摘要:

AbstractN‐Fmoc‐Protected (Fmoc = (9H‐fluoren‐9‐ylmethoxy)carbonyl) β‐amino acids are required for an efficient synthesis of β‐oligopeptides on solid support. Enantiomerically pure Fmoc‐β3‐amino acids β3: side chain and NH2at C(3)(= C(β)) were prepared from Fmoc‐protected (S)‐ and (R)‐α‐amino acids with aliphatic, aromatic, and functionalized side chains, using the standard or an optimizedArndt‐Eistertreaction sequence. Fmoc‐β2‐ Amino acids (β2side chain at C(2), NH2at C(3)(= C(β))) configuration bearing the side chain of Ala, Val, Leu, and Phe were synthesizedviatheEvans' chiral auxiliary methodology. The target β3‐heptapeptides5–8, a β3‐ pentadecapeptide9and a β2‐heptapeptide10were synthesized on a manual solid‐phase synthesis apparatus using conventional solid‐phase peptide synthesis procedures (Scheme 3). In the case of β3‐peptides, two methods were used to anchor the first β‐amino acid: esterification of theortho‐chlorotrityl chloride resin with the first Fmoc‐β‐amino acid2(Method I, Scheme 2) or acylation of the 4‐(benzyloxy)benzyl alcohol resin (Wangresin) with the ketene intermediates from theWolffrearrangement of amino‐acid‐derived diazo ketone1(Method II, Scheme 2). The former technique provided better results, as exemplified by the synthesis of the heptapeptides5and6(Table 2). The intermediate from theWolffrearrangement of diazo ketones1was also used for sequential peptide‐bond formation on solid support (synthesis of the tetrapeptides11and12). The CD spectra of the β2‐ and β3‐peptides5,9, and10show the typical pattern previously assigned to an (M)31helical secondary structure (Fig.). The most intense CD absorption was observed with the pentadecapeptide9(strong broad negativeCottoneffect atca.213 nm); compared to the analogous heptapept

ISSN:0018-019X    

DOI:10.1002/hlca.19980810202

出版商:WILEY‐VCH Verlag GmbH    

年代:1998

数据来源: WILEY

摘要:

AbstractThe synthesis and characterization of tripodal dodecadentate ligands with salicylamide and bipyridine binding sites for iron(II) and iron(III) are presented.

ISSN:0018-019X    

DOI:10.1002/hlca.19980810203

出版商:WILEY‐VCH Verlag GmbH    

年代:1998

数据来源: WILEY

摘要:

AbstractThe reaction of CuO'Bu with CO2, andiPr2NH in the presence of PPh3, gives the dialkylcarbamato complex [Cu(O2CNiPr2)(PPh3)2] (1). The CO2/R2NH system (R = Me, Et) in an appropriate organic medium reacts with Ag2O giving the correspondingN,N‐dialkylcarbamato complexes of analytical formula [Ag(C2CNR2)] (R = Me,2; R = Et,3). The methyl derivative2was characterized by X‐ray diffraction methods. Crystal data of2: for [Ag2(O2CNMe2)2], C6H12Ag2N2O4, mol. wt. 391.9; monoclinic, space groupP21/c,a= 12.08(1),b= 3.797(2),c= 11.316(7) Å, β = 113.37(6)°,V= 476.3 Å3,Z= 2,Dc= 2.732 g cm−3; μ(MoKα) = 40.64 cm−1,F(000) = 376.0;R= 0.059,Rw= 0.067; g.o.f. 1.27. The structure consists of dinuclear [(Ag2OCNMe2)2] units with slightly distorted linearly two‐coordinated Ag‐atoms containing bridging carbamato groups to form a substantially planar eight‐membered ring with an intra‐annular AgAg distance of 2.837(2) Å; the dinuclear units are further joined by AgO bonds to form an infinite array. Compound3, which is presumably dinuclear, as suggested by cryoscopic measurements in benzene, undergoes a structural fission with PPh3, giving the mononuclear triphenylphosphine derivative [Ag(O2CNEt2)(PPh3)2] (4). The amine‐catalyzed conversion of Ag2O into Ag2CO3, in the presence of theiPr2NH/CO2system, is also reported. Cl‐Exchange from [AuCl(PPh3)] with [Ag(O2CNEt2)] (3) gives the firstN,N‐dialkylcarbamato complex of gold, namely [Au(O2CNEt2)(PPh3)] (5), which crystallizes in the monoclinic system: C23H25AuNO2P · 0.5 C7H16, mol. wt. 625.5, space groupP21/c;a= 13.212(5),b= 12.25(1),c= 16.795(6) Å, β = 109.09(2)°,V= 2568(2) Å3,Z= 4,Dc, = 1.618 g cm−3; μ(AgKα) = 31.40 cm−1,F(000) = 1236.0;R= 0.058;Rw= 0.064; g.o.f. 2.121. The compound contains two‐coordinated Au‐atom, namely to the P‐atom and to the O‐atom of the monodentate carbamato group, the PAuO bond angle being 174.7(3)°. The reaction with MeI showed these compounds to react predominantly at the carbamato O‐atom giving the corresponding urethanes R2NCO2Me. Evidence w

ISSN:0018-019X    

DOI:10.1002/hlca.19980810204

出版商:WILEY‐VCH Verlag GmbH    

年代:1998

数据来源: WILEY

摘要:

AbstractMethyl (E)‐2‐(acetylamino)‐3‐cyanoprop‐2‐enoate (2a), and its 2‐benzoyl analog2bere prepared from the corresponding methyl (Z)‐2‐(acylamino)‐3‐(dimethylamino)propenoates1Multifunctional compounds2are versatile synthons for preparation of polysubstituted heterocyclic systems such as pyrroles4, pyrimidines5and6, pyridazines7, pyrazoles8,9, an

ISSN:0018-019X    

DOI:10.1002/hlca.19980810205

出版商:WILEY‐VCH Verlag GmbH    

年代:1998

数据来源: WILEY

摘要:

AbstractTo get informations on both the structure and dynamics of hydrogen chelates1of heteroaromatic systems, a great variety of quinazoline‐2‐acetonitrile chelates were synthesized (see2–4). Similarly to the situation of the corresponding H‐chelates in the pyrimidine‐2‐acetonitrile series, the investigation of these new derivatives2–4by NMR spectroscopic methods (DNMR, COSY, NOESY, ROESY, EXSY, HMQC, HMBC) confirms the presence of an equilibrium of the two possible H‐chelate structures (two ‘rotamers’IandII,i.e., (E)/(Z) isomers; seeScheme). The corresponding equilibriaI⇌IIwere determined by complete1H‐NMR signal assignment at low temperatures (after freezing the rotational processes). In addition, the tautomer equilibriaA⇌B(relative energies of the two minima of the nonsymmetrical double‐well potential) for both ‘rotamers’ are ascertained by H,H and C,H couplings. The results are an important basis for the interpretation of both the UV/VIS absorptions and the dependence of fluorescence and fluorescenc

ISSN:0018-019X    

DOI:10.1002/hlca.19980810206

出版商:WILEY‐VCH Verlag GmbH    

年代:1998

数据来源: WILEY

摘要:

AbstractOxy anions3generated from 1,2‐dihydrocyclobutabenzen‐1‐ones1through addition of a charged nucleophile or from 1‐hydroxy‐1,2‐dihydrocyclobutabenzenes2by deprotonation with base lead to stable products throughdistaland/orproximalcleavage of the strained four‐membered ringviabenzyl carbanion4and/or aryl carbanion5. A systematic study of this process reveals the relative stability of the two isomeric carbanions4and5as a key factor in determining the course of the ring‐cleavage reaction. While benzyl carbanions4can be trapped with carbon electrophiles, attempts at trapping aryl carbanions5with electrophiles other than H+failed. In protic solvents, the magnesium salt of the tertiary alcohol2shows an increased rate ofproximalcleavage as compared to its alkali salts. From this, we conclude that, in contrast to benzyl carbanions4, free aryl carbanions5are of transient existence only.ProximalC,C‐bond cleavage seems to occur either through protonation of5from a fast, reversible equilibrium3⇌5in which3strongly predominates, or in protic solvents possibly even through a rate‐limiting protonation of3at the aromatic C‐atom, bypassing free anion5altogether. Thus, additional factors other than just the relative stability of isomeric carbanions4and5are of importance in determining the regiochemistry of the base‐induced C,C‐bond cleavage

ISSN:0018-019X    

DOI:10.1002/hlca.19980810207

出版商:WILEY‐VCH Verlag GmbH    

年代:1998

数据来源: WILEY

摘要:

AbstractStereoselective reactions of phthalimido‐substituted radicals derived from (±)‐threonine with different radical traps are reported (Scheme 3, Table 1). A strong influence of the nature of the radical trap on the stereoselectivity was noticed. Small nucleophilic radical traps gave preferentially thesynproducts. The observed selectivities are explained with theA1,3strain model and depend on steric and electronic effects (Fig. 2). Reactions with electrophilic radical traps such as diphenyl diselenide gave theantidiastereoisomers with moderate stereocontrol, presumably due to stereoelectronic effects. The same stereochemical outcome,i.e., preferential formation of theantiproducts, was observed for the reactions of the relatedN‐phthaloyliminium ion (Scheme 5, Table 2). The stereochemistry of the ionic reaction is rationalized by aFelkin‐Anhmodel

ISSN:0018-019X    

DOI:10.1002/hlca.19980810208

出版商:WILEY‐VCH Verlag GmbH    

年代:1998

数据来源: WILEY

摘要:

AbstractThe reactions of α‐diazo ketones1a,bwith 9H‐fluorene‐9‐thione (2f) in THF at room temperature yielded the symmetrical 1,3‐dithiolanes7a,b, whereas1band 2,2,4,4‐tetramethylcyclobutane‐1,3‐dithione (2d) in THF at 60° led to a mixture of two stereoisomeric 1,3‐oxathiole derivativescis‐ andtrans‐9a(Scheme 2). With 2‐diazo‐1,2‐diphenylethanone (1c), thio ketones2a–das well as 1,3‐thiazole‐5(4H)‐thione2greacted to give 1,3‐oxathiole derivatives exclusively (Schemes 3and4). As the reactions with1cwere more sluggish than those with1a,b, they were catalyzed either by the addition of LiClO4or by Rh2(OAc)4. In the case of2din THF/LiClO4at room temperature, a mixture of the monoadduct4dand the stereoisomeric bis‐adductscis‐ andtrans‐9bwas formed. Monoadduct4dcould be transformed tocis‐ andtrans‐9bby treatment with1cin the presence of Rh2(OAc)4(Scheme 4). Xanthione (2e) and1cin THF at room temperature reacted only when catalyzed with Rh2(OAc)4, and, in contrast to the previous reactions, the benzoyl‐substituted thiirane derivative5awas the sole product (Scheme 4). Both types of reaction were observed with α‐diazo amides1d,e(Schemes 5–7). It is worth mentioning that formation of 1,3‐oxathiole or thiirane is not only dependent on the type of the carbonyl compound2but also on the α‐diazo amide. In the case of1dand thioxocyclobutanone2cin THF at room temperature, the primary cycloadduct12was the main product. Heating the mixture to 60°, 1,3‐oxathiole10das well as the spirocyclic thiirane‐carboxamide11bwere formed. Thiirane‐carboxamides11d–gwere desulfurized with (Me2N)3P in THF at 60°, yielding the corresponding acrylamide derivatives (Scheme 7). All reactions are rationalized by a mechanism via initial formation of acyl‐substituted thiocarbonyl ylides which undergo either a 1,5‐dipolar electrocyclization to give 1,3‐oxathiole derivatives or a 1,3‐dipolar electrocyclization to yield thiiranes. Only in the case of the most reactive 9H‐fluorene‐9‐thione (2f) is the thiocarbonyl ylide trapped by a

ISSN:0018-019X    

DOI:10.1002/hlca.19980810209

出版商:WILEY‐VCH Verlag GmbH    

年代:1998

数据来源: WILEY

摘要:

AbstractOn irradiation (350 nm) in the presence of excess 2,3‐dimethylbut‐2‐ene, the newly synthesized title compound5affords as main products the unexpected cyclopropylpyrrolidine10(50%) and the spiro‐oxetan

ISSN:0018-019X    

DOI:10.1002/hlca.19980810210

出版商:WILEY‐VCH Verlag GmbH    

年代:1998

数据来源: WILEY

摘要:

AbstractThe cyclopalladation of 1,1′‐azonaphthalene (= di(naphthalen‐1‐yl)diazene;2) with bis(hexafluoroacetyl‐acetonato)palladium(II) (3; [Pd(hfa)2]) yields theortho‐palladated complex (1,1,1,5,5,5‐hexafluoropentane‐2,4‐dionato‐κ2O,O′)[1‐(naphthalen‐1‐ylazo‐κN2)naphthalen‐2‐yl‐κC2]palladium(II) (4) as well as theperi‐palladated complex (1,1,1,5,5,5‐hexafluoropentane‐2,4‐dionato‐κ2O,O′)[8‐(naphthalen‐1‐ylazo‐κN2)naphthalen‐1‐yl‐κC1]‐palladium(II) (5); their structures were corroborated by X‐ray analyses. The formation of the novelperi‐metallated product5containing

ISSN:0018-019X    

DOI:10.1002/hlca.19980810211

出版商:WILEY‐VCH Verlag GmbH    

年代:1998

数据来源: WILEY