Chiral γ-(tetrahydropyranyloxy)allylstannane: a new chiral reagent for the asymmetric synthesis of syn 1,2-diols

(2R,3S)-3-Allyloxy-3,4,5,6-tetrahydro-2H-pyran-2-methanol was prepared in high yield from tri-O-acetyl-d-glucal, an easily available and cheap chiral source. The hydroxyl group was protected as the t-butyldiphenylsilyl ether, and the allyl group was converted to the corresponding allyl stannane via formation of allyl carbanion and subsequent trapping with tributylstannyl chloride. The γ-alkoxy allyl stannane 1 bearing the (2R,3S)-2-[(t-butyldiphenylsilyloxy)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 · OEt₂ or AlCl₃ gave the corresponding syn adducts 2 with high diastereoselectivities; the ratio of syn/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%. Removal of the protecting group of 2 gave the (R,R) diols 3. The high diastereo- and enantioselectivity in the formation of the (R,R) isomer is accounted for by an anti s′_E transition state 18.