REMOTE CONFORMATIONAL BIAS EFFECTS ON DIASTEREOFACIAL SELECTIVITY IN S(E)2' ADDITIONS OF GAMMA-OXYGENATED ALLYLIC STANNANES TO CHIRAL ENALS

The enal 10 derived from (R,R)-diethyl tartrate shows matched/mismatch ed characteristics in BF3-promoted additions of the chiral gamma-oxyge nated allylic stannanes S1, S2, R1, and R2. Both S2 and S1 afford a si ngle syn adduct 11 and 14 with enal 10, whereas R2 and R1 give mixture s of syn and anti products 12/13 and 15/16. Racemic stannane RS2 affor ds a 82:18 mixture of syn adducts 11 and 12; RS1 gives the two syn add ucts 14 and 15 as a 77:23 mixture. The observed facial bias in these a dditions is attributed to conformational effects engendered by the vic inal syn OTBS substituents which cause enal 10 to adopt a chair-like c onformation. The matched additions proceed by attack on the ''outside' ' face of the carbonyl grouping in the s-cis orientation of this chair -like arrangement.