Cyclopentanepentone, also known as leuconic acid, is a hypothetical organic compound with formula C5O5, the fivefold ketone of cyclopentane.

It would be an oxide of carbon (an oxocarbon), indeed a pentamer of carbon monoxide.

As of 2000, the compound had yet to be synthesized in bulk, but there have been reports of trace synthesis.

[2][3][4] Cyclopentanepentone can be viewed as the neutral counterpart of the croconate anion C5O2−5.

The compound referred to in the literature and trade as "cyclopentanepentone pentahydrate" (C5O5·5H2O) is probably decahydroxycyclopentane (C5(OH)10).