Retene, methyl isopropyl phenanthrene or 1-methyl-7-isopropyl phenanthrene, C<sub>18</sub>H<sub>18</sub>, is a polycyclic aromatic hydrocarbon present in the coal tar fraction, boiling above 360 ðC. It occurs naturally in the tars obtained by the distillation of resinous woods. It crystallizes in large plates, which melt at 98.5 ðC and boil at 390 ðC. It is readily soluble in warm ether and in hot glacial acetic acid. Sodium and boiling amyl alcohol reduce it to a tetrahydroretene, but if it heated with phosphorus and hydriodic acid to 260 ðC, a dodecahydride is formed. Chromic acid oxidizes it to retene quinone, phthalic acid and acetic acid. It forms a picrate that melts at 123-124 ðC.
Retene is derived by degradation of specific diterpenoids biologically produced by conifer trees. The presence of traces of retene in the air is an indicator of forest fires; it is a major product of pyrolysis of conifer trees. It is also present in effluents from wood pulp and paper mills.
Retene, together with cadalene, simonellite and ip-iHMN, is a biomarker of vascular plants, which makes it useful for paleobotanic analysis of rock sediments. The ratio of retene/cadalene in sediments can reveal the ratio of the genus Pinaceae in the biosphere.
A recent study has shown retene, which is a component of the Amazonian organic PM10, is cytotoxic to human lung cells.