Heteroannulation of Pyrido[2,3-d]Pyrimidines. Synthesis and Spectral Characterization of Pyridotriazolopyrimidines, Pyridopyrimidotriazine and Pyridopyrimidotriazepine Derivatives

Number of pyridotriazolo-, pyridothiazolo-, pyridotetrazolopyrimidines and pyrido-pyrimidotriazepine derivatives were prepared using the readily obtainable starting material pyrido[2,3-d]pyrimidinthione and its hydrazino derivative. The antimicrobial screening of selected synthesized compounds was done using the agar diffusion assay. The IR, H NMR and mass spectra of the synthesized compounds were investigated.


Results and Discussions
The reaction of equimolar portions of m-chloro-α-cyanocinnamonitrile 1 with 6-aminothiouracil 2 in refluxing ethanol in the presence of a catalytic amount of piperidine afforded 7-amino-5-(3-chlorophenyl)-4-oxo-2-thioxo-1H, 3H-pyrido [2,3-d]pyrimidin-6-carbonitrile 3. The structure 3 was established as pyridopyrimidine rather than thiazinopyrimidine 4 on the basis of IR and 1 H-NMR spectra which revealed a pattern completely in accord with the structure 3. The formation of 3 was assumed to proceed via nucleophilic addition of thiouracil C-5 to the β-carbon of the activated double bond in cinnamonitrile followed by cycloaddition of the acyclic Michael adduct and aromatization as shown in scheme 2.
Compelling chemical evidence for the structure 3 is forthcoming from the reaction with methyl iodide, 1,2-dichloroethane and ethylchloroacetate under different conditions and in all cases S-alkylated products were isolated (Scheme 3).
Refluxing compound 3 with methyl iodide in ethanolic

EI-MS of compound 6
Ethyl-S-[7-amino-5-(3-chlorophenyl)-6-cyano-4-oxo-3,4dihydropyrido[2,3-d] pyrimidin-2-yl]thioacetate 7 was obtained in a fairly good yield as the sole product upon alkylation of 3 with ethylchloroacetate in boiling pyridine. IR spectrum of 7 displayed ν NH2 at 3470, 3300, 3170 cm -1 , ν C≡N at 2208cm -1 and ν C=O at 1667 cm -1 (lower than the expected value for the ester group due to chelated H-bonding). Hydrazinolysis of pyridopyrimidin-2-thione 3 with hydrazine hydrate (80%) in refluxing pyridine (38 hrs) afforded the sulphur free compound in pour yield (22.6%) which identified as 7-amino-2-hydrazino-5-(3-chlorophenyl)-4-oxo-3,4-dihydro-pyrido[2,3-d]pyrimidin-6-carbonitrile 8. The same product was isolated in fairly good yield (58.2%) via nucleophilic displacement of thio methyl group for compound 5 with hydrazine hydrate in boiling ethanol (Scheme 4). The structure feature of 8 was deduced from the correct analytical and spectral data IR, 1 H-NMR and mass spectra. Thus, the highest recorded peak at m/z = 327 (100%) represent the molecular ion and the base peak.  As a part of this work, the utility of readily obtainable hydrazino derivatives in heterocyclic synthesis was investigated. The work was resulted in development of several new approaches for the synthesis of otherwise difficulty accessi-ble multifunctional heterocyclic derivatives of utility for further chemical transformations and for biological activity evaluation. We report here the results of our investigation on the behaviour of 8 towards varieties of electrophilic reagents.
Upon treatment of compound 8 with ethoxymethylene malononitrile in boiling pyridine, a yellow solid product with molecular formula C 18 H 10 ClN 9 O [403] was obtained as the sole product. Three structures for this product seemed possible 9-11. (Scheme 5) The i.r spectrum of this product displayed the absorption bands for NH 2 , NH at 3406, 3323, 3222, 3172 cm -1 and one sharp stretching absorption band for C≡N at 2225 cm -1 , ν C=O at 1685 cm -1 and ν C=N at 1641 cm -1 . The alcoholic solution of this product does not show the characteristic colour of the pyrazole ring upon treatment with ferric chloride solution, this eliminate structure 10. The mass spectrum of this product show the correct molecular ion peak at m/z = 403 (62.7%) together with M+1 and M+2 at m/z = 404 (39.2%), 405 (20.7%), respectively. The base peak represented at m/z = 402 (M-1, 100%). All these data agree well with structure 11. (Scheme 5)

Scheme 7
When compound 8 was reacted with penta-2,4-dione and/or 1-phenyl-buta-1,3-dione in molar ratio (1:1) in refluxing pyridine afforded 7-amino-5-(3-chlorophenyl)-2-(3,5-disubstituted pyrazol-1-yl)-3,4-dihydro-pyrido [2,3-d] pyrimidin-6-carbonitrile 14a,b. Furthermore, the reaction of compound 8 with ethylacetoacetate in refluxing pyridine yielded 7-amino -5 -(3-chlorophenyl)-2-(3-methyl pyrazol-5-on-1-yl)-4-oxo-3,4-dihydro-pyrido [2,3-d]pyrimidin-6-carbonitrile 15. Structures 14a,b and 15 were deduced from the full analysis of IR, 1 H-NMR and mass spectra (C.f Exp.). (Scheme 5) The formation of the pyrazole derivatives 14a,b and 15 from the hydrazino derivative 8 could be visualized via the nucleophilic addition of the amino group of the hydrazino on the carbonyl group of the acetyl group rather than that of the benzoyl and/or carboethoxy group which less positively charged and more crowded through tetrahedral mechanism with elimination of water to give the condensation product (not isolated) followed by 1,5-exo-trig cyclization (Scheme 8  [1,2,4]Triazoles represent a class of heterocyclic compounds of significant importance in medicine 26 . They used in metalloorganic chemistry as polyfunctional ligands 27 and exhibit a broad spectrum of biological activity 28 . Pronounced pharmacological and biological activities are also intrinsic for pyridopyrimidines 29 . This stimulated us to combine the above pharmacophoric fragments in a single molecule with the aim of finding new chemotherapeutic agents. The hydrazine derivative 8 was used as the key intermediate for exclusive synthesis of pyridotriazolopyrimidines through the reaction with one carbon donors such as phenyl isothiocyanate, carbon disulfide, formic acid, ethyl chloroformate and acetyl chloride. (Scheme 9) Thus, the reaction of 8 with phenyl isothiocyanate in refluxing pyridine yielded the pyridotriazolopyrimidine derivative 16. The same product 16 was obtained upon treatment of a solution of 8 in pyridine with carbon disulphide for 18hrs on water-bath. Identity was confirmed by m.p, mixed m.p, IR and MS spectra. The formation of 16 is assumed to proceed via nucleophilic addition of nitrogen nucleophile of amino group of the hydrazino group to the activated double bond -N=C=S in the phenyl isothiocyanate followed by 1,5-exo-trig. cyclization through loss of aniline molecule while, in case of addition to carbon disulphide, the nucleophilic attack occurred at the C=S group of carbon disulphide followed by cyclization and elimination of H 2 S molecule.

Scheme 10
The structure 16 was confirmed from the correct analytical and spectroscopic data (C.f Exp.). The EI-MS shows the molecular ion peak which is the base peak at m/z = 369 (100%).
Upon refluxing the hydrazino derivative 8 with formic acid, a yellow solid product was obtained which identified as pyridotriazolopyrimidine derivative 17. The infrared spectrum of 17 displayed the well defined absorption bands at 3312 (br.), 3165 (br.) cm -1 (ν NH2 ), 2227 cm -1 (ν C≡N ), 1706 cm -1 (ν C=O ) higher than that of 8 which indicates the absence of enolic form and 1634 cm -1 (ν C=N ). Full analysis of the mass spectrum of 17 shows the correct molecular ion peak at m/z = 338 (57.3%) and the base peak at m/z = 337 (100%) attributable to M-1 peak.
The reaction of 8 with ethyl chloroformate in refluxing pyridine yielded the pyridotriazolopyrimidine derivative 18 whose structure was confirmed by analytical and spectroscopic data. Thus, the mass spectrum of 18

EI fragmentation pattern for compound 18
Surprisingly, the reaction of 8 with phenyl isocyanate in refluxing pyridine afforded the same compound 18 (identity m.p, mixed m.p, TLC, IR comparison). Other product was formed when the mother liquor was diluted with water which detected to be the diphenyl urea by identity with authentic sample and the mass spectrum of this compound show the molecular ion peak at m/z = 212 (10.9%).
Refluxing of 8 with acetyl chloride in boiling pyridine for 30 min. afforded a product with molecular formula C 18  The i.r spectrum of 19 devoid the absorption bands for ν NH2 which suggest the N-acetylation for this product, furthermore, the higher value for ν C=O (pyrimidone) at 1720 cm -1 than the starting material (1692 cm -1 ) together with bands at 1704 cm -1 for acetyl group were in accordance with the proposed structure. 1 H-NMR spectrum of 19 (DMSO-d 6 ) revealed signals at δ 11.06 (s, 1H, exchangeable with D 2 O), 7.6-7.3 (m, 4H arom. ), 3.3 (br.s, 1H, exchangeable with D 2 O), 2.4 (s, 3H) and 2.1 (s, 3H). Full analysis for the mass spectrum of 19 shows the correct molecular ion peak at m/z = 393 (8.31%) together with the base peak at m/z = 351 (100%) characteristic for the radical cation resulted from loss of a ketene molecule.
When compound 8 was subjected to react with 2-cyano-4-chloro-cinnamonitrile in refluxing pyridine for 6hrs until no more substrates (TLC), pyridotriazolopyrimidine derivative 20 was obtained in fairly good yield.
The mass spectrum of 20 showed a fragmentation pattern which in harmony with the proposed structure. The molecular ion peak displayed at m/z = 450 (28.3%) together with a peaks characteristic for M+1 and M+2 at m/z = 451

Experimental
Melting points are uncorrected and were measured by an electric melting point apparatus (G-K). The IR spectra were recorded on a Pye-Unicam SP1200 spectrophotometer using KBr Wafer technique. The ¹H-NMR spectra were deter-mined on a Varian GEMINI 200 MHz NMR spectrophotometer using CDCl3 or DMSO-d 6