Synthesis of α- and β-Glycosyl Asparagine Ethylene Isosteres (C-Glycosyl Asparagines) via Sugar Acetylenes and Garner Aldehyde Coupling^†

A convergent approach has been developed for the synthesis of C-glycosyl amino acids in which the glycinyl moiety CH(NH₂)CO₂H is connected to the anomeric center of the sugar residue by a three carbon atom tether. Essentially, these compounds are isosteres of N-glycosyl asparagines in which the amide group has been replaced by an ethylene bridge. Following the coupling of α- or β-d-linked lithium C-glycoside acetylides with N-Boc d-serinal acetonide (Garner aldehyde), the resulting adducts were transformed into the final N-Boc-C-glycosyl-α-aminopentanoic acids via reduction of the triple bond, deoxygenation, and oxidative cleavage of the oxazolidine ring. By this protocol, 12 C-glycosyl asparagines, six pairs of α- and β-anomers, have been prepared incorporating the gluco, galacto, manno, and the corresponding 2-acetamido-2-deoxy residues.