Preparation and Transmetallation of a Triphenylstannyl PD - Glucop yranoside : A Highly Stereoselective Route to p - D - C - Glycosides via Glycosyl Dianions

The triphenylstannyl p-u-glucopyranoside 4 was synthesized in one step from the 1.2-anhydro-a-uglucopyranose 3 with (hiphenylstannyl)lithium (Scheme 1) . Transmetallation of 4 with excess BuLi, followed by quenching the dianion 7 with CD,OD gave (1s)1 ,5-anhydro-3,4,6-tri-O-benzyl-f 1 -2H]-u-glucitol (8) in 81% yield (Scheme 2). Trapping of 7 with benzaldehyde, isobutyraldehyde, or acroleine gave the expected p-Dconfigurated products 11,12, and 13 in good yields. Preparation of C-acyl glycosides from acid chlorides, such as acetyl or benzoyl chloride was not practicable, but addition of benzonitrile to 7 yielded 84% of the benzoylated product 14. Treatment of 7 with Me1 led to 15 (30%) along with 40% of 18, C-alkylation being accompanied by halogen-metal exchange. Prior addition of lithium 2-thienylcyanocuprate increased the yield of 15 to 50% and using dimethyl sulfate instead of Me1 led to 77% of 15. No a-u-anomers could be detected, except with ally1 bromide as the electrophile, which yielded in a 1:l mixture of the anomers 16 and 17.