Photochemical cyclization reaction in cis-tetramethoxystilbene

trans-3,3′,5,5′-Tetramethoxystilbene (TMST) underwent cis–trans isomerization on photoirradiation. However, cis-3,3′,5,5′-tetramethoxystilbene efficiently underwent cyclization reaction to give dihydrophenanthrene (DHP) type compounds followed by oxidation to give 2,4,5,7-tetramethoxyphenanthrene (TMP), but the isomerization to the trans isomer did not practically take place.