fulvenes

thumb|right|upright|Chemical structure of fulvene Fulvenes are the class of hydrocarbon obtained by formally cross-conjugating one ring and methylidene through a common exocyclic double bond.

In addition to compounds with substituent group on the fulvene skeleton, structural analogs in this class include heteroatom replacements and variations of ring-size. Thus, methylenecyclopropene can be called triafulvene and cyclopropenone is an oxafulvene. Analogs in which certain ring carbons are replaced by other elements, sometimes entailing changes of the double-bonding pattern of the heterocyclic core as in dithiafulvene, lead to changes in the electronic properties of the ring.