2- Chloro-5-nitro-6-hydroxypyridine

Azine approach. The parent cation and substituted derivatives are available by acid-catalyzed cyclization of 2-/3-oxoalkylthiopyridines (401) using an acid such as sulfuric, phosphoric or PPA. Chloro or nitro substituents in the pyridine ring do not seriously interfere (66JHC27). The cyclization of 3-hydroxypyridine analogues (402) is also at the nitrogen to yield the thiazole derivatives. The cyclization, however, is sensitive to the peri interaction between 3- and 5-substituents. In 3,5-dimethyl derivatives (403 R2 = R3 = Me) the steric repulsion is apparent by the unusually low field signals for the methyl protons <81H(15)1349>. 

---