(2S,3S)-3-Phenylbutan-2-yl tosylate is solvolysed in acetic acid. Assume that the reaction goes exclusively through phenyl participation (a bridged phenonium ion) and ignore elimination. The product 3-phenylbutan-2-yl acetate is

The phenyl attacks C2 from the back (inversion), giving a phenonium ion that is chiral (its two methyl groups are trans on the three-membered ring, so it has no mirror plane). Acetate then opens it at the same carbon from the back (second inversion) or, equivalently, at the other carbon; both modes give the same compound, the retained (2S,3S) acetate. A configuration check with the CIP rules shows that the descriptors do not change. The diastereomer (2S,3R) would, in contrast, give a symmetrical phenonium ion and a racemic product.