A Novel Approach to the Synthesis of Taxol. A Synthesis of Optically Active 3,7-dibenzyloxy-4,8-di-<i>t</i>-butyl-dimethylsiloxy-5,5-dimethyl-6-<i>p</i>-methoxybenzyloxy-2-octanone by Way of Stereoselective Aldol Reactions
of diastereoselective aldolreaction between 4-benzyloxy-5-t-butyldimethylsiloxy-2,2-dimethyl-3-p-methoxy-benzyloxypentanal (3) and ketene silyl acetal 4 using MgBr2·OEt2 as a catalyst. The chiral pentanal 3 was synthesized either by asymmetric aldolreaction of both prochiral aldehyde 5 and ketene silyl acetal 4 using a chiral Lewis acid or by diastereoselective aldolreaction between the chiral aldehyde
thoxybenzyloxy)–8β,15,15-trimethyl-4-methylene-trans-bicyclo[6.4.0]dodecan-9-one 2 that corresponds to BC ring system of Taxol was prepared from 8-membered ring enone 3 in high yield via stereoselctive Michael addition and successive intramolecular aldol cyclization. The above 8-membered ring enone 3 was synthesized from the linear optically active polyoxy-compound 4 by SmI2-mediated intramolecular
Optically active 7-t-butyldimethylsiloxy-4,8-dibenzyloxy-6,6-dimethyl-5-p-methoxybenzyloxy-2-cycloocten-1-one (1) was synthesized from 3,7-dibenzyloxy-4,8-di-t-butyldimethylsiloxy-5,5-dimethyl-6-p-methoxybenzyloxy-2-octanone (2) by way of intramolecular Reformatsky-type reaction using SmI2.