Synthesis of some substituted pyridylsydnones

Synthesis of the I‐oxide ( 2 ) of the photochromic N ‐(3‐pyridyl) sydnone ( 1 ), of N ‐(5‐bromo‐3‐pyridyl) sydnone ( 3 ), and the I‐oxide ( 4 ) of 3 were undertaken in order to study the effect on photochromism exerted by substituents on the pyridine ring. Compounds 2 and 3 were prepared via the corresponding aminopyridines and N ‐pyridylglycines by the general procedure used earlier to prepare 1 . The required amines, 3‐aminopyridine I‐oxide and 3‐amino‐5‐bromopyridine, were obtained by Hofmann rearrangement of the corresponding amides. An excellent preparation of 5‐bromonicotinamide was developed involving bromination in thionyl chloride followed by reaction of the bromoacid chloride with ammonia in chloroform. Proof of structure for 2 and 3 was accomplished by acid hydrolysis to the corresponding hydrazines, which were isolated, respectively, as acetophenone 3‐pyridylhydrazone I‐oxide and as 5‐bromo‐3‐pyridylhydrazine hydrochloride. These products were identical with samples prepared by reduction of the respective diazotized amines. Sydnone 4 eluded preparation by this general procedure. 3‐Amino‐5‐bromopyridine I‐oxide was prepared conveniently from 5‐bromonicotinamide but attempts to prepare the corresponding glycine by catalytic hydrogenation of a mixture of the amine and butyl glyoxylate afforded, in acid solution, N ‐(3‐pyridyl)glycine and, in neutral or alkaline solution, the I‐oxide of N ‐(3‐pyridyl)glycine. Both products resulted from the reductive cleavage of the bromine atom. Neither 2 nor 3 was photochromic.