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

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Abstract
Total syntheses of ent-5-epi-montecristin (1a) and of (−)-montecristin (1b) were accomplished. The stereocenters of compounds 1a and 1b were established by asymmetric dihydroxylations of the trans-configurated β,γ-unsaturated 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 stereogenic CC bonds stem from the carbocuprations 48 → 49 and 50 → 51 (Scheme 9). Treating hydroxylactones 27 (Scheme 7), 3a (Scheme 12) and 3b (Scheme 13) with PPh3 and DEAD, we found a racemization-free dehydration giving butenolide 26 and epimerization-free dehydrations giving butenolides 2a and 2b . Relating the [α]D values of synthetic 1a and 1b to the [α]D value of natural (+)-montecristin, the absolute configuration of its side-chain stereocenters was determined to be R.

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