Total synthesis of the cytotoxic threo, trans, threo, trans, threo annonaceous acetogenin asimin and its C-10 epimer:: Unambiguous confirmation of absolute stereochemistry

A convergent synthesis of asimin (1) and its C-10 epimer 33 is reported. The essential features of this synthesis include (a) the addition of an enantioenriched gamma-OMOM allylic indium reagent to a core C-23 aldehyde precursor to install the C-24-C-34 segment with concomitant introduction of the C-24 and C-23 stereocenters; (b) the addition of an enantioenriched gamma-OMOM allylic indium reagent to a core C-16 aldehyde to install the C-10-C-15 segment with formation of the C-15 and C-16 stereocenters, (c) the addition of a dialkyl zinc reagent, catalyzed by a chiral triflamide-Ti(O-i-Pr)(4) complex, to introduce the C-1-C-9 segment with creation of either the 10(R) or 10(S) stereocenters; and (d) aldol condensation of the foregoing C-1-C-34 segment with OTBS-protected lactic aldehyde to incorporate the C-35-C-37 butenolide segment. Removal of the three MOM protecting groups was achieved with aqueous HCl in THF. The 10(R) diastereomer was found to correspond to natural asimin.