AN ALTERNATIVE ROUTE TO ENANTIOENRICHED ALPHA-ALKOXYALKYLSTANNANES BYSTEREOSELECTIVE OPENING OF CHIRAL ALPHA-STANNYLACETALS WITH ORGANOMETALLIC REAGENTS

alpha-Tributylstannylacetals derived from chiral C-2 symmetry axis dio ls were reacted with miscellaneous organometallic reagents to give chi ral alpha-oxygenated organotins in high yields. The Lewis acid promote d ring opening of the these chiral alpha-tributylstannylacetals by org anocopper reagents, allyltins or silylenol ethers has been considered to occur mainly according to an anti process (d.r. = 70/30 to 93/7), t he absolute configuration of the newly created centre being S when the reaction was performed with Me2CuLi/BF3 on the alpha-stannylacetal de rived from (2S,4S)-2,4-pentanediol. Of interest is the reverse stereoc hemical trend obtained using organo-aluminium reagents (d.r. = 30170 t o 15/85) since it becomes possible to reach selectively the new chiral centre with a preferential R or S configuration starting from the sam e precursor. The obtained alpha-alkoxyalkylstannanes can be transmetal lated with n-butyllithium (THF, -78 degrees C) to give configurational ly stable alpha-alkoxyalkyllithiums, Furthermore, if desired, the enan tioenriched alpha-alkoxyalkylstannanes derived from 2,4-pentanediol ca n be converted into enantioenriched alpha-hydroxyalkylstannanes (subse quently protected as MOM derivatives) with retention of the configurat ion at the asymmetric carbon using an appropriate oxidation-beta-elimi nation sequence. (C) 1997 Elsevier Science Ltd.