Iron–Carbene Activation Enables Stereocontrolled Glycosylations From Conventional Thioglycosides

Stereocontrolled formation of glycosides remains one of the most persistent challenges in carbohydrate chemistry. Existing methods often require strongly acidic promoters, cryogenic temperatures, precious‐metal catalysts, or specially prefunctionalized donors. Here, we report a mild, neutral, and earth‐abundant iron–carbene‐mediated activation of readily accessible conventional thioglycoside donors that delivers 1,2‐ cis and 1,2‐ trans glycosides with excellent stereoselectivity. The access of 1,2‐ cis selectivity is induced by utilizing conformationally locking protecting groups, and 1,2‐ trans selectivity by neighboring group participation to direct the anomeric outcome and functions effectively with D‐glucose, D‐galactose, D‐mannose, D‐xylose, and acid‐sensitive functional group carrying glycosyl donors. A broad range of alcohol acceptors, including primary, secondary, tertiary, and bioactive motifs such as menthol, adamantanol, cholesterol, and oleanolic acid, is well tolerated. Furthermore, iterative coupling of furanosyl and pyranosyl thioglycosides enables streamlined assembly of a well‐defined, stereo‐enriched tetrasaccharide.