Enantioselective synthesis of the papulacandin ring system: conversion of the mannose diastereoisomer into a glucose stereoisomer.

[structure] An enantioselective synthesis of three diastereoisomers of the C-arylglycoside tricyclic spiroketal nucleus of the papulacandins has been achieved, in which the initial asymmetry was introduced via a Sharpless dihydroxylation of substituted 5-aryl-2-vinylfurans. A selective oxidation-reduction sequence converted the mannose isomer into the glucose isomer. This sequence can conveniently produce both the papulacandin ring system along with its enantiomer and diastereomers in only 10-14 steps from 3,5-dimethoxybenzyl alcohol in 5-8% overall yield.