3- Hydroxypyridines, tautomerism

Pyridones and other six-membered compounds (functional tautomerism). The pyridone /hydroxypyridine tautomerism (76AHCS1, p. 87), especially 2-pyridone (15a)/2-hydroxypyridine (15b), has received more attention from theoreticians than any other example of tautomerism, probably in part because it is a simple model for biologically important molecules such as thymine, cytosine, and uracil (Scheme 8). 

The overall influences favoring OH forms in pyridinone-hydroxypyridine tautomerism have been well summarized (76AHC(S1)71>. 

Chou. P.-T, Wie, C.-Y. and Hung, F.-T. (1997) Conjugated dual hydrogen bonds mediating 2-pyridone/2-hydroxypyridine tautomerism. J. Phys. Chem. B, 101, 9119-9126. 

Pyrazol-3-one 392 reacts with arylidene malononitriles 393a-c in basic ethanolic medium to yield the 6-(3-oxopyrazol-4-yl)-2-oxo-l,2,3,4-tetrahydropyridine/ 2-hydroxy-6-(3-oxopyrazol-4-yl)-3,4-dihydropyridine adducts 397 398a-c in a 1 1 ratio (87AP140) (Scheme 109). The mechanism is assumed to proceed via intermediate 396 formed from Michael adduct 394 or possible isomer 395. The pyran derivative 396 rearranges to the pyrid-2(l//)-one/2-hydroxypyridine tautomeric mixture 397/398. 

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