A Versatile Synthesis of Functionalized Pentahelicenes

A general synthetic methodology for the preparation of functionalized (hetero)helicenes has been developed. It employs the sequence of a double propargyl organometallics (Li, Mg, Ga/In) addition to a tolan-2,2′-dialdehyde-type intermediate, a cobalt-catalyzed/cobalt-mediated [2 + 2 + 2] cycloisomerization of a triyne intermediate, and a double silica gel-assisted acetic acid elimination to receive pentahelicene, 1,14-diazapentahelicene, and 3,12-dichloro-, 3,12-dichloro-7-trimethylsilyl-, and 3,12-di-tert-butylpentahelicene. 3,12-Dichloropentahelicene undergoes a Suzuki−Miyaura coupling with aryl boronic acids (or ester) under palladium catalysis to afford 3,12-diarylpentahelicenes.