4a,4b-Dihydrophenanthrene photocyclization

Z-Stilbene also undergoes photocyclization to 4a,4b-dihydrophenanthrene via an electrocyclization.  

When stilboestrol (diethylstilbestrol, 62) was photolysed in aqueous methanol at 254 nm it gave the stable 4a,4b-dihydrophenanthrene dione (63). The mechanism required a photo trans-cis isomerization, photocyclization and spontaneous enol-keto tautomerism [53,54]. Previous workers had carried out a similar irradiation in dilute acetic acid and obtained the expected aromatic product (64) [55], 

In addition to the definite conclusions of the theoretical studies (see Sect. VIC) which rule out a hot ground state process there are numerous experimental find-it which clearly suggest that 4a,4b-dihydrophenanthrene-like photocyclizations (with few exceptions, see below) take place in the first excited singlet state of diaryl-ethylenes Among those findings we should note the complementary 

In many cases, the CTI process competes with rearrangements and cyclizations that occur during the radiationless transition. Z-Stilbene is well known to give dihydrophenanthrene as a photoproduct along with F-stilbene (Fig. 2.4A). Reversible photocyclization is even the dominant reaction in fulgide [12] and merocya-nine [13] systems [3] (Fig. 2.4B,C). 

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