(S)-3-Methylpentan-2-one, $\mathrm{CH}{}_3-\mathrm{CO}-\mathrm{CH}(\mathrm{CH}{}_3)\mathrm{CH}{}_2\mathrm{CH}{}_3$ (stereocentre at C3), is treated with a peroxy acid. The major product is

The secondary alkyl group migrates in preference to methyl, and migration occurs with retention of configuration. The product is $\mathrm{CH}{}_3\mathrm{COO}-\mathrm{CH}(\mathrm{CH}{}_3)\mathrm{CH}{}_2\mathrm{CH}{}_3$. The oxygen replaces the $\mathrm{C}(=\mathrm{O})\mathrm{CH}{}_3$ group at the stereocentre, and in both compounds the priority order is that group (or O) > ethyl > methyl > H, so the descriptor stays (S).