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 das's 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. S tarting from 1,2-diarylacetylenes with two terminal or methyl-substitu ted tethered acetylene moieties (1a,b,4, 5, and 7), the target product s (8a,b, 11, 12, and 14) are obtained in 64-89% yield. The triynes bea ring terminal trimethylsilyl groups (Ic, 2, and 3) are less reactive a nd afford the products (8c, 9a,b, and 10) in 23-71% yield. The ability of the trimethylsilylated derivatives to cycloisomerize strongly depe nds 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 exhibit s a considerable versatility and allows the preparation of a variety o f helical objects.