Deprotonation 2- hydroxy-, tautomers

Dioxygenated pyrones exist as the 4-hydroxy tautomers. Such molecules are easily substituted by electrophiles, at the position between the two oxygens (C-3) and can be side-chain deprotonated using two mole equivalents of strong base. ... 

Hydroxyindole does not exist as such the stable form is the carbonyl tautomer the hydroxy tautomer cannot be detected. There is nothing remarkable about the reactions of oxindole, for the most part it is a typical 5-membered lactam, except that deprotonation at the /3-carbon (p fg 18) occurs more readily than with simple amides, because the resulting anion is stabilised by an aromatic indole canonical contributor. This anion will react with electrophiles like alkyl halides and aldehydes at the /3-carbon, the last with dehydration and the production of aldol condensation products. It is interesting that the 3-position is three times more reactive than the 1-position. Oxindoles can be effectively oxidised to isatins (section 17.14.3) via easy 3,3-dibromination, then hydrolysis. Bromination of oxindole with A -bromosuccinimide gives 5-bromo-oxindole. ... 

Tile ID and 2D NMR experiments for 5-aza-7-deaza-2 -deoxyguanosine concluded that the oxo-amino tautomer 59a is preferable in DMSO-dg whereas the oxo-imino form 59b dominates in D2O (87JOC5136). Evidence for the hydroxy-imino tautomer 59c was not found. Poor solubility of the parent compound, 5-aza-7-deazaguanine (60) did not allow study of its tautomerism, but the pK values of protonation and deprotonation for 60 are identical with those for 59. 

Theoretical estimations at the B3LYP level for the 298 K gas-phase acidities of all possible deprotonation sites of the most stable tautomers of cytosine predicted the almost equal acidity of the oxo and hydroxy forms (03JPC(A)4893). Relative stabilities... 

The compound HOtpy is of interest as the free ligand exists as the 2,2 6, 2"-terpyridin-4 (l //)-one tautomer, but coordination to a metal forces it to adopt the tautomeric 4 -hydroxy-2,2 6 ,2 -teipyridine form. These complexes are acidic, and deprotonated forms are readily accessed by treatment with mild bases.. 

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