A Non-Aldol Preparation of Enantiopure Propionate-Derived Motifs with the Assistance of Chiral Sulfoxides: Application to a Convergent Synthesis of the Lactone Core of Octalactins

摘要

Propionate-derived fragments were prepared by a non-aldol method using chiral sulfoxide chemistry. A methacrylate ester, assisted by a chiral sulfoxide as an auxiliary agent, was easily transformed into an optically pure allylic alcohol, which was then subjected to diastereoselective hydroboration with the bulky dialkylborane 9-BBN. The diastereofacial selectivity was governed by a preferred spatial arrangement of a staggered conformation model, as suggested by Houk, taking into account of the various A(1,2) allylic strains. Thus, the the two adjacent stereocentres in the enantiopure allylic alcohol were installed with an anti configuration. As an application of this work, we have described an enantioselective synthesis of the unusual eight-membered lactone ring of the octalactins. Two propionate-derived fragments were coupled to give precursor 18, which was subjected to Yamaguchi's macrolactonisation to give the required lactone ring.