ENANTIOSELECTIVE SYNTHESIS OF (+)-MURICATACIN AND (-)-MURICATACIN THROUGH S(E)2' ADDITION OF NONRACEMIC GAMMA-SILYLOXY ALLYLIC STANNANES TO ALDEHYDES

The acetogenins (R,R)-(+)- and (S,S)-(-)-muricatacin were synthesized through the highly stereoselective addition of gamma-silyloxy allylic stannanes (S)-6 and (R)-6 to (E)-ethyl 8-formylpropenoate (12) in the presence of BF3.OEt(2). The resulting adducts (R,R)-13 and (S,S)-13 afforded the aforementioned acetogenins upon catalytic hydrogenation and subsequent deprotection-lactonization with aqueous HF. Interestingly, additions of stannanes 6 (racemic) to the saturated aldehydes RCH(2)CH(2)CHO (8, R = CO(2)Et, R = CN, R = Cl) in the presence of BF3.OEt(2) afforded ca. 70:30 mixtures of syn and anti adducts 9a-c. In contrast, the n-alkyl aldehyde 8 (R = n-C4H9) underwent highly selective syn addition (97:3) with the crotyl analogue 7 of stannane 6.