C-Glycosyl compounds, naturally occurring

Glycosuloses, 3-deoxy-, and the degradation of carbohydrates, 19, 181-218 Glycosylamines, 10, 95-168 Glycosyl azides, 16, 85-103 C-Glycosyl compounds, naturally occurring, 18, 227-258 20, 357-369 Glycosyl esters,... 

Glycosylamines, 10,95-168 Glycosyl azides, 16,85-103 C-Glycosyl compounds, naturally occurring, 18, 227-258 ... 

C-Glycosyl compounds have a carbon atom ia place of the exocycHc oxygen atom of the acetal group and, therefore, are branched cycHc ethers. An example is the naturally occurring anthroquiaone dye, carminic acid [1260-17-9] (Cl Natural Red 4). 

The addition of organometallic reagents to the carbonyl group of conveniently substituted aldonolactones constitutes a viable chain-extension method. The reaction leads to the formation of hemiacetals of glyculoses, 1-methylene sugars, and C-glycosyl compounds, which are precursors of, or occur as subunits of, a variety of natural products. 

C-Glycosyl compounds are important molecular targets as they occur in Nature and have interesting biological properties (for reviews on C-glycosylation, see [72-75]). Chain extensions of aldonolactones have been employed to create C-C bond formation at the anomeric center. Claisen-type reactions of aldono-1,4-lactones (e.g., 9, 55) with acetone or acetophenone (Scheme 17) generate hemi-acetals of type 56a-c [76]. Similarly, lactone 55b reacts with CHsCN/NaH to give hemiacetal 57. 

C-Glycosyl derivatives, in which the anomeric carbon atom is attached to a carbon atom of a phenolic or heterocyclic nucleus to form a C-C linkage, are rather widespread, natural products formed in plants. Naturally occurring C-glycosyl compounds have been treated in this Series by Haynes,366,367 and the synthesis of naturally occurring C-nucleosides and their analogs has been the subject of an article by Hanessian and Pernet368 in this Series. 

Naturally occurring C-glycosyl compounds were reviewed in Volume 18 of this Series, but progress in this field has been so rapid that it has seemed desirable to bring the earlier review up to date. The pattern of the present article follows that of the earlier one. 

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