Photochemistry of Stilbenes. 8. Eliminative Photocyclization of o-Methoxystilbenes

Article abstract of DOI:10.1021/jo00280a032

The synthetic value of the eliminative photocyclization of o-methoxystilbenes to give phenanthrenes with loss of the element of methanol has been enhanced by the use of tert-butyl alcohol as the solvent and sulfuric acid as a catalyst. 2-Methoxy-5-X-stilbenes and 2-methoxy-3-X-stilbenes undergo this photoreaction to produce the corresponding 2-X-phenanthrenes and 4-X-phenanthrenes, respectively.This regioselective photochemical route to these particular types of substituted phenanthrenes represents an improvement synthetically over the well-known oxidative photocyclization method with meta-substituted stilbenes, from which approximately 1:1 mixtures of 2-substituted and 4-substituted phenanthrenes usually are obtained.An attempt to extend the scope of this eliminative photocyclization method to the synthesis of benzanthracene by the ultraviolet irradiation of 3-methoxy-2-styrylnaphthalene was not successful, but this synthetic objective was achieved in an alternative way by the eliminative photocyclization of 5,6,7,8-tetrahydro-3-methoxy-2-styrylnaphthalene followed by oxidation of the resulting 8,9,10,11-tetrahydrobenzanthracene with DDQ.