(2S,3S)-3-Bromobutan-2-ol is treated with aqueous NaOH. An intramolecular SN2 reaction occurs in which the alkoxide displaces bromide. The product is

The alkoxide attacks the carbon bearing bromine from the side opposite to Br, so that carbon is inverted while the other stereocentre is untouched. Working out the CIP labels, the carbon that loses Br changes from S to R and the other stays S. A $(2S,3R)$ oxirane with the methyls cis is the meso compound, the same as $(2R,3S)$. No diol forms because water competes poorly with the intramolecular reaction.