Diastereoselective Ni-catalyzed Negishi cross-coupling approach to saturated, fully oxygenated C-alkyl and C-aryl glycosides.

A Ni-catalyzed Negishi cross-coupling approach to C-glycosides is described with an emphasis on C-aryl glycosides. The combination of NiCl2/PyBox in N,N'-dimethylimidazolidinone (DMI) enabled the synthesis of C-alkyl glycosides under mild reaction conditions. Moderate yields and beta-selectivities were obtained for C-glucosides, and good yields and high alpha-selectivities were the norm for C-mannosides. For C-aryl glycosides, reactions employing Ni(COD)2/(t)Bu-Terpy in N,N-dimethylformamide (DMF) were typically high yielding and provided C-glucosides with high beta-selectivities (1:>10 alpha:beta) and C-mannosides in moderate alpha-selectivities (3:1 alpha:beta); alpha-C-aryl glycosides could be obtained by the combination of Ni(COD)2/PyBox in DMF (>20:1 alpha:beta). The collective studies suggest that stereochemical control of the C-glycosides is dependent on the substrate and catalysts combination. The Negishi protocol displays excellent functional group tolerance, as demonstrated by its use in the first total synthesis of the natural product salmochelin SX.