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$$\text{R--CH=CH}_2 \xrightarrow{\text{Cold dil. alkaline KMnO}_4} \text{R--CH(OH)--CH}_2\text{OH}$$
$$\text{R--CH=CH}_2 \xrightarrow{1) \, \text{OsO}_4 \quad 2) \, \text{NaHSO}_3 / \text{H}_2\text{O}} \text{R--CH(OH)--CH}_2\text{OH}$$
Concept: Conversion of an alkene into a 1,2-diol (vicinal glycol) through the addition of two hydroxyl ($$-\text{OH}$$) groups across the double bond from the same face of the molecule. Proceeds via a cyclic ester intermediate (cyclic manganate ester or osmate ester). Because the cyclic intermediate forms on one face of the double bond, it results strictly in syn-addition of both hydroxyl groups.
