DIASTEREOSELECTIVE ADDITIONS OF ENANTIOENRICHED (GAMMA-ALKOXYALLYL)STANNANES TO ALPHA-ALKOXY ALDEHYDES - A SYNTHETIC ROUTE TO CARBOHYDRATES

BF3.OEt2-promoted additions of the (S)-(gamma-alkoxyallyl) stannane 3-(S) to the (R)-alpha-alkoxy aldehydes 13 and 18 affords the syn adducts 14 and 19 with greater than 90:10 diastereoselectivity. With MgBr2 as the catalyst addition to the (S)-alpha-alkoxy aldehyde 4 is most selective (97:3) with the (S)-(gamma-alkoxyallyl)stannane 3-(S) whereas BF3-promoted addition to 4 is most selective (92:8) with the R enantiomer 3-(R).