Photoracemization of Blestriarene C and Its Analogs

Two analogs of blestriarene C (4,4'-dimethoxy-1,1'-biphenanthrene-2,2',7,7'-tetraol) bearing no 7,7'-dihydroxy (3) and 4,4'-dimethoxy groups 4 were prepared. Unlike blestriarene C (1), compounds 3 and 4, as well as 1,1'-biphenanthrene-2,2'-diol (5), do not racemize under fluorescent lamp illumination. Cyclic voltammetry analysis reveals that compound 1 has a lower half-wave potential (E1/2) than compounds 3, 4, 5, suggesting that a redox cycle is involved in the racemization. Compound 1 racemizes by absorbing UV light corresponding to the 1Lb band. During the reaction, no side products are observed. The racemization is significantly inhibited under nitrogen. Based on these observations, we propose a feasible mechanism for the easy racemization of compound 1, which is mediated by a cation radical generated in situ by a reversible photo-induced oxygen oxidation. Chirality 27:479–486, 2015. © 2015 Wiley Periodicals, Inc.