Trans-1,2-Glycosides

In 1964, Kochetkov, Khorlin and Bochkov reported that the reaction of 1,2-alky-lorthoacetates with alcohols in the presence of catalytic amounts of HgBr2 and pTsOH furnished acetylated 1,2-trans glycosides or isomeric orthoesters depending on the reaction conditions [4]. Polar solvents (nitromethane, acetonitrile) and large amounts of catalyst promoted glycosylation (a, Scheme 5.5), whereas solvents of low polarity (dichloroethane) and the use of small amounts of catalyst favored transorthoesterification (b, Scheme 5.5) [16]. 

The lipoarabinomannan (LAM) capsule is the major vimlence factor of M. tuberculosis [88]. The multifaceted architecture, shown as 143 (Figure 5.2) by Turnbull ct al. [89], anticipates the complexity of any synthetic scheme aimed to its conquest. However, a simple analysis of the molecule shows that the mannan and arabino segments possess 1,2-trans glycosidic linkages, which point toward a retrosynthesis in which NPOEs could be the donors of choice. 

The anomeric configuration of the newly formed bond should be reasonably well controlled (i.e., major 1,2-cis or 1,2-trans glycosides). 

As expected, unprotected MOP donors containing a 2-acetamido group give 1,2-trans-glycosides (Scheme 3), presumably by the intermediacy of an oxazolinium ion 15. 

Scheme 7 shows examples of 1,2 trans-glycosides synthesized from MOP glycosyl donors that have neighboring participating groups. In the presence of Cu(OTf)2, the... 

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