Preparation and Properties of New Methano‐Bridged Dibenzo[ c,g ]phenanthrenes

Intramolecular Friedel‐Crafts alkylation of 1‐(hydroxymethyl)[5]helicenes 1‐R and 15‐Me leads to 10,11‐methano‐bridged dibenzo[ c,g ]phenanthrenes 2‐R and 16‐Me , respectively. These molecules are shaped like the back of a turtle as shown by X‐ray crystal structure analysis. Their nonplanarity, however, is not rigid, as revealed by the temperature dependence of their 1 H‐NMR spectra. With inversion barriers of about 62.4 and 67.1 kJ mol –1 2‐H and 2‐Me are surprisingly flexible. A remarkable diastereoselectivity is observed in the addition of methyl‐ and phenylmagnesium bromide to 1‐formyl[5]helicenes 14‐Me leading to secondary alcohols 15a, b‐Me , which cyclize to bridged hydrocarbons 16a, b‐Me , in which the substituents R=Me, Ph are in the thermodynamically less favorable endo ‐position. Upon heating, endo ‐ 16a,b‐Me isomerize to exo ‐ 6a, b‐Me irreversibly.