Helicene, tetrahydro

Fig. 15.13 Nickel(O)-catalyzed asymmetric synthesis of tetrahydro[6]helicene 38.

Keywords 3-amino-5,6,9,10-tetrahydro[5]-helicene-7,8-dicarboxylic anhydride... 

The most ideal synthesis of helicenes and helicene-like molecules is apparently catalytic enantioselective synthesis. In 1999, Stard, Stary, and co-workers developed the catalytic enantioselective synthesis of tetrahydro[6]helicene (R = H) by the Ni(cod)2/(5)-MOP complex-catalyzed complete intramolecular [2 - - 2 - - 2] cycloaddition of a triyne (Scheme 10.18) [5a]. A functionalized tetrahydro[6]helicene (R = OMe) was also synthesized using (5)-BOP as a ligand (Scheme 10.18) [5b]. Although both the product yields and ee values were moderate, this method enabled the one-step synthesis of the enantioenriched helicene precursors. 

Star was first to carry out the asymmetric syntheses of (- -)-tetrahydro[6]helicens 2.121a,b in the presence of Ni(0) catalyst and a phosphine ligand by asymmetric [2-I-2-I-2] cycloisomerization of triynes 2.120a,b (Scheme 2.44) [88]. This route provides an efficient one-step formation of helicens in a total yield of 53-74% but with a stereoselectivity of only 42-48% ee. Nevertheless, this route is the basis of a helical-controlled strategy of asymmetric synthesis of [7]helicen-type molecules [88, 89]. 

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