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

The reaction of N,N-diethyl-2-(dimethylsulfuranylidene)acetamide (1) with p rotected monosaccharides has been extended to several O-benzyl- and 4,6-O-B enzylidene-D-gluco- and -D-galactopyranose derivatives. When the monosaccha ride is 2,3,4,6-tetra-O-benzyl-D-glucose (6) or D-galactose (9), eliminatio n of the 3-benzyloxy substituent occurs, to give the unsaturated epoxyamide s 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-glucopyr anose derivatives 11, 14, and 17 yielded the corresponding epoxyamides 12, 15a:15b, and 18a:18b in good yields and variable stereoselectivity. In acco rdance with previous studies concerning the configurational assignments for the epoxide derivatives 3, 5a, and 21,(6) obtained from 2, 4, and 20, resp ectively, 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 unprote cted (4, 11, 20), the major epoxide formed has the configuration 2S, 3R (ep oxyamides 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 epo xides the corresponding 2R, 3S isomers (3, 15b, and 18b).