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Crich beta-mannosylation
The Crich β-mannosylation is a synthetic strategy which is used in carbohydrate synthesis to generate a 1,2-cis-glycosidic bond. This type of linkate is generally very difficult to make, and specific methods like the Crich β-mannosylation are used to overcome these issues.
== Background ==
The development of facile chemical glycosylation protocols is essential to synthesizing complex oligosaccharides. Among many diverse type of glycosidic linkages, the 1,2-''cis''-β-glycoside, which exists in many biologically relevant glycoconjugates and oligosaccharides, is arguably one of the most difficult to synthesize.〔Gridley, J. J.; Osborn, H. M. I. ''J. Chem. Soc., Perkin Trans. 1'' 2000, 1471.〕 The challenges in constructing β-mannose linkage have been well documented in several reviews.〔Kaji, E.; Lichtenthaler, F. W. ''Trends Glycosci. Glycotechnol.'' 1993, ''5'', 121.〕〔Banoub, J. ''Chem. Rev.'' 1992, ''92'', 1167.〕 To date, a few laboratories have devised efficient methodologies to overcome these synthetic hurdles, and achieved varying degrees of success. Of those elegant approaches, a highly stereoselective β-mannosylation protocol developed by Crich and co-workers was realized as a breakthrough in β-mannoside synthesis.〔Crich, D.; Sun, S. ''J. Org. Chem.'' 1996, ''61'', 4506.〕〔Crich, D.; Sun, S. ''J. Org. Chem.'' 1997, ''62'', 1198.〕〔Crich, D.; Sun, S. ''J. Am. Chem. Soc.'' 1998, ''120'', 435.〕 This strategy is based on the initial activation of α-mannosyl sulfoxides 1 with triflic anhydride (Tf2O) using DTBMP (2,6-di-''tert''-butyl-4-methylpyridine) as a base, followed by nucleophilic substitution of glycosyl acceptors (HOR3) to provide the 1,2-''cis''-β-glycoside 2 in good yield and selectivity (Scheme 1).
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