Stereopure 1,3-butadiene-2-carboxylates and their conversion into non-racemic α-alkylidenebutyrolactone natural products by asymmetric dihydroxylation

Dienoic esters 1 with the four possible permutations of the CC configurations were prepared in two steps via non-stereoselective aldol additions followed by stereospecific β-eliminations. Sharpless dihydroxylations of these esters yielded natural and unnatural α-alkylidene-β-hydroxybutyrolactones 2. Among these were synthetic dihydromahubanolide B (cis,Z-2a), isodihydromahubanolide B (cis,E-2a) and, for the first time, litsenolide D1 (ent-trans,Z-2b) and the enantiomer trans,E-2b of litsenolide D2. Competitively, dihydroxyesters 10 were formed.