ENANTIOSELECTIVE AND DIASTEREOSELECTIVE ADDITIONS OF ALLYLIC STANNANES TO ALDEHYDES PROMOTED BY A CHIRAL (ACYLOXY)BORANE CATALYST

A modified Yamamoto Lewis acid (CAB), prepared from the 2,6-dimethoxyb enzoic ester of (R-R)-tartaric acid, and 1.5 equiv of BH3.THF was empl oyed in additions of crotyltributyltin (6) and allyltributyltin (9) to representative achiral aldehydes in the presence of 2 equiv of (CF3CO )(2)O. The crotyltin additions proceeded with good to excellent diaste reoselectivity and enantioselectivity affording the syn adducts 7a-e o f 70-90% ee as major products (78:22-92:8). Addition of allylstannane 9 to cyclohexanecarboxaldehyde (1a) afforded the (R)-adduct of only 55 % ee. In contrast, the use of Keck's BINOL catalyst gave 10, the allyl adduct of la, of 87% ee. However, addition of the crotylstannane to 1 a with this catalyst led to a 65:35 mixture of syn and anti adducts 7a and 8a of 95% and 49% ee. Additions of crotylstannane 6 to (R)- and ( S)-2-methyl-3-ODPS-propanal [(R)-11 and (S)-11] promoted by the modifi ed CAB Lewis acid afforded the syn,syn and syn,anti products 12 and 14 in large predominance (98:2 and 90:10), indicative of effective compl ex control in the transition state. The results are consistent with th e Corey H-bonded aldehyde transition-state proposal.