(Z)-GAMMA-SILYLOXY AND (E)-GAMMA-SILYLOXY ALLYLIC STANNANES - HIGHLY SYN SELECTIVE REAGENTS FOR SE' ADDITIONS TO ALDEHYDES

The (E)-gamma-silyloxy allylic stannane 2E, available in one step through addition of Bu(Bu3Sn)Cu(CN)Li2 to crotonaldehyde and subsequent in situ quenching of the enolate with t-BuMe2SiCl, undergoes BF3-promoted addition to representative aldehydes 3a-e, affording syn adducts 4a-e with >99:1 diastereoselectivity. The (Z)-gamma-silyloxy allylic stannane 2Z can be prepared by treatment of the adduct from Bu3SnLi and crotonaldehyde with TBSOTf in the presence of i-Pr2NEt. Stannane 2Z also affords syn adducts upon BF3-promoted addition to aldehydes 3a-e but with somewhat lower diastereoselectivity (93:7-99:1).