Glycosyl isonitriles

Table 4.5 Ugi and Passerini reactions with glycosyl isonitriles...

It was found that a variety of stabilized carbon-centred radicals such as glycosyl, benzyl, a-amino, a-alcoxy, a-carbalcoxy and aryl derivatives, generated from the corresponding organotellurium compounds, are effectively trapped by the isonitriles. 

The high stereoselectivity can be explained again by means of the zinc complex A (see Scheme 3). The nucleophilic isonitrile attacks the glycosyl imine from the steri-cally less shielded Re-side. The bulky pivaloyl group at the 2-position and the formed zinc complex block the Si-side efficiently. The exchange of the pivaloyl for the acetyl group decreases the selectivity of the reaction (10 1 instead of 30 1) [13]. 

In an elegant and different approach, an array of C-linked, glycopeptide-like mimics of SLex were synthesized in parallel by an Ugi four-component reaction. Reaction of different anomeric two- or three-carbon extended C-glycosyl aldehydes or acids with resin-bound amine, isonitriles and with other acids or aldehydes, respectively, yielded an array of C-linked analogues. The method is easy to perform in good yield on solid phase. How-... 

The synthesis, reactions and spectroscopic properties of carbohydrate isonitriles, including both glycosyl and ring-substituted... 

A mixture of the P-D-mannofuranosyl isonitrile (46 76%) and nitrile (47 8%) and small proportions of their a-anomers was obtained on treatment of the corresponding glycosyl fluoride with diethylaluminium cyanide. Similar results were obtained with the l,2 4,6-di-0-isopropylidene-D-... 

Photochemical addition of a nitrene to the isonitrile (6) led to the carbodiimide (7), which added water to give urea (8) and malonic acid to give the potential glycosyl barbiturate synthon (9)... 

When the donor is correctly adjusted to the acceptor, yields are high (64-97%). However, acidic conditions of the glycosylation reaction may induce partial anomerization at the reducing end of the obtained disaccharide. In that case, more reactive A -phthaloylglucosamine donors can be used a sequential treatment converts then the A -phthaloyl group into the required Af-formyl substituent. Radical deamination at C-2 through an isonitrile delivered 2 -deoxy-P-disaccharides 132 in 65 89% overall yields (Scheme 29). 

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