15N labeling and analysis of 13C–15N and 1H–15N couplings in studies of the structures and chemical transformations of nitrogen heterocycles

This review provides a generalization of effective examples of 15N labeling followed by an analysis of 13C–15N (JCN) and 1H–15N (JHN) coupling constants in solution as a tool to study the structural aspects and pathways of chemical transformations (e.g., rearrangements and ring-chain tautomerisms) in monocyclic and fused nitrogen heterocycles. This approach allows us to significantly expand and supplement the scope of NMR techniques for heterocyclic compounds. Moreover, methods for the incorporation of 15N atoms into the cores of various N-heterocycles have been collected in this work.


Introduction
Nitrogen heterocycles are a ubiquitous class of organic compounds that include azirine, azetidine, azole, azine and azepine derivatives and their fused analogs. These structures are inherent in drug design [1][2][3][4][5][6][7] and natural compounds, 8,9 in catalysis for cross-coupling and asymmetric synthesis reactions, 10,11 in materials science as metal complexes with luminescent properties, [12][13][14] in ligands for the separation of lanthanides 15 and in high-density energy materials. 16,17 At rst glance, it seems that establishing the structure of poly-nitrogen-containing compounds can be achieved by general NMR methods suitable for carbon-containing compounds; however, this statement is not correct. It should be noted that heterocycles have low densities of hydrogen and carbon atoms. Therefore, the application of conventional NMR methods (1D 1 H and 13 C spectroscopy, 2D HMQC, HMBC, INADEQUATE, etc.) might be ineffective for the structural estimation of poly-nitrogen compounds. Another important consequence of an increased nitrogen atom content is a decrease in the aromaticity and the tendency to undergo ringchain transformations. Thus, the conrmation of the structure can be complicated by the azide-tetrazole equilibrium, ANRORC (Addition of Nucleophile, Ring Opening, Ring Closure) reaction, Dimroth rearrangement, etc., which are observed in the azolo and azine series under the action of nucleophilic reagents and solvents. In this case, researchers usually use X-ray crystallography or comparison of UV-vis and 1 H and 13 C NMR spectra with the data of model compounds with unambiguously conrmed structures. Unfortunately, the rst approach gives information about the structure of compounds only in the solid state and is not suitable for the analysis of mixtures of compounds. The second method allows for the determination of structural characteristics in solution, but it can lead to incorrect conclusions. 18 There are several approaches to solve these issues. For example, the use of 2D H-(C)-N multiple bond correlation (HCNMBC) experiments was described for the structural conrmation of N-alkylated azolo derivatives based on the natural 15 N abundance. 19,20 However, these NMR procedures rely on magnetization transfer through 13 C-15 N J-coupling, and although they allow the determination of some other structures of simple monocyclic azoles, they cannot be considered as general.
The selective 15 N-labeling of organic molecules leads to the appearance of additional 1 H- 15 N and 13 C-15 N spin-spin coupling constants (SSCCs) that signicantly expand the application of NMR methods in the determination of molecular structures. This approach is widely used in the chemistry of proteins and nucleic acids. [21][22][23][24] Although 15 N-labeling and the subsequent analysis of J HN and J CN couplings in structural studies of nitrogen heterocycles have been described in a few articles, such a method can be effective for the determination of the structure and studying of the mechanism of chemical transformations of azoles, azines and their fused derivatives. Most of these articles were presented in early published review papers and books that described the highlight achievements in various areas of chemistry.
However, the incorporation of a 15 N atom into structures leads to the appearance of isotope shis. 25 These data may only be considered as additional features to conrm the structure. The chemical shis of labeled nitrogens in 1D 15 N NMR spectra can also be used as additional characteristics of enriched compounds. 26 However, information on chemical shis is relative and can only be used in the context of the already obtained detailed data. In contrast, the measurement of 13 C-15 N and 1 H- 15 N coupling constants provides unambiguous information about the molecular structures.
Despite the great opportunities for selective 15 N incorporation followed by an analysis of J HN and J CN couplings, no systematic review devoted to this tool for structural studies of heterocycles has been presented until today. This is the rst attempt to generalize the literature data on 15 N-labeling and show the capacity of the usage of J HN and J CN couplings for the determination of the molecular structure and the chemical transformations of nitrogencontaining heterocycles. The relevance of the topic has been conrmed by a recently published article describing the Dimrothtype ring transformation of an azine fragment in the imidazo [1,2-a] pyrimidine scaffold, which is of signicant interest for modern medicinal chemistry. 27 The authors showed the efficiency and universality of the use of 15 N-labeling and the analysis of 1 H-15 N coupling constants for ring-chain transformations, which require more detail to determine the structures of heterocyclic compounds. It was this work that drove us to summarize the literature data on the incorporation of 15 N isotopes and the use of J HN and J CN couplings in the chemistry of heterocycles. Analysis of the literature showed that this approach is general and covers different series of heterocyclic systems.
This review includes three sections. The rst section describes the application of 1 H- 15 N and 13 C-15 N constants for conrmation of the ways to fuse azole, azine and azepine rings during the synthesis of bicyclic, tricyclic and polycyclic compounds and the use of J HN and J CN in the studies of tautomeric rearrangements of heterocyclic derivatives. The next section reports the 15 N-labeling and analysis of 1 H- 15 N and 13 C-15 N spin-spin interactions in the determination of the mechanism of ring transformation rearrangements. The last section includes information about the use of J HN and J CN couplings for the determination of the sites and mechanisms of interaction of heterocycles with electrophilic and nucleophilic reagents, which occurs without the transformation of the heterocyclic framework.
2. Use of J HN and J CN for establishing methods for the heterocyclization of azole, azine or azepine to azole and azine fragments and studying ringchain tautomerism in a series of heterocycles A combination of 15 N-labeling and an analysis of J HN and J CN couplings is one of the approaches for the conrmation of the structure of a fused heterocyclic system that can be formed by different methods for the cyclization of an azole, azine or azepine ring to various veor six-membered heterocycles. Moreover, this method may be effective for the study of ring-chain tautomerisms that are observed in various classes of heterocycles.

Determination of methods of heterocyclization based on an analysis of 1 H-15 N and 13 C-15 N spin-spin interactions
The measurement of 13 C-15 N and 1 H-15 N spin-spin interactions can be used for the determination of alternative heterocyclization pathways, including monocyclic derivatives. This method can be efficient even for the analysis of isotopomer mixtures. For example, the interaction of 4,5-dicyanoimidazole 1 with 15 N-guanidine 2* led to compounds 3*a and 3*b (Scheme 1). 28 It should be noted that the reaction could give tautomers 3*a-i and isomers 4*a-c. Nonetheless, the registration of the signals of the two labeled nitrogen nuclei as a triplet and singlet in the 15 N NMR spectrum provided solid evidence of the formation of isotopomers 3*a and 3*b and excluded the formation of tautomers 3*c-i. Furthermore, structure 3*a was conrmed by observation of the single 1 H-15 N spin-spin coupling ( 1 J HN ¼ 90 Hz) between a 15 N atom and the protons of an NH 2 group in the 1 H NMR spectrum. If a mixture of compounds 4*a-c were produced, additional 1 J HN splitting for the 15 NH imino group would appear in both the 1 H and 15 N NMR spectra. However, such 1 H-15 N SSCCs were absent, which excluded the formation of 4*a-c. This result also showed that the measurement of 1 J HN couplings can be effective in the determination of the structure of prototropic tautomers instead of an analysis of the chemical shis of 13 C and 15 N atoms in the NMR spectra. [29][30][31] An analysis of direct 13 C-15 N coupling constants allowed for the determination of the method of cyclization in the reaction of guanosine with glycidaldehyde. 32 Two methods were used for the incorporation of 15 N atoms in the pyrimidine fragment of guanosine. The rst procedure was based on the treatment of compound 5 with [ 15 N]-ammonia (Scheme 2). This approach gave an isotopomeric mixture containing 75% 6*a and 15-20% 6*b. Another synthesis of 6 # a and 6 # b that included obtaining the 15 N-labeled amide 5 # by the interaction of [ 15 N]-benzoyl isothiocyanate and imidazolecarboxamide derivative 7 is depicted in Scheme 3. The reaction of compound 5 # with ammonia yielded a mixture containing 6 # a (15-20%) and 6 # b (75%).
The values of 1 J C7-N8 and 1 J C6-N5 were obtained from the carbon spectra of compounds 8*a and 8*b with 75% 15 N enrichment. This characteristic permitted the determination of sites for the attachment of the hydroxymethyl group in compounds 8*a and 8*b. However, the alternative isomers 9*a/ 9 # a and 9*b/9 # b were not observed (Fig. 1).
The values of the 1 H-15 N and 13 C-15 N couplings can be applied as diagnostic features for conrmation of the ring closure of heterocycles. This approach was described in 1986 by Villarasa et al., where a 15 N-labeled derivative of diazoazole 11* was prepared. 33 The synthesis comprises the diazotation of 2aminoimidazole 10 with Na 15 NO 2 having 25% 15 N enrichment (Scheme 5). The following reaction between compound 11* and 1,1-dimethoxyethene 12 can lead to either open-chain azoalkene 13* or fused azoloazine 14*. It should be noted that compound 14* is the product of the cyclization of 13*. The observed 1 J CN of 2.3 Hz and 2 J HN of 14.2 Hz for the obtained compound were in good agreement with the NMR spectral data of pyridine and other azines. Thus, they conrmed the formation of bicycle 14* in the reaction of imidazole 11* with dimethoxyethene compound 12.
Compound 18* ($87%, 15 N) with one equivalent of unlabeled 3-amino-1,2,4-triazole was used in a condensation reaction with 2-benzylidene-2-uoroacyl ester 19 (Scheme 6). 35 The analysis of the J CN couplings showed that a 13 C-15 N spin-spin interaction was observed for the C7 atom bonded to a phenyl fragment ( 2 J CN ¼ 4.7 Hz). Moreover, a 2 J H2-N1 of 15.3 Hz was detected in the 1 H NMR spectrum of compound 20*. These data unequivocally prove the formation of structure 20* ($43%, 15 N). In the case of obtaining the alternative product 21*, 13 C-15 N splitting should be observed for the signal of the carbon atom coupled with a CF 3 group.
The use of 15 N-phenylhydrazine 23*a in the interaction with nitroenamine 22 allowed for the determination of a reaction pathway that can be route A or B (Scheme 7). 36 According to pathway A, product 25* is formed through 24*, while the alternative pathway B involves the formation of compound 26* that transforms into resulting pyrazole 27*. The comparison of the detected J HN of 9.2 Hz with the early described amplitudes of the 1 H-15 N SSCCs for the pyrazole series showed that the observed coupling is a vicinal one. Thus, it was found that the reaction between 22 and 23*a gave compound 25*. Moreover, the formation of azole 27* would lead to the appearance of 1 J C5-N1 with values of 8-11 Hz, whereas a carbon coupled with a hydrogen did not show the splitting of 13 C-15 N due to the small amplitude of 2 J C3-N1 (>2 Hz). The weak 13 C3-15 N1 constant was used as an additional criterion for the conrmation of the structure of 25*.

Study of ring-chain tautomerism using J CN and J HN coupling constants
Aminoguanidine bicarbonate 17* ( 15 N, 86%) was also applied as a labeled starting material for the incorporation of 15 N atoms in aminotetrazole 28* (pathway A, Scheme 8). The synthesis of 28* was based on the Thiele method, 37 which includes the interaction of aminoguanidine salts with nitrous acid produced in situ from potassium nitrite and nitric acid.
The reaction of 15 N-potassium nitrite ( 15 N, 86%) with unlabeled aminoguanidine 17 is an alternative method that is suitable for obtaining compound 28* (pathway B, Scheme 8).
A similar result was obtained by the reaction of 2-hydrazinopyrimidine 31 with 15 N-enriched nitrous acid that was generated from labeled potassium nitrite in acetic acid (Scheme 11). 37 A mixture of isotopomers 30*aT and 30*bT in a 5 : 1 ratio was obtained. The Dimroth rearrangement is one of the most likely sources of the observed isomerization.
The NMR spectra of the isotopomer mixture in DMSO-d 6 solution showed that the tetrazole forms 30*a,bT prevail over the azides 30*a,bA, which were detected in trace amounts ($5%). The detection of J CN couplings for the C2, C5, and C6 nuclei (Scheme 11 and Table 1) proved the formation of a fused tetrazole (Scheme 10). Interestingly, small amounts of the azide forms 30*a,bA ($5%) were detected in both the 13 C and 1 H NMR spectra. Similarly, J CN splitting, which was only observed for the C2 nucleus, conrmed the azide structure of 30*a,bA. In TFA solution, the azide-tetrazole equilibrium was rapidly shied toward the azides 30*a,bA. As expected for the azide forms 30*a,bA, the presence of 13 C-15 N J-couplings was detected only for the C2 nucleus (Scheme 11 and Table 2).
Compounds 37*a,bT were only registered in the tetrazole form in DMSO-d 6 . The observation of the 13 C-15 N splitting for the C2 and C5 carbon signals in the 1D 13 C NMR spectra of 37*a,bT unambiguously conrmed the [1,5-b] fusion between the tetrazole and 1,2,4-triazine rings.
A small amount of the azide forms 40*a,bA ($4%) in DMSOd 6 solution was detected in the 13 C and 1 H NMR spectra for the sample that was obtained by the interaction of compounds 34* and 38 ( Table 2). The low concentration of the azide form did not allow for the detection or measurement of the corresponding J CN couplings. This form was only characterized by a relatively large downeld shi of the C2 resonance. Moreover, the main tetrazole isomers 40*a,bT were characterized by J CN couplings for signals C2 and C5. These characteristics allowed for the determination of the type of fusion between the tetrazole and 1,2,4-triazine rings in heterocycles 40*a,bT as [1,5-b].  The NMR spectral parameters of compounds 37*a,bT were determined in TFA immediately aer dissolving the tetrazole form and 30 days aer the preparation of the solution. During this period of time, the relative population of the azide forms of 37*a,bA increased from 0% to 60%. Signicant broadening of the C2 signal observed in the 13 C spectra of 37*a,bT in TFA did not allow for the identication of the J CN couplings from this carbon atom (Table 1). The 3 J C5-N7 and 4 J C5-N8 values for the C5 nucleus were measured easily. The obtained values of 1.6 and 0.7 Hz, respectively, correspond nicely to the J-couplings observed for the C5 carbon of 37*a,bT in DMSO-d 6 solution (1.6 and 0.9 Hz, respectively). This similarity conrms the retention of the bicyclic structure with a [1,5-b] type of fusion between the azole and azine fragments for 37*a,bT in TFA (Scheme 12). An analysis of the 13 C multiplets of 37*a,bA in the 1D NMR spectra revealed the presence of J CN interactions only for the C2 nucleus, which conrmed the azide structure of this compound ( Table 2 and Scheme 12).
The rearrangement of 40*a,bT to 40*a,bA in TFA solution was relatively fast when compared to that of compounds 40*a,bT. The 13 C NMR spectra were measured 1, 2 and 12 h aer dissolving 40*a,bT in TFA, and the concentrations of the azide forms 40*a,bA were 18%, 40% and 97%, respectively. The J CN interactions for the C2 and C5 carbon atoms of tetrazole 40*a,bT were observed in the 13 C NMR spectra (Scheme 12 and Table 1), thus proving that the tetrazole structure was retained and that the [1,5-b] type of fusion for 40*a,bT occurred. The precise measurement of the J CN values for 40*a,bT was impossible due to the fast conversion of tetrazoles to azides. A simplied line-shape analysis, however, revealed that the 2 J C2-N7 and 2 J C2-N8 interactions in 40*a,bT have signicantly lower values in TFA than in DMSO-d 6 ( Table 1). The azides 40*a,bA in TFA solution were characterized by the presence of 13 C-15 N Jcoupling constants for only the C2 carbon (Scheme 12 and Table  2).
The use of double-labeled aminotriazole 28** is one way to avoid the formation of isotopomeric mixtures. For example, the

Compound
Solvent a Unless otherwise stated, the listed J CN values represent the average between two independent measurements using 13 C line-shape analysis and amplitude-modulated spin-echo experiments. b The mixture of isotopomers 30*a/30*b (5 : 1) synthesized by Scheme 11 was used for the J CN measurements. c The J CN value was measured only by 13 C line-shape analysis. d The measurement of the J CN coupling was impossible due to the low abundance of the 30*b isotopomer. e Signals from the azide forms 37*a,bA were not observed in DMSO-d 6 solution. f The measurement of J CN was impossible due to the low concentration of the azide forms 40*a,bA in DMSO-d 6 solution.
reaction of diazonium salt 34**, obtained from 28**, with ethyl a-formylphenylacetate 35 gave compound 36**, which underwent cyclization in acetic acid (Scheme 13). 39 As a result, 15 N 2tetrazolo[1,5-b][1,2,4]triazine 37**T was synthesized. An analysis of the J CN couplings conrmed the type of fusion between the tetrazole and 1,2,4-triazine rings in compound 37**T (Table  1 and Scheme 13). A similar approach was applied for the incorporation of two 15 N atoms in the structure of tetrazolo[1,5-a]pyrimidine derivatives. The interaction of 28** with benzoylacetone 41 led to product 42**T (Scheme 14). 38 The structure of compound 42**T in DMSO-d 6 was determined by the analysis of the long-range 1 H-15 N coupling constants, which were measured by spinecho experiments with the selective inversion of 15 N nuclei. The observed n J HN values are presented in Table 3. The 4 J H6 0 -N7 and 5 J H6 0 -N8 couplings can only be observed in the proton spectrum of compound 42**T (Scheme 14). In the case of the formation of tetrazoloazine 42**T 0 , these spin-spin interactions should be absent.
This result indicated that the analysis of 4 J HN and 5 J HN can be used for the determination of the method of fusion between azole and azine fragments in tetrazolo[1,5-a]pyrimidines.
It was also found that tetrazole isomer 42**T underwent complete rearrangement into the azide form 42**A in TFA solution. This was conrmed by the disappearance of the longrange 1 H-15 N7/8 correlations in the proton spectrum.
In contrast to 2-hydrazinopyrimidine 31, the reaction of compound 43 with 15 N-potassium nitrite ( 15 N, 86%) in phosphoric acid led to the single isotopomer 44*T (Scheme 15). 37 Registration of the 1D 13 C NMR spectra of 44*T in DMSO-d 6 showed J CN splittings for the signals from the C2 and C5 nuclei (Scheme 15 and Table 1).
These spin-spin interactions conrmed the formation of a cyclic structure with a [1,5-b] type of fusion between the tetrazole and 1,2,4-triazine fragments. If the alternative isomer 44*T 0 was present, then additional J CN splitting for the C4 signal should be observed in the carbon spectrum (Scheme 15). The azide-tetrazole equilibrium was shied to the azide form 44*A in TFA solution. The rearrangement of tetrazole 44*T to the azide did not occur quickly.
The NMR spectra for compound 44*T were measured in TFAd solution immediately aer dissolving the tetrazole form, aer 16 days, and again aer 30 days. During this period, the relative population of isomer 44*A increased gradually from 0% to 87%. An analysis of the J CN couplings allowed for the detection of both 44*T and 44*A in TFA-d (Tables 1 and 2).
Similar to compound 43, the treatment of hetarylhydrazines 45 and 47 with labeled sodium nitrite ( 15 N, 98%) in acetic acid selectively yielded compounds 46*A and 48*A, respectively (Schemes 16 and 17). 40 The selective incorporation of a 15 N label into the azolo core of the tetrazoloazine leads to the appearance of long-range 1 H-15 N J couplings, which can be easily measured in a quantitative fashion using 1D spin-echo experiments.
Compound 46* in DMSO-d 6 underwent a transformation to an isomeric mixture of 1,2,4-triazine derivatives 46*T 0 , 46*A and 46*T in a ratio of 79 : 12 : 9, respectively. The addition of TFAd to DMSO-d 6 solution led to an increase in the relative concentration of the minor forms A and T.
In pure TFA, compound 46* underwent an almost complete rearrangement to yield the open-chain azide 46*A. In this case, the minor isomer 46*T 0 was also found with a concentration of approximately 1%. The measurement of 4-6 J HN for tetrazole 46*T 0 was possible in DMSO-d 6 , TFA-d and different mixtures of these solvents ( Table 3). The detection of 15 N7-1 H13 and 15 N7-1 H12 spin-spin interactions provides evidence for the [5,1-c] type of fusion between the azole and azine rings in 46*T 0 . It should be noted that other protons signals (H11 and H12) can have J H-N7 couplings. The disappearance of the J H13-N7 and J H12-N7 couplings and the measurement of the values of J H10-H7 and J H11-N7 allowed the conrmation of the structure of 46*T (Table  3 and Scheme 16).
Two tetrazole forms, 48*T 0 and 48*T, in a ratio of 1 : 1 were detected upon dissolution of 48*A in DMSO-d 6 . The signals from azide 48*A in the NMR spectra became detectable only in the mixture DMSO-d 6 /TFA-d at a concentration of the acid of more than 50%. For example, in DMSO-d 6 /TFA-d (3 : 1) solution, a 24 : 29 : 47 concentration ratio of 48*T 0 : 48*A : 48*T was found. In TFA-d, a mixture of the isomers 48*A and 48T (96 : 4) was obtained. The observed J H5-N7 and J H6 0 -N7 patterns enable the unambiguous determination of the cyclization method of the azole fragment in tetrazolopyrimidines 48*T 0 and 48*T in Scheme 13 The azide-tetrazole equilibrium in double-labeled 1,2,4triazine 37**. The observed 13 C-15 N coupling constants in DMSO-d 6 solution are shown by red arrows.
The treatment of nitrobenzofuroxan 49 with 15 N-nitric acid ( 15 N, 98%) gave enriched compound 50* (Scheme 18). 41 The incorporation of the isotope label in the 6-position of dinitrobenzofuroxan 50* permitted the determination of the specicity of the reaction with indene 51. The appearance of 3 J H5-N6 ¼ 1.9 Hz, 3 J H7-N6 ¼ 3.1 Hz and 3 J H1 0 -N6 ¼ 7.1 Hz in the proton spectrum showed that the expected s-adduct 52* underwent a transformation into N-oxide 53*. If the isomerization of 52* into 53* had not occurred, the 3 J H1 0 -N6 coupling would have been absent.
The incorporation of the 15 N isotope in structure 55* was achieved by the nitration of compound 54 with labeled nitric acid (Scheme 19). 42 The appearance of the 15 N atom provided the opportunity to study the interaction of 55* with sulte ions (Scheme 19). It was found that the initial reaction occurs at the 5-position to give the s-adduct 56*a, which was characterized by a geminal 1 H- 15    afforded the formation of a mixture of isomers 58b,c,e,f and 59b,c,e,f. In the cases where Ar 1 ¼ Ar 2 , the reaction gave a single product. 43 The application of 15 N-enriched p-chlorobenzonitrile in these experiments allowed the authors to study the structures of the obtained compounds and the mechanism of formation of these products. The treatment of 3-(4-chlorophenyl)-5-methylisothiazole with LDA and 15 N-labeled p-chlorobenzonitrile afforded a mixture of isotopomers 58*d and 59*d containing 15 N atoms in the amino group and thiazole fragment, respectively (Scheme 21). The positions of the isotope label in heterocycles 58*d and 59*d were provided by the measurement of the corresponding direct 13 C-15 N coupling constants (11.7 Hz and 6.9 Hz for 58*d and 59*d, respectively). The value of 3 J HN ¼ 4.3 Hz for the vinyl proton in isotopomer 58*d showed that the obtained compound had a trans geometry with respect to the two aromatic rings Ar 1 and Ar 2 .
For an explanation of the formation of the isotopomeric mixture 58*d and 59*d, the selectively labeled compound 58*d was prepared, and the ring-chain transformation for this labeled structure was studied by NMR. Compound 58*d was obtained by treatment of 60 with 15 N-labeled pchlorobenzonitrile.
This procedure led to compound 61*, which then underwent desilylation to form isotopomer 58*d (Scheme 22). Compound 60 was obtained by the interaction of methylisothiazole with tert-butyldimethylsilyl chloride in the presence of LDA. Then, heating the C 6 D 6 solution of 58*d at 50 C for 50 h resulted in an equilibrium of a 1 : 1 mixture of isotopomers 58*d and 59*d.
Hence, two possible mechanisms (A and B) for the reversible equilibrium between 58*d and 59*d were presented (Scheme 23). It should be noted that the key intermediate in each path is sulfurane-2, 62*b. Pathway A involved the formation of sulfurane-1, 62*a, which transformed into structure 62*b via a [1,5]-sigmatropic hydrogen shi. Alternative pathway B suggests that thiazoles 63* and 64* are involved in the isomerization process.
Thus, the incorporation of 15 N labels and the analysis of the J CN and J HN couplings allow the determination of the formation mechanism of different heterocyclic systems and the observation of ring-chain tautomerism in a series of heterocycles.
3. Analysis of J CN and J HN couplings as a method for the study of ring-chain transformations under the action of nucleophilic and electrophilic reagents Some reactions of nitrogen heterocycles are simple at rst glance, but they can hide a complex and completely unobvious mechanism. Isotopic labels allow a look deep inside into the details of such reaction mechanisms. Several heterocycles undergo ring transformations under the action of nucleophilic reagents (ANRORC, Dimroth rearrangement, etc.). Another type of recyclization is associated with intramolecular attack by the electron-decient terminal atoms of the open-chain form on the ring system heteroatoms bearing a lone pair of electrons. It should be noted that ring opening or recyclization processes in both cases can occur and lead to the formation of a new type of heterocyclic structure. Additionally, determination of the structure of the obtained product always remains an important issue in organic chemistry. Moreover, solution of this task can help provide evidence for the mechanisms of ring-chain transformations.

Study of ANRORC and Dimroth rearrangements and similar chemical transformations by analysis of J CN and J HN couplings
The analysis of J CN and J HN is an efficient method for studying different rearrangements that include ring-chain and ring Scheme 18 The study of the interaction between dinitrofurazone 50* and indene using a 15 N label. The observed 1 H-15 N6 J coupling constants are indicated by the blue arrows.

Scheme 19
The study of the interaction between nitrofurazone 55* and SO 3 2À using a 15   The application of double-labeled 13 C, 15 N-ethyl cyanoacetate 38* permitted the detection of the Dimroth rearrangement for nitro-1,2,4-triazolo[1,5-a]pyrimidine 71 (Scheme 25). 45,46 It was found that 13 C, 15   Recently, a representative study of a Dimroth-type ring-chain rearrangement in the imidazo[1,2-a]pyrimidine series using 15 N-labeled samples and an analysis of 1 H-15 N couplings was described. 27 The incorporation of a 15 N atom in molecule 79*a involved the substitution at C2 in compound 77 using 15  A similar result was obtained from the treatment of compound 79*a with a solution of sodium hydroxide in a mixture of EtOH and THF. In this case, the rearrangement was accompanied by hydrolysis of the ester group.
An unusual transformation was found from the study of Vorbrüggen glycosylation in a series of aminopyrimidines 82a,b and 89a-e using 15 N-labeled regents (Schemes 28 and 29). 47 It was revealed that the silylated 4-aminopyrimidines 83a,b or 90a-e aer interaction with 84 yielded compounds 88a,b and 94b-e, which are products of the Dimroth rearrangement of nucleosides 85a,b and 91a-e in basic conditions. The mechanisms of these processes involve nucleophilic addition, ring opening and ring closure. Furthermore, the formation of structures 88a and 94a-e from 91a-e presumes the nucleophilic substitution of a chlorine atom in open intermediates 92a-e.
The synthesis of double-labeled diaminopyrimidine 82*b and the use of this compound in the glycosylation reaction led to product 88*b (Scheme 30). The observed 1 J H8-N8 ¼ 93 Hz, 2 J H2-N1 ¼ 16 Hz and 3 J H7-N1 ¼ 3.9 Hz unambiguously conrmed the structure of compound 88*b. These characteristics showed that the pyrimidine derivative 88*b was formed by the Dimroth rearrangement of the ribosylated azine 85*b. The mechanism presented in Scheme 29 was conrmed by the application of 15  showed that the nucleophilic attack mainly occurs at the C4 atom of the uridine ring. 48 Moreover, this interaction afforded ring-chain intermediate 97*, which transformed into compound 98* in the presence of K 2 CO 3 (Scheme 32). The production of structure 97* was conrmed by analysis of the 13 C-15 N coupling constants. Indeed, the J CN splittings were only detected for C4 ( 1 J CN ¼ 16.4 Hz) and C5 ( 2 J CN ¼ 7.7 Hz) atoms, which unambiguously proved the production of the ring opening product 97*. The formation of intermediate 97* was monitored by an NMR experiment in CDCl 3 solution. This reaction is one example that allows the incorporation of a 15 Nlabeled atom in the core of a uridine derivative.
A previously mentioned article 48 also describes the study of the features of the interaction of N-nitroinosine with an amine by using 15  The three differently 15 N-enriched 1,4-diphenylthiosemicarbazides 103*a-c in reaction with two different aroyl chlorides 104 and 105 were used as labeled starting materials. Thiosemicarbazide 103*a was synthesized by treatment of phenylhydrazine with 15 N-phenyl isothiocyanate, which was the product of the interaction between 15 N-aniline, carbon disulde and ethyl chloroformate (equivalent quantities). To obtain compound 103*b/103*c, it was necessary to use labeled hydrazine obtained by the diazotization of aniline/ 15 N-aniline with Na 15 NO 2 /NaNO 2 in the reaction with phenyl isothiocyanate and perform the subsequent reduction in a mixture of Na 2 SO 3 -Na 2 S 2 O 3 . The formation of structures 106*a-c/107*a-c from compounds 108*a-c/109*a-c was observed by reuxing in pyridine solution. The measured values of 1 J CN for heterocycles 106*a-c-and 109*a-c were collected in Table 4.
The main characteristics conrming the formation of structures 108* and 109* were the appearance of three direct 13 C5-15 N1 coupling constants that were observed in the carbon spectra of isotopomers 108*a and 109*a. The starting heterocycles 106*a and 107*a were only characterized by two 13 C 2 -15 N exo and 13 C6-15 N exo spin-spin interactions. The detection of the 1 J C5-N1 couplings veried the transformation of compound 106*/107* into 108*/109* under reux in pyridine.

Analysis of J CN and J HN couplings in the study of other ring-chain rearrangements and transformations occurring with changes to the heterocyclic scaffold
The analysis of the 1 J CN value was used for the determination of the structure of the nitrosation product obtained from compound 110 (Scheme 35). 50 The data from the 1D 1 H and 13 C NMR spectra did not permit the identication of the ring opening of the pyrrole fragment in 110 under the action of sodium nitrite in acetic acid. The use of the 15 N-labeled sodium nitrite and analysis of the direct 13 C-15 N coupling constant showed that the tetrazole derivative 111* underwent  Table 4. a transformation into the open form 112*. The conclusion was based on comparison of the 13 C-15 N spin-spin interaction ( 1 J CN ¼ 81 Hz) that was observed in the carbon spectrum of compound 112* to that ( 1 J CN ¼ 77 Hz) obtained for the earlier structure 113*. 51 The use of 15 N-phenylhydrazine 23*b in the reaction of 2,3dihydrofuro[3,2-c]coumarin-3-one 114 is another example of the use of labeled compounds to study chemical conversions in a series of heterocyclic compounds (Scheme 36). 52 The analysis of the 1 H-15 N coupling constants allowed the determination of the positions of the labeled atoms in product 118* and proved the mechanism of the transformation. In the proton spectrum of compound 118*, direct and long-range 1 H-15 N spin-spin interactions were detected for two signals of 15 NH-groups ( 1 J HN 89.7 Hz, 4 J HN $4 Hz and 1 J HN 93.4 Hz, 4 J HN $4 Hz). Moreover, additional splitting was observed for the protons from the CH] N and NH-Ph fragments. Thus, compound 118* was formed by the double addition of 15 NH 2 -NH-Ph (23*b) and the elimination of aniline in the last step of the reaction. This mechanism involves the formation of intermediates 115*-117* (Scheme 36).
The revision of the result for the reduction reaction of diazonium salt 120 that was obtained by the diazotation of amine 119 was described in another work. 53 Previously, it was considered 54 that the product of this transformation is 1,2,3triazine 121 (Scheme 37), while the determination of structure 121 was based on the detection of two direct 1 H-15 N coupling constants ( 1 J H-N12 ¼ 107 Hz and 1 J H-N6 ¼ 97 Hz) in the 1D 15 N NMR spectrum of the unlabeled sample. However, using 15 Nlabeled diazonium salt 120* in the reaction with sodium sulte shed light on the real method of this transformation. The signal from the 15 N-nitrogen atom in the resulting compound was only characterized by one 1 H-15 N coupling constant, 2 J HN ¼ 8.2 Hz. Therefore, the 1,2,3-triazine structure of 121* was rejected, and it was shown that the reduction of compound 120* led to heterocycle 122*.
An unusual transformation was detected from the use of 15 Nenriched compound 125* in a reaction with phenyltetrazole 126 (Scheme 38). 55 The synthesis of chloride 125* included the interaction of benzoyl chloride and labeled ethylammonium chloride ( 15 N, 99%) in the presence of triethylamine in dichloromethane. Then, amide 124* was treated with thionyl chloride. The interaction of 125* and 126 led to heterocycles 129* and 131*, which were characterized by 13 C-15 N and 1 H-15 N SSCCs.
These data conrmed the structures of compounds 129* and 131*. In the proton NMR spectra of 129* and 131*, the signals of the N-ethyl fragments showed additional splittings ( 2 J HN ¼ 1.2 Hz, 3 J HN ¼ 2.9 Hz and 2 J HN ¼ 0.5-1.0 Hz, and 3 J HN ¼ 3.2 Hz, respectively). Moreover, in the 13 C NMR spectra, the signals of the carbons of the tetrazole and 1,2,4-triazine fragments were detected as doublets with magnitudes of 12.2 Hz and 12.1 Hz, respectively. These results indicate that the formation of structures 129* and 131* occurred over two different mechanisms (pathways A and B, Scheme 38). However, these transformations started with the common intermediate 127*. Pathway A included the elimination of benzonitrile, obtaining azide 128*, which underwent cyclization into tetrazole 129*. Compound 131* may be formed according to the mechanism demonstrated by route B. In this case, structure 127* transformed into 130* by the elimination of nitrogen. Then, the isomerization of intermediate 130* led to 1,2,4-triazine 131*.
The incorporation of the 13 C and 15 N atoms in compound 136* allowed determination of the method of the photosensitized oxidation of imidazole derivatives (Scheme 39). 56 This interaction with singlet oxygen was considered a model reaction for natural and biologically active structures containing the imidazole fragment (guanosine, xanthine, theophylline, histidine, etc.). The production of 136* involved heating a mixture of 15 N 2 -urea 132* ( 13 C, 99% and 15 N, 98%) and 13 Cformic acid 133* ( 13 C, 99%) at 150 C for 4 h (Scheme 39). Then, 13 C-enriched benzoin 135* was introduced, and the reaction mixture was heated at 180 C.
The synthesis of labeled benzoin 135* was based on coupling 2 mol of 13 C-benzaldehyde ( 13 C, 99%) in the presence of NaCN. The photosensitized oxidation of 13  It should be noted that the estimation of the 13 C-15 N and 1 H-15 N coupling constants with analysis of the chemical shis for 1 H, 13 C and 15 N NMR spectra and the measurement of the 1 H-13 C spin-spin interaction allowed for the determination of the structures of these compounds. For example, the detection of only one 1 J CN ¼ 2.3 Hz for C2 in the carbon spectra of 143* showed that one bond of C2-15 N was broken. The chemical shi of C4 was characteristic of a sp 3 -carbon connected to two heteroatoms. These data indicated that C4 was bonded to an oxygen and the N1 nitrogen. Moreover, the signal from the amino group was observed as a triplet split by two bonded hydrogens (   In the case of the reaction between azirine derivative 146* and phthalimide, the tricyclic product 153* 57 was isolated. The structure of 153* was identied by measurements of 13 C-15 N SSCCs ( 1 J C9b-N2 ¼ 7 Hz and 1 J C3-N2 ¼ 8 Hz), which were registered for sp 3 -hybridized carbon atoms (Scheme 41). Additionally, the 1 J HN -couplings were not detected in the 1D 1 H and 15 N NMR spectra. The authors suggested that compound 153* is the product of the transformation of 2,3-dihydro-2,5benzodiazocine-1,6-dione 152*. Moreover, the analysis of chemical shis for the carbon atoms and 13 C-15 N coupling constants of signals C2 ( 1 J CN ¼ 5 Hz) and C4 ( 1 J CN ¼ 5 Hz) veried the formation 2-(4H-imidazol-2-yl)benzoic acid derivative 154*, which was obtained by treatment of 153* with methanol.
An interesting example of the use of 15 N-labeled compounds for determining the ring open reaction pathway in a series of diazirines was reported by Creary and co-workers. 58 The incorporation of 15 N in structure 157* was based on the reaction of methyl benzimidate 155 with enriched ammonium chloride ( 15 N, 99%) (Scheme 42). Then, oxidation of the resulting benzamidine 156* with NaOBr yielded 15 N-phenylbromodiazirine 157*. Compound 157* reacted with tetrabutylammonium azide to give a mixture of benzonitriles 160 and 160* in a ratio of 1 : 1. The 13 C NMR spectrum conrmed partial 15 N labeling for the obtained product. The nitrile carbon of 160* appeared as a doublet ( 1 J CN ¼ 17.8 Hz), while the carbon of the cyano group of 160 registered as a singlet. A comparison of the intensity of these signals showed a 50% excess of the 15  4. 13 C-15 N and 1 H-15 N coupling constants as an approach to the determination of structural reaction/ isomerization products occurring without changes to the heterocyclic core Nitrogen-enriched heterocycles are molecules containing several reaction centers. This class of compounds can exhibit dual behaviors, expressed by the fact that they can react with both nucleophilic and electrophilic reagents. As a result, such interactions oen give a mixture of isomers even when the heterocyclic scaffold does not undergo rearrangement. This situation requires that researchers use different methods for the determination of structures obtained from similar products. One of the effective approaches that can be used to solve these problems is the measurement of the J CN and J HN couplings in 15  couplings were only registered in the proton spectra of 164*a-c, and the expected geminal coupling ( 2 J HN ) was not detected by 1D 1 H NMR spectroscopy due its small amplitude (approximately 1.2 Hz). For compound 164*a/164*c, 3 J H1 0 -N2 / 3 J H1 0 -N2 was measured ( Table 5). The detection of an additional splitting 3 J H2 0 -N3 (3.7 Hz) for the methyl signal of 164*b unambiguously conrmed the structures of the products obtained by the interaction of compound 164*a-c with EtO 3 + BF 4 because the formation of the alternative isomers 164*d,e should lead to the appearance of 3 J H2 0 -N2 and 3 J H2 0 -N4 couplings, respectively (Fig. 2). Moreover, the authors developed a selective synthesis for isotopomer 164*c. This approach included the interaction of unlabeled 2-ethylthiobenzoylhydrazine 162 with 15 N-nitric acid. Then, the obtained 163*c underwent alkylation with triethyloxonium tetrauoroborate (Scheme 44). The detection of 2 J C1 0 -N2 ¼ 5.0 Hz was further evidence for the binding of the N-ethyl fragment with the N2 atom in compounds 164*a-c.
Another example of the application of 13 C-15 N SSCC has been presented for the determination of the structures of Nalkylated derivatives 172*a and 172*b that were obtained by the interaction of 15 Table 5. The use of 171* in the N-benzylation reaction led to the formation of two isomers, 172*a and 172*b, that were separated by column chromatography. The determination of the site of the N-alkyl fragments in compounds 172*a and 172*b was based on data from the proton-decoupled 13 C NMR spectra. In the case of isomer 172*a, a signal of atom C1 0 of a benzyl moiety was registered as a singlet. The benzylic carbon of 172*b showed splitting and appeared as a doublet ( 1 J CN ¼ 8.6 Hz) due to the spin-spin interaction 13 C1 0 -15 N1 (Scheme 45).
The obtained results allowed the authors to reach the conclusion that this method for determining the site of alkylation may well prove applicable in other heterocyclic systems where 15 N can be selectively incorporated.
Indeed, the selective 15 N-labeling of compound 178* and analysis of the 13  The methylation of compound 178* yielded isomers 179*a and 179*b, which were separated. The detection of the direct 13 C-15 N coupling of 11.0 Hz for the signal of the carbon of the Nmethyl group conrmed the formation of betaine-like structure 179*b in the alkylation reaction of imidazo [2,1-c] [1,2,4]triazin-4ones and their analogs.
It should be noted that today, for the determination of the site(s) of the benzylation/methylation of an unlabeled analog 171*/178*, it is possible to use conventional 1 H, 13 C-NMR methods because for the resulting products, the 1 H-13 C spinspin interactions between the protons of the N-CH 2 /N-CH 3 fragment and the carbons of the heterocyclic part (8-methylthioimidazo [4,5-g]quinazoline/imidazo [2,1-c] [1,2,4]triazin-4-one) can be identied. That is, these cases do not require 15 N labeling and the analysis of J CN couplings. However, the determination of N-adamantylation site(s) in unenriched analogs of heterocycle 178* such as azolo [5,1-c] [1,2,4]triazine, azolo[1,5-a]pyrimidine and related azoloazines using well-established 1 H and 13 C NMR methods (such as 1D, 2D COSY, HMQC, HMBC, and INADE-QUATE spectra) are difficult because the heterocyclic moiety is covalently attached to the adamantane tertiary carbon that has no bond with hydrogen atoms. Nuclear Overhauser effect spectroscopy (NOESY or ROESY) also does not provide unequivocal structures for N-adamantylated derivatives. In this case, the application of 15 N labeling and the measurement of 13 C-15 N and 1 H-15 N coupling constants allows for the determination of the Nadamantylation sites in the azolo-1,2,4-triazine and 1,2,4-triazolo [1,5-a]pyrimidine series.
Then, the resulting sodium salt 182* was transformed into azoloazine 183*, which was treated with adamantanol 184 in a solution of sulfuric acid. The adamantylation of 183* gave a mixture of isomers 185*a and 185*b. The appearance of the 15 N label in structure 185*a permitted the determination of the conformation of the additional adamantane fragment in the N3-atom azole rings by the observation of the 3 J CN couplings (0.5 Hz) in the carbon spectrum.
The structure of the adamantylated derivative 185*b was determined by 13 C NMR spectroscopy via comparison with a model compound, N-methylated 1,2,4-triazolo [5,1-c] [1,2,4] triazin-7-one. Moreover, the use of 15 N-enriched azoloazine 183* in the investigation of adamantylation showed that compound 185*b is a product of the reversible isomerization of 185*a; this rearrangement occurs via the formation of an adamantyl cation and heterocyclic base 183*.
The selective incorporation of two 15 N atoms in different positions of the 1,2,4-triazolo [5,1-c] [1,2,4]triazin-7-ones and other nitrogen-containing heterocycles together with a combined analysis of the J HN and J CN coupling constants was more effective for the structural determination of heterocyclic N-adamantylated derivatives. 39 For example, the use of this approach allowed for the determination of the structures of  It should be noted that the formation of an isomeric mixture in the adamantylation reaction of 186** could explain the rearrangement of the N3-isomer 170**a into the N4-isomer 170**b. This process could include the production of an adamantyl cation and the base of 186**. The structures of compounds 187**a,b were unambiguously conrmed by analysis of the J CN and J HN couplings. The detection of a single 3 J C1 0 -N1 coupling (0.4 Hz) for the adamantane carbon in compound 187**a indicated that the substituent group is attached to the N3 atom of the 1,2,4-triazole ring (Table 6 and Scheme 48).
The attachment of the adamantane fragment to the N4 atom of the triazine ring in compound 187**b led to a large set of observable J CN couplings, including geminal ( 2 J C1 0 -N5 ¼ 5.0 Hz) and vicinal ( 3 J C2 0 -N5 ¼ 1.7 Hz) ( Table 6 and Scheme 45). The 15 N-HMBC spectra of compounds 187**a,b allowed for 3 J HN couplings of the nitrogen atoms at a natural isotopic abundance. The analysis of the 1 H-15 N 3 / 15 N 5 spin-spin interactions also conrmed the structures of the products of adamantylation of 186** (Table 7 and Scheme 48). Moreover, the appearance of the 15 N isotope in the azine ring of 187**b permitted the observation of the long-range 1 H2 0 -15 N5 coupling constant (Table 7 and Scheme 48). This characteristic was additional evidence for the attachment of the adamantane substituent to the unlabeled N4 atom.
In the abovementioned work, 39 the selective incorporation of two 15 N atoms and combined analysis of J CN and J HN were also used for the determination of the adamantylation site of 1,2,4triazolo[1,5-a]pyrimidine and tetrazolo [1,5-b] [1,2,4]triazine derivatives (Schemes 49 and 50).
A similar situation with the determination of the adamantylation site in isomer 192**b had arisen. The detected 13 C-15 N spin-spin interactions of the 2 J C1 0 -N2 (2.7 Hz), 3 J C2 0 -N2 (1.1 Hz), 3 J C1 0 -N3 (0.3 Hz) and 4 J C3 0 -N2 (0.3 Hz) couplings in the 1D 13 C NMR spectra of 192**b revealed the attachment of the adamantane fragment to the N1 atom of the tetrazole ring (Table 6 and Scheme 50). However, an analysis of the 1 H-15 N coupling constants in the 15 N-HMBC spectrum ( 3 J H2 0 -N1 and 5 J H2 0 -N3 ) and the obtained values of J HN from spin-echo experiments in 1D 1 H NMR did not allow the unambiguous establishment of the structure of 192**b (Table 7 and Scheme 50).

J CN and J HN coupling constants as an approach for the conrmation of the mechanisms of nitration
The application of 15 N-labeled compounds and the measurement of the 1 H-15 N and 13 C-15 N coupling constants permitted the investigation of the mechanism for nitration in a series of different heterocycles. An unusual method for the incorporation of the nitro group into the core of 6-chloro-9-Boc-purine was found by using a mixture of triuoroacetic anhydride (TFAA) with labeled tetrabutylammonium nitrate (Bu 4 -N +15 NO 3 À ) and freezing the nitration reaction (Scheme 51). 63 The transformation of intermediate 194* was detected at À50 C by NMR spectroscopy. In the corresponding 1 H spectrum, the signal of proton H8 was split ( 3 J HN ¼ 2.7 Hz).
Moreover, in the carbon spectrum of an experiment with Bu 4 N +15 NO 3 À , the 2 J CN (1.8 Hz) coupling was observed for C8, and the signal of the carbon of the carbonyl group for the tri-uoroacetoxy fragment was characterized by a 1 H8-13 C spinspin interaction (2.   Table 6 J CN couplings (Hz) observed from the signals of the adamantyl fragment in the spectra of azoloazines 187**a,b, 191** and 192**a,b a The use of 15 N-enriched nitric acid in the nitration of nitroimidazole 197 allowed the detection of the rearrangement occurring in this process. 64 It was found that the interaction of 197 with 15 N-nitric acid led to N-15 NO 2 dinitroimidazole 198*, which underwent isomerization into compound 199* in chlorobenzene at 115 C (Scheme 52). It should be noted that the carbon NMR spectrum of dinitroimidazole 199* was only characterized by one direct 13 C-15 N coupling constant at the C2 atom with a magnitude of 30.6 Hz (Scheme 52). Thus, the fact that a doublet was observed for the 13    The selective incorporation of the 15 N-label into diamino-2quinoxalinol 210* permitted the analysis of the 1 H-15 N coupling constants for the conrmation of the structure of product 212*, which was isolated from the reaction of 210* with salicylaldehyde 211 (Scheme 54). 66 A synthesis of compound 210* was based on several steps. Initially, an interaction occurred between compound 207 and 15 N-labeled ammonium hydroxide ( 15 N, 98%), and then 208* was transformed into 209*.
Oxidation of 209* led to diamine 210*, which had two amino groups with different reactivities. Next, reaction of compound 210* with aldehyde 211 could give an imine 212*a or the isomer 212*b. The observation of the 1 H-15 N coupling constant for the proton signal of the imine group ( 2 J HN ¼ 3.0 Hz) conrmed the formation of structure 212*a.

Conclusion
We have reviewed examples of the incorporation of 15 N atoms and subsequent analyses of the J HN and J CN couplings in labeled samples. The above approach can be considered an effective, general, and convenient tool for establishing realistic structures of nitrogen-containing compounds in solution, including mixtures of compounds and equilibrium mixtures. The measurement of the 1 H-15 N and 13 C-15 N coupling constants allows the investigation of ring-chain tautomerism, the mechanisms of chemical transformation and other structural aspects of azoles, azines, azepines and their fused derivatives. Although this approach is not widely used, 15 N labeling and estimation of the 1 H-15 N and 13 C-15 N spin-spin interactions may be a single method that can be efficient for the determination of the molecular structure or method of chemical transformation in the chemistry of poly-nitrogen heterocycles. In structural studies, two types of coupling constants are used, namely, longrange and near (direct, geminal, vicinal) constants. While early publications used only near constants, modern works exploit both long-range and near constants, as well as 2D spectra. For example, 2D 1 H-15 N HMBC spectra and spin-echo experiments were used for the determination of the N-adamantylation site of azolo-1,2,4-triazines and conrmation of the structure of Scheme 52 An example of the application of 15 N-labeled compounds for establishing a nitration mechanism in a series of imidazoles. The observed direct J CN coupling is shown by a red arrow.
Scheme 53 The use of 15 N-labeled propyl amine to determine the oxidative amination mechanism. The observed direct J CN and J HN couplings are shown by red and blue arrows, respectively. imidazo [1,2-a]pyrimidine. This situation is closely related to the development of the NMR technique. Thus, it can be expected that this approach will be expanded in the chemistry of nitrogen-containing compounds since 15 N labeling and the analysis of the 13 C-15 N and 1 H-15 N spin-spin coupling constants shines light on new and known chemical transformations, rearrangements and ring-chain tautomerism in a series of nitrogen heterocycles.

Conflicts of interest
The authors declare no conict of interest.