Decarboxylative glycosylations

An alternative linker which can serve as both a leaving group and a latent glycosyl acceptor is a mixed carbonate [103]. In this case, suitable activation with a Lewis acid causes extrusion of carbon dioxide and formation of the reactive glycosyl acceptor, which can then be trapped by the glycosyl donor. The mixed carbonate precursor 299 for this decarboxylative glycosylation strategy was prepared as a mixture of stereoisomers... 

Scheme 10-96 Formation of a mixed carbonate allows a decarboxylative glycosylation pathway.

Decarboxylative Glycosylation. Acyl-protected mixed carbonates were effectively decarboxylated by catalytic amounts (2-5 mol %) of Hf(OTf)4 in dichloromethane at room temperature to give the corresponding -glycosides in excellent yields (eq 6).l ... 

I (i) glycosylation of 7-hydroxyl (ii) hydrolysis of ester (Hi) decarboxylation of 10-acid (iv) oxidation to 13-ketone T (v) methylation of 4-hydroxyl... 

Combination of Cp2HfCl2 and AgC104 in the ratio 1 2 catalyzes the decarboxylation of glycosyl carbonates, thereby giving rise to glycosides (Eq. 3). The stereochemical outcome is solvent-dependent—a-selectivity is obtained in Et20, yS-selectivity in CH2CI2 [13]. 

Fig. 4. Pathways for the catabolic degradation of lAA. The non-decarboxylation pathways include (A) the oxIAA pathway of maize and pine (C) the diOxIAA pathway of rice (D) the direct oxidation of lAA-aspartate from broad bean Viciafaba) (E) the direct N-glycosylation of lAA-aspartate as also occurs in pine and (F) oxidation followed by N-glycosylation in tomato. The decarboxylation pathway catalyzed by peroxidase has two possible routes, as shown in (B).

Intramolecular substitutions. A ring expansion at the expense of cyclopropane ring scission has been shown. A glycosyl carbonate extrudes COj on exposure to Me SiOTf, as a result of ionization, decarboxylation of the carbonic acid monoester, and the return of the alcohol residue to the anomeric site. ... 

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