Synthetic studies on annonaceous acetogenins .3. Synthesis of two possible diastereoisomers of the epoxy lactone proposed for epoxyrollin A

Syntheses of two possible stereoisomeric epoxy lactones (20S,21R)- and (20R,21S)-1 that have been proposed for epoxyrollin A, a structural representative of biosynthetic precursors of tetrahydrofuran annonaceous acetogenins, are described. Protected dihydroxy compound 4, which is the latent epoxy moiety of (20S,21R)- and (20R,21S)-1, was prepared by an 8-step sequence via Sharpless asymmetric dihydroxylation, starting from allyl alcohol 6. Preparation of gamma-lactone moiety 5 was conducted by the method reported earlier, A palladium-catalyzed cross coupling reaction of 4 with 5 gave enyne 3, which, by a 3-step sequence, was converted into (20S,21R)- and (20R,21S)-1. The C-13-NMR and mass spectral data of the two synthetic diastereoisomers are not in accordance with those recorded for epoxyrollin A. Consequently, the structure of epoxyrollin A needs to be revised.