Scheme 5

Scheme 6

Scheme 6

with DBU in acetonitrile to afford the product of elimination 21 in good yield (Scheme 5). The same chemistry was carried out on the d-manno and d-galacto derivatives which gave the analogous compounds in similar yields [9].

Work by Magnusson dealt with the application of allylic sulfones as radical acceptors for allylic carbon-carbon bond formation at the radical center. Various acceptors were utilized, and the products are shown in Scheme 6 [10].

The Kolbe electrolysis of sugar acids to give radicals and their subsequent trapping with a radical source gave the chain-extended C-glycoside 27 as the major product (Scheme 7). Compounds such as 27 are useful as stable nonionic tensides and are expected to show liquid crystalline properties [11].

2. Radical Addition to Sugar Olefins

Vismara, Nicotra, and their colleagues used glycoexenitol, 30 and 32 as radical acceptors in the preparation of precursors for isosteres of glycolipids and glycoproteins. In this case, the formed radical is anomeric, and reduction by tin hydride from the axial direction should then give the ^-C-glycoside. The additions proceeded cleanly to give only 31 and 33 as the major products of (a:fi = 10:4) noting that the yields

Scheme 7

Scheme 7

34: R = SAC, 83%

Scheme 8

Glycosyl Amino Acids

are quantitative when based on the amount of recovered glycoexenitol. Gervay has added thiolacetic acid to 32 to obtain 34 in good yield, (Scheme 8) [12].

Motherwell published the full paper corresponding to his initial work on radical additions to difluoromethylene-linked C-glycosides. Scheme 9 shows an application of the atom transfer work developed by Curran with iodide 34 to give the bis-C,C-dialkylglycoside in fair yield [13].

C. Intramolecular Approaches

Fraser-Reid has published a full account of his work on serial radical cyclizations with carbohydrates. The example shown (38 ^ 40, Scheme 10) illustrates the complexity of the frameworks that can be accessed by careful application of this methodology [14].

Work by Beau, Skrydstrup, and their colleagues has focused on the generation of anomeric radicals from anomeric sulfones by treatment with samarium iodide [15]. If a radical trap is present, the second electron transfer to form the C1 anion does not have time to occur to any appreciable extent, and the formed radical can then

Good Carb Diet

Good Carb Diet

WHAT IT IS A three-phase plan that has been likened to the low-carbohydrate Atkins program because during the first two weeks, South Beach eliminates most carbs, including bread, pasta, potatoes, fruit and most dairy products. In PHASE 2, healthy carbs, including most fruits, whole grains and dairy products are gradually reintroduced, but processed carbs such as bagels, cookies, cornflakes, regular pasta and rice cakes remain on the list of foods to avoid or eat rarely.

Get My Free Ebook

Post a comment