The Diao group later developed a method to synthesize C-acyl furanosidesvia the nickel-catalyzed cross-coupling of glycosyl ester with carboxylic acid (Scheme 25). [34] This method enables the coupling of a wide range of carboxylic acids with various DHP esters derived from furanoses and pyranoses, delivering acyl C-glycosides in useful yields and with good selectivity. Notably, functional groups such as alkene (183g, 183l, 183r ) and sulfide (183q ) were intact in the reaction. Moreover, glycosylation of biologically active substrates proceeded smoothly, producing desired acyl C-glycosides in moderate-to-good yields (183l, 183n, 183o, 183q, 183r ). In all examined reactions of DHP-derived furanosyl esters, the 1,2-trans product was isolated as the predominant product, presumably owing to the approach of an incoming catalyst from the opposite face of the C2-substituent. Not surprisingly, the 2-deoxy-D-ribose substrate, lacking C2-substituent, generated products with a modest α-selectivity (183af ). With respect to the stereoselectivity observed for pyranosyl coupling partners, the kinetic anomeric effect was suggested to determine the stereoselectivity of acyl C-pyranosides. Of note, low efficiency and diminished anomeric diastereoselectivity were observed in reactions of D-galactopyranose (183ao ) and D-glucopyranose (183ap ), which could be attributed to the contradictory preferences of the steric hindrance of C2-substituent and the kinetic anomeric effect. To further demonstrate the generality of this protocol, the successful synthesis of biologically relevant molecules, a thymidine analog, and diplobifuranylones B, was realized (187 , Scheme 26-A).
Scheme 25 Synthesis of acyl C-glycoside via nickel/photoredox catalyzed coupling of DHP esters with carboxylic acids