Recent Advances in the Chemical Synthesis of C-Glycosides

Advances in the chemical synthesis of C-pyranosides/furanosides are summarized, covering the literature from 2000 to 2016. The majority of the methods take advantage of the construction of the glycosidic C-C bond. These C-glycosylation methods are categorized herein in terms of the glycosyl donor precursors, which are commonly used in O-glycoside synthesis and are easily accessible to nonspecialists. They include glycosyl halides, glycals, sugar acetates, sugar lactols, sugar lactones, 1,2-anhydro sugars, thioglycosides/sulfoxides/sulfones, selenoglycosides/telluroglycosides, methyl glycosides, and glycosyl imidates/phosphates. Mechanistically, C-glycosylation reactions can involve glycosyl electrophilic/cationic species, anionic species, radical species, or transition-metal complexes, which are discussed as subcategories under each type of sugar precursor. Moreover, intramolecular rearrangements, such as the Claisen rearrangement, Ramberg-Bäcklund rearrangement, and 1,2-Wittig rearrangement, which usually involve concerted pathways, constitute another category of C-glycosylations. An alternative to the C-glycosylations is the formation of pyranoside/furanoside rings after construction of the predetermined glycosidic C-C bonds, which might involve cyclization of acyclic precursors or D-A cycloadditions. Throughout, the stereoselectivity in the formation of the resultant C-glycosidic linkages is highlighted.