$(S)$ -2-Methyloxirane is treated with sodium methoxide in methanol and then neutralised. Select the correct statements. (Priorities at the stereocentre of the epoxide: ring O $>$ ring $CH X_{2}$ $>$ $CH X_{3}$ $>$ H.)

AThe product is $(R)$ -2-methoxypropan-1-ol, formed by inversion at the stereocentre.
BThe product is 1-methoxypropan-2-ol.
CThe stereocentre is not attacked, so the product is $(S)$ -1-methoxypropan-2-ol.
DMethoxide attacks the less substituted ring carbon ( $CH X_{2}$ ).

Answer: B, C, D

Under basic conditions, ring opening is SN2 at the less hindered carbon, here

CH X_{2}

. The stereocentre keeps all four of its bonds, so its geometry is retained. Its priorities become

OH > CH X_{2} OCH X_{3} > CH X_{3} > H

, the same order as before, so the descriptor remains

S

. The last option corresponds to attack at the more hindered carbon, which is characteristic of acidic conditions.