Phenanthridines photochemical synthesis

The cyclic borates (758) are useful alternatives to benzanilides in the photochemical synthesis of phenanthridines. Irradiation, followed by reduction with lithium aluminum hydride, gives phenanthridines in good yield, the borate ring maintaining the correct stereochemistry (78CC884). 

A photochemical ring construction similar to that described in Scheme 5 was used in the synthesis of the aromatic lycorine alkaloid ungeremine (51) (Scheme 6). Photocyclization of the somewhat inaccessible imine (48) gave the phenanthridine derivative (49) in 21 % yield which upon metal hydride reduction and cyclization with phosphorus tribromide afforded the quaternary salt (50) which was shown to be identical with the methyl ether of ungeremine (51) obtained as shown. 

Photochemical methods of synthesis of the phenanthridine system involve the cyclisation of benzanilides and Ifl-benzylideneanilines. The unfavourable geometry of these species and the competing n —> it excitation of the latter, generally result in low yields of the heterocycle. These adverse steric and electronic effects have been largely overcome in the photocyclisation of the boron complexes of A -arylbenzolydroxamic acids, a process which is both fast and high yielding (S. Prabhakar, A.M. Lobo and M.R. Tavares, Chem. Comm., 1978, 884). 

Although the synthesis of phenanthridine from iminyl radicals, photochemical processes, and microwave-mediated cyclizations has been developed, further exploration of convenient, efficient, and milder protocols are still desirable due to the broad applications of phenanthridines. Jiao group successfuUy realized an intermolecular nitrogenation reaction toward the synthesis of phenanthridines 64 (Scheme 7.43) [113]. This transformation can proceeds under metal-free... 

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