C-Allyl glycopyranosides

C-AUylated glycopyranosides a-Glycopyranosides or a-glycopyranosyl chlorides arc converted stereoselectively into oc-C-allylated glycopyranosides on reaction with al-lylsilanes catalyzed by trimethylsilyl triflate. lodotrimethylsilane is less effective as the catalyst. 

Notes and discussion. Methyl a-D-gluco- and mannopyranosides and a-D-glycopyranosyl chlorides can react with allylsilanes via activation with trimethylsi-lyl trifluoromethanesulfonate or iodotrimethylsilane to stereoselectively afford the corresponding a-C-allylated glycopyranosides in excellent yields. The yields of the reactions depend on the solvent and the amount of catalyst employed. Acetonitrile is the most suitable solvent whereas dichloromethane, the most commonly used solvent for allylation reactions using allylsilanes, does not afford satisfactory results. The reactions proceed very slowly when promoted by less than 5 mol% of the catalyst, however, the use of 50 mol% of catalyst is sufficient to force the reactions to completion. 

Highly stereoselective C-allylation of glycopyranosides with allylsilanes, catalysed by silyl triflate or iodosilane [48]. 

Alkylation of dihydropyranylacetates. The Pd(0)-catalyzed alkylation of allylic acetates by stabilized carbanions originally reported by Trost and Verhoeven (8,475 9, 451 -457)4 has been extended to alkylation of dihydropyranyl acetates as a route to natural C-glycopyranosides.5 Again the displacement is regioselective and results in net retention of configuration in reactions with most carbanions. However, alkylation with phenylzinc chloride or vinylzinc chloride occurs with inversion. 

Sakurai et al. as well as Sandhoff et al. used this approach for the synthesis of allyl-C-glycopyranosides 21 from readily accessible peracetylated or perbenzylated glycopyranoses 19 [12, 13]. Addition of allylsilane to oxonium 20 proceeds with an axial/equatorial selectivity that is considerably affected by the polarity of the solvent (Scheme 13.9). 

Kozikowski, A P, Sorgi, K L, A mild method for the synthesis of anomerically allylated C-glycopyranosides and C-glycofuranosides, Tetrahedron Lett., 23, 2281-2284, 1982. 

C-Glycopyranosides may be obtained from glycopyranosyl halides via intermolecular addition of glycopyranosyl radicals [129]. In a more useful example, the a-aminoacrylate 192 was used as the radical acceptor for preparation of C-glycosyl amino acids 193 and 194 [130] (Scheme 66). In a concise synthesis of showdomycin (197), Barton utilized the trigger reaction of the 7V-hydroxy-2-thiopyridone derivative and the exceptional radicophilicity of tellurides in concocting the conditions for the conversion from the anisyl telluride 195 to the intermediate 196 after oxidative elimination [131] (Scheme 67). In Keck s synthesis of (-t-)-pseudomonic acid C (201), the intermediate 200 was prepared via stereocontrolled intermolecular addition of the radical generated from the iodide 198 to the allylic sulfone 199 [132] (Scheme 68). 

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