Stereoselective bromination of dehydroamino acids with controllable retention or inversion of olefin configuration
摘要:
Dehydroamino acid esters react with brominating reagents to produce syn-a-bromo imines as the major products, which undergo tautomerization to mixtures of the diastereomeric (E)- and (Z)-beta-bromo-alpha,beta-dehydroamino acids upon treatment with base. Herein, we examine the characteristics of dehydroamino acid substrates 1 that affect both the diastereofacial selectivity of the bromination and the configuration of the resulting olefin product(s) 3. In these studies, we demonstrate E- and Z-diastereoselectivity for this reaction using kinetic and thermodynamic reaction conditions. Z-Vinyl bromides were shown to be the thermodynamically more stable products and were formed from the kinetic E-isomers by DABCO-induced isomerization. High levels of kinetic E-selectivity were obtained by treatment of the intermediate alpha-bromo imines with hindered amine bases such as 2,2,6,6-tetramethylpiperidine or sodium hexamethyldisilazide. The reaction conditions detailed herein allow for the stereodivergent preparation of both E- and Z-vinyl bromide products from either diastereomeric E- or Z-olefin starting material with useful levels of diastereoselectivity.
Stereoselective bromination of dehydroamino acids with controllable retention or inversion of olefin configuration
摘要:
Dehydroamino acid esters react with brominating reagents to produce syn-a-bromo imines as the major products, which undergo tautomerization to mixtures of the diastereomeric (E)- and (Z)-beta-bromo-alpha,beta-dehydroamino acids upon treatment with base. Herein, we examine the characteristics of dehydroamino acid substrates 1 that affect both the diastereofacial selectivity of the bromination and the configuration of the resulting olefin product(s) 3. In these studies, we demonstrate E- and Z-diastereoselectivity for this reaction using kinetic and thermodynamic reaction conditions. Z-Vinyl bromides were shown to be the thermodynamically more stable products and were formed from the kinetic E-isomers by DABCO-induced isomerization. High levels of kinetic E-selectivity were obtained by treatment of the intermediate alpha-bromo imines with hindered amine bases such as 2,2,6,6-tetramethylpiperidine or sodium hexamethyldisilazide. The reaction conditions detailed herein allow for the stereodivergent preparation of both E- and Z-vinyl bromide products from either diastereomeric E- or Z-olefin starting material with useful levels of diastereoselectivity.
AN ASYMMETRIC SYNTHESIS OF α-BENZYLOXY ALDEHYDES HAVING A CHIRAL TERTIARY CENTER—AN APPLICATION TO THE ASYMMETRIC SYNTHESIS OF<i>exo</i>-(+)-BREVICOMIN—
作者:Masatoshi Asami、Teruaki Mukaiyama
DOI:10.1246/cl.1983.93
日期:1983.1.5
α-Benzyloxy aldehydes having a chiral tertiary center at α-carbon atom are synthesized in high enantiomeric excess by successive treatment of 2-methoxycarbonyl-3-phenyl-1,3-diazabicyclo[3.3.0]octane with diisobutylaluminum hydride (DIBAL-H) and Grignard reagents. The asymmetric reaction is applied to the total synthesis of exo-(+)-brevicomin.
Stereoselective bromination of dehydroamino acids with controllable retention or inversion of olefin configuration
作者:Robert S. Coleman、Andrew J. Carpenter
DOI:10.1021/jo00068a047
日期:1993.7
Dehydroamino acid esters react with brominating reagents to produce syn-a-bromo imines as the major products, which undergo tautomerization to mixtures of the diastereomeric (E)- and (Z)-beta-bromo-alpha,beta-dehydroamino acids upon treatment with base. Herein, we examine the characteristics of dehydroamino acid substrates 1 that affect both the diastereofacial selectivity of the bromination and the configuration of the resulting olefin product(s) 3. In these studies, we demonstrate E- and Z-diastereoselectivity for this reaction using kinetic and thermodynamic reaction conditions. Z-Vinyl bromides were shown to be the thermodynamically more stable products and were formed from the kinetic E-isomers by DABCO-induced isomerization. High levels of kinetic E-selectivity were obtained by treatment of the intermediate alpha-bromo imines with hindered amine bases such as 2,2,6,6-tetramethylpiperidine or sodium hexamethyldisilazide. The reaction conditions detailed herein allow for the stereodivergent preparation of both E- and Z-vinyl bromide products from either diastereomeric E- or Z-olefin starting material with useful levels of diastereoselectivity.