CHIRAL GAMMA-(TETRAHYDROPYRANYLOXY)ALLYLSTANNANE - A NEW CHIRAL REAGENT FOR THE ASYMMETRIC-SYNTHESIS OF SYN 1,2-DIOLS

-3-Allyloxy-3,4,5,6-tetrahydro-2H-pyran-2-methanol was prepared in hig h yield from tri-O-acetyl-D-glucal, an easily available and cheap chir al source. The hydroxyl group was protected as the t-butyldiphenylsily l ether, and the allyl group was converted to the corresponding allyl stannane via formation of allyl carbanion and subsequent trapping with tributylstannyl chloride. The gamma-alkoxy allyl stannane 1 bearing t he lyloxy)methyl]-3,4,5,6-tetrahydro-2H-pyran-3-yloxy auxiliary at the allyl terminus was prepared by this procedure. The reaction of 1 with aldehydes in the presence of AlCl3 . OEt(2) or AlCl3 gave the corresp onding syn adducts 2 with high diastereoselectivities; the ratio of sy n/anti > 97:3. The diastereomeric excess of the syn adducts 2, that is , the ratio 2/(2 + the (S,S) isomer of 2), varied from 85 to 94%. Remo val of the protecting group of 2 gave the (R,R) diols 3. The high dias tereo- and enantioselectivity in the formation of the (R,R) isomer is accounted for by an anti S'(E) transition state 18.