Chemistry of dioxenium cations. Synthetic and mechanistic studies on the stereocontrolled formation of tetrahydropyrans from homoallylic alcohols and ortho esters
作者:Francoise Perron-Sierra、Michele A. Promo、Van A. Martin、Kim F. Albizati
DOI:10.1021/jo00021a043
日期:1991.10
Despite their long history, dioxenium cations are underutilized reactive synthetic intermediates. It was found that ortho esters and homoallylic alcohols in the presence of Lewis acids provide 4-heterosubstituted pyranosides in a stereoselective manner. The mechanistic course of events was supported by control experiments and synthesis of a putative mixed ortho ester intermediate which exhibited identical reactivity. A transition state for termination of the dioxenium cation-olefin cyclization is proposed, involving intramolecular delivery of chloride by a coordinated tin species. Structure-reactivity relationships indicate that a cation-stabilizing substituent (alkyl or alkoxy) at the internal position of the olefin is required for cyclization. A variety of 3-alkyl-substituted homoallylic alcohols cyclize cleanly to substituted 2-alkoxytetrahydropyrans in good yield. beta-silyloxy silyl enol ethers were found to smoothly provide 4-oxotetrahydropyranosides when subjected to the same reaction conditions. For these substrates, the course of the cyclization proceeds in a different manner involving a rapid intermolecular Mukaiyama aldol condensation followed by transacetalization.
US3996303A
申请人:——
公开号:US3996303A
公开(公告)日:1976-12-07
A highly stereoselective synthesis of trans-trisubstituted olefinic bonds
作者:Stephen F. Brady、Marcia A. Ilton、William S. Johnson
DOI:10.1021/ja01013a027
日期:1968.5
Nonenzymic, biogenetic-like cyclization of a tetraenic acetal to produce the D-homosteroid nucleus
作者:William S. Johnson、Koenraad Wiedhaup、Stephen F. Brady、Gary L. Olson