A simple method for avoiding alkylthio group migration during the synthesis of thioglycoside 2,3-orthoesters. An improved synthesis of partially acylat
✍ Scribed by Vince Pozsgay
- Publisher
- Elsevier Science
- Year
- 1992
- Tongue
- English
- Weight
- 508 KB
- Volume
- 235
- Category
- Article
- ISSN
- 0008-6215
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✦ Synopsis
The potential usefulness of alkyl and aryl 1-thioglycosides as intermediates in oligosaccharide synthesis was recognized many years ago'. Since the revival of the concept of direct activation of alkyl thioglycosides by Liinn* in 1984, such compounds became familiar building blocks in oligosaccharide syntheses either as glycosyl acceptors or glycosyl donors3.
Although thioglycosides have often been used with success in oligosaccharide syntheses, a number of undesirable transformations were also observed such as glycal formation', alkylthio group transfer2*4*5, and decomposition5.
Recently, Auzanneau and Bundle6 reported an additional, serendipitous observation involving a thiogiycoside. They found that major structural changes of ethyl 1-thio-a-r.-rhamnopyranoside7 (7) occurred when preparation of its cyclic, 2,3-orthoester (A, Scheme 1) was attempted, using trialkyl orthoacetates or trialkyl orthobenzoates as reagents, according to the protocol of Garegg and Hultbergs that became a standard procedure for the O-rhamnopyranosides' (Scheme 1). These changes included the stereospecific migration of the anomeric, ethylthio group to C-2 and the attack by the alkoxy group, derived from either the reagent or the product orthoester, at the anomeric carbon to yield the L-gluco derivative B. The migration could be suppressed by employing an optimized set of reaction conditions. Thus, treatment of the trio1 7 under catalysis by 4-toluenesulfonic acid, in NJV-dimethylformamide as the solvent' at 50" for two days, followed by acetic