The study of the influence of the nitro substituent on the hydrazinolysis of certain substituted 5-nitropyrimidines Public Deposited

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  • The unique reaction between 4, 6-dimethoxy-5-nitropyrimidine and methylhydrazine was investigated. The reaction product was identified as 4-hydrazino-6-hydroxypyrimidine from (a) spectral data, (b) by direct synthesis of the product via 4-chloro-6-hydroxypyrimidine and hydrazine, and (c) by conversion of the product to the known 4-amino-6-hydroxypyrimidine using Raney nickel. The product was not produced by the nucleophilic substitution of methylhydrazine with 4, 6-dimethoxy-5-nitropyrimidine. The product was the result of an initial rearrangement of the methoxypyrimidine (presumably a methyl migration) to an intermediate which then reacted with the methylhydrazine. When the methoxypyrimidine was refluxed in pyridine, it was converted into at least two non-interconvertible products, one soluble and the other insoluble in cold pyridine. The soluble product yielded 4-hydrazino-6-hydroxypyrimidine when treated with methylhydrazine at room temperature while the insoluble one did not. The insoluble product was tentatively identified from spectral data as an N-methylpyridinium N-methylpyrimidinate salt. The reason that 4, 6-dimethoxy-5-nitropyrimidine does not respond to direct nucleophilic substitution with methylhydrazine (as in the case of hydrazine at both the 4- and 6-positions) apparently stems from steric hindrance imposed by the attacking methylhydrazine. Among the effects caused by the steric crowding is the forcing of the nitro substituent out of the plane of the pyrimidine ring thus losing the important resonance contribution of the nitro substituent in making the 4- and 6-positions more electrophilic. Treatment of the 2-methyl- and 2-phenyl-derivatives of 4, 6-dimethoxy-5-nitropyrimidine with methylhydrazine likewise does not yield products as a result of direct nucleophilic substitution but rather the corresponding 4-hydrazino-6-hydroxy-2-substituted pyrimidine. Reaction of 4, 6-dimethoxy-5-nitropyrimidine with either 1,1-dimethyl- or 1,2-dimethylhydrazine results in decomposition of the pyrimidine ring; with phenylhydrazine there is no apparent reactivity. In the course of this investigation it was demonstrated that in most cases methoxy-nitropyrimidines are good intermediates for the preparation of hydrazino-nitropyrimidines. The yields, when reaction occurred, were excellent and the product of analytical purity. This is important as most hydrazino-nitro compounds are thermally unstable and oxidized readily when heated in hot solvents. Some pyrimidine derivatives prepared by this method were 4-amino- 6-hydrazino- 5-nitropyrimidine, and 2-R-4, 6-dihydrazino-5-nitropyrimidine (where R = H, methyl or phenyl). Some pyrimidines which did not react with hydrazine are; 4-methoxy-6-(1-methylhydrazine)-5-nitropyrimidine and 4-dimethylamino-6-methoxy-5-nitropyrimidine. Thus a methoxy substituent in the 4-position can only be replaced by hydrazine if there is no large group (e. g. methylhydrazino) adjacent to the nitro substituent (6-position). Also some substituted hydrazino-nitropyrimidines were prepared via displacement of chloro substituents. The reaction occurred under mild conditions (e.g. 0-20°C in methanol) even with the less reactive hydrazines (e.g. phenylhydrazine). Generally the yields were good and occasionally the derivative needed no further purification. Some of the pyrimidine derivatives that were prepared by this method included: 2-R-4, 6-di(l-methylhydrazino)-5-nitropyrimidine (where R = H, methyl or phenyl), 4-methoxy-6-(1-methylhydrazino)-5-nitropyrimidine, 4-dimethylamino-6-(1-methylhydrazino)-5-nitropyrimidine and 2-R-4, 6-(2-phenylhydrazino)-5-nitropyrimidine (where R = H or phenyl). The it and uv spectral data are' included for all previously unreported compounds which were prepared.
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