Reaction Under Lewis Acid Catalysis

The reaction of glycals with allylsilanes, enol ethers, or silyl acetylenes has received considerable attention over recent years. In 1996 Toshima et al. employed environmentally friendly montmorillonite K-10 (clay) to catalyze the addition of allylsilanes to sugar glycals for the purpose of obtaining allyl C-glycosides. Reaction of the galactal derivative 323 as shown in Scheme 58 gave a good yield of the a- and allyl C-glycosides, with the 324 as the major isomer [87]. Work from the same group has also shown that the glycal does not have to be protected for the allylation to work, at least with conventional Lewis acid catalysts. This is exemplified by the conversion of 325 to 326 (Scheme 58) [88].

Scheme 59

Panek and Schaus continued their work with crotyl silanes and their addition to glycals (319 ^ 320) as shown in Scheme 59 [89]. Grieco et al. utilized [LiCo(B9C2Hn)2] as a catalyst for the condensation of an allylic acetate with glycal 331 to give the C-glycosides 332. The use of this catalyst is interesting because it mimics the effect of highly polar media (5 M LiClO4 in Et2O) for reactions of these types [90]. Portella et al. showed that the acylsilanes and the glycal 309, in the presence of CF3SiMe3 and TBAF, react to give the C-glycosides 334 as a mixture of anomers [91]. Hayashi et al. cyanated (335 ^ 336) unprotected glycals in the presence of a catalytic amount of palladium(II) acetate (Scheme 59) [92].

Csuk et al. also examined some reactions of various glycals with a hindered silyl enol ether under Lewis acid catalyzed conditions (not shown) [93]. Isobe has continued his explorations of the reactivity of silyl acetylenes with glycals. The sugar-based acetylene 338 is condensed with glycal 337 to give the disaccharide 339 [94]. This area was reviewed in 1998 [95]. Steel et al. used actinium perchlorate to catalyze the dimerization of glucal. Three products were isolated in the yields shown. Separate treatment of 342 with boron trifuluoride etherate and triethylsilane then gave 343 in good yield (Scheme 60) [96].

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