Elucidation of the stereostructure of the annonaceous acetogenin (+)-montecristin through total synthesis

Total syntheses of ent-5-epi-montecristin (1a) and of (-)-montecristin (1b) were accomplished. The stereocenters of compounds 1a and 1b were establish ed by asymmetric dihydroxylations of the trans-configurated beta,gamma -uns aturated esters 6 (-->4, up to 80% ee; Scheme 3; improved procedure with up to 94% ee: Scheme 7) and 56 (--> 55, 97% ee: Scheme 9) while the stereogen ic C2C bonds stem from the carbocuprations 48 --> 49 and 50 --> 51 (Scheme 9). Treating hydroxylactones 27 (Scheme 7), 3a (Scheme 12) and 3b (Scheme 1 3) with PPh3 and DEAD, we found a racemization-free dehydration giving bute nolide 26 and epimerization-free dehydrations giving butenolides 2a and 2b. Relating the [alpha](D) values of synthetic 1a and 1b to the [alpha](D) va lue of natural (+)-montecristin, the absolute configuration of its side-cha in stereocenters was determined to be R.