A Novel Strategy for the Synthesis of Molecules with Helical Chirality. Intramolecular [2 + 2 + 2] Cycloisomerization of Triynes under Cobalt Catalysis

A straightforward synthetic approach to a new class of molecules with helical chirality has been developed involving an intramolecular [2 + 2 + 2] cycloisomerization of triynes under CpCo(CO) 2 /PPh 3 catalysis. The cyclization reaction is promoted by visible light irradiation. Starting from 1,2-diarylacetylenes with two terminal or methyl-substituted tethered acetylene moieties ( 1a, b, 4, 5, and 7 ), the target products ( 8a, b, 11, 12, and 14 ) are obtained in 64−89% yield. The triynes bearing terminal trimethylsilyl groups ( 1c, 2, and 3 ) are less reactive and afford the products ( 8c, 9a, b, and 10 ) in 23−71% yield. The ability of the trimethylsilylated derivatives to cycloisomerize strongly depends on the tether length. The cyclized products ( 8 − 14 ) with five, six, and seven ortho-fused rings structurally resemble classical helicenes. Several representatives ( 8b, 11, 12, and 14 ) have been resolved into enantiomers by HPLC on a chiral column. Thus, the methodology exhibits a considerable versatility and allows the preparation of a variety of helical objects.