Enaminones tautomerism

The enol form 10 of pyrrolam A 10 could not be detected despite its aromatic character <2004JOC6105>. Also, its pyrrolinone analogues 34 exist mainly as /3-enaminones <1985TL833> the tautomeric hydroxypyrrole 34 was only observed when R = C02Et (Scheme 1) <1988JOC5680>. 

Fig. 11.8. Formation and tautomeric equilibria of enaminones (11.63) as potential prodrugs...

The behaviour of compounds 49 where R = H and CH3 from is even more complicated. For example, 4-methylamino-3-(4-X-phenylazo)pent-3-ene-2-one (49) contains two isomers. The tautomeric form of both isomers was confirmed with the help of 2D H-15N GHMQC spectra. The minor isomer is very similar to the tautomeric form of azo-enamine ( j(15N, H) = 89.6 Hz) the major isomer possesses a somewhat stronger character of hydrazono-ketimine form ( /(15N,1H) = 74.5 Hz about 80% azo and 20% hydrazone). These conclusions are confirmed by the values of <5(15NH2) in the minor isomer it is 6(15N) = —269.0 ppm (i.e. almost the same as in the starting enaminone with... 

A theoretical study65 based on the semi-empirical AMI method was carried out in order to account for the tautomeric equilibria of enaminones 35. The results obtained were consistent with previous X-ray observations66 and showed the enamine form of acyclic primary and secondary amines with a hydrogen bond between the N and O atoms to be ca 5 kcal mol"1 more stable than its tautomer. 

IR, UV and NMR spectra obtained so far are compatible with the structure of the secondary enamines. The tautomeric enamine structure of some enaminones in the solid state has also been established unambiguously by single-crystal X-ray diffraction analyses21. In agreement with the experimental evidence, theoretical calculations22,23 performed on enaminones 6-8 and nitroenamine 9 indicate that the enamine forms are... 

Very recently, an enamine-imine tautomerism of ketene aminals has been reported by Huang and coworkers64-67. Due to competition by increased aromaticity, very stable enaminone isomers 63-65 equilibrate to their imine isomers 63-65 (equation 10). It is... 

Table 2. The geometrical (in A and degrees) and energetic (kcal/mol) parameters for two tautomeric forms of the simple enaminones the results for transition states of corresponding proton transfer reactions are also included there is also the difference in energies between the considered system and the most energetically stable system (hence 0 designates the most stable system)...

Extensive studies have shown that the enamine-imine tautomeric equilibrium of equation 1 shifts almost completely to the enamine side when an unsaturated electron-withdrawing group including nitrile nitro °, carbonyl, thiocarbonyP etc. ° is attached to the -carbon. Reviews related to these enamines which include enaminoni-triles 2, nitroenamines 3, enaminones 4 and the enaminothioketones 5, have appeared in the literature, ... 

A detailed investigation of the photolysis mechanism of enamines has been conducted by Hoffmann and Eicken The rearrangement proceeds through radical processes. When A -acylenamine 73 was irradiated at the wavelength corresponding to the n n -transition of the amide at approximately 200 nm, the amide bond was cleaved to the radical pair. This radical pair could either recombine and revert back to the reactant or undergo a [l,3]-acyl shift to give the imine 74. In turn 74 underwent rapid tautomerization to the enaminone 75, which was in photochemical equilibrium with its isomer 75 (Scheme 6). 

The stable enaminone 168 undergoes an aza-Cope reaction 170 at nearly 200 °C in nitrobenzene. It may seem odd that a linear alkyne should take part in such a reaction but in fact this is not unusual. The product is an allene 171 that undergoes tautomerism to the more stable enaminone 172 then a [1,5] sigmatropic H shift before cyclising to a dihydropyridine 174. The cyclisation is formally a six electron disrotatory electrocyclic reaction 173. The solvent PhN02 oxidises 174 to the pyridine 169. 

On the other hand, unsubstituted ethyl (2-pyridyl)pyruvate 113 (R = COOEt) was reported to exist predominantly in the enaminone form 113c in methanol, but as enol form in cyclohexane (94JCS(P2)2461). The tautomeric equilibrium between 113b... 

No enaminone tautomer 124b was observed in CDC13 solutions of substituted 4-(acylmethyl)quinolines 124. The position of the tautomeric equilibrium is greatly affected by both R1 and R2 substituents. Quinolines 124 (R1 = alkyl R2 = H) exist in solution exclusively as keto tautomers 124a, whereas only tautomeric form 124c was observed for diesters 124 (R1 = COOEt R2 = COOMe, COOEt). Both taut-... 

A tandem sulfide-contraction and Robinson annelation reaction was used for the synthesis of a me-sembrine analog, A -mesembrenone (133). The aryl-substituted thiobutyrolactam (130) was treated with chloromethyl vinyl ketone followed by diisopropylethylamine to afford A -mesembrenone (133) in good yield (Scheme 29). The reaction probably proceeds through initial sulfide contraction to the intermediate vinyl enaminone (131), which tautomerized to the enamine (132) and underwent intramolecular Michael closure to produce (133). 

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