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

In the Baeyer-Villiger reaction the group that migrates is the one better able to stabilise positive charge, here the secondary isopropyl group rather than methyl. Oxygen is inserted between the carbonyl carbon and isopropyl, giving the ester $\mathrm{CH}{}_3-\mathrm{CO}-\mathrm{O}-\mathrm{CH}(\mathrm{CH}{}_3){}_2$, propan-2-yl ethanoate.