Transformation of oxygen-bridged pyrimidines with nitrogen nucleophiles and characterization of resulting products

The reactivity of oxygen-bridged Biginelli pyrimidines, such as 2,6-methano-1,3,5-benzoxadiazocines, towards nitrogen nucleophiles such as hydrazine hydrate, N,N - dimethylhydrazine, and benzylamine was studied. While all these reagents caused opening of the oxygen-containing ring, only the reaction with hydrazine furnished the corresponding pyrazolopyrimidine and pyridopyrimidine condensation products. 1 H– 15 N HMBC correlation spectroscopy was used for structure confirmation.


Introduction
Recently we found that the use of dimethyl acetone-1,3-dicarboxylate in the Biginelli reaction with salicylaldehyde and urea or thiourea resulted in the production of 2,6-methano-1,3,5benzoxadiazocine 1. 1 We reported a similar formation of oxygen-bridged pyrimidine 2 for the classical Biginelli condensation under standard conditions.The presence of two neighbouring ester groups in compounds 1, provides an opportunity for the elaboration of a further nitrogen heterocycle, fused onto the pyrimidine ring, without altering the parent bridged system.Here, we describe transformation of oxygen-bridged Biginelli pyrimidines in reactions with various nitrogen-containing reagents.

Results and Discussion
First we decided to test the behaviour of the compound 2a towards hydrazine.However, a literature search revealed that an analogous reaction of the closely related acetyl derivative 3 had been already studied. 3The transformation proceeded with the rupture of the oxa-cyclic fragment to give the corresponding pyrazolo [3,4-d]pyrimidine 4. 3 Nevertheless, the authors described the relative stereochemistry only at two of the three chiral centres in the resulting bicyclic skeleton, e.g. at C-3a and C-4.In line with the reported article 3 , our ester 2a underwent a compararable type of conversion to yield the heterocycle 5. Prior to an NMR stereochemical analysis, a full assignment of 1 H and 13 C signals was unambiguously accomplished using 2D COSY and HMBC techniques (Table 1).J (H-4,NH-5) = 2.8 Hz.
The relative configuration at all stereogenic atoms was readily be established by 1D NOESY experiments.Selective NOE transfer from the CH 3 resonance at δ H 0.88 led to significant enhancements of the signal intensities for the adjacent hydrogens NH-1, NH-7 and H-3a.Hence, the methyl group at C-3 is on the same side of the ring as the H-3a methine.Furthermore, a weak NOE interaction with the remote aromatic H-6' was also found, which was indicative of a 1,3syn relationship between the methyl and phenyl moieties.The resulting stereochemical arrangement was further confirmed by the strong interaction between H-3a and the methyl hydrogens and, additionally, by a weak NOE between H-3a and H-4.This showed that the stereochemical outcome of the ring closure resulting in pyrazolopyrimidine 5 was related to the relative stereochemistry of the starting O-bridged pyrimidine 2a.
With the aforementioned data at hand, we carried out the planned cyclocondensation of diester 1a with hydrazine under identical conditions (EtOH, reflux 30 h).Although this reaction was accompanied by some decomposition, we were able to isolate a small amount of a product.Considering the nucleophilic nature of both hydrazine nitrogens and also in view of the preceding transformation, we considered two structures with fused ring systems, pyrido[4,3d]pyrimidine 6 and pyrimido [5,4-d]diazepine 7, for the reaction product.ESI-MS showed an [M+H] + ion at m/z 289 (C 13 H 13 N 4 O 4 ) which was compatible with both 6 and 7.However, the absence of a methylene in the 1 H and 13 C NMR spectra ruled out these two structures.Instead, the spectra indicated the presence of a methine in the product.Additionally, integration of the hydroxyl and amine protons, revealed 6H, i.e. one H atom more than in structures 6 and 7.The other signals in the NMR spectrum pointed to a 5,6-disubstituted 4-(2hydroxyphenyl)-pyrimidin-2-one moiety.These findings indicated that the product was another stable isomer related to heterocycles 6 or 7, e.g., enol tautomers 6A or 7A.
Because of the low solubility of the condensation product, the solvent dependence of the tautomeric equilibrium could not be studied by NMR.Nevertheless, the presence of an oxo-form was not observed in DMSO-d 6 .To assign a definite structure to the new product, we used 1 H-15 N heteronuclear chemical shift correlation spectroscopy. 1H-15 N HSQC and HMBC experiments are based on direct or multiple bond N-H couplings.In particular, the gradientenhanced HMBC technique correlating remote 1 H and 15 N nuclei provides valuable information for molecular structure elucidation and determination of 15 N chemical shifts of non-protonated nitrogens. 4The HSQC spectrum showed only two cross-peaks, between δ N -270.9 and NH-1 (δ H 8.77) and between δ N -288.6 and NH-3 (δ H 7.13).This suggested structure 6A rather then structure 7A.The lack of 1 H-15 N correlation with the primary amino group is due to a fast amine proton exchange in solution.On the contrary, structure 7A should exhibit four correlations for all protonated amide-type (not proton exchanging) nitro gen atoms.The assignment of the urea 15 N chemical shifts from HSQC measurements was supported by the long-range coupling pathways detected in the HMBC spectrum (Figure 1).Since the signal at δ N -270.9correlates with the olefinic methine, it must be N-1.On the other hand, because of twobond coupling to the benzylic methine, the other 15 N resonance at δ N -288.6 corresponds to N-3.Moreover, the mutual 3 J(H-N-CO-N) interactions were consistent with this assignment.The third, non-protonated, nitrogen at δ N -198.4 is significantly coupled to the olefinic proton (H-8) and displays a weak four-bond correlation with benzylic H-4 and thus can be attributed to the nitrogen ring atom N-6.Finally, the last 15 N nucleus of the exocyclic amino group was assigned to δ N -314.3through its 4 J NH with H-8.The chemical shift value conforms with previous data 5 reported for hydrazine derivatives (~ -320 ppm).To summarize, data from 1 H-15 N correlation spectroscopy allowed us to establish the new product as 6-amino-7-hydroxy-4-(2hydroxyphenyl)-4,6,dihydro[4,3-d]pyrimidine-2,5-(1H,3H)-dione (6A) and to exclude alternative structures such as pyrimidine [5,4-d]diazepine derivative 7A.In order to further investigate the formation of structure of 6A, we used the reaction of N,Ndimethylhydrazine with 1a.This hydrazine reagent which has only one condensable nitrogen would presumably force the formation of a 6-6 fused system.Unfortunately, the desired cyclocondensation failed and we were able to isolate only a ring-open dihydropyrimidine 1,8 with a free phenolic moiety 8.The same product 8 was also obtained in the reaction of 1a with benzylamine as a result of transesterification and breaking of the oxygen-bridge.Interestingly, when Biginelli compound 9 was treated with PhCH 2 NH 2 , only acetate transesterification was observed forming compound 10.