Vanadium(V)-Catalyzed Oxidative Bromination of Acid Labile Alkenols and Alkenes in Alkyl Carbonates

Abstract

Molecular bromine is formed from bromide and tert -butyl hydroperoxide under mild and neutral conditions. The oxidation is catalyzed by vanadium(V)-complexes and requires bromide- and proton- aliquots that are slowly released from a 3-bromopropionic acid-bromide buffer in solutions of alkyl carbonates. In such an environment, bromocyclization of acid labile alkenols occurs without hydrolytically cleaving acetal- or ester-protecting groups. 4-Pentenols having methyl- and/or phenyl-groups attached to the terminal carbon atom of the alkenol double bond undergo 6- endo -selective ring closures if subjected to oxidative bromination and provide bromotetrahydropyrans in synthetically useful yields. Application of the new procedure affords a hexasubstituted tetrahydropyran-building block en route to synthesis of the marine natural product aplysiapyranoid A.