Enamides, photocyclisation

The analogous photocyclisation of enamides is well known and is of particular value in synthesis. Photocyclisation of the enamides (29) to the furopyridones (30), for example, has been used in the preparation of key intermediates for alkaloid synthesis. Similar approaches have been employed in the synthesis of corynanthe alkaloids ° and in the preparation of trans-hexahydro-indolo[43-ab]phenanthridines as selective dopamine agonists. The benzoylmethylenebenzothiazepines (31) can be converted in the same way into the B-homo-5-thiaprotoberberines (32). Enamide photocyclisation has also been employed in the syntheses of halo-benzofuro[2,3-c]quinolines,benzo[h][l]- and benzo[/j[1]-benzo-... 

Several reviews whose topics include photochemical reactions of aromatic compounds have appeared during the year. The preparation of indole alkaloids by enamide photocyclisation reactions has been surveyed. A very useful four volume work dealing with the theory and experimental practicalities of light-induced electron transfer reactions has been published the third volume of this set is... 

In a reaction which is mechanistically related to the stilbene-dihydrophenanthrene photocyclisation, the anilide derivative of (178), compound (179), photocyclises to give (180). As mentioned in the Introduction to this chapter, such enamide cyclisatlons have now been reviewed. Other new examples of this reaction are the formation of (181) from (182) and the formation of the regioisomers (183) and (184) from (185) by irradiation of the enamldes in the presence of sodium borohydride. These enamide photocyclisation reactions can be viewed as an electrocyclic closure of the amide contributing structure (186) to give (187) in the presence of borohydride the iminlum function in (187) is reduced to the observed products. 

The enamide photocyclisation can also proceed oxidatively or, if a potential leaving group is present in an ortho position, rearomatlsation by elimination of the ortho substituent in the dihydroaromatic intermediate can occur. This is the case with (188) which cyclises to (189). ... 

The photocyclisation of enamides provides a notably successful approach to this group. It is based on the ready formation of enamides by acylation of the imines derived from 1-tetralone and primary amines (I. Ninomiya et at., J. chem. Soc. Perkin I, 1979, 1723). The photocyclisation is non—oxidative and stereospecific, affording the trons-BC-tetrahydrobenzo[c]phenanthridone (121). Under oxidative conditions, the del dro... 

The application of the photocyclisation reaction of enamides of benzoic acid to the synthesis of alkaloids and their analogues is exemplified this year by the preparation of (251) from (252), (253) from (254), and (255) from (256). ... 

Ninomiya, 1., Kiguchi, T, Yamamoto, O., and Naito, T., Photocyclisation of enamides. Part 13. Substituent effects in the photocyKsation of N-benzoylenamides, /. Chem. Soc., Perkin 1,1723-1728,1979. Couture, A., Grandclaudon, R, and Hooijer, S.O., Photocyclization in an alcohol solution containing dissolved base. A New development in enamide photochemistry, /. Org. Chem., 56, 4977-4980,1991. 

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