Cyclization of stilbenes

Photochemical cyclization of heterocyclic stilbenes. The photochemical oxidative cyclization of stilbenes to phenanthrenes6 proceeds in low yield when applied to heterocyclic analogs. Cava et al have examined a number of oxidants other than 02 for in situ oxidation of the dihydroheterocycle formed initially and find that 5% Pd on carbon is the reagent of choice. In addition, a hydrogen acceptor, p-nitrobenzoic acid, can improve yields. 

A number of examples are known where elimination gives six membered rings (Table 4). A carbocyclic ring is formed in the cyclization of stilbene to phenanthrene 13>. In a similar way stilbazol forms azaphenanthrene 19>. 

Gilbert, A. (2004) Cyclization of stilbene and its derivatives, in Handbook of Organic Photochemistry and Photobiology, 2nd edn (eds W. Horspool and... 

The cyclization of stilbene occurs through an anti-Hiickel transition state analogous to anti-Hiickel phenanthrene. Photodimerizations of polycyclic aromatic systems are also very common. The dimerization of anthracene derivatives is a classical example ... 

A key basis for modern helicene cyclization chemistry occurred in 1964 when Wood and Mallory effectively prepared phenanthrene via photochemical induced cyclization of stilbene [81]. The new method effectively provided efficient pathways to substituted phenanthrene derivatives, easier access to existing and new helicene structures, as well as sufficient quantities for property studies. In the mid 1960s, Martin and coworkers successfully isolated the previously made hexahelicene as well as new hepta- 99, octa- 100, and nonahelicenes 101 via highly efficient syntheses (Scheme 1.22) [82]. These preparations culminated in photo-induced cyclization of bisarylethylenes with an I2 oxidant and Hg-lamp radiation source. The key improvements over previous efforts included production of a phenanthrene derivative via photo-induced cyclization of intermediate 102 and subsequent conversion to Wittig ylide 103, a common intermediate to the four helicenes. 

---