1,6-Anhydrosugars glycosides

The stereoselective glycosidation of the secoaglycon 1338 was achieved by reaction of the sodium salt of 1338 with the a-l,2-anhydrosugar 1339 to give the p-glucopyranoside 1340 in 48% yield. After the removal of the SEM protecting group with TBAF, the photocyclization of compound 1341 afforded the indolocarbazole... 

Scheme 25 C-Glycoside synthesis via the reductive opening of 1,2-anhydrosugars...

The effect of a free OH group is usually much more pronounced than that of the protected one. For example, oxiranes 58 and 59 were obtained with high stereoselectivity (directed by the free OH), although both oxy-substituents act in the opposite directions [ 1,44] (O Fig. 12). When the epoxidation is performed not on allylic alcohols but allylic esters the selectivity of this process is low both possible oxiranes are obtained in comparable amounts. The stereoselective formation of the 2-3-epoxides can be achieved also by the Sn2 process. Recently, Lowary proposed an efficient synthesis of 2,3-epoxy-arafci o-furanoside 61 from the parent glycoside 60 in a sequence of reactions presented in Scheme 20 [45]. Such anhydrosugars are convenient precursors for further functionalization at either the C-2 or C-3 position. 

A subsequent study, reported by Dunkerton, et al.,21 showed that the above described chemistry was also applicable to l-O-acetyl-2,3-anhydrosugars. The results, directly analogous to the use of phenolic glycosides shown in Scheme 4.3.2, are illustrated in Scheme 4.3.4. As shown, excellent yields and high anomeric selectivity was observed with a net retention of the anomeric configuration. Although complementary to similar C-glycosidations discussed thus far, it should be noted that the applications of this specific reaction type is limited with low yields observed when substituents are present at C4 of the substrate. 

Further research into the types of substrates available for transition metal mediated C-glycosidations demonstrated the utility of glycosyl carbonates. As shown in Scheme 4.3.5, Engelbrecht, et al.,22 effected the formation of glycosyl carbonates on reaction of anhydrosugars with isobutyl chloroformate and pyridine. Under these conditions, the a glycosyl carbonate was isolated in 75% yield. Subsequent treatment with diethyl malonate in the... 

Bellosta V, Czemecki S (1989) Stereocontrolled synthesis of C-glycosides by reaction of organocuprates with protected 1,2-anhydrosugars, and their transformation into 2-deoxy-C-glycosides. J Chem Soc Chem Commun 1989 199-200... 

Deprotection of the l°-hydroxyl initiates formation of the intramolecular glycoside (1,6-anhydrosugar), resulting in simultaneous protection of the anomeric centre and the 6-OH. 

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