2-Hydroxycyclohexanone is treated with one equivalent of $\mathrm{HIO}{}_4$. The product is

An alpha-hydroxy ketone is cleaved between the carbonyl carbon and the carbinol carbon. The ring opens, the former $\mathrm{C}=\mathrm{O}$ carbon becomes $-\mathrm{COOH}$ and the former $\mathrm{CHOH}$ carbon becomes $-\mathrm{CHO}$. The product is $\mathrm{OHC}-(\mathrm{CH}{}_2){}_4-\mathrm{COOH}$, 6-oxohexanoic acid.