Enamines nitroenamines

Nitropyrimidinone 3 has also electrophilic sites, the 2-and the 6-positions, which are attacked by amines to yield nitro enamines having a carbamoyl group 11 in good yields [28]. Nitroenamine 11 is converted to polyfunc-tionalized pyridone 12a upon treatment with sodium enolate of ethyl 3-oxobutanoate 13a [29]. 

Important classes of enamines are those having electron-withdrawing substituents, R2 and/or R3, at C(2), particularly when the substituent(s) act(s) by a +R effect, as in aminoenones and nitroenamines. The stronger and more extensive electron delocalization and the concomitant changes in bond orders of the resulting push-pull systems is well reflected in the NMR spectra and have been extensively investigated in this way. 

The 13C-, 15N- and -NMR spectral data for a series of simple l-amino-2-nitroalkenes are collected in Table 30. The greater electron-withdrawing character of the nitro group as compared with carbonyl results in downfield shifts of the resonances of C(2) ( + 10 to +14 ppm), the enaminic nitrogen ( + 23.2 to + 27.4 ppm) and the olefinic protons [ + 0.8 to +1.0 ppm for H(l) +1.5 to = 1.8 ppm for H(2)] of 2-nitroenamines with respect to enaminones. This deshielding does not affect C(l), which has in fact a lower chemical shift ( — 5.5 to — 10.2 ppm) in 2-nitroenamines. The NMR spectra show that 2-nitroenamines with a primary or a secondary amino groups exist in solution as solvent-dependent equilibrium mixtures of E- and Z-isomers. The... 

The 13C- and H-NMR spectral data of a set of 2-acyl-2-nitroenamines are collected in Tables 31 and 32. The stereochemistry around C(1)=C(2) has been assigned by correlation with the NMR spectral properties of the corresponding enamines with a single electron-withdrawing group at C(2), or by comparison with compounds of fixed configuration, as those in Table 32. In the case of 3-amino-2-nitroacrylic esters (554... 

An important class of enamine derivatives are the nitroenamines, that have been considered as suitable starting materials for further synthetic manipulations. For some reviews see References 566-571. 

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... 

Deuterium isotope effects on 13C and 15N nuclear shielding in enaminones, enamine esters and nitroenamines which often exist in solution as a mixture of E (1) and Z (2) isomers have been recently investigated1. When labelled with deuterium E and Z isotopomers are obtained. 

The reaction of dibromocarbene with A-nitroenamines, cyclic enamides or enamines ... 

IR spectra and isomerism of -nitro enamines 250 and 251 have been studied recently by Gomez-Sanchez and coworkers in combination with dynamic and high-field NMR spectroscopy. The isomeric behaviour of these compounds is similar to that of j -enaminones. / -Nitroenamines with primary or secondary amino groups 250a,b and 251a,b exist as solvent-dependent equilibrium mixtures of the intramolecularly hydro-gen-bonded c -form 251 and the fraras-form 250. The latter isomer can adopt an additional Z or conformation around the —N single bond to the amino substituent... 

Nitroenamines with a primary or secondary amino group can exist also in two tautomeric forms the imino-nitro form (511) and the imino-nitronic acid form (512). Although the enamine form represents the thermodynamically more stable tautomer, the imino-nitro form has been observed when severe steric hindrance prevents planarity of the conjugated system Compounds 513, derived from 2-nitrocycloalkanones, have been reportedto exist exclusively as the imino-nitronic acid tautomer, although the spectroscopic evidence presented is also consistent with the enamine form,... 

There are few N-NMR spectral data for nitroenamines ". As already mentioned, the enaminic nitrogen resonates at a lower field than in enaminones, and exhibits the same dependence on the double-bond configuration (see Section III.A). The resonance of the nitro group appears at 0.6-3.3 ppm for the compounds included in the Table,... 

The isomeric equilibria of 2-acyl-2-nitroenamines with primary or secondary amino group are strongly solvent-dependent, as already seen for other enamines. An increase in the polarity of the solvent increases the population of the isomer(s) with Z-configuration, as deduced from the H-NMR spectra . In 3-amino-2-nitroacry-lic esters (567-574), the IR spectra show that an increase of solvent polarity increases the population of the ZE isomer , which has the highest calculated dipole moment. The effect of solvent polarity on the isomeric equilibria of some model 2-acyl-2-nitroenamines (554, R = H, Me R = Me, OMe R = R = H) has been qualitatively predicted by quantum-mechanical calculations using the self-consistent reaction field approach . 

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, ... 

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