Scheme

on the 2-oxygenated compounds. Aza sugars have also been found to be reactive toward electrophilic C-glycosidation upon conversion to the anomeric fluorides. Several nucleophiles were examined, and Scheme 71 shows the allylation and reaction with silyl enol ethers of 413 [121].

Veyrieres et al. found that the allylsilane 417 reacts with a variety of pyranose derivatives to give good yields of allylated and functionalized C-glycosides (Scheme 72) [122].

2-Thioglycosyl chlorides, derived from ionic addition of RSCl to glycals, have been found to react with Grignard reagents to give the ^-gluco C-glycosides 426. The same chlorides also react with silyl enol ethers and allyltrimethylsilane to give the corresponding C-glycosides, 425 (Scheme 73) [123,124].

Intramolecular C-glycosidations have also been explored. For example, exposure of 427 to AgBF4 with strict exclusion of water gave a 74% yield of 428. The benzylic positions were oxidized to furnish 429, and ester cleavage then gave the free C-glycoside 430 (Scheme 74) [125].

Lopez and Gomez found that treatment of pyranosyl chlorides with a suitable organolithium gave acceptable yields of the corresponding C1-glycals 432-434 (Scheme 75). No mechanistic explanation was offered. One possibility could be elimination of the benzyloxy group to generate the C1-chloro glycal, followed by addition of the organometallic to the double bond at C1 and subsequent elimination of chloride [126].

Scheme 74
0 0

Post a comment