A total synthesis of (-)-eserethole (2) has been accomplished using the Diels-Alder reaction of (S)-(-)-2-methyl-2-[2-(E)-nitroethenyl]-δ-valerolactone (3) and Danishefsky's diene as a key step. The synthesis of optically pure (-)-eserethole (2) constitutes a formal synthesis of naturaly occurring Calabar bean alkaloids, such as (-)-physostigmine (1), (-)-physovenine (17) and (-)-geneserine (18).
An efficient asymmetric synthesis of nitroolefinic lactones with chiral nitroenamines possessing bulky chiral leaving groups
作者:Xiaowu Yang、Rui Wang
DOI:10.1016/s0957-4166(97)00452-7
日期:1997.10
An efficient asymmetric nitroolefination of enolates 4a-c, 5 afforded multifunctional group nitroolefinic lactones 7a-e and 8a,b containing stereogenic quaternary carbon centers using bulky chiral nitroenamines 1a,b–3a,b as chiral auxiliaries. Studies on the effect of the bulkiness of leavinggroup showed that bulky nitroenamines 1a,b gave higher ees and yields than those of the less bulky 2a,b–3a
Zinc enolates derived from an ester and a lactone, 1 and 4, were found to have enhanced reactivity over the corresponding lithium enolates in asymmetric nitroolefination.
New chiral nitroenamines 4a,b having (S)-2-t-butyldimethylsiloxymethylpyrrolidine as an auxiliary were found to be very effective for asymmetric nitroolefination of alpha-alkyl-gamma- and delta-lactones. The enantiomeric excess of the product increased remarkably in the reaction with gamma-lactones compared with previous nitroenamines 1a,b. A possible chelation model for the transition state of the asymmetric nitroolefination is discussed.