HIGHLY STEREOSELECTIVE SE' ADDITIONS OF ALPHA-ALKOXY ALLYLSTANNANES TO CHIRAL ALDEHYDES - SYNTHESIS OF A C-1-C-9 SUBUNIT OF TYLONOLIDE

The racemic alpha-alkoxy allylic stannane RS4 adds to the 2S aldehyde 24 to afford the homoaldol adduct 25 (45%) derived exclusively from the S enantiomer S4 along with isomerized (R)-gamma-alkoxy allylstannane 26 of > 80% ee (50%) and unreacted aldehyde (40%). Use of the nonracemic alpha-alkoxy allylstannane 28 (1:1 mixture of diastereomers) in excess leads to the homoaldol adduct 29, which is transformed in two steps to lactol ether 31, an intermediate in Nicolaou's synthesis of O-micinosyl tylonolide.