Features of the intramolecular Hosomi–Sakurai reaction of allylic cyclopentenylsilane containing an enol ether moiety

Abstract

A new intramolecular variant of the Hosomi–Sakurai reaction of 5-(2-methoxyvinyl)-4-((phenylthio)methyl)cyclopent-2-en-1-yl)trimethylsilane is presented, which is a by-process between hydrolysis to aldehyde and acetalization to dimethylacetal. The key steps are the generation of methoxy methylene carbocation from enol ether during acid catalysis and its subsequent attack on the allylsilane moiety with formation of norbornene derivative. In turn, the corresponding aldehyde undergoes Hosomi–Sakurai cyclization by interaction with BF3·Et2O in acetonitrile, as well as by reaction with TBAF in THF through various transition states, leading to epimeric alcohols of the same norbornene structure. At the same time, the starting enol ether by treatment with TBAF undergoes exclusively protodesilylation and hydrolysis, while with BF3·Et2O it gives mixtures of epimeric products of topology both bicyclo[2.2.1]heptane and bicyclo[3.2.0]heptane.