GB726378A - Improvements in the production of compounds of the pyridine series - Google Patents

Abstract

Compounds of the pyridine series including halogenated pyridine bases are prepared by reacting glutaric dialdehydes or d -ketoaldehydes, as such or in the form of their acetals; in particular in the form of their cyclic enol acetals, i.e. the 2-alkoxy-2,3-dihydropyranes, or the corresponding 2-acyloxy-2,3-dihydro-pyranes, at elevated temperatures in the presence of oxidizing agents with aqueous solutions of ammonium salts (e.g. ammonium sulphate, phosphate and halides). Suitable starting materials are glutaric and b -methylglutaric dialdehydes, 2-methoxy-, 2-methoxy-4-methyl-, 2-ethoxy-4-methyl-, 2-ethoxy-4-ethyl-5-methyl-, 2-methoxy-5-methyl-, 2-ethoxy-5-methyl-, 2-methoxy-6-methyl-, and 2-acetoxy-2,3-dihydropyranes. Suitable oxidizing agents are salts of metals in a higher valency state which are reducible to a lower valency state, e.g. salts of trivalent iron or manganese and divalent copper, and also hydrogen peroxide in the presence of iron or copper salts. Ammonium ferric sulphate may be used simultaneously as the ammonium salt and the oxidizing agent. Mono- and di-halogenated pyridine bases are obtained by using as the ammonium salt and/or as the oxidizing agent the corresponding halides (e.g. ammonium chloride or bromide on the one hand and ferric, cupric or mercuric chlorides on the other hand). It is also possible to work in presence of free chlorine, bromine or iodine and these are preferably added to the glutaric dialdehydes or dihydropyrane derivatives at low temperature before the addition of the oxidizing agent and the ammonium salt, halogenated pyridines being obtained exclusively when salts containing halogen ions are also added. The process is carried out by heating the reactants together in aqueous solution or by allowing glutaric dialdehyde or the dihydropyrane derivative to flow into a heated solution of ammonium salt and oxidizing agent and may also be carried out under increased pressure and/or continuously. Mono-and di-halogenated pyridine bases are preferably separated from each other and from nonhalogenated pyridine compounds (formed simultaneously) by distillation of the reaction mixture at different pH values. The examples describe the preparation of pyridine, b -picoline, g -picoline; 3-chloro-, 3,5-dichloro-, 3-chloro-4-methyl-, and 3,5-dichloro-4-methyl-pyridine.