摘要:

AbstractChiralp‐nitrophenyl esters derived from the amino acid phenylalanine are cleaved by various histidine‐containing tripeptides and higher oligopeptides as catalysts at a micellar interface. It is assumed that the oligopeptides adopt an internally hydrogen‐bonded C7conformation when they dissolve into the micellar hydrocarbon phase. Chiral recognition is attributed to the formation of a hydrogen bond in the micellar hydrocarbon phase between one enantiomer of the ester and the peptide. It is important for the activity that the imidazolyl NH moiety of the His residue remains in the aqueous phase. The hydrophilic/hydrophobic balance, which determines the location of each of the amino acid residues of the peptide in the two‐phase system, can be controlled by varying the length and stereochemistry of the peptide chain and by attaching hydrophobic groups. The most selective catalyst is a tripeptide with the structure C4H9OC(O)‐L‐Phe‐L‐His‐L‐Leu. It cleaves thep‐nitrophenyl esters of N‐protected L‐ and D‐phenylalanine with an enantioselective ofkI/kD≈︁ 40. Further lengthening of the peptide chain with L‐Leu and L‐Ala residues and lengthening of the N‐protecting group of the catalyst decreases the enantioselectivity down tokI/kD≈︁ 6 for the penta‐peptide C12H25OC(O)‐L‐Phe‐L‐His

ISSN:0165-0513    

DOI:10.1002/recl.19921111101

出版商:WILEY‐VCH Verlag    

年代:1992

数据来源: WILEY

摘要:

AbstractThe synthesis is described of a series of eighteen 16‐dehydro‐20‐isocyano‐20‐sulfonylpregnanes (5and8–14) by C‐20 alkylation of 17‐[isocyano(sulfonyl)methylene]androstanes1–3. The geminal isocyano and sulfonyl groups at C‐20 (compounds5, 8–14) are removed by acid hydrolysis to provide a new entry into 20‐oxo steroids (6, 15–19). The C‐20 alkylation also includes halomethylation and alkoxymethylation to form 21‐halo‐ and 21‐alkoxy‐16‐dehydro‐20‐oxopregnanes, respectively. As an attractive alternative to acid hydrolysis, the isocyano group is first oxidized with Pb(OAc)4to an isocyanato group prior to hydrolysis (of the geminal isocyanato and sulfonyl groups) to the same 20‐oxo steroids. The latter conversion is carried out under non‐acidic conditions at room temperat

ISSN:0165-0513    

DOI:10.1002/recl.19921111102

出版商:WILEY‐VCH Verlag    

年代:1992

数据来源: WILEY

摘要:

AbstractThe reactions of the alkenoic acid derivatives1a–19awith sulfur trioxide were studied in the temperature range −60 to 25°C using dichloromethane as solvent and 1.5 equiv. of dioxane as reactivity moderator. The alkenoic acids13aand14a, having a ‐(CH2)4‐ and ‐(CH2)7‐ linkage between the C = C and CO2H moieties, respectively, react as simple alkenes; at low temperatures, they yield the β‐sultones13d, 14cand14d, which at 25°C are slowly converted into the corresponding carbyl sulfates13h, 14gand14h. Reactions of the alkenoic acids4a–12awith 1.2 equiv. of SO3at −60°C leads to the quantitative formation of the γ‐lactones4e–9eand11e, and in some cases to the γ‐lactones10fand12f. The stereochemistry of this sulfo‐lactonization is dependent on the SO3concentration, which is clearly demonstrated for the reactions of (E)‐ (5a) and (Z)‐3‐hexenoic acid (6a) with 1.0 equiv. and 10.0 equiv. of SO3. For reactions of5aand6awith ≤ 1.0 equiv. of SO3, the β‐sultones5cand6care the presumed sole initial products undergoing subsequent lactonization. Upon using an excess of SO3, in addition, the corresponding carbyl sulfates5gand6gwill undergo sulfocyclization to give thecis‐ (6e) andtrans‐γ‐lactones (5e), respectively.Reaction of 5‐norbornene‐endo‐2,endo‐3‐dicarboxylic anhydride (15a) with 1.2 equiv. of SO3at 25°C yields slowly but quantitatively the β‐sultone15c. This β‐sultone is surprisingly stable and it is very slowly converted into the γ‐lactone15eonly in the presence of H2O. Reactions of the norbornenecarboxylic acids and derivatives16a–19awith SO3yield the γ‐lactones16e, 18e

ISSN:0165-0513    

DOI:10.1002/recl.19921111103

出版商:WILEY‐VCH Verlag    

年代:1992

数据来源: WILEY

摘要:

AbstractThe enzymatic hydrolysis of a series of carboxylic esters by carboxylesterase NP has been investigated in order to determine the scope and limitations of this enzyme. 2‐Substituted propionates were hydrolyzed with high enantioselectivity when an aromatic moiety was part of the 2‐substituent. Enantioselective hydrolysis could be accomplished with several 2‐arylpropionates, 2‐(aryloxy)propionates andN‐arylalanine esters. The propionate esters yielded propionic acids as (S) enantiomers, whereas the alanine esters yielded the (R) enantiomers. Without a 2‐aryl substituent, the enzymatic hydrolysis of the propionates occurred at a lower rate without acceptable enantioselectivity. In addition to 2‐substituted propionates, only a few other esters were hydrolyzed with high enantioselectivity by carboxylesterase NP, such as some prochiral disubstituted malonates. 1‐Phenylethyl acetate was the only substrate with chirality in the alcohol part of the ester that was found to be hydrolyzed enantioselectively.Carboxylesterase NP proved to be a powerful enzyme for kinetic resolution of propionate esters with an aromatic ring containing

ISSN:0165-0513    

DOI:10.1002/recl.19921111104

出版商:WILEY‐VCH Verlag    

年代:1992

数据来源: WILEY

ISSN:0165-0513    

DOI:10.1002/recl.19921111106

出版商:WILEY‐VCH Verlag    

年代:1992

数据来源: WILEY

摘要:

AbstractZinc enolates of phenyl‐ andtrans‐styryl‐acetic acid methyl ester react with excellent diastereoselectivety and good enantioselectivety (e.e. 64 to 91.5%) with imines in high yields totrans‐2‐azetidinones. The corresponding lithium enolates reacted with much lower selectivity a

ISSN:0165-0513    

DOI:10.1002/recl.19921111107

出版商:WILEY‐VCH Verlag    

年代:1992

数据来源: WILEY