SYNTHESIS OF ENANTIOENRICHED HOMOPROPARGYLIC ALCOHOLS THROUGH DIASTEREOSELECTIVE-SE' ADDITIONS OF CHIRAL ALLENYLSTANNANES TO ALDEHYDES

Allenylstannanes (S)-4 and (R)-4, available in ca. 90% ee from alkynones 1 through reduction with the LiAlH4-Darvon alcohol or -ent-Darvon alcohol complex, followed by S(N)2' displacement on the derived mesylates (R)-3 or (S)-3 with Bu3SnLi.CuBr.Me2S, readily add to various aldehydes under Lewis acid catalysis to afford optically active homopropargylic alcohols with good to excellent syn diastereoselectivity. With 2-(benzyloxy)propanal (48), MgBr2-catalyzed reactions are highly stereoselective, affording the syn adduct 49 from the (S)-stannane (S)-4 and the anti adduct 52 from the (R)-stannane (R)-4. BF3-promoted additions give mainly or exclusively the syn adducts 49 and 51. Additions of (S)- and (R)-4 to (R)-3-(benzyloxy)-2-methylpropanal (61) yield the syn adducts 62 and 64 as major or exclusive products.