Photocyclization electrocyclic -ring closure

The photocyclization of enamides to quinolines or isoquinolines has become an important reaction in the synthesis of alkaloids 219,358). It has recently been applied in the preparation of the isoquinoline alkaloid polycarpine 359). The principle of the reaction is demonstrated in the preparation of dihydroquinolines 360> (3.39) and of spirocyclohexaneisoquinoline derivatives (3.40) 361K In each case the electrocyclic ring closure product undergoes a subsequent 1,5-hydrogen shift. 

Electrocyclic closure of butadiene units encased within cycloheptane rings has been used to obtain bicyclo[3.2.0]heptene systems (Scheme 5). For example, irradiation of eucarvone 21 led to the formation of adduct 22 in 52% yield via a disrotatory ring closure . This adduct was used as a key intermediate in the synthesis of the pheromone grandisol, 23, which proceeded in 20% overall yield from 22. in their synthesis of a-lumicolchicine. Chapman and coworkers utilized a photochemically initiated four-electron disrotatory photocyclization of colchicine to produce j6-lumicolchicine 24a and its y-isomer 24b in a 2 l ratio . These adducts were then converted, in a second photochemical step, to the anti head-to-head dimer or-lumicolchicine 25. 

Two classes of photocyclization reactions of indoles are well exemplified in the literature. One is based on a 6-ti electrocyclic closure of trienes to cyclohexadienes where the indole C2-C3 bond forms part of the triene. The other involves photolysis of m-haloalkylindoles whereupon the halogen-substituted carbon becomes attached to an indole ring position. The first example of the former reaction was reported by Szmuszkovicz [75], who found that UV light irradiation of 2,3-bis-(para-methoxyphenyl)indole in the presence of iodine gave the carbazole 71 (Scheme 37). By analogy with the photocyclization reaction of other 1,2-diarylethenes [76], this reaction presumably proceeds via conrotatory closure of the indole singlet excited... 

---