An efficient synthetic approach to substituted penta- and hexahelicenes

A two-step synthetic approach to penta- and hexahelicenes substituted at the terminal aromatic rings has been studied. This approach is based on the Diels-Alder reaction of 5,5',8,8'-tetramethyl-3,3',4,4'-tetrahydro-[1,1']- binaphthalene (2b), 3-vinyl-1,2-dihydronaphthalene (5a), 5,8-dimethyl-3- vinyl-1,2-dihydronaphthalene (5b) and 3-vinyl-1,2-dihydrophenanthrene (15) followed by the aromatization of the cycloadducts. This method is flexible, efficient and of wide application to the synthesis of several benzenoid and nonbenzenoid penta- and hexahelicenes. The racemization energy barriers of helicenes 3, 9, 10, 13, 14, 17 and 18 were computed with the semiempirical quantum method AM1. The computed values show that, when methyl groups are introduced at the inner position of the terminal aromatic rings, the racemization barriers increase markedly. A structure analysis of the reaction products by ¹H- and ¹³CNMR spectroscopy is also presented.