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

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Abstract

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/PPh3 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.

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