DOUBLE STEREODIFFERENTIATION IN THE LEWIS ACID PROMOTED ADDITION OF CHIRAL (E)-CROTYLSILANES WITH (S)-2-(BENZYLOXY)PROPANAL - EFFECT OF LEWIS ACID ON REACTION DIASTEREOSELECTION

Methyl alpha-methyl-beta-(dimethylphenylsilyl)-(E)-hex-4-enoates (28,3R)-1a undergo BF3.OEt2-Promoted additions with (S)-2-(benzyloxy)propanal (2) producing nearly enantiomerically pure cis-2,5-substituted tetrahydrofurans 3 whereas the reactions run under chelation controlled conditions employing SnCl4 resulted in the formation of the complementary trans-2,5-disubstituted tetrahydrofuran. Additions to the enantiomeric silane (2R,3S)-1b utilizing BF3.OEt2 or SnCl4 afforded the cis-2,5-disubstituted furan stereochemistry in both cases.