Synthesis and self-aggregation of cyclic alkynes containing helicene

All stereoisomers of a cyclic alkyne containing three helicene units, 1,12-dimethylbenzo[c]phenanthrene, are synthesized using a building block. Isomeric [3 + 3]cycloalkynes aggregate in organic solvents, Vapor pressure osmometry reveals dimer formation of (M,M,M)-[3 + 3] cycloalkynes in chloroform and benzene at concentrations above 2 mM. No higher aggregation is observed. The chirality of helicenes plays an important role in self aggregation, and diastereomeric (M,P,M)-[3 + 3]cycloalkyne forms a dimer only above 15 mM. Aggregation of racemic (M*,M*,M*)-[3 + 3]cycloalkyne or (M*,P*,M*)-[3 + 3]cycloalkyne is much weaker than that of a single enantiomer.