Amine-imine tautomeric forms

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... 

There are various forms of tautomerism which operate in the different purine species. (1) Prototropy which involves attachment of the proton to any one of the four ring nitrogen atoms (Scheme 5). Corresponding CH tautomers, for example (52), seem to be of little significance. (2) Amine-imine tautomerism which operates in the aminopurines such as adenine (Scheme 6). (3) Lactam-lactim tautomerism as in the hydroxypurines such as hypoxanthine (Scheme 7) and the related thioxo-thiol tautomerism (53) and (54) in the biologically imporfant mercaptopurines (Scheme 8). The subject has recently been discussed in some detail <76AHC(Si)502>. 

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. ... 

A comparison of the tautomeric constants in the series of N4-substituted cytosines indicates an interesting trend. As seen above, in the case of cytosine itself, the ratio of the tautomers of the amine type 2 to those of the imine type 6 is about 105. In the case of jV4-amino-cytosine134 the amine form 22 predominates by a factor of about 30, while N4-hydroxy compounds have mainly the imine form 21 with tautomeric constant 10 [i.e., Kt (amine/imine) = — 10 1]. Another study136 of this constant gave the value of 25 [Kt (amine/imine) = 4 x 10-2] in favor of the imine. 

Horvath et reported that the tricyclic formyl derivatives 390 n = 0-2) exhibit triple tautomerism between forms 451 and 453. The imine-enamine-enol tautomerism depends greatly on the ring size ( ). A significant solvent dependence was observed only for the pyirolo[2,l-i ]-quinazolinones 390 n = 0), which in ethanolic solution exist predominantly in the imino form (451), and in chloroform in the enol form (453). H-NMR spectroscopy showed that in chloroform the proportion of the imine tautomer 451 was about 5%. The fast equilibrium between the en-amine and enol forms 452 and 453 was shifted considerably toward the enamine tautomer 452 in the case of the pyrido[2,l-fi]quinazolinones 390 ( = 1). The azepino[2,1 -6]quinazoline ( = 2, R = 2-NO2) is a 2 3 mixture of the imine and the enamine tautomers 451 and 452. 

Equation 9.85 shows the reaction of a similar rhodium-amido complex with unstrained olefins. In this case, the product is a free imine. The inverse order of the reaction in added phosphine and the zero order of the reaction in any free amine imply that the imine is formed by the sequence shown in Equation 9.86, involving olefin insertion, 3-hydrogen elimination (see Chapter 10), and tautomerization of the resulting enamine to the imine. 

Nuclear magnetic resonance studies were also made of Schiff bases obtained from amines and aliphatic /9-diketones. These compounds can exist in any of three tautomeric forms, the keto-imine (10), the keto-enamine (11) and the enol-imine (12) . It was found that in... 

Figure 13.17. The four bases of DNA in their normal form (top row) and tautomeric form (bottom row). The tautomers involve a change in the position of a H atom from an NH group (keto form) to an OH group (enol form) or from an NH2 group (amine form) to an NH group (imine form). The change in position of the H atom is accompanied by changes in bonding of the atoms related to it.

On the basis of IR and NMR spectral data it was shown that of the three possible tautomeric forms (bis-keto-imine, bis-enol-imine, bis-keto-enamine), quinoxaline 208a adopts the bis-keto-enamine form. Of the six possible tautomeric forms (every two of keto-imine/keto-enamine, hydroxyl-ene-imine/keto-ene-amine, keto-enamine/enol-imine forms) in quinoxaline 210 the hydroxyl-ene-imine/keto-ene-amine form is adopted (Waring et al. 2002). The structures of compounds 208a and 210 were also confirmed by X-ray analysis and deduced from theoretical calculations of the possible limiting stractures (Fig. 2.19) (Waring et al. 2002). 

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