Too bad you did not put an image, but based on the title, a pentagon refers to a cyclic molecule with five atoms, and the only one that fulfills this criterion is deoxyribose.
Deoxyribose (C5H10O4), or more exactly 2-deoxyribose, is a pentose derived from ribose by substitution of hydrogen to replace the hydroxyl group in position 2, which involves the loss of oxygen.