Glycosidation with Lewis acids

Ramesh, N.G. and Balasubramanian, K.K. 1992. Reaction of 2,3-unsaturated aryl glycosides with Lewis acids A convenient entry to C-aryl glycosides. Tetrahedron letters, 33 3061-64. 

Kusumoto and coworkers have found that the treatment of hemiacetal 1 with trifluoro- or trichloroacetic anhydride 94 (1 equiv) and trimethylsilyl perchlorate (0.2 equiv) selectively provides the corresponding anomeric ester intermediate 91 [152], Hemiacetal acylation occurs even in the presence of the alcohol acceptor. With Lewis acid assistance, the glycosyl ester intermediate is displaced to provide disaccharide products in good yields. This transformation allowed the synthesis of disaccharides 98 (81%) and 99 (91%). In some cases, acetic anhydride has been used as the electrophilic activator of hemiacetal donors and the reaction with thiol acceptors yields S-linked glycosides [153,154],... 

Furthermore some O- and. S -glycosylated pyropheophorbide-a derivatives have been prepared.75 The O-glycosylated pyropheophorbide derivative 97 was obtained by carbohydrate per-acetate glycosylation acid catalyzed method of derivative 96 (Fig. 10). The reagent proportions and the reaction times have been shown to rule the reaction yield. The best results were found when the reagents ratio was 2 1 2 (glycoside pyropheophorbide Lewis acid). Under these conditions the reaction yield was 36% with 83% of anomeric purity. The glycosylated derivatives 97 are diastereomeric mixtures. 

Thioglycosides and 4-pent-l-yl glycosides are inert to activation with Lewis acids. Consequently, all donors that can be activated by treatment with a suitable Lewis... 

A new way of converting acylated glycals into 2,3-unsaturated glycosides involves treatment with alcohols in acetonitrile in the presence of catalytic amounts of DDQ (Scheme 4). Efficiencies are good and the anomeric selectivities (mainly a) seem to be very similar to those obtained when this type of reaction is conducted with Lewis acid catalysts. Di-O-acetyl-L-rhamnal is the source of tfie glycosylating agents 11, suitable furanoid compounds for making ristosamine related products. ... 

The most important of such processes is the reaction of glycals with alcohols catalyzed with Lewis acids known as Ferrier-I rearrangement [1,2] an example is shown in Figure 10.1. Reaction of glycals with allylsilanes catalyzed by Lewis acids is also a convenient method for the synthesis of C-allyl glycosides (e.g., 3) [2,3],... 

The first total synthesis of the marine dolabellane diterpene (+)-4,5-deoxy-neodolabelline (70) was accomplished by D. R. Williams et al. [58]. The trans-disubstituted dihydropyran moiety in key intermediate 69 was efficiently prepared from mixed acetal 66 by RCM with second-generation catalyst C and subsequent Lewis acid-catalyzed allylation of ethyl glycosides 67 with allylsi-lane 68 (Scheme 12) [59]. 

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