Phenanthridines => aminobiphenyls

Phenanthridine cyclization by dehydrative ring closure of acyl-o-aminobiphenyls with phosphorus oxychloride in boiling nitrobenzene. 

Phenanthridines and related systems are obtained by the following methods 1. Gyclodehydration of acylated 2-aminobiphenyls 41 42 (Scheme 20). 

Earlier antitrypanosomiasis agents included the aminonaphthalenes trypan blue and Afridol violet. They were followed by the amide suramin79 (see Chapter 1). Heterocyclic aromatic amines include quinapyramine (138) and puromycin (139). Starting with 2-aminobiphenyl (140), a five-step reaction yields the phenanthridine dimidium (141) (Scheme 28). 

On acid catalysis, 2-aminobiphenyl-2 -carboxylic acids 31 cyclize affording phenanthridones 32, which are reduced by zinc or LiAlH4 to phenanthridines 34. The aminobiphenylcarboxylic acids 31 are accessible from the corresponding nitro compounds 33 by reduction. 

The Morgan-Walls reaction is a variant of the Pictet-Hubert reaction where the phenanthridine cyclization was accomplished by dehydrative ring closure of acyl-o-aminobiphenyls on heating with zinc chloride at 250—300 °C. 

In the presence of halides, yields of heterocyclisation products (carbazole (2) and 5H-phenanthridin-6-one (5)) were ca. 75-85 %, with a preference for 2 in the order Cl > Bf s F f, whereas the selectivity towards 5 was the reverse, F > Br = F > Cr. The yield of orf/to-aminobiphenyl (3), probably derived from the protonation of the intermediate nitrene complex, was strongly decreased. 

Aminobiphenyl-2 -carboxylic acids 32 undergo acid-catalyzed cyclization to phenanthridones 33, which can be reduced (by zinc or LiAlH4) to phenanthridines 34 ... 

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