Glycosylation trisaccharides

Total synthesis of (+)-validamycins A and B starting from a common synthetic intermediate was elaborated by the following sequence. Tetra-(9-benzyl-(-l-)-valienamine (370), derived from 211, and the di-O-benzyl derivative (371) of the epoxide were coupled in 2-propanol to produce the protected dicarba compound (374), the structure of which was confirmed by conversion into (-1-)-validoxylamine B nonaacetate. Concurrently, compound 372 was glycosylated and the product oxidized with a peroxy acid, to afford a mixture of products from which the desired epoxide (373) was obtained in 70% yield. Coupling of 370 with 373 in 2-propanol at 120° afforded two carba-trisaccharides, and the major product (47%) was depro-tected and characterized as the dodecaacetate of validamycin B. The pro-... 

Trisaccharide 80 was synthesized by a similar approach, in which a rhamnoside was formed in 86% yield, transformed into its trichloroacetimidate, submitted to the second glycosylation step promoted by H2S04-silica (84% yield), and subsequently deprotected to 80.91... 

In a synthesis of a partial structure of the GPI anchor of Trypanosoma brucd, three of the four glycosylations were realized with the zirconocene activator (Cp2ZrCl2/AgC104, 1 1 Scheme 8.8). First, the coupling of a 1-D-myo-inositol acceptor with a disaccharide donor gave a mixture (93 %) of the desired a-linked trisaccharide and its (3-isomer in a ratio... 

The glycosylation process by using the trisaccharide fluoride is repeated twice to give trimeric Lex61 after deprotections. 

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