes under Lewis acid catalysis, leading to four-membered rings 24 bearing an exocyclic trisubstituted alkene.20 Ph EtAICI2 c r8Tic12cp21 -78 "C AcOH I HZO ph& 24 A number of metal-catalysed [2 + 21 methods have also been reported, of which the most interesting is probably that of Narasaka in which sulfur-bearing alkenes, allenes, or alkynes may be reacted with electron-deficient alkenes in the presence of a chiral titanium catalyst to give the corresponding cyclobutanes 25, exo-methylene cyclobutanes 26, and cyclobutenes 27 with good to excellent enantioselectivity.2' The use of ketene thiocetals, followed by desulfurization of the resulting adduct, makes for a useful synthesis of cyclobutanones 28, as does the work of Wulff, which utilizes the reaction of 1,Genynes with Fischer carbene complexes to give cis-[3.2.0]bicycloheptanones 29 in good yields and with good diastereoselectivity for positions a- to the ring junction.*, Where a is *;to 0 0 25 (%%, >98% e.e.) SiMe, d N K o + =*< SMe Me02C \ u , a, TiCI2(0Pi)p PhMe, 0 "C 1 OOV0, >9Wo e.e.I 26 Boden and Pattenden: Saturated and partially unsaturated carbocycles 4350 0 'NAO R2 SMe 27 S M e 28 2.2 Photochemical and free radical methods Wagner et al. have reported an interesting synthesis of bicyclo[4.2.0]cyclo-octadienes, based on the intramolecular photoaddition of tethered alkenes to triplet benzenes 30,23 and Galatsis et al.have devised a modification of the de Mayo reaction which allows the synthesis of bicyclo[4.2.0]cyclooctenones.24 hv Two unusual examples of cyclobutane ring formation by radical methods have appeared. The first, described by Jung et al., constitutes a route to cyclobutanones 3 1 via the corresponding ketal; 4- exo-trig cyclization of the radical onto an alkenoate occurs in the presence of a di-alkoxy substituent but with the free ketone only the product of reduction is formed.25 The second example relies on electronic rather than steric effects, with the stabilizing effect of methythio- and sulfone-substituents on an intermediate radical promoting 4-ex0 cyclization, to 32, rather than to the 5-endo cyclization products.26 Bu&nH, AlBN PhH, rdlux \C02E1 \CO2Et 31 32 2.3 Other routes to four-membered carbocycles Barber0 et al. have reported a remarkable synthesis of both endo and ex0 cyclobutenes, based on the fast transmetalation of ally1 and vinyl stannanes with alkyl-lithiums at low temperature (Scheme 55% / TsO R Tsd R=HorPh 61 -72% Scheme 6 Tin-lithium exchange is so rapid that the reaction can be carried out with Bu"Li in the presence of ketone groups, viz.33 --* 34. Nagasawa and Suzuki have published an effective method for the synthesis of cyclopropyl cyclobutanes and cyclobutenes based on the high stability of cyclopropyl carbocations (Scheme 7).28 Finally, Fukumoto et al. have extended their work on tandem intramolecular Michael-aldol reactions to allow the synthesis of polycyclic cyclobutane derivatives 35 which are commonly found in nature.29 0 33 34 (65%) 3 Five-membered rings A very wide range of methods leading to isolated and to ring-fused substituted cyclopentanes, published 436 Contemporary Organic SynthesisL R = alkyl -76% 1 NuH R ?? 8048% Nu = OH, OMe Scheme 7 Me02C OTMS TMS-I, (TMS)ZNH Mm2cw 1 ,Pdbhbroethane, 25 O C ' 35 during the period under review, have been based on free radical or transition metal mediated cyclizations.Furthermore, many of these methods have been applied in the area of total synthesis amongst naturally occurring polyquinanes. 3.1 Free radical cyclizations The enormous scope for oxidative free-radical cyclizations of /3-keto esters, 1,3-diones, and 1,3-diesters containing proximate alkene residues, in the presence of Mn( OAc), and Cu( OAc), has been further delineated, e.g.36 -+ 37,30 and the effects of solvent and ligand on the efficacy of the process have been discussed.31 Mn(OAc), - 36 1 37 Consecutive treatment of appropriately substituted alkenyl malonates with sodium hydride and iodine in THF provides excellent yields of cyclopentanes, e.g. 38 -+ 39.32 The reaction also works well in three- and six-, but not four-membered ring formations. Further developments of cobalt( 1)-mediated radical cyclization~~~ leading to cyclopentanes have been described, and Schafer et al. 34 have described more details of their novel use of CrC1, [and Cr( OAC),] in the radical cyclizations of /3-halo esters. Thus, after performing the desired radical cyclization, the product radical can be trapped to produce an organochromium intermediate which can then react with electrophiles, e.g. aldehydes, leading to substituted adducts (Scheme 8).EtO2C Ezu N;: t Et02C-f1 38 30 Reagents: (i) CrC13, LiAIH4, 0 "C; (ii) PhCHO, N2 25 "C Scheme 8 The relatively unexplored area of the use of phosphorus-centred radicals in cyclization reactions involving 1,6-diene systems has now been investigated, viz. 40 -+ 41;35 so too has the use of arenesulfonyl hydrazones as radical acceptors in the synthesis of unsaturated five-membered carbocycles, 4 2 -+ 4 3.36 EtO2C PhPPH pph2% ,CO2Et AIBN, C&ls A 40 41 BuSnH I NHS02Ar 42 / 43 A beautiful illustration of the scope for 5-exo-dig radical cyclizations in meaningful target synthesis is contained in the synthesis of the intermediate 45 from 44, en route to the anti-tumour agent fredericamycin A 46.,' In addition, the scope for cyclizations of unsaturated ketyl radical anions in natural product synthesis has been neatly summarized by COSSY.~~ Tellurium derivatives, viz.47, have featured in approaches to substituted cyclopentanes from carbohydrates described by Barton et aZ.,39 and other approaches to carbocycles from carbohydrates have been published during the period under review.40 a range of substituted five-membered ring synthesis. Thus, enone radicals generated from the corresponding iodoenones have been found to undergo intramolecular cyclization to tethered trimethylsilyl acetylene side-chains, leading to highly unsaturated cyclic products, viz.48 -. 49.41 Allenes and acetylenes have featured prominently in Boden and Pattenden: Saturated and partially unsaturated carbocycles 437OMe MeO *;h SePh \\ i)SiPh,Bu' 44 Ph3SnH 1 E~,B 46 6COMe 47 7 . 76% q, (fast :r addition) - (q fMS 48 1 TMS 49 (85%) Intramolecular reductive coupling reactions involving carbonyl and allene units have been carried out using either samarium( 11) iodide4* or cathodic redu~tion:~ leading to cyclopentenes, e.g. 50 .+ 5 1 and 52 .+ 53. U 50 51 (82%) 52 53 (96%) In addition, Blechert et u Z . ~ ~ have shown that allenylic radicals, generated from propargylic precursors undergo cyclizations onto proximate alkene electrophores, providing a facile synthesis of vinylidene-substituted cyclopentanes, e.g.54 .+ 5 5 . 55 (44%) 54 A neat combination of epoxide ring fragmentation triggered from a ketyl radical, followed by a 1,5-hydrogen abstraction and cyclization provides the basis of a synthesis of ring-fused carbocycles from cyclic a$-epoxyketones described by R a ~ a l ~ ~ and by Kim46 and their respective collaborators (Scheme 9). 0 OSnBu3 OSnBu3 OH OH 57 Scheme 9 OH In extensions of their studies with radical induced epoxide-fragmentations, Rawal and his colleagues have also shown that atom-transfer cyclizations of iodo-epoxides lead to similar angular hydroxy substituted cyclopentanes, whereas Kim and colleagues have demonstrated that treatment of epoxy silyl enol ethers derived from 56 with Bu,SnH-AIBN also provides a useful synthesis of cis-fused bicycles of constitution 57.A 1,5-hdyrogen abstraction protocol, involving a vinyl radical intermediate and an ally1 rnethylene residue, has featured in an approach to 5-ring fused bicycles described by Parsons et al.49 (Scheme 1 0 ) , and Motherwell et al. 50 have summarized their novel approach to the construction of bicyclic systems, based on a tandem free radical cyclopropylcarbinyl rearrangement- cyclization strategy, highlighted in Scheme 1 1. y p """'\p NaCNBH3 __c @ OH OH OH 63% Scheme 10 438 Contemporary Organic SynthesisS Scheme 11 A number of other interesting radical-induced cyclizations, leading to five-membered carbocycles, have been published during the period under review. These include the double ring expansion of the allylidenecyclopropane 58 to 59 at 1 70"C,51 the intramolecular cyclization/ring expansion/radical trap sequence 60 --+ 6 1 --* 62 whereby the cyclopropanol60 is converted into 62 in the presence of manganese( 111) tris-2-pyridine carboxylate5* or ferric chloride,53 and the cyclization of the tosylhydrazone derived from 63 in the presence of NaBH,CN-ZnCl, leading to the bicyclic ester 64.54 A further interesting development in cyclopentane ring synthesis is shown by the divergent reaction pathways followed by o-bromo acylgermanes and acylsilanes in their radical induced cyclizations (Scheme 1 2).55 58 59 60 Mn3* DMF, 0 "C - w H 62 (i) TsNH,NH, (ii) NaBH,CN H ', 63 64 OSiPh2Me MR3 = GePhs MR, = SiPh,Me Bu3SnH/AIBN (X = I or Br) Scheme 12 3.2 Transition metal mediated cyclizations In recent years the Pauson-Khand reaction has emerged as one of the most powerful methods for the synthesis of cyclopentenones.The reaction has also undergone substantial development since its initial report in 1973, including the introduction of new promoters and new metal carbonyl precursors for the source of CO. Dimethyl sulfoxide can now be added to the list of promoters for the reaction,56 so too can W(CO),.THF,57 MO(CO),,~~ and MO,C~,(CO),~~ as precursors to the intermediate alkyne complexes and source of CO. The effects of coordinating ligands in the homo and bis-homopropargylic position of a 1,6-diene precursor have also been exarnined;,O so too has a solid phase variant of the Pauson-Khand reaction.6 Reports of successful Pauson-Khand reactions with electron-deficient alkynes have been p~blished,h~,~~ and the reaction has played a pivotal role in the recent synthesis of ( - )-a-kainic acid 6564 and of ( k )-loganin 6665 (Scheme 13).0 I I 65 76% o+$ OH C 0 2 k 66 Scheme 13 Applications of palladium-catalysed reactions leading to polycycle constructions are ubiquitous in contemporary organic synthesis. Thus, Trost and his colleagues have published more details of their atom-economical cyclo-isomerizations of enediynes,66 and of their zipper reactions67 leading to ring-fused cyclopentanes, under palladium catalysis. They have also demonstrated the compatibility and effect of Boden and Pattenden: Saturated and partially unsaturated carbocycles 439carbonyl, carboxylic acid, and ketal functionality in these catalysed enyne cyclizations.68.6y Overman has published a useful review of his early work on Heck-type polyene cyclizations of organopalladium intermediate^,^^ and has also described a neat total synthesis of ( k )-scopadulcic acid A 67 using an intramolecular double-Heck cyclization to form the B-D rings in this complex tetracyclic diterpene structure, with complete stereochemical control (Scheme 1 4).7 1, 72 Grigg et aZ. have also published full details of their contemporaneous synthetic studies on the intramolecular Heck reaction, featuring anion capture,73 which adds further credence to this powerful method for the construction of novel polycycles. 67 Scheme 14 10% Pd(0AC)Z PPh3, \ THF. rdlux 1 OR TBAF, THF A OH Buchwald et 75 have highlighted the use of a combination of Cp,TiCl, with two equivalents of EtMgBr as an effective cocktail for the reductive cyclization of enynes to bicyclic titanacyclopentanes; reactions of the latter with CO then lead to cyclopentenones in good yield (Scheme 15).The cyclocarbonylation of acyclic 1,3-dienes via their tricarbonyl iron complexes has also resurfaced as a route to cyclopentenones, viz. 68 -+ 69.76 TiCp, CO2Et CPlTiCh 2EtMgBr EtO2C E t O 2 C w 0 Scheme 15 440 Contemporary Organic Synthesis (0c)3Fx A 20 "C j& 0 68 69 (>95%) Further details on the scope and limitations of the synthesis of cyclopentenones involving reactions between c yclopropylcarbene-chromium complexes and alkynes have been disclosed,77 and highly diastereo- and enantio-selective cyclizations of substituted penten-4-als using a chiral rhodium( I) complex leading to cyclopentanones have been published (Scheme 16).78 Rh'C104 d BlNAP Scheme 16 A number of [ 3 + 21 type cycloaddition reactions bear witness to the use of this strategy in the synthesis of cyclopentenones (Scheme 1 7),7y-81 and the use of the molybdenum alkylide 70 in effecting the novel olefin metathesis/carbonyl olefination route to cyclopentenes is, to say the least, intriguing.82 Ni(CO), base, MeOH I R' i"" C02Me Scheme 17 Q L O ( C H 2 ) a P h 06%3.3 Other routes to five-membered carbocycles The scope for intramolecular additions of the carbon-to-lithium bond into unactivated & Zn/Et20t & - I2 & 20 "C, 20 min.carbon-to-carbon double bonds, leading to attention in recent years. 5-exo-dig Cyclizations are also possible, and these reactions can be effected in tandem with 5-exo-trig processes (Scheme 18).83 cyclopentylmethyl lithiums, has received considerable 73% Scheme 21 OBu OBu N Bu'Li U Scheme 18 I (I) +20 "C, 2h (ii) MeOH (iniii) 65% Reagents: (i) Et2Zn(2 eq.), PdCln(dppf)(P d%), 20 "C.5-20h: (ii) CuCN, 2LiCI; (iii) 85% Scheme 22 In a similar vein Krief et al. 84 have shown that o-alkenyl allyl-lithiums also undergo facile cyclizations with high regio- and stereo-control (Scheme 19), and usefully functionalized cyclopentanes are produced when stabilized organolithiums are added intramolecularly to alkoxyacetylenes (Scheme 20).8s A number of methods are available for the synthesis of cyclopentane derivatives via ring expansion processes of three- and four-membered ring systems.To add to this list are the rearrangement of cobalt complexed alkynyl cyclopropanols shown in (Scheme 23)90,91 and the ring expansions of alk- 1 -enyl-cyclobutan- 1-01s under Hg2 + catalysis illustrated in the conversion of 71 into 72.36 Ph Ph P r 2 s i t co CO), P r ~ ~ i ~ k c o 2 ~ c o ) 6 reflux 4 2( - Ph # , DME or THF ; BuYi MF. -1 10 "c. then warm c Scheme 19 # , Scheme 20 Recent advances in the accessibility of organozinc compounds have prompted Normant, and others, to examine the intramolecular cyclizations of a range of suitably functionalized alkenyl zinc derivatives, with some interesting and useful results emerging (Scheme 2 1 ).86, 87 Related studies by Knochel et al. 88, 89 have shown that intramolecular carbozincation reactions of alkenes can be dramatically accelerated by the addition of small amounts of Pd'I or Ni" complexes.These novel ring closures, which are probably radical in nature, lead to organometallic intermediates which can subsequently be trapped with a range of electrophiles (Scheme 22). Scheme 23 0 71 72 The novel use of an anionic oxy-Cope rearrangement, from 73, in combination with a transannular cyclization has culminated in a neat synthesis of the 5,7-ring-fused terpene ( k )-africanol 74, described by White et al. (Scheme 24).92393 Danheiser and his colleague^^^^ 95 have extended their studies of [3 + 21 annulation reactions involving allenylsilanes, and now shown that ally1 and propargyl silanes can take part in the reactions, leading to usefully functionalized cyclopentanes, e.g.75 -, 76. Boden and Pattenden: Saturated and partially unsaturated carbocycles 441SnBu3 73 rh" 1st.. 74 Scheme 26 Reagents: (i) KH, ether, 25 "C, 3h; (ii) Na/CloHe, THF; (iii) CH212, EtpZn, 0 "C Scheme 24 OSiMe:, I 0 OHo 70 OMe 75 76 3.4 Polyquinanes Interest in the biological activity and structural novelty of natural products like hirsutic acid, pentalenene, and modhephene has sustained activity in the synthesis of linear, angular, and propellane-type triquinanes. Furthermore, a number of the strategies used towards these intriguing compounds have been very much based on the burgeoning interests in free radical and transition metal mediated cyclopentane ring-forming reactions, discussed earlier in this section. Thus, Curran and Shen have published full details of their approach to ( f )-modhephene 77 based on tandem transannular radical cyclizations (Scheme 25),96 and Weinges et al.have demonstrated the scope for radical cyclizations in the presence of samarium iodide in their approach to coriolin 78 (Scheme 26).97 Reagents: (i) CICOCOCI; (ii) Scheme 25 (iii) 100 "C; (iv) steps A Pd2+ promoted cyclization provides a cornerstone in Fukumoto's synthesis of ( +_ )-hirsutene (79, Scheme 27),"," and an asymmetric Heck-reaction/anion-capture process features in an approach to capnellenols, via 80, presented by Shibasaki et al. (Scheme 28).Io0 Both the H H H H 79 Scheme 27 89% (80% e.e.) -OH H Scheme 28 80 Pauson-Khand'"' reaction and tandem cyclizations of 5-hexenyl lithiums"'* have been used in other approaches to angular and linear polyquinanes.The interesting ring cleavage reaction 8 1 --, 82 is the key step in a new synthesis of ( f )-pentalenene 83 presented by Franck-Neumann et al.,103 and an unusual oxyradical fragmentation radical-transannulation-cyclization sequence, i. e. 84 --, 85 -, 86, has been investigated as an alternative approach to 83.1°4 A range of other routes to polyquinane constructions have been published during the period under review,lo5-lo9 and a total synthesis of ( f )-crinipellin B 87 has also been described.lIo 442 Contemporary Organic SynthesisSiMelBu' & yAi0A- * CO2Et H H 81 82 OAc i steps 83 84 05 H 86 mx 89 90 COMe X, Y = H,CHO, CO,Me, difficult to obtain. Welker has reported similar results with cobalt-substituted butadienes.' l 3 Konopelski has extended his work on vinylketene acetals to 3-substituted cases 9 1, although quite reactive dienophiles are still needed.' The useful 2-pyrone equivalent 92 has been prepared and investigated in Diels-Alder reactions, where the conditions for the cycloadditions are much milder than is usual for pyrone additions.' The conversion of the product 94 obtained on acidolysis of the adduct 93 into an intermediate in the synthesis of a vitamin D, analogue has also been described.An example of the use of tethered vinylallenes and dienes (which allows the construction of two rings simultaneously) has been reported.' I h OH 0 1 n 87 An interesting new approach towards the tricyclo [5.3.1.01.s]undecane ring system found in a-cedrene 88 relies on the tandem radical cyclization reaction shown in Scheme 29, which proceeds via an addition-elimination mechanism.' 91 X = SPh, OTBS NC4HaO 80-95% single isomer 'FSph Bu3SnH 'fl @CHO, then NaBH,, 25 MeOH "C, 96h 0 AIBN, A, CeHe I 88 Scheme 29 4 Six-membered rings 4.1 Diels-Alder reactions The Diels-Alder reaction retains its central role in the synthesis of six-membered rings, with several new modifications being developed, particularly in the field of asymmetric synthesis.reported, of which the most generally useful may prove to be the butadienyl boronate species 89 described by Miyaura and Suzuki.' l 2 This highly reactive diene leads to adducts of the type 90, and provides a useful route into cyclic alkenyl boronates which are otherwise A number of new diene variants have been CSA, MeOH 25 "C, 15h I 94 As with dienes, the use of boron-substituted dienophiles such as 95, followed by appropriate manipulation of the adducts 96, has been reported, most notably by both this and the related work of Vaultier'21 are beset by problems of regioselectivity, although there are indications that these problems can be overcome.If this is indeed the case the method provides a useful route to cyclic boranes. Saigo and his co-workers have reported a novel method for producing highly electron-deficient dienophiles whereby the acrylate-acetal97 is first converted into the species 98, which then reacts readily with a range of dienes under milder conditions Boden and Pattenden: Saturated and partially unsaturated carbocycles 443,25"C,THF &OH BX, then oxidative work-up \ 95 96 (75%, single regioisomer) than are usually needed for Lewis acid catalysed acrylate cycloadditions.122 The related cationic species 100 may be formed in situ from the ketone 99 by the action of TMS-OTf and TMS-OMe and then used in a similar manner.123 Sieburth et al.have described the use of in situ formed vinylsilane dienophiles such as 10 1 in intramolecular Diels-Alder reactions; by careful choice of the alkyl groups on silicon, excellent exo:endo selectivity can be obtained.i24 Roush has prepared the unusual dienophile 102 as an intermediate in the synthesis of kijan01ide.l~' Lewis add CHzCl OMe OMe -78% Me0 97 98 6649% &.ex0 2 25:l I 0 Me0 v v 1 % SiRa A Two examples of the use of the allenes 103 and 104 as dienophiles in natural product synthesis have been reported,126y127 and Kim has investigated the scope for cyclic sugar-derived dienophiles such as 105.These dienophiles react with five- or six-membered ring cycloalkadienes to give single stereoisomers of the resulting adducts, which can then be converted into highly substituted cyclopentanols or cyclohexanols such as 106.128 1 02 103 1 05 A 104 ACO Hd 106 A number of new chiral auxiliaries for dienophiles have been reported.12Y,130 Thus, Hoffmann et al. have described the use of N-sulfonyloxazolidines in conjunction with cyclopentanones, illustrated by 107 -. 108, and Nouguier et al. have used arabinose derivatives such as 109. In addition, Yamamoto et al. have described a strategy for increasing the effectiveness of chiral menthyl esters by the use of bulky Lewis acids.131 101 108 109 0- SiR2 endo:exo = 1:4 (R = Me), 1:20 (R = But) Interestingly, the less usual course of placing chiral auxiliaries on the diene component has also seen significant progress this year.Enders has used chiral, proline-derived 2-amino- 1,3-butadienes, such as 1 10, 444 Contemporary Organic SynthesisO y O M . N 110 for both carbo- and hetero-cycl~additions,~ 32 whilst Jones and Aversa have studied the use of chiral sulfoxide-bearing d i e n e ~ . ' ~ ~ The latter researchers obtained excellent results in the optimization of Lewis acid conditions in the case of 1 1 1. 0- & Ho@ ' L e I 111 70%, enhexo = 100: 0 Liclo,, CH2C12, 25 oc 9% e.e. (after sulfoxide removal) Q e c o 2 M e " d V C O 2 M e OMe Perhaps one of the most interesting general developments in the area of Diels-Alder reactions is the demonstration by Pandey that reactions between cyclic dienes and dienophiles can be forced to give predominantly the product of exo-addition ( e.g.1 13) by carrying out the reaction under photolytic ~0nditions.l~~ If this method proves to have general application it should be of considerable value, given the normal tendency towards predominantly endu-addition products, e.g. 1 12. A number of new or improved Lewis acid catalysts for Diels-Alder reactions have been reported; these include the bulky MAD and MABr aryloxyaluminium reagents of Yamarnot~,'~~ examples of catalysis on solid support^,^^^^ 137 and the use of scandium triflate.138 Asymmetric catalysis has seen little genuinely new work; however, the basic staples of metal-complexed chiral binaphth~ls'~'~ 140 and the chiral 0 0 + Q 0 1 111:llB > 4Q:l (cf.c 1 :49 under thermal conditions) hv, EtsN, EtOH v 112 113 oxaborolidinone work of Corey141 have seen further development. The latter has been extended to reactions with furan, providing a useful route into chiral cycl~hexanols.~ l 4.2 Transition metal mediated cyclizations As in previous years palladium has been the dominant metal used in transition-metal based syntheses of six-membered rings published during the period covered by this report, with several descriptions of coupling to sp2 centres. Trost has published a study of intramolecular palladium-catalysed intramolecular carbametalations of 1,6-enynes, wherein the regioselectivity of addition is shown to be dependent on the substitution pattern of the alkyne, with monosubstituted alkynes such as 114 giving exclusively the product 1 15 of 6-endo cy~lization.'~~ Another example of endo cyclization of alkenyl palladium intermediates has been reported by Negishi, viz.116 -+ 11 7, although evidence is presented which indicates that the reaction in fact proceeds via the exo-mode of bicyclization followed by cyclopropanation and then ~earrangement.'~~ 114 TPh H 115 - :y PdCI2(PPH&, EtpNH-EtsN, DMF, 80 "C 94% E EcBun 116 117 Bun The recent revival of synthetic interest in the vitamin D carbon skeleton (following the discovery of immunosuppressant and anti-cancer properties in analogues) has led several research groups to approach the A-ring (with its two exo-alkene groups) via palladium-based methodology of similar type to that described above for 1 14 -+ 1 15.144 Interestingly, attempts to form the triene system in 119 via the cycloisomerization of a suitable enyne 1 18 followed by trapping of the intermediate vinyl palladium species with a vinyl halide were hampered by a tendency towards isomerization in the product, viz.1 19 -. 120; however, less ambitious approaches based on initial formation of the exo-diene structure followed by triene Boden and Pattenden: Saturated and partially unsaturated carbocycles 445synthesis by more conventional methods have been more s u c c e s s f ~ l . ' ~ ~ - ' ~ ~ Other metal-mediated research of note includes Kim's ring expansion method for 1 -alkenyl cyclopentanols 12 1 via the corresponding mercurinium ion 122.'48 The method may also be applied to the synthesis of five-, seven-, and eight-membered rings (see Section 3.2).Br p Pd(OAc),, PhaP Et3N - PhMe, reflux then desilylation 5 HO" 119 118 m s o G P h 121 120 122 & Ph 4.3 Free radical cyclizations Cyclizations of simple heptenyl radicals are, in general, an unreliable method of forming six-membered rings, owing to a tendency for competing side-reactions (7-endo cyclization, 1,Shydrogen abstraction, and simple reduction) to both decrease the chemical yield and possibly introduce problems in purification. However, by attaching suitable substituents to either the radical centre of the acceptor site, it is possible to minimize or even eliminate such complications.For example, the propensity of acyl carbon atoms, acting as either radical centres or radical acceptors, to selectively form six-membered-ring cyclic ketones is emerging as a useful addition to more established methods of annulation. Crich has described the use of the acyl radial cyclization route in an alternative approach, viz. 123 -* 124, to the vitamin D A-ring discussed in Section 4.2.'49 The clean 6-ex0 addition to the alkene 123 seen here is also apparent in other work. so 0 123 0 Bu3SnH, AlBN PhH. reflux 125 In a related study, Crich has demonstrated that (at least in arylacyl cases) acyl tellurides are a viable alternative to acyl selenides as a source of acyl radi~a1s.l~~ Another case in point is the use of copper-manganese reagents to generate radicals at P-dicarbonyl centres.This method has been investigated by Snider, yielding some fascinating results. Such radicals favour 6-endo cyclization over 5-exo in many circumstances, and as such are a potentially useful tool for the synthesis of cyclohexanones. In the case of 126 a tandem process occurs, giving the product 127 via the mechanism indicated in Scheme 30.15* By careful choice of the chiral auxiliary 'X' in 126 synthetically useful yields and diastereomeric excesses can be obtained. Snider has also described the control of regioselectivity by the use of chloroalkenes for cases where either 7-endo or 5-exo cyclizations are serious ~ide-reacti0ns.l~~ The case of the latter is exemplified by the conversion of 128 into 129.Finally, an unusual route to six-membered rings has been reported by Kilburn,lS4 based on radical addition to methylene cyclopropanes such as 130. 0 I 126 1 0 127 90%, fwX= 86% d.e. & 0 Scheme 30 446 Contemporary Organic Synthesis0 R 128 0 COpEt 129 Bu$nH,AlBN Ph PhMe. reflux Ip,,*1 130 L J I.'. ph+ 4.4 Other routes to six-membered rings Anionic methods remain popular in six-membered ring constructions, with the familiar Michael and Dieckmann reactions seeing further development. Periasamy has described a general method for the introduction of cyclohexyl rings at the a-methylene centre in ketones, esters, lactones, and nitriles, utilizing a one-pot combination of the two, e.g. 131 + 132.155 Na2Fe(CO),, H,C=CHCO,Me, THF 53% 131 132 New ring-expansion methods have also been reported. Thus, McNelis has shown that treatment of alkynyl cyclopentanols 133 with iodine and Koser's reagent gives cleanly and in good yield the methylene cyclohexanone 134 with a doubly halogenated alkene.Subsequent selective reduction of the iodine moiety in 134 gives the stereodefined bromide 135.15h Schick has reported an unusual ring-expansion of the 1,3-~yclopentanediones 136 to the corresponding 1,4-~yclohexanediones 137;' 57 the reaction is unusual in that cleavage of 1,3-~yclopentanediones is usually seen under alkaline conditions. OH 133 12. HTlB MGN, 25 "C 55-7% NaOMe. MeOH (j+ -70% * 0 0 136 137 (R = 1 O alkyl) Cationic methods, particularly those mediated by Lewis acids, are increasing in popularity.Thus Booker-Milburn has reported the preparation of cyclohexenones 140 from cyclopentenones 138 by iron-catalysed ring expansion of siloxybicyclo- [ 3.1 .O]cyclohexane intermediates 1 39,15* and Overman has demonstrated that the Prins-pinacol rearrangement can be usefully applied to the synthesis of quite complex structures such as 142 following simple treatment of the vinyl (siloxycylopentane) 14 1 with tin tetrachl~ride.~~' Finally, Lewis-acid catalysed asymmetric enelh0 and carbonyl ene reactions have seen further development, the latter forming part of a remarkable (and highly stereoselective) tandem process with the Sakurai reaction (Scheme 3 l ) . I h 1 (I) RMgX, CuI, Me3SiCI (ii) CH& Etsn, Et20 R = alkyl, allyl, benzyl THF-HMPA, -78 "C R 139 138 or aryl FeClS DMF, 0 "C NaOAc, MeOH 1 45-71 % 0 R 140 OMe OMe -78OCt0-23"C SnCI.,, CH&IR & 57% OMe 141 142 Scheme 31 4.5 Fused six-membered rings The majority of the methods used for six-membered-ring annulation are equally applicable to the synthesis of six-membered-ring containing bicycles and polycycles, by the simple expedient of using a cyclic substrate.Such simplicities are not Boden and Pattenden: Saturated and partially unsaturated carbocycles 447covered in this section, which instead highlights cases of 'one-pot' formation of two or more rings, where at least one is six-membered. Transannular ring contractions of medium-rings provide one such approach, as illustrated by the work of White,162 in which treatment of the cyclododecenone 143 with acid gives the trans-decalin system 144.Note that under radical conditions (Bu,SnH or SmI,) the 7,5-ring system is favoured. Other transannular work includes the continuing research of Deslongchamps into intramolecular Diels-Alder syntheses of polycycles, which has been self-su~nmarized.~~~ Roush has also made a recent contribution to this field.164 An increasingly popular alternative to transannulation reactions for polycycle synthesis is the use of 'cascade' cyclizations, as described for five-membered rings in Section 3. An excellent example of this has been reported by D e m ~ t h , ' ~ ~ wherein the polyene 145 is cyclized to the tricycle 146 under single-electron transfer conditions. This fascinating process is suggested to proceed via radical cation intermediates (as would be expected from the reaction conditions), and represents an alternative to the classical cationic means of achieving 'biomimetic' polycyclization. Although the yield of 146 is low, there remains considerable scope for optimization.02CCF3 1 C02Bu' BunLi, THF, -78 Dct I 1 94% single isomer 147 c02Bu' BunLi, THF. -78 "c I 148 p C02Me CF&O2H, CH& 0 "C 64% (single isomer) 0 143 144 OH 63 OH 145 146 18%. + 6% of c-14 epimer A 'tandem' approach to decalins 147 and spiro 6,5-systems 148 has been described by Cooke.166 The method is dependent on the rapid halogen-metal exchange (presumably via single-electron transfer) between alkyl iodides and Bu"Li, and appears to be remarkably efficient. Another route (asymmetric in this case) to spiro-bicycles has been reported by Sakai,167 utilizing chiral diol auxiliaries to allow the synthesis of 149 in chral form.A method for the selective formation of cis or trans decalins via samarium-mediated c yclization has been reported, 68 where the stereocontrol is proposed to arise by intramolecular samarium chelate formation between the carbonyl and hydroxyl groups; thus the hydroxyl groups in 150,151, and 152 are all syn, to each other. I 149 (86% yield 85% e.e.) Me0& p Sda THF, MeOH, -78 "C El%, single isomer (6H c0,Me 150 c02Me \ Sd2, THF, MeOH, -78 "C 151 + * 70%. 151:152 = 5 1 152 'Appendage' bicyclization ( i.e. attaching a bifunctional synthon to a monocycle and then closing a second ring in situ) continues to attract interest. Fuchs has developed the four carbon appendage 153 as a means of preparing highly functionalized allylic stannanes such as 154 via the classical Robinson annulation.169 The survival of 154 under the alkaline conditions of the annulation is noteworthy (the equivalent silane is hydrolyzed). Finally, Piers has 153 154 448 Contemporary Organic Synthesisreported a simple method of exo-alkenyl cyclohexane synthesis based, like that of Pulido mentioned in Section 2, on the fast transmetalation of vinyl stannanes with alkyl-lithiums.' 70 5 Seven-membered rings Without doubt one of the most effective methods for the synthesis of seven-membered rings to be utilized in recent years has been the Cope rearrangement of cis-divinylcyclopropanes, e.g.155 -+ 156. The only limitation to this method of synthesis is the availability of the cis-divinylcyclopropanes.In a concise review Davies17' has now drawn together the main features of the conversions to demonstrate how the cis-divinylcyclopropanation/Cope-rearrangement can be effected in tandem, leading to highly functionalized seven-membered rings, often with excellent control of both relative and absolute stereochemistry. Me0 LOBn 155 156 (8Wo) The equally familiar Claisen rearrangement, but used in the form of a ring-contraction strategy, has been used in a most elegant fashion to elaborate the novel 7,7-fused bicyclic ring portion 159 in the ingenane family of natural products, viz. 157 -, 158 -, 159.17* 157 E=C02Me 158 ! I 1 (i) LiHMDS. Bu'M+SK;i (ii) 95C.3h (iii) HF. 0 "c 43 H02C 159 (88%) Cycloaddition reactions based on [4 + 31 and [5 + 21 annulations leading to seven-membered rings have featured prominently in the recent literature, and two further examples of these reactions described during the period under review are collected in Scheme 32.173-176 The unusual reaction between 2-aminobuta- 1,3-dienes and vinylchromium Fischer TBSO 145 'c 1 Raney-Ni 0 OpJH Scheme 32 type carbenes, which can be regarded as a [4 + 31 cycloaddition, also provides an interesting route to seven-membered rings (Scheme 33).' 77 Furthermore, in a modification of the more familiar Dotz reaction, Herndon et al.17*, 17y have now shown that cycloheptadienones can be obtained by the reaction of cyclopropylcarbene molybdenum or tungsten complexes with alkynes according to Scheme 34.MeCN 25 'c c IEF] OM* Fu = furan Scheme 33 D-tM0(c0)5 O(CHJ& ECPh Scheme 34 81 Yo 1 0 q 0 55% Boden and Pattenden: Saturated and partially unsaturated carbocycles 449Ring-enlargement reactions are often used in synthesis, and two new sequences used in seven-membered ring constructions are the rhodium( 1)-catalysed ring fusion 160 -+ 16 1 free-radical ring expansion of fused cyclobutanones described by Dowd et ~ 1 . ' ~ ' and highlighted in Scheme 35. and the Majetich et al. L8s have published full details of their allylsilane-based annulation strategy to construct perhydroazulenes, viz. 164 -+ 165, and Jones et al. 18h have described an approach to the same ring system based on some novel cyclizations involving sulfones, e.g. 166 - 167. H (i) THF (ii) aq.HCI . 160 Cu (C N) L 161 H 0 Scheme 35 A further illustration of the scope for tandem free-radical reactions in seven-membered ring synthesis is shown by the cascade fragmentation-transannulation process triggered by treatment of the bicyclic dienol 162 with iodosylbenzene diacetate/iodine, leading to the 7,5,5,-tricycle 163 in 80% yield (Scheme 36).IX2 Other radical-based procedures, involving acyl radical' x3 and organocobalt intermediate^,'^^ leading to the synthesis of linear fused seven-membered ring systems have also been described. 162 163 (81%) SiMes 164 165 I . -0 graveolide 166 167 Ring-opening reactions of oxabicyclo[ 3.2.1 ]octanes leading to functionalized cycloheptenes and cycloheptadienes have been summarized,' 87 and the neat metathesis reaction 168 -, 169 has been highlighted as a route to h.ydroazulenes.'8x 169 (58%) 168 6 Eight-membered rings Much of the recent spate of activity in approaches to the synthesis of eight-membered rings stems from interest in taxol 170 and its promising anticancer activity.A concise review of approaches to taxanes has recently been published in Contemporary Organic Synthesis.lX' Recent additions to these approaches are Scheme 36 170 450 Contemporary Organic Synthesisthe intramolecular Ni"-Cr" mediated coupling reaction shown by 171 -+ l72,lYo and the intramolecular nucleophilic allylic bromide-aldehyde addition reaction 173 -+ 174 promoted by a Zn-Cu couple."' 0 It 1%NiCIfirC12 DMSO, 2 0 d 171 172 (50%) 173 174 OR 1 C02Me C02Me OMOM The Claisen rearrangement of 175 -.+ 176 has featured in Paquette's approach to the 5,8,5-ring fused diterpene ( + )-ceroplastol I 177,ly2 and Myers and CondroskilY3 have outlined a neat radical-based transannular strategy in their novel synthesis of the tobacco isolate ( k )-7,8-epoxy-4-basnien-6-one 178 (Scheme 37).constructions published during the period covered by this review, efficient intramolecular [4 + 41 photocycloadditions between 2-pyrones and furan have been pre~ented,"~ Funk has extended his work on Claisen rearrangements to enol phosphates,' y5 and Inoue et al. Iy6 have used a modified de Mayo photochemical reaction in an approach to ( f )-precapnelladiene found in marine coral. In other approaches to eight-membered ring 175 176 i steps \ 177 178 Reagent: ( i ) N-methylcarbazole, 1,6cyclohexadiene, THF-H,O, hv Scheme 37 7 Ten-membered and larger rings In so-called higher order cycloaddition reactions, Rigby and co-worker~~~' have summarized numerous examples of [6n + 4n] cycloadditions which can be effected either thermally, photochemically, or by employing a chromium metal catalyst (Scheme 38); related [6n + 2271 cycloaddition reactions have also been highlighted by Rigby and co-workers.198 6 + OAc 0 % A xylene 59% 66% 0 52% Scheme 38 The directed ring-opening of epoxycarbinyl radicals set in decalin ring systems, followed by fragmentation of the resulting oxy-centred radicals, has been used as an approach to substituted cyclodecanones (Scheme 39),"'? 2oo and related work involving radical intermediates from homoallylic alcohols has been presented.*O' from farnesol has been described by Corey et al.202 which incorporates the trick of utilizing an allylic A biogenetically inspired synthesis of humulene 180 Boden and Pattenden: Saturated and partially unsaturated carbocycles 45 1cb] COpEt C02Et c3 CO2Et Scheme 39 Bu,Sn to lower the activation energy in the cyclization of 179 -+ 180 in the presence of dimethylaluminium chloride at - 78°C. The cyclization of 181 to the eleven-membered ring intermediate 182 has been used in the biogenetically patterned synthesis of the clavularanes 183 described by Williams et al. 203 SFBU:, toluene, -78 "C 179 180 I Po-& ' 1 e i 182 i steps i 183 In Paquette's synthesis of the fourteen-membered furanocembrane acerosolide 185, the crucial cyclization step was achieved by treatment of the allylic bromide 184 with the reducing cocktail of CrCI, and LiAlH, .204 8 General carbocyclic ring synthesis In addition to those reviews already sited, Tietze and Beifuss2OS have produced a useful review of sequential (tandem/domino/cascade) ring-forming reactions in organic synthesis, which nicely complements the book written by T.-L.Ho covering the same area.2o6 Roxburgh has reviewed the synthesis of medium-sized rings by ring expansion reactions,2o7 and Paquette and CCl* THF 20% c 0 0 184 I 0 185 Stirling208 have presented a useful expose of the intramolecular S,l reaction and some of its applications to ring synthesis. A general account of stereo-electronic effects in the formation of five- and six-membered rings, and the role of so-called Baldwin's rules, has been presented,20y and Petasis and Patane2l0 have provided an excellent account of the synthesis of eight-membered carbocycles. 9 References 1 Y.Ukaji, K. Sada, and K. Inomata, Chem. Lett,, 1993, 2 A.B. Charette and J.-F. Marcoux, Tetrahedron Lett., 3 A.B. Charette and B. Cbte, J. Org. Chem., 1993,58, 4 W.B. Motherwell and L.R. Roberts, J. Chem. SOC., 1227. 1993,34,7157. 933. Chem. Commun., 1992,1582; cf. R.M. Vargas, R.D. Theys, and M.M. Hossain, J. Am. Chem. SOC., 1992, 114,777. 5 K.C. Brown and T. Kodadek, J. Am. Chem. Soc., 1992, 114,8336. 6 K. Ito and T. Katsuki, Tetrahedron Lett., 1992,34, 2661; Synlett, 1993,638. 7 G. Jommi, R. Pagliarin, G. Rizzi, and M. Sisti, Synlett, 1993,833. 8 S.F. Martin, C.J. Oalmann, and S.Liras, Tetrahedron Lett., 1992,33,6727. 9 A.M.P. Koskinen and H. Hassila, J. Org. Chem., 1993, 58,4479. 10 D. Romo and A.I. Meyers, J. Org. Chem., 1992,57, 6265. 11 A. Krief and P. Lecomte, Tetrahedron Lett., 1993,34, 2695. 12 K. Tanaka and H. Suzuki, J. Chem. SOC., Perkin Trans. I, 1992,2071. 13 A.R. Katritzky and M.F. Gordeev, Synlett, 1993,213. 14 J.D. White and M.S. Jensen, J. Am. Chem. SOC., 1993, 15 M.Y. Chu-Moyer and S.J. Danishefsky, J. Am. Chem. 16 D. Montgomery, K. Reynolds, and P. Stevenson, J. 115,2970. SOC., 1992, 114,8333. Chem. SOC., Chem. Commun., 1993,363. 452 Contemporary Organic Synthesis17 K. Takeda, J. Nakatani, H. Nakamura, K. Sako, E. Yoshii, and K. Yamaguchi, Synlett, 1993,841. 18 M. Ihara, T. Taniguchi, N. Taniguchi, and K.Fukomoto, J. Chem. SOC., Chem. Commun., 1993,1477. 19 W. Zhang and P. Dowd, Tetrahedron Lett., 1992,33, 7307. 20 A.E. Harms and J.R. Stille, Tetrahedron Lett., 1992,33, 6565. 2 1 K. Narasaka, Y. Hayashi, H. Shimadzu, and S. Niihata, J.Am. Chem. SOC., 1992,114,8869. 22 O.K. Kim, W.D. Wulff, W. Jiang, and R.G. Ball, J. Org. Chem. 1993,58,5571. 23 P.J. Wagner, M. Sakamoto, and A.E. Madkour, J. Am. Chem. SOC., 1992,114,7298. 24 P. Galatsis, K.J. Ashbourne, J.J. Manwell, P. Wendling, R. Dufault, K.L. Hatt, G. Ferguson, and J.F. Gallagher, J. Org. Chem., 1993,58,1491. 25 M.E. Jung, I.D. Trifunovich, and N. Lensen, Tetrahedron Lett., 1992,33,6719. 26 K. Ogura, N. Sumitami, A. Kayano, H. Iguchi, and M. Fujita, Chem. Lett., 1992, 1487. 27 A. Barbero, P. Cuadrado, A.M. Gonza'lez, F.J.Pulido, R. Rubio, and I. Fleming, Tetrahedron Lett., 1992,33, 5841. 28 T. Nagasawa and K. Suzuki, Synlett, 1993,29. 29 M. Ihara, T. Taniguchi, K. Mamita, M. Tukano, M. Ohnishi, N. Taniguchi, K. Fukumoto, and C. Kabuto, J. Am. Chem.Soc., 1993,115,8107. 30 B.B. Snider, L. Armanetti, and R. Baggio, Tetrahedron Lett., 1993,34, 1701; for a review see: G.G. Melikyan, Synthesis, 1993,833. 31 B.B. Snider and B.A. McCarthy, J. Org. Chem., 1993, 58,62 17. 32 A.L.J. Beckwith and M.J. Tozer, Tetrahedron Lett., 1992,33,4975. 33 B.Giese, P. Erdmann, T. Gobel, and R. Springer, Tetrahedron Lett., 1992,33,4545. 34 T. Lubbers and H.J. Schafer, Synlett, 1992,743. 35 J.E. Brumwell, N.S. Simpkins, and N.K. Terrett, Tetrahedron Lett., 1993,34, 1215. 36 S. Kim and J.R. Cho, Synlett, 1992,629.37 D.L.J. Clive, Y. Tao, A. Khodabocus, Y.-J. Wu, A.G. Angoh, S.M. Bennett, C.N. Boddy, L. Bordeleau, D. Kellner, G. Kleiner, D.S. Middleton, C.J. Nichols, S.R. Richardson, and P.G. Vernon, J. Chem. SOC., Chem. Commun., 1992,1489. 38 J. Cossy, Pure Appl. Chem., 1992,64,1883. 39 D.H.R. Barton, J. Camara, X. Cheng, S.D. Gero, J.C. Jaszberenyi, and B. Quiclet-Sire, Tetrahedron, 1992, 48,926 1. 40 J. Marco-Contelles, P. Ruiz, L. Martinez, and A. Martinez-Grau, Tetrahedron, 1993,49,6669. 41 C.-K. Sha, C.-Y. Shen,T.-S. Jean, R.-T. Chiu,and W.-H. Tseng, Tetrahedron Lett., 1993,34,7641. 42 T. Gillmann, Tetrahedron Lett., 1993,34,607. For a concise review on mechanistic insights into radical reactions using samarium( 11) iodide see; D.P. Curran, T.L.Fevig, C.P. Jasperse, and M.J. Totleben, Synlett, 1992,943. Synlett, 1992,72 1. Tetrahedron Lett., 1993,34,5251. Tetrahedron Lett., 1993,34,2899. 1992,1377. 4687. 43 S. Toni, H. Okumoto, M.A. Rashid, and M. Mohri, 44 F.-H. Wartenberg, H. Junga, and S. Blechert, 45 V.H. Rawal, V. Krishnamurthy, and A. Fabre, 46 S. Kim and J.S. Koh, J. Chem. SOC., Chem. Commun., 47 V.H. Rawal and S. Iwasa, Tetrahedron Lett., 1992,33, 48 S. Kim and J.S. Koh, Tetrahedron Lett., 1992,33,7391. 49 A.D. Borthwick, S. Caddick, and P.J. Parsons, Tetrahedron, 1992,48,10655. 50 R.A. Batey, J.D. Harling, and W.B. Motherwell, Tetrahedron, 1992,48,8031. 51 C.A. Shook, M.L. Romberger, S.-H. Jung, M. Xiao, J.P. Sherbine, B. Zhang, F.-T. Lin, and T. Cohen, J. Am. Chem. SOC., 1993,115,lO 754. 52 N.Iwasawa, M. Funahashi, S. Hayakawa, and K. Narasaka, Chem. Lett., 1993,545. 53 K.I. Booker-Milburn, Synlett, 1992,809; K.I. Booker-Milburn and D.F. Thompson, ibid., 1993,592. 54 D.F. Taber, Y, Wang, and S.J. Stachel, Tetrahedron Lett., 1993,34,6209. 55 D.P. Curran, W.-T. Jiaang, M. Palovich, and Y.-M. Tsai, Synlett, 1993,403; D.P. Curran and M. Palovich, ibid., 1992,631. 56 Y.K. Chung, B.Y. Lee, N. Jeong, M. Hudecek, and P.L. Pauson, Organometallics, 1993,12,220. 57 T.R. Hoyeand J.A. Suriano,J. Am. Chem. SOC., 1993, 115,1154. 58 N. Jeong, S.J. Lee, B.Y. Lee, and Y.K. Chung, Tetrahedron Lett., 1993,34,4027. 59 C. Mukai, M. Uchiyama, and M. Hanaoka, J. Chem. SOC., Chem. Commun., 1992,1014. 60 M.E. Krafft, I.L. Scott, and R.H. Romero, Tetrahedron Lett., 1992,33,3829.6 1 D.P. Becker and D.L. Flynn, Tetrahedron Lett., 1993, 34,2087. 62 T.R. Hoye and J.A. Suriano, J. Org. Chem., 1993,58, 1659. 63 M.E. Krafft, R.H. Romero, and I.L. Scott, J. 0%. Chem., 1992,57,5277. 64 S. Yoo, S.-H. Lee, N. Jeong, and I. Cho, Tetrahedron Lett., 1993,34,3435. 65 N. Jeong, B.Y. Lee, S.M. Lee, Y. K. Chung, and S.-G. Lee, Tetrahedron Lett., 1993,34,4023. 66 B.M. Trost and Y. Shi, J. Am. Chem. SOC., 1993,115, 12491. 67 B.M. Trost and Y. Shi, JAm. Chem. SOC., 1993,115, 942 1. 68 B.M. Trost and O.J. Gelling, Tetrahedron Lett., 1993, 34,8233. 69 B.M. Trost and L.T. Phan, Tetrahedron Lett., 1993,34, 4735; see also F.-H. Wartenberg, B. Hellendahl, and S. Blechert, Synlett, 1993,. 539. 70 L.E. Overman, M.M. Abelman, D.J. Kucera, V.D. Tran, and D.J. Ricca, Pure Appl.Chem., 1992,64,1813. 71 L.E. Overman, D.J. Ricca, and V.D. Tran, J. Am. Chem. Soc., 1993,115,2042. 72 D.J. Kucera, S.J. O'Connor, and L.E. Overman, J. Org. Chem., 1993,58,5304. 73 B. Burns, R. Grigg, V. Santhakumar, V. Sridharan, P. Stevenson, and T. Worakun, Tetrahedron, 1992,48, 7297. 74 R.B. Grossman and S.L. Buchwald, J. Org. Chem., 1992,57,5803. 75 S.C. Berk, R.B. Grossman, and S.L. Buchwald, J. Am. Chem. SOC., 1993,115,4912. 76 D. Schinzer, J. Kabbara, and K. Ringe, Tetrahedron Lett., 1992,33,80 17. 77 S.U. Turner, J.W. Herndon, and L.A. McMullen, J. Am. Chem. SOC., 1992,114,8394. 78 X.-M. Wu, K. Funakoshi, and K. Sakai, Tetrahedron Lett., 1992,33,6331; ibid., 1993,34,5927. 79 A. Llebana, F. Camps, and J.M. Moreto, Tetrahedron, 1993,49,1283. 80 M.Yamaguchi, M. Sehata, A. Hayashi, and M. Hirama, J. Chem. SOC., Chem. Commun., 1993,1708. 8 1 J. van der Louw, J.L. van der Baan, C.M.D. Komen, A. Knol, F.J.J. de Kanter, F. Bickelhaupt, and G.W. Boden and Pattenden: Saturated and partially unsaturated carbocycles 453Klumpp, Tetrahedron, 1992,48,6105. 115,3800. Am. Chem. SOC., 1993,115,3080. 907. Ellestad, and J.B. Stallman, J. Am. Chem. SOC., 1993, 115,7023. 86 C. Meyer, I. Marek, G. Courtemanche, and J.-F. Normant, Synlett, 1993,266. 87 C. Meyer, H. Marek, G. Courtemanche, and J.-F. Normant, Tetrahedron Lett., 1993,34,6053. 88 H. Stadmiiller, C.E. Tucker, A. Vaupel, and P. Knochel, Tetrahedron Lett., 1993,34,7911. 89 H. Stadtmuller, R. Lenz, C.E. Tucker, T. Studemann, W. Dorner, and P. Knochel, J. Am.Chem. SOC., 1993,115, 7027. 90 N. Iwasawa and T. Matsuo, Chem. Lett., 1993,997. 91 N. Iwasawa and M. Iwamoto, Chem. Lett., 1993,1257. 92 W. Fan and J.B. White, J. Org. Chem., 1993,58,3557. 93 W. Fan and J.B. White, Tetrahedron Lett., 1993,34, 957. 94 R.L. Danheiser, B.R. Dixon, and R.W. Gleason, J. Org. Chem., 1992,57,6094. 95 R.L. Danheiser, T. Takahashi, B. Bertok, and B.R. Dixon, Tetrahedron Lett., 1993,34,3845. 96 D.P. Curran and W. Shen, Tetrahedron, 1993,49,75. 97 K. Weinges, R. Braun, U. Huber-Patz, and H. 98 M. Toyota, Y. Nishikawa, K. Motoki, N. Yoshida, and K. 99 M. Toyota, Y. Nishikawa, K. Motoki, N. Yoshida, and K. 82 G.C. Fu and R.H. Grubbs, J. Am. Chem. SOC., 1993, 83 W.F. Bailey and T.V. Ovaska, Chem. Lett., 1993,819; J. 84 A. Krief, D. Derouane, and W.Dumont, Synlett, 1992, 85 R.L. Funk,G.L.Bolton, K.M. Brummond, K.E. Imgartinger, LiebigsAnn. Chem., 1993, 1133. Fukumoto, Tetrahedron Lett., 1993,34,6099. Fukumoto, Tetrahedron, 1993,49,11 189. 100 K. Kagechika, T. Ohshima, and M. Shibasaki, Tetrahedron, 1993,49,1773. 101 E.G. Rowley and N.E. Schore, J. Org. Chem., 1992,57, 6853. 102 W.F. Bailey, A.D. Khanolkar, and K.V. Gavaskar, J. Am. Chem. SOC., 1992,114,8053. 103 M. Franck-Neumann, M. Miesch, and L. Gross, Tetrahedron Lett., 1992,33,3879. 104 G. Pattenden and D.J. Schulz, Tetrahedron Lett., 1993, 34,6787. 105 D.L.J. Clive and H.W. Manning, J. Chem. SOC., Chem. Commun., 1993,666. 106 M.-C.P. Yeh, B.-A. Sheu, H.-W. Fu, S.-I. Tau, and L.-W. Chuang, J.Am. Chem. SOC., 1993,115,5941. 107 T. Shono, N. Kise, T.Fujimoto, N. Tominaga, and H. Morita,J. Org. Chem., 1992,57,7175. 108 T. Cohen, K. McNamara, M.A. Kuzemko, K. Ramig, J.J. Landi, Jr., and Y. Dong, Tetrahedron, 1993,49,7931. 109 T.K. Sarkar, S.K. Ghosh, P.S.V. Subba Rao, T.K. Satapathi, and V.R. Mamdapur, Tetrahedron, 1992,48, 6897. 110 E. Piers and J. Renaud, J. Org. Chem., 1993,58, 11. 11 1 Y.-J. Chen, C.-M. Chen, and W.-Y. Lin, Tetrahedron Lett., 1993,34,2961. 1 12 A. Kamabuchi, N. Miyaura, and A. Suzuki, Tetrahedron Lett., 1993,34,4827. 113 J.T.L. Smalley, M.W. Wright, S.A. Garmon, M.E. Welker, and A.L. Rheingold, Organometallics, 1993, 12, 998. 1993,34,4587. 33,7839. Chem. Commun., 1993,270. 114 J.P. Konopelski and R.A. Kasar, Tetrahedron Lett., 1 15 K. Afarinkia and G.H. Posner, Tetrahedron Lett., 1992, 116 J.P.Dulcere, V. Agati, and R.J. Faure, J. Chem. SOC., 11 7 D.A. Singleton, J.P. Martinez, J.V. Watson, and G.M. Ndip, Tetrahedron, 1992,48,5831. 118 D.A. Singleton, J.P. Martinez, and G.M. Ndip, J. Org. Chem., 1992,57,5768. 119 D.A. Singleton and S.-W. Leung, J. Org. Chem., 1992, 57,4796. 120 D.A. Singleton, K. Kim, and J.P. Martinez, Tetrahedron Lett., 1993,34,3071. 121 N. Noiret, A. Youssofi, B. Carboni, and M.J. Vaultier, J. Chem. SOC., Chem. Commun., 1992,1105. 122 Y. Hashimoto, T. Nagashima, K. Kobayashi, M. Hasegawa, and K. Saigo, Tetrahedron, 1993,49,6349, 123 Y. Hashimoto, T. Nagashima, M. Hasegawa, and K. Saigo, Chem. Lett., 1992, 1353. 124 S.M. Sieburth and L. Densterbank, J. Org. Chem., 1992, 57,5279. 125 W.R. Roush and B.B. Brown, J. Org. Chem., 1993,58, 2152.126 M.E. Jung, C.N. Zimmerman, G.T. Lowen, and S.I. Khan, Tetrahedron Lett., 1993,34,4453. 127 J. De Schrijver and P.J. De Clerq, Tetrahedron Lett., 1993,34,4369. 128 K.S.Kim,I.H. Ch0,Y.H. Joo,I.Y.Yoo, J.H. Song,and J.K. KO, Tetrahedron Lett., 1992,33,4029. 129 H. Hoffmann, M. Bolte, B. Berger, and D. Hoppe, Tetrahedron Lett., 1993,34,6537. 130 R. Nouguier, J.-L. Gras, B. Giraud, and A. Virgili, Tetrahedron 1992,48,6245. 13 1 K. Maruoka, K. Shiohara, M. Oishi, S. Saito, and H. Yamamoto, Synlett, 1993,42 1. 132 D. Enders, 0. Meyer, and G. Raabe, Synthesis, 1992, 1242. 133 H. Adams, D.N. Jones, M.C. Aversa, P. Bonaccorsi, and P. Giannetto, Tetrahedron Lett., 1993,34,6481. 134 B. Pandey and P.V. Dalvi, Angew. Chem., Znt. Ed. Engl., 1993,32,1612. 135 K.Maruoka, M. Oishi, and H. Yamamoto, Synlett, 1993,683. 136 C. Cativiela, J.M. Fraile, J.L. Garcia, J.A. Mayoral, E. Pires, A.J. Royo, F. Figueras, and L.C. de Menorval, Tetrahedron, 1993,49,4073; see also idem., Tetrahedron, 1992,48,6467. 137 A.K. Saha and M.M. Hossain, Tetrahedron Lett., 1993, 34,3833. 138 S. Kobayashi, I. Hachiya, M. Araki, and H. Ishitani, Tetrahedron Lett., 1993,34,3755. 139 Y. Hong, B.A. Kunz, and S. Collins, Organometallics, 1993, 12,964. 140 K. Maruoka, A.B. Concepcion, and H. Yamamoto, Bull. Chem. SOC. Jpn., 1992,65,3501. 141 E.J. Corey and T.P. Loh, J. Am. Chem. SOC., 1991,113, 8966; E.J. Corey and T.-P. Loh, Tetrahedron Lett., 1993,34,3979. 142 B.M. Trost and J. Dumas, Tetrahedron Lett., 1993,34, 19. 143 Z . Owczarczyk, F. Lamaty, E.J.Vawter, and E.-I. Negishi,J.Arn. Chem. SOC., 1992,114,10091. 144 B.M. Trost, J. Dumas, and M.J. Villa, J. Am. Chem. SOC., 1992,114,9836. 145 J.M.Nuss, M.M. Murphy, R.A. Rennels, M.H. Haravi, and B.J. Mohr, Tetrahedron Lett., 1993,34,3079. 146 T. Takahashi and M. Nakazawa, Synlett, 1993,37. 147 J.L. Mascareiias, A.M. Garcia, L. Castedo, and A. Mouriiio, Tetrahedron Lett., 1992,33,4365. 148 S. Kim and K.H. Uh, Tetrahedron Lett., 1992,33,4325. 149 C. Chen and D. Crich, Tetrahedron, 1993,49,7943. 150 D. Batty and D. Crich, J. Chem. Soc., Perkin Trans I , 1992,3205. 15 1 C. Chen, D. Crich, and A.J. Papadatos, J. Am. Chem. Soc., 1922,114,8313. 454 Contemporary Organic Synthesis152 B.B. Snider and Q. Zhang, Tetrahedron Lett., 1992,33, 5921. 153 B.B.Snider, Q. Zhang, and M.A.Dombroski, J. Org. Chem., 1992,57,4196. 154 C.Destabe1, J.D. Kilburn, and J. Knight, Tetrahedron Lett., 1993,34,3151. 15 5 M. Periasamy, M.R. Reddy, U.Radhakrishnan, and A. Devasagayaraj, J. Org. Chem., 1993,58,4997. 156 P. Bovonsombat and E. McNelis, Tetrahedron, 1993, 49, 1525. 157 S. Schramm, B. Roatsch, E. Grundemann, and H. Schick, Tetrahedron Lett., 1993,34,4759. 158 K.I. Booker-Milburn and D.F. Thompson, Tetrahedron Lett., 1993,34,7291. 159 G.C. first, T.O. Johnson, Jr., and L.E. Overman, J. Am. Chem. Soc., 1993,115,2992. 160 K. Hiroi and M. Umemara, Tetrahedron, 1993,49, 1831. 16 1 L.F. Tietze and M. Rischer, Angew. Chem., Znt. Ed. Engl., 1992,31, 1221. 162 D. Colclough, J.B.White, W.B. Smith, and Y.J. Chu, J. Org. Chem., 1993,58,6303. 163 P.Deslongchamps, Pure Appl. Chem., 1992,64,1831. Ten other related papers have been published by this author. Lett., 1993,34,4427. Warzecha, C. Druger, H.D. Roth, and M. Demuth, J. Am. Chem. SOC., 1993,115,10358. 164 W.R. Roush, J.S. Warmus, and A.B. Works, Tetrahedron 165 U. Hoffmann, Y. Gao, B. Pandey, S. Hinge, K.-D. 166 M.P. Cooke, Jr,J. Org. Chem., 1993,58,2910. 167 H. Suemune, Y. Takahashi, and KJ. Sakai, J. Chem. 168 M. Kito, T. Sakai, K. Yamada, F. Matsuda, and H. 169 S. Kim and P.L. Fuchs, J. Am. Chem. SOC., 1993,115, 170 E. Piers, B.W.A. Yeung, and F.A. Fleming, Can. J. 171 H.M.L. Davies, Tetrahedron, 1993,49,5203. 172 R.L. Funk,T.A. Olmstead, M. Parvez, and J.B. Stallman, J. 0%. Chem., 1993,58,5873. 173 M. Harmata and B.F. Herron, Tetrahedron Lett., 1993, 34,5381.174 M. Harmata and B.F. Herron, J. Org. Chem., 1993,58, 7383. 175 M. Harmata, C.B. Gamlath, and C.L. Barnes, Tetrahedron Lett., 1993,34,265. 176 A. Rumbo, L. Castedo, A. Mourifio, and J.L. Mascerefias, J. Org. Chem., 1993,58,5585. 177 J. Barluenga, F. Aznar, A. Martin, S. Garcia-Granda, M.A. Salvado, and P. Pertierra, J. Chem. SOC., Chem. Commun., 1993,319. 178 J.W. Herndon,M. Zora, P.P. Patel, G. Chatterjee, J.N. Matasi, and S.U. Turner, Tetrahedron, 1993,49, 5507. 179 J.W. Herndon and M. Zora, Synlett, 1993,363. 180 M.A. Huffman and L.S. Liebeskind, J. Am. Chem. SOC., 1993,115,4895. 181 P. Dowd and W. Zhang, J. Org. Chem., 1992,57,7171.. 182 C.E. Mowbray and G. Pattenden, Tetrahedron Lett., 1993,34,127. 183 D. Batty and D. Crich, J. Chem. SOC., Perkin Trans. I , 1992,3 193. Soc., Chem. Commun., 1993,1858. Shirahama, Synlett, 1993,158. 5934. Chem., 1993,71,280. 184 A. Ali, D.C. Harrowven, and G. Pattenden, Tetrahedron Lett., 1992,33,2851. 185 G. Majetich, J.-S. Song, A.J. Leigh, and S.M. Condon, J. Org. Chem., 1993,58,1030. 186 D.N. Jones, M.W.J. Maybury, S. Swallow, and N.C.O. Tomkinson, Tetrahedron Lett., 1993,34,8533. 187 (a) M. Lautens, Synlett, 1993, 177; (b) M. Lautens and C. Gajda, Tetrahedron Lett., 1993,34,4591; (c) M. Lautens, C. Gajda, and P. Chiu, J. Chem. SOC., Chem. Commun., 1993, 1193; (d) M. Lautens, P. Chiu, and J.T. Colucci,Angew. Chem., Znt. Ed. Engl., 1993,32,281. 188 H. Junga and S. Blechert, Tetrahedron Lett., 1993,34, 3731. 189 A.N. Boa, P.R. Jenkins, and N.J. Lawrence, Contemp. Org. Synth., 1994,1,47. 190 M.H. Kress, R. Ruel, W.H. Miller, and Y. Kishi, Tetrahedron Lett., 1993,34,6003. 191 Z. Wang, S.E. Warder, H. Perrier, E.L. Grimm, and M.A. Bernstein,J. Org. Chem., 1993,58,2931. 192 L.A. Paquette, T.-Z. Wang, and N.H. Vo, J. Am. Chem. SOC., 1993,115,1676. 193 A.G. Myers and K.R. Condroski, J. Am. Chem. SOC., 1993,115,7926. 194 F.G. West, C.E. Chase, and A.M. Arif, J. Org. Chem., 1993,58,3794. 195 R.L. Funk, J.B. Stallman, and J.A. Wos, J. Am. Chem. SOC., 1993,115,8847. 196 Y. Inoue, M. Shirai, T. Michino, and H. Kakisawa, Bull. Chem. SOC. Jpn., 1993,66,324. 197 (a) J.H. Rigby,K.M. Short, H.S. Ateeq, and J.A. Henshilwood, J. Org. Chem., 1992,57,5290; (b) J.H. Rigby and S.V. Cuisiat, J. 0%. Chem., 1993,58,6286; (c) J.H. Rigby, H.S. Ateeq, and A.C. Kruegar, Tetrahedron Lett., 1992,33,5873; (d) J.H. Rigby, Ace. Chem. Rex, 1993,26,579; (e) J.H. Rigby and V.P. Sandanayaka, Tetrahedron Lett., 1993,34,935. Tetrahedron Lett., 1993,34,5397; (b) J.H. Rigby, H.S. Ateeq, N.R. Charles, J.A. Henshilwood, K.M. Short, and P.M. Sugathapala, Tetrahedron, 1993,49,5495. 199 V.H. Rawal and H.M. Zhong, Tetrahedron Lett., 1993, 34,5 197. 200 D.A. Corser, B.A. Marples, and R. Kinsey Dart, Synlett, 1992,987. 20 1 A. Boto, C. Betancor, T. Prange, and E. Suarez, Tetrahedron Lett., 1992,33,6687. 202 E.J. Corey, S. Daigneault, and B.R. Dixon, Tetrahedron Lett., 1993,34,3675. 203 D.R. Williams, P.J. Coleman, and S.S. Henry, J. Am. Chem. SOC., 1993,115,11654. 204 L.A. Paquette and P.C. Astles, J. Org. Chem., 1993,58, 165. 205 L.E. Tietze and U. Beifuss, Angew. Chem., Znt. Ed. Engl., 1993,32, 131. 206 T.-L. Ho, ‘Tandem Organic Reactions’, Wiley Interscience, 1992. 207 C.J. Roxburgh, Tetrahedron, 1993,49,10 749. 208 L.A. Paquette and C.J. M. Stirling, Tetrahedron, 1992, 209 C.D. Johnson, Acc. Chem. Res., 1993,26,476. 210 N.A. Petasis and M.A. Patane, Tetrahedron, 1992,48, 198 (a) J.H. Rigby, G. Ahmed, and M.D. Ferguson, 48,7383. 5757. Boden and Pattenden: Saturated and partially unsaturated carbocycles 455