Amides tautomers

Clearly, in the case of (66) two amide tautomers (72) and (73) are possible, but if both hydroxyl protons tautomerize to the nitrogen atoms one amide bond then becomes formally cross-conjugated and its normal resonance stabilization is not developed (c/. 74). Indeed, part of the driving force for the reactions may come from this feature, since once the cycloaddition (of 72 or 73) has occurred the double bond shift results in an intermediate imidic acid which should rapidly tautomerize. In addition, literature precedent suggests that betaines such as (74) may also be present and clearly this opens avenues for alternative mechanistic pathways. 

Acylation of l,2,4-triazolo[l,5-c]quiiiazoliii-5-oiie (141) with ethyl chlo-roformate occurred at N6 to afford the 6-ethoxycarboiiyl derivative 142 (75CB3799) (Scheme 55). Tliis result indicates that the amide tautomer (lactam tautomer) 141 prevailed over the corresponding imidic acid tautomer (lactim tautomer). 

The cationic species 4 thus formed reacts with water to give the iminol 5, which tautomerizes to a more stable amide tautomer, the N-substituted carboxylic amide 2. Those steps correspond to the formation of amides by the Schmidt reaction. A side reaction can give rise to the formation of nitriles. 

This area has received much less attention in the literature than the reactivity of conjugated systems. As noted in CHEC-II(1996) <1996CHEC-II(8)345>, most of the examples studied contain one or two oxo groups in the six-membered ring and are cyclic amide tautomers of the corresponding hydroxyl compounds. 

T. Gloster, S. J. Williams, C. A. Tarling, S. Roberts, C. Dupont, P. Jodoin, F. Shareck, S. G. Withers, and G. J. Davies, A xylobiose-derived isofagomine lactam glycosidase inhibitor binds as its amide tautomer, Chem. Commun. (2003) 944-945. 

In common with many other nitrogen heterocycles azepin-2-ones and their benzo and dibenzo derivatives exist solely as the amide tautomers (76AHC(S1)554). 

Presumably, the oxidation of the iron(n) diimine complex derived from glyoxal and methylamine (encountered previously) by cerium(iv) occurs by a related mechanism (Fig. 9-37). This also provides an interesting example of the metal ion stabilising a particular tautomer of a ligand. The free ligand would be expected to exist as the amide tautomer 9.22. 

Pyrimidones exist as the amide tautomer 60 rather than the enol tautomer 59 unlike the case with phenols 61. The enol tautomer of a phenol 61 is aromatic but the keto tautomer 62 is not. 

Reprotonation occurs on the nitrogen to produce the amide. As was the case with the carbonyl—enol tautomerization, the stability of the carbon—oxygen double bond causes the amide tautomer to be favored at equilibrium. 

The [(dien)Pt(OH2)] complex reacts with urea or 1,1-dimethylurea (dmu) in acetone to initially give the O-bound compound (39) as the kinetically preferred product (Scheme 6). This compound then rearranges (t j 5h for urea and 18 min for dmu at 22°C) to the slightly more thermodynamically favored N-bound linkage isomer (Kj o = [N isomer]/[0 isomer] 10 for urea and 3 for dmu) (87, 88). An X-ray crystal structure confirms the presence of the amide tautomer 40b, not the iminol tautomer 40b (Fig. 7). Apparently, in the urea complex the O atom is less basic than the N atom. 

The mechanism of this reaction involves the formation of an amide tautomer. Two tautomers can be drawn for any carbonyl compound, and those for a 1° amide are as follows ... 

The imidic acid and amide tautomers are interconverted by treating with acid or base, analogous to the keto-enol tautomers of other carbonyl compounds. In fact, the two amide tautomers are exactly the same as keto-enol tautomers except that a nitrogen atom replaces a carbon atom bonded to the carbonyl group. 

Piperazine-2,5-dithiones, thio analogues of cyclic dipeptides, have been shown to have a flattened boat configuration (1643), and infrared spectroscopic studies indicate that in the trans tautomer (120) the C-N bond has a more pronounced double bond character than that of the trans amide tautomer (1644). 

C-Hydroxy-8-azapurines do not exist as such but as equilibrium mixtures (e.g., 8 9) in which the cyclic amide tautomers greatly preponderate over the hydroxy tautomer. This behavior parallels what was found in the pyridine, pyrimidine, and purine series on evidence from ionization constants and the UV spectra of C- and A -methyl derivatives. A formal name for9 is 1,6-dihydro-8-azapurin-6-one but such specification of the hydrogen atom s position is, in the absence of data, risky for example, pyrimidin-4-one is an equilibrium mixture in which tautomers with mobile hydrogen on N-3 and N-1 preponderate in a 5 2 ratio, respectively. Hence the simpler names, such as 8-azapurin-6-one, will be used in this review. 

In the solid state, infrared absorption data indicated the preponderance of the 3-oxo 25 an the 5-oxo 92 structures (amide tautomers) over the 3-... 

In basic hydrolysis, a hydroxide ion attacks the nitrile carbon atom, and subsequent protonation leads to the amide tautomer. Further attack by the hydroxide ion leads to hydrolysis in a manner analogous to that for the basic hydrolysis of an amide (Section 17.8F). (Under the appropriate conditions, amides can be isolated when nitriles are hydrolyzed.)... 

In base-catalyzed nitrile hydrolysis, direct attack of hydroxide gives the anion of the amide tautomer, which protonates on nitrogen. The remaining proton on the oxygen is then removed by base and a second N-protonation gives the amide. Hydrolysis is completed as... 

The triclinic form [29] was obtained by crystallization from ethanol or by reflux in ethanol with the addition of copper(II) chloride in a 1 IM ratio. The sulfasalazine in this form (QIJZOY) is present as an amide tautomer forming the supramolecular dimer, stabilized by two R2 (8) hydrogen bonds, namely synthon R between the carboxylic part of one molecule and pyridine N and amide N-H groups from the other molecule (Scheme 13.12). The contacts with other dimers are realized mostly through weaker hydrogen bond C-H- -O and C-H- -N... 

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