LEWIS-ACID PROMOTED ADDITIONS OF GAMMA-ALKOXY-)CROTYLSTANNANES AND GAMMA-(SILYLOXY)CROTYLSTANNANES TO (S)-2-(BENZYLOXY) PROPANAL

Additions of the gamma-oxygenated allylic stannane MOM ethers (R)-1a, (S)-1a, and silyl ethers (R)-1b, (S)-1b to (S)-2-(benzyloxy)propanal (2) in the presence of BF3.OEt(2) and MgBr2 were examined in order to establish matched and mismatched preferences. In the BF3 reactions, stannanes (R)-1a and (R)-1b were matched with 2. The former gave the syn adduct 3 and the cyclopropane 4 as a 93:7 mixture. The OTBS stannanes (R)-1b and (S)-1b gave only the syn adducts 7 and 9, respectively. However, in each case considerable cyclopropane adduct, 8 and 10 respectively, was also formed. When aldehyde 2 was treated with excess racemic stannane, (RS)-1a, adduct 3 derived from (R)-1a was the major product, whereas with (RS)-1b, products 7 and 8 derived from (R)-1b were formed preferentially. Aldehyde 2 showed a preference for MOM ether (S)-1a in the MgBr2-promoted reaction, but each of the enantiomeric OTBS stannanes (R)-1b and (S)-1b afforded a single product, the (Z)-syn,syn alcohol 13 from the former and the (E)-syn,syn. alcohol 9 from the latter. With excess (RS)-1b, aldehyde 2 reacted fastest with (R)-1b, in contrast to the MOM analogue(RS)-1a. The racemic (E) stannanes (E)-1a and (E)-1b were likewise examined. In the BF3 reaction, the major products (5 from the MOM ether (E)-1a and 9 from the silyl ether (E)-1b) were derived from the (R)-enantiomer. With MgBr2, the OMOM stannane (E)-1a gave rise to a mixture of four adducts arising from both the (R) and (S) enantiomers of the stannane. The OTBS stannane (E)-1b, on the other hand, afforded mainly adduct 9 derived from the (R) enantiomer of stannane (E)-1b.