A General Approach to Optically Pure [5]-, [6]-, and [7]Heterohelicenes†

The lack of a general method for the effective synthesis of nonracemic helicenes and their analogues has been a major hurdle that has limited a wider exploitation of these helically chiral aromatic systems in enantioselective catalysis, molecular recognition, self-assembly, surface science, chiral materials, and other branches of science. Ideally, a practical asymmetric synthesis would be independent of both the length of the helical backbone and the presence of functional groups. Since the pioneering studies by Martin et al. and Katz et al. , who successfully used diastereoselective photodehydrocyclization of stilbene-type precursors, various concepts of the asymmetric synthesis of helicenes have been explored but no general procedure for obtaining optically pure helicenes or their analogues with a wide structural diversity has yet been reported. Recently, we demonstrated diastereoselective [2+2+2] cycloisomerization of centrally chiral triynes to receive nonracemic helicene-like compounds with incorporated dihydrooxepine or dihydroazepine ring(s). This promising approach, however, has not yet reached the merit of being general and practical. Herein, we present fundamental progress in this endeavor to receive optically pure and functionalized [5]-, [6]-, and [7]heterohelicenes by means of asymmetric synthesis. With the aim of keeping the molecular shape of the helicene analogues as close as possible to that of the parent helicenes, such as 1 (Figure 1), we proposed embedding two