Patent GB-734381-A (GB)
Title: Production of compounds of the pyridine series
Filed: 1953-04-17
Abstract: A process for the production of compounds of the pyridine series consists in introducing glutaric dialdehydes or d -keto-aldehydes, 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-dihydropyranes, into aqueous or alcoholic solutions of hydroxylammonium salts at temperatures above 90 DEG  C., or introducing the corresponding glutaric dialdehyde or d -ketoaldehyde mono- or di-oximes into excess strong acids at the said temperatures or heating said oximes as rapidly as possible with excess strong acids to the said temperatures.  The said temperatures may be between 90 DEG  and 220 DEG  C., preferably between 100 DEG and 180 DEG  C.  The 2-alkoxy-2.3-dihydropyranes can be obtained by the diene synthesis from vinyl ethers and a .b -unsaturated carbonyl compounds, for example 2-methoxy-2.3-dihydropyrane from acrolein and methyl vinyl ether, 2-ethoxy-4-methyl-2.3-dihydropyrane from crotonaldehyde and ethyl vinyl ether, 2-methoxy-5-methyl-2.3-dihydropyrane from a  - methylacrolein and methyl vinyl ether, or 2-methoxy-6-methyl-2.3-dihydropyrane from vinyl methyl ketone and methyl vinyl ether.  The 2-acyloxy-2.3-dihydropyranes are obtained similarly from vinyl esters and a .b -unsaturated carbonyl compounds, e.g. 2-acetoxy-2.3-dihydropyrane from vinyl acetate and acrolein.  The 2-acyloxy- or 2-alkoxy-2.3-dihydropyranes can be hydrolysed to the free glutaric dialdehydes or d -keto aldehydes.  As hydroxylammonium salts there may be used the hydrochloride, hydrobromide, or sulphate and also compounds which are converted into such salts under the reaction conditions.  The process may be carried out continuously by passing the reactants through heated pressure tubes. Hydrochloric, sulphuric or organic sulphonic acids such as toluene or paraffin sulphonic acids are suitable strong acids.  In examples: (1) pyridine is prepared by introducing 2-ethoxy-2.3-dihydropyrane with stirring into aqueous hydroxylammonium chloride solution in an autoclave heated to 120 DEG  C.; (2) g -picoline is prepared similarly from 2-ethoxy - 4 - methyl - 2.3 - dihydropyrane; (3) g -picoline is prepared by slowly adding 2-ethoxy-4-methyl-2.3-dihydropyrane into a refluxing solution of hydroxylamine mono- and disulphonic acids; (4) 2-methyl glutaric dialdehyde (from the hydrolysis of 2-ethoxy-4-methyl-2.3-dihydropyrane with hydrochloric acid at 30 DEG C.) is introduced into boiling hydroxylammonium chloride solution to give g -picoline; (5) glutaric dialdehyde is introduced into boiling hydrochloric acid to yield pyridine; (6) b -methylglutaric dialdehyde mono-oxime (obtained by dissolving 2-ethoxy-4-methyl-2.3-dihydropyrane in dilute hydrochloric acid containing hydroxylammonium chloride) is passed through a coil heated in xylene vapour to give g -picoline.  Similarly to (1) there are prepared (7) b -picoline from 2-methoxy-5-methyl-2.3-dihydropyrane, (8) a -picoline from 2-methoxy-6-methyl - 2.3 - dihydropyrane, (9) pyridine from 2 - acetoxy - 2.3 - dihydropyrane and (10) 3-methyl-4-ethyl pyridine from 2-ethoxy-4-ethyl-5-methyl-2.3-dihydropyrane (obtained by the diene synthesis from a -methyl-b -ethyl-acrolein and ethyl vinyl ether).