Morgan-Walls

The Pictet-Hubert reaction describes the construction of the phenanthridine nucleus (2) by dehydration of acyl-o-xenylamines (1). The application of zinc chloride at high temperature facilitates the dehydration/ This reaction is also referred as the Morgan-Walls Reaction. 

There have also been reports of adding zinc chloride or tin chloride to the Morgan-Walls conditions to catalyze the reaction (see experimental section). ... 

Pictet-Hubert Reaction / Morgan-Walls Reaction... 

Badger and Sasse have described the preparation of 2-, 3-, and 8-bromophenanthridine by the cyclization of the appropriate bromo-formamidobiphenyl with polyphosphoric acid.25 In the case of 2-bromo-2 -formamidobiphenyl a higher concentration of phosphorus pentoxide in the acid was necessary to effect ring closure, and a simple steric effect was invoked.26 Nevertheless, the Morgan-Walls reaction has been used to obtain several overcrowded compounds in which unfavorable steric factors operate. Following the original report of the preparation of the 1,10-dimethylphenanthridine (5a) by this procedure,27 several other examples have been described, notably the synthesis of the related phenanthridine (5b),28 the l,2-(6)29 and 9,10-benzophenanthridines (7),30 and the 1,2 9,10-dibenzophenanthridine (8).29... 

The Morgan-Walls cyclization has also been employed in eases where the acylamino group is attached to a reduced ring. For example, the amides (13a-c,41 and d42) are readily cyclized by phosphoryl chloride and the tetrahydro-l,2-benzophenanthridine (14) has been prepared... 

The utility of the above method is again evident in the synthesis of 7,8-dimethoxy-phenanthridine which is not obtained by Morgan-Walls reaction. 

ZnO 2. Pictet-Hubert conditionX POCI3 Morgan-Walls condition] R = H, alkyl, aryl... 

The reaction mechanism under the initial Pictet-Hubert condition is displayed in Scheme 1 and the modified Morgan-Wall condition is outlined in Scheme 2. It is believed that the complexation of Lewis acid (e.g., ZnCh) with oxygen polarizes the carbonyl group, which facilitates the electrophilic substitution as shown below. 

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. 

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