Cyclopentanone is treated with Mg-Hg and water to give X. X is warmed with dilute $\mathrm{H}{}_2\mathrm{SO}{}_4$ to give Y, which is reduced with $\mathrm{NaBH}{}_4$ (then $\mathrm{H}{}_3\mathrm{O}{}^{+}$) to give Z. Z is

Bimolecular reduction gives 1,1'-bi(cyclopentyl)-1,1'-diol (X). Pinacol rearrangement with ring expansion gives spiro[4.5]decan-6-one (Y), the six-membered ring containing the C=O. $\mathrm{NaBH}{}_4$ reduces the ketone to the secondary alcohol spiro[4.5]decan-6-ol (Z).