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