Synthesis and structure of optically active 1,12-diethyl- and 1,12-diisopropylbenzo[c]phenanthrenes:: An isopropyl group can be smaller than a methyl group

1,12-Diethyl- and 1,12-diisopropylbenzo[c]phenanthrene5,8-dicarboxylic acids were synthesized, and resolved. The dihedral angles formed by the A and D rings increased in the order of diisopropyl < diethyl < dimethyl as indicated by X-ray analysis, which showed that the strain decreased with the increase of the bulkiness at the 1,12-groups.