Sulfuric Acid → 2-Hydroxynicotinic acid

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Synthetic Scheme II describes the three step preparation of 2-halopyridines from the oxo-pyridinecarbonitriles 8 (corresponding to oxopyridine 5 where R2 is cyano). In step 1, the oxo-3-pyridinecarbonitrile 8 is hydrolyzed by treating with a strong acid such as 85% sulfuric acid and heating at a high temperature, such as about 200° C. to give the oxopyridine-3-carboxylic acid 9. In step 2, the oxo-3-pyridinecarboxylic acid 9 from Step 1 is decarboxylated such as by heating at a high temperature such as about 330° C. to give the 1,2-dihydro-oxopyridine 10. In step 3, the 1,2-dihydro-oxopyridine 10 from Step 2 is halogenated with a reagent such as phosphorous oxychloride and heating in an autoclave at a high temperature, such as about 200° C., to give the desired 2-halopyridines 11 of the present invention. Alternatively, the 1,2-dihydro-oxopyridine 10 can be halogenated by treatment with a halogenating agent such as phenylphosphonic dichloride at an elevated temperature. A suitable temperature for this reaction is 200°. ##STR5##

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