Stereospecific Oxidative Contraction of Anomeric Nucleophiles.

We report a single-flask oxidative ring-contraction that edits hexoses into pentono-1,4-γ-lactones, delivering access to arabino- and lyxono-configured scaffolds. C1-Metal(loid) glycosides are treated with an oxidant to generate a transient Cr(VI) ester that undergoes stereospecific C1-C2 bond cleavage. For α-silyl glucopyranosides, the process expels a silanol and yields γ-lactones in up to 61% isolated yield, whereas β-isomers diverge to α-silyl C2-ketones, highlighting a stereoelectronic dichotomy. The transformation tolerates a wide range of silyl groups, extends to galactose, arabinose, and lactose derivatives, and proceeds likewise with anomeric trifluoroborates, but not stannanes, which instead afford glycals. Collectively, these results introduce a concise route from common hexoses to pentose frameworks, furnish handles for oligosaccharide diversification, and uncover new reactivity patterns for organosilicon and organoboron functions under oxidative conditions.