Steric strain and tumorigenicity: the molecular structure of 3,6-dimethylcholanthrene

The crystal structure of 3,6-dimethylcholanthrene, a highly tumorogenic polycyclic aromatic hydrocarbon, has been solved and refined, from X-ray diffractometric data collected at room temperature, to an R index 0.050 over 1684 independent reflections. The steric strain arising from the presence of the bay-methyl substituent distorts the molecule almost as much as in the moderately carcinogenic 1- and 12-methylbenz[a]anthracenes, with the benzo A ring inclined at approximately 16 degrees to the furthest C and D rings. The 6-methyl carbon atom is displaced 0.7 A from the molecular plane and the beach bond length of the bay region is 1.480(6) A, with beach C-C-C angles of 124.1 and 123.5 degrees. The shortest carbon-carbon bond is C(5)-C(6) = 1.331(7) A at the K region, with the next shortest C(8)-C(9) = 1.362(7), C(10)-C(11) = 1.366(7), and C(3)-C(4) = 1.369(8) A.