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U.S. Pat. No. 4,785,113 discloses the preparation of the title compounds from ethyl 2-methylacetoacetate by reaction with NH3 to give ethyl 3-amino-2-methylcrotonate, reaction with diethyl monomethylmalonate to give 2,3,5-trimethyl-4,6-dihydroxypyridine, chlorination to give 2,4-dichloro-3,5,6-trimethylpyridine, selective cleavage of the 2-Cl substituent and exchange of the 4-Cl substituent for a methoxy group, and then oxidation to give the N-oxide. The subsequent reactions are carried out analogously to the process described in U.S. Pat. No. 4,472,409.

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