Benzo systems photoisomerization

The photocyclization of enamides has been widely employed in the construction of heterocyclic systems the N-acryloyl-2-aminopyridines 37, for example, are converted on irradiation to the lactams 38.36 Numerous benzylisoquinoline alkaloids have been prepared using this approach, and in particular, the syntheses of benzo[c]phenanthridine alkaloids have been reviewed.37 Thus, irradiation of the [Z]-l-ethylidene-2-benzoyltetra-hydroisoquinoline 39 affords the corresponding 8-oxoberberine 4038 competing photoisomerization to the E-isomer is observed but cyclization occurs only via the Z-isomer. Examples of syntheses of Amaryllidaceae and indole alkaloids have also been reported. In this way, the precursor 41 of ( )-lycoran has been obtained by oxidative cyclization of the enamide 42.39... 

Later, Falk [66] used TET to explain the photoisomerization of pyrrole pigments. Markov [67] applied the theory to photoketonization of dicarbonylic compounds. Hamanoue [68] showed that TET predictions were consistent with the relative reactivity of (n, n ) and (n, Tt ) states of an-thraquinones. Following a suggestion by Isaacs [69], Okamoto [70] claimed that the increase in the KIE of the H abstraction of methanol by benzo-phenone was evidence for a nuclear tunnelling mechanism, but this claim is not entirely consistent with the predictions of Isaacs. Shizuka [71] proposed TET could be used to explain excited state proton transfers, but this may only be the case for intramolecular proton transfers or reactions in apolar solvents, otherwise the system effective reduced mass will be too high [72]. 

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