ARMED-DISARMED EFFECTS IN GLYCOSYL DONORS - RATIONALIZATION AND SIDETRACKING

: A general rationalization for armed/disarmed effects, first recognized in n-pentenyl glycosides but recently extended to a variety of other glycosyl donors, is postulated. Reaction of a glycosyl donor with an appropriate electrophile gives a positively charged intermediate which is less favorable when there is an adjacent electron-withdrawing group (for example OCOR, as in a disarmed donor) than when there is an adjacent alkoxy group (as in the armed counterpart). The latter therefore reacts faster and if, in the reaction medium, there is a disarmed species carrying a free hydroxyl group, a pathway based on Le Chatelier's principle can be envisaged that leads to products of cross-coupling with (virtually) none of the self-coupled analogue. In n-pentenyl glycosides activation of the anomeric center involves two preequilibrium steps, the second of which can be sidetracked to afford a vicinal dibromo derivative. The ability to prepare such derivatives in near quantitative yields allows the normal armed/disarmed protocol for saccharide assembly to be reversed. © 1990, American Chemical Society. All rights reserved.