Reaction of O-benzyl- and 4,6-O-benzylidene-D-gluco- and D-galactopyranose derivatives with amide-stabilized sulfur ylides:: Stereoselectivity and reactivity

The reaction of N,N-diethyl-2-(dimethylsulfuranylidene)acetamide (1) with protected monosaccharides has been extended to several O-benzyl- and 4,6-O-Benzylidene-D-gluco- and -D-galactopyranose derivatives. When the monosaccharide is 2,3,4,6-tetra-O-benzyl-D-glucose (6) or D-galactose (9), elimination of the 3-benzyloxy substituent occurs, to give the unsaturated epoxyamides 7 and 10, respectively, in reasonable yields and poor stereoselectivity. On the other hand, the, reaction of 1 with the 4,6-O-benzylidene-D-glucopyranose derivatives 11, 14, and 17 yielded the corresponding epoxyamides 12, 15a:15b, and 18a:18b in good yields and variable stereoselectivity. In accordance with previous studies concerning the configurational assignments for the epoxide derivatives 3, 5a, and 21,(6) obtained from 2, 4, and 20, respectively, the present article confirms the role of the hydroxyl group at C-2 on the stereselectivity of the reaction. Thus, when the C-2 OH is unprotected (4, 11, 20), the major epoxide formed has the configuration 2S, 3R (epoxyamides 5a, 12, and 21). Conversely, derivatives with the hydroxyl group protected at C-2, or a-deoxy sugars (2, 14, and 17), yield as the major epoxides the corresponding 2R, 3S isomers (3, 15b, and 18b).