SYNTHETIC STUDIES ON ANNONACEOUS ACETOGENINS .3. SYNTHESIS OF 2 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 r epresentative of biosynthetic precursors of tetrahydrofuran annonaceou s acetogenins, are described. Protected dihydroxy compound 4, which is the latent epoxy moiety of (20S,21R)- and (20R,21S)-1, was prepared b y an 8-step sequence via Sharpless asymmetric dihydroxylation, startin g from allyl alcohol 6. Preparation of gamma-lactone moiety 5 was cond ucted by the method reported earlier, A palladium-catalyzed cross coup ling reaction of 4 with 5 gave enyne 3, which, by a 3-step sequence, w as converted into (20S,21R)- and (20R,21S)-1. The C-13-NMR and mass sp ectral data of the two synthetic diastereoisomers are not in accordanc e with those recorded for epoxyrollin A. Consequently, the structure o f epoxyrollin A needs to be revised.