A Route to Enantiopure (O-Methyl)6-2,6-Helic[6]arenes: Synthesis of Hexabromo-Substituted 2,6-Helic[6]arene Derivatives and Their Suzuki–Miyaura Coupling Reactions

A route to enantiopure ( O-methyl)₆-2,6-helic[6]arenes (+)- P-2 and (-)- M-2 has been provided. By the reaction of enantiopure triptycene precursors (+)-1 and (-)-1 in refluxed o-DCB for 12 h in the presence of catalytic amount of FeCl₃, and then followed by treatment of the obtained oligomers under the same conditions, (+)- P-2 and (-)- M-2 could be obtained in 51% and 53% total yield, respectively. It was also found that racemic and enantiopure ( O-methyl)₆-2,6-helic[6]arenes could be easily brominated by Br₂ to give the corresponding hexabromo-substituted helic[6]arene derivatives rac-4, (+)- P-4, and (-)- M-4 in high yields. The crystal structure of (+)- P-4 further confirmed the absolute configuration of the helic[6]arenes and their derivatives. Moreover, a series of hexaaryl-substituted helic[6]arene derivatives 5a-f with deepened cavities could be conveniently synthesized in 55-71% yield by Suzuki-Miyaura coupling reactions of 4 and arylboronic acids. rac-5a could encapsulate chloroform and exhibit self-sorting stacking in solid state. Enantiopure (+)- P-5a-f and (-)- M-5a-f showed mirror images in their CD spectra.