Remote Participation of Protecting Groups at Remote Positions of Donors in Glycosylations

Abstract

Effects of potentially participating groups at remote positions of glycopyranosyl and glycofuranosyl donors on the glycosylation stereochemistry are reviewed. Substantial evidences in favor of the remote participation by protecting groups are presented and also included are a few reports opposed to the remote participation. A. IntroductionT h e p r o t e c t i n g g r o u p s i n t h e o l i g o s a c c h a r i d e synthesis are used to selectively block interfering functions as well as influence the reactivity and stereoselectivity in the glycosylation steps. The anchimeric assistance of a neighboring participating group such as an acyl group at the O-2 position of the glycosyl donor has been one of the most useful strategies for the stereoselective formation of 1,2-trans glycosides (1−5). The carbonyl functionality at the O-2 position of the glycosyl donor can attack an incipient oxonium ion to give a dioxonium ion. Nucleophilic attack by an acceptor to the opposite face of the dioxonium ion provides the 1,2-trans glycoside. When potentially participating groups are located at remote positions other than the O-2 position of glycosyl donors, the existence of the remote participation by the remote protecting groups in the glycosylation step has been controversial. Although numerous examples for glycosylations with glycosyl donors possessing potentially participating groups at remote positions can be found in the literature, most of them do not provide any information on the remote participation by protecting groups in glycosylations. There have been reports both in favor of and opposed to the remote participation by protecting groups in glycosylations. Herein we review the effects of potentially participating protecting groups at remote positions of glycopyranosyl and glycofuranosyl donors on the glycosylation stereochemistry. Further, we present substantial evidence in favor of the remote participation by protecting groups and also include a few reports that are opposed to remote participation.

On a so-called “kinetic anomeric effect” in chemical glycosylation

“…Thirdly, multiple examples to examine the scope of "interesting" (which may be synonymous with "unusual") diastereoselectivities may result in their being overrepresented in the literature. Fourthly, the effect of solvents 5 on glycosylation diastereoselectivity, and the influence of protecting groupsthe possible participation of C2 esters is well documented, but remote protecting groups [6][7][8] may also play a roleresult in a very complex picture.…”

The intriguing dual-directing effect of 2-cyanobenzyl ether for a highly stereospecific glycosylation reaction

“…On the other hand, neighbouring hydroxyl groups of the donor could show influence on the anomeric preference through their protecting substituents, many of which were routinely exploited in various glycosylation methods, such as 2-O-ester-type [8][9][10] or 2-O-picolyl-type [11][12][13] for 1,2-trans glycosylation, and several sulfide auxiliaries [14][15][16][17][18] as well as intramolecular aglycon delivery IAD-type 19,20 for 1,2-cis glycosylation reactions.…”

Anomeric spirocycles by solvent incorporation: reactions of O-peracylated (glyculopyranose and glyculopyranosyl bromide)onamide derivatives with ketones

“…Formation of the by-products 4 and 7 are to be explained by the water content of the solvents attacking on glycosyliumion B1. The finding that from the D-gluco configured 5 only the formation of 6 was observed, 2 while 1 of the D-galacto configuration gave both epimers 2 and 3, may be indicative of a remote participation 29 of the axial 4-O-acetyl group of 1 facilitating the formation of 3.…”