Imine tautomeric form

Knowledge of these rules was used to isolate individual tautomers of compound 429 [431] (Scheme 3.140). This heterocycle is crystallized from alcohols and chloroform in the imine tautomeric form B, while in trifluoroacetic acid solution the dimethylamino group undergoes a protonation leading to... 

In derivatives of dihydroazolopyrimidines containing the ortho-oxyaryl substituent (for instance, compounds 431) a sufficient factor stabilizing the imine tautomeric form is an intamolecular hydrogen bond [428, 429, 430] (Scheme 3.142). 

This is where we meet the famous double helix immortalised by many books, articles, television programmes and of course films. The DNA double helix is the key element of DNA three dimensional structure. In the 1950s, when James Watson and Francis Crick first proposed the double helix as the key piece of DNA three dimensional structure, they generated enormous scientific and popular excitement, since for the first time the inheritance of genetic information could be understood explicitly in terms of a real chemical structure In order to appreciate this structure, there is a requirement to understand more about the heterocyclic bases (see Section 1.4.1) and their unrivalled capacity for specific hydrogen bonding. All these bases are aromatic but paradoxically prefer keto/amine to enol/imine tautomeric forms... 

Analogous compounds with a secondary amino group (a,j8-unsaturated secondary amines) can, in principle, exist in either the form of imines (6) or the tautomeric form of enamines (7). As they practically occur and react in the former structure, it is more convenient to use the group designation imines. ... 

The aminomethylenemalonates (1 and 2) may exist in imine or enamine tautomeric forms of R1 or R2 is a hydrogen atom. In some early papers, the imine tautomer (see below) was given for the structure of the products, usually without any explanation or evidence (e.g., 1885CB319 37JCS867 49JIC171 88KGS931). 

Amine-imine tautomerism in 3-acyl-substituted aminopyrazines has been examined by H, C, and N spectral analysis as well as X-ray crystallography <2005JST67>. In the same way as the parent aminopyrazine, those aminopyrazines have been shown to exist in the amino form 11 (R = H, Me, Ph) (Scheme 2) in contrast to an expectation that the electron-withdrawing acyl groups adjacent to the amino substituent would stabilize the imino tautomers 12 and 13. Thus, all NMR spectra showed only existence of the amino tautomer 11, and additionally the... 

Garner et al. (90,320) used aziridines substituted with Oppolzer s sultam as azomethine ylide precursors. The azomethine ylide generated from 206 added to various electron-dehcient alkenes, such as dimethyl maleate, A-phenylmalei-mide, and methyl acrylate, giving the 1,3-dipolar cycloaddition product in good yields and up to 82% de (for A-phenylmaleimide). They also used familiar azomethine ylides formed by imine tautomerization (320). Aziridines such as 207 have also been used as precursors for the chiral azomethine ylides, but in reactions with vinylene carbonates, relatively low de values were obtained (Scheme 12.59) (92). 

UV, 1H, and 13C NMR investigations revealed that 5,7,8,9-tetrahydro and 1,2,3,4,5,7,8,9-octahydro tautomeric forms are predominant for 6-ethoxalyl derivatives 23 and 24, while the lower homologs, the pyrrolidino[2,l-Z>]-quinazolin-10-ones, exist in the enol-imine forms [89JCS(P2)1613]. 

The tautomeric studies of azacytosines are not so complete as those of cytosine itself. The contribution of other tautomeric forms of azacytosines to the tautomeric equilibrium has not been evaluated (because of lack of model tautomeric compounds). It appears that 6-aza-substi-tution causes a tautomeric shift from form 3 of cytosine toward its imine form, 6. Thus, upon 6-aza-substitution the contribution of the imine tautomers to the tautomeric equilibrium of cytosine increases, while that of tautomers 3 decreases. 

Condensation of an amino acid-derived anilide and a /3-ketoamide afforded l,4-benzodiazepin-2-ones in which the initially formed imine tautomerizes to an exocyclic enamide (Scheme SO) <2001TL3227>. Only the (Z)-isomer of the enamide was isolated, assigned based on NOE data, and presumably reflecting stabilization by an intramolecular H-bond between the ring NH and exocyclic amide carbonyl. 

A high density of electrons associated with atoms C(3) and C(5) of 1,4-dihydropyridines and 1,4-dihydropyrimidines is also observed when these heterocycles undergo electrophilic substitutions such as Friedel-Crafts [315, 316, 317, 318, 319, 320] and Vilsmeier [297, 321] reactions (Scheme 3.99). In [315] it was shown that treatment of dihydropyridines 371 with aroyl or acyl chlorides 372 in the presence of SnCl4 leads to acylation of the heterocycle at position 3 (compounds 373). Dihydropyridines 374 and dihydroazolopyrimidines 376 undergo Vilsmeier reaction with the formation of the corresponding derivatives 375 and 377. It is interesting that imine heterocycle 376 after Vilsmeier reaction exists in the enamine tautomeric form. The tautomerism of dihydroazines and factors influencing it will be discussed in detail in Sect. 3.8. 

Associated with this scheme is, however, the observation that if a base happens to be in one of its rare tautomeric forms, imine for... 

PPP calculations have been performed148 for a number of other tautomeric forms of guanine (imine and lactim). No discussion of the calculated results has, however, been given. 

The first of these questions has, of course, to be considered, in a broader way, for the probability of tautomerism in both the purines and the pyrimidines. In this respect, the general theoretical prediction, made by us on the basis of simpler calculations as early as 1962178 and confirmed later by other authors (e.g., Danilov179) and by the present refined calculations, is that it is the guanine and cytosine constituents of the nucleic acids which should have the greatest tendency to exist in their respective (lactim and imine) rare forms. These are therefore the bases which should have the greatest probability of being involved in spontaneous mutations insofar as tauto-merization may be considered as a cause of such mutations. The transformation G-C -> A-T should thus be more frequent than the reverse one. 

One of the most important condensed ring systems is indole. Whether the indole nitrogen is substituted or not, the favored site of attack is C-3 of the heterocyclic ring. Bonding of the electrophile at that position permits stabilization of the intermediate by the nitrogen without disruption of the benzene aromaticity. Indole can exist in two tautomeric forms, the more stable enam-ine and the 3-H-indole or imine forms. C-2 to C-3 pi-bond of indole is more capable of cycloaddition reactions then the other pi bonds of the molecule. Inter molecular cyclo additions are not favorable, whereas intramolecular variants are often high-yielding. 

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