The Grignard reaction represents one of the most powerful carbon-carbon bond forming reactions and is the subject of continual study. Investigations of alkyl magnesium halide additions to β-hydroxy ketones identified a unique effect of the magnesium halide on diastereoselectivity, with alkylmagnesium iodide reagents demonstrating high levels of selectivity for the formation of 1,3-syn diols. Density functional theory (DFT) calculations and mechanistic studies suggest that the Lewis acidity of a chelated magnesium alkoxide can be tuned by the choice of halide, with the highest levels of diasteroselectivity achieved using alkyl magnesium iodide reagents. Exploiting this finding, we demonstrate that the diastereoselective addition of alkyl magnesium iodide reagents to ketofluorohydrins enables rapid access to naturally configured C4'-modified nucleosides. This work provides a platform to support antiviral and anticancer drug discovery and development efforts.