**4. Materials and Methods**

**General Methods.** All commercially available starting materials and solvents were of reagent grade and used without further purification. Chemical reactions were monitored with thin-layer chromatography using precoated silica gel 60 (0.25 mm thickness) plates. Flash column chromatography was performed on silica gel 60 (SDS 0.040–0.063 mm). 1H NMR spectra were recorded at 298 K in CDCl3 using the residual signals from CHCl3 ( 1H: = 7.26 ppm) as internal standard. 1H peak assignments were made by first order analysis of the spectra, supported by standard 1H-1H correlation spectroscopy (COSY) (see Supplementary Materials).

**General process A for synthesis of 2-OH 1-thioaryl glycosides from glycals.** *Step 1*. To a cooled (0 ◦C) solution of a per-protected glycal (1 mmol) in DCM (4 mL) were added acetone (0.4 mL) and saturated aqueous NaHCO3 (7 mL). The mixture was stirred vigorously, and a solution of oxone (2 mmol) in H2O (2.5 mL) was added dropwise over 10 min. The mixture was stirred vigorously at 0 ◦C for 30 min and then at rt until TLC indicated consumption of the starting material. The organic phase was separated, and the aqueous phase was extracted with DCM (2 × 10 mL). The combined organic phases were dried (MgSO4) and concentrated in vacuo to obtain the crude 1,2-anhydro sugar.

*Step 2*. To a mixture of phenyl disulfide (or phenyl diselenide) (0.7 mmol) and NaBH4 (53 mg, 1.4 mmol) was added acetonitrile (5 mL). The mixture was stirred at rt for 30 min to 2 h until TLC indicated full conversion of the phenyl disulfide (or phenyl diselenide). The mixture was then added to the crude α-1,2-anhydro sugars. The reaction was stirred at rt for 5–60 min until TLC indicated full conversion of the starting material. The mixture was diluted with DCM and washed with water. The aqueous phase was re-extracted with DCM, and collected organic phases were dried and evaporated under vacuum. The residue was purified by silica gel flash chromatography.
