HETEROCYCLES

Short Paper | Regular issue | Vol. 92, No. 12, 2016, pp. 2244-2251
Received, 8th September, 2016, Accepted, 21st October, 2016, Published online, 10th November, 2016.

■ Methodology to Access Thiazo[3’,2’:2,3]pyrido[4,5-d]thiazolo[3,2-a]pyrimidinones

Dao-Lin Wang,* Dong Wang, Qiao-Min Li, and Jian-Hua Qian

Liaoning Key Laboratory of Synthesis and Application of Functional Compound, College of Chemistry & Chemical Engineering, Bohai University, Jinzhou 121001, China

Abstract

A synthesis of model thiazolo[3’,2’:2,3]pyrido[4,5-d]thiazolo[3,2-a] pyrimidin-5-ones (5), based on the classical Pictet-Spengler method, is described. The key intermediate, 7-(3-amino-5-phenylaminothiazol-2-yl)-5H-thiazolo [3,2-a]pyrimidin-5-one (3), was synthesized from 7-chloromethyl-5H-thiazolo [3,2-a]pyrimidin-5-one (1) with potassium N-phenyl-N'-cyanoimidothiocarbonate (2) by Thorpe-Ziegler isomerization. Cyclocondensation of the intermediate amine with aromatic aldehydes, using sulfamic acid under Pictet-Spengler reaction conditions, delivered the target compounds 5a-m. 

Pyrimidines represent a class of heterocyclic compounds of great importance in biological chemistry. Moreover, fused pyrimidines have drawn the attention of medicinal chemists as chemotherapeutic agents, where several members of this class have earned valued places in chemotherapy as effective agents.1
Besides, some derivatives of thiazolo[3,2-a]pyrimidines are known to exhibit versatile biological activity such as anticancer,2 antitumor,3 antiinflammatory,4 antinociceptive,5 antiviral,6 and antibiofilm properties,7 while the thiazolopyrimidine skeleton is present in drugs proposed as immunomodulators (TEI 3096).8 Owing to these remarkably broad pharmacological properties, a variety of synthetic methods have been reported for the preparation of thiazolo[3,2-a]pyrimidinone derivatives.9-11
Recently, we reported the synthesis of novel fused benzofuro- and pyridothieno-fused thiazolo[3,2-a]pyrimidinones12 via the Pictet-Spengler reaction.13 In continuation of our interest on the construction of complex thiazolo[3,2-a]pyrimidine skeletons, herein we report the synthesis of some new fused heterocyclic systems: thiazolo[3’,2’:2,3]pyrido[4,5-d]thiazolo[3,2-a]pyrimidin-5-ones by the appli- cation of Pictet-Spengler reaction (Scheme 1).

To access the target thiazolo[3’,2’:2,3]pyrido[4,5-d]thiazolo[3,2-a]pyrimidin-5-ones, we envisioned a strategy by which the Pictet-Spengler cyclization key reaction step consists of a condensation reaction of amine 3 with various aromatic aldehydes.
The key intermediate amine, 7-(3-amino-5-phenylaminothiazol-2-yl)-5H-thiazolo[3,2-a]pyrimidin-5-one (3) was obtained by the condensation of 7-chloromethyl-5H-thiazolo[3,2-a]pyrimidin-5-one (1) with potassium N-phenyl-N'-cyanoimidothiocarbonate (2) via Thorpe-Ziegler isomerization,14 in 85% yield. Elemental analysis (C15H11N5OS2) and spectral data supported its structure. Its IR spectrum contains absorption peaks at 3426, 3356 and 1682 cm-1, demonstrating the presence of NH and C=O funcitions, respectively. Its 1H NMR spectrum (DMSO-d6) shows the presence of a D2O exchangeable broad singlet at δ 7.67 (2H) and 10.62 ppm (1H) which can be attributed to the NH2 and NH protons, respecticely. The singlet peak at δ 5.45 corresponding to C6-H of thiazolo[3,2-a]pyrimidine nucleus. The multiplet between 7.02-7.89 ppm (7H) corresponding to the aromatic protons of benzene and thiazole nucleus.
In an initial endeavor, we selected benzadehyde 4a as model aromatic aldehyde to react with equimolar amounts of intermediate amine 3a for the preparation of 6-phenyl-9-phenylamino-5H-thiazolo[3’,2’:2,3]- pyrido[4,5-d]thiazolo[3,2-a]pyrimidin-5-one 5a and investigated the optimal reaction conditions. The reaction was carried out under neat conditions at 120 oC without and with different acid catalysts such as p-toluenesulfonic acid (p-TsOH), trifluoroacetic acid (TFA), and sulfamic acid (SA) each 10 mol% in HOAc. The maximum yield was obtained using SA. It can be seen that the reaction did not proceed even after 24 h in the absence of this catalyst (Table 1, entry 1). Although a lower catalyst loading of 5 mol% accomplished this condensation, 10 mol% of SA was optimal in terms of reaction time and isolated yield (Table 1, entry 4). Increasing the amount from 10 to 15 mol% has no effect on the product yield and reaction time (Table 1, entry 6).
In addition, various solvents such as DMF, DMSO, toluene, and MeCN were screened for the optimal reaction conditions. The best catalytic activity was observed in HOAc compared to other organic solvents (Table 1, entries 7-10).

With these optimized reaction conditions in hand, we then planned to examine the versatility of the methodology for the preparation of thiazolopyridine fused thiazolo[3,2-a]pyrimidines. The substrate scope of the SA catalyzed coupling of 1 with aromatic aldehydes 4 is shown in Table 2 and it was found that this protocol could be applied not only to the aromatic aldehydes with either electron-donating groups (e.g., methyl, methoxy, hydroxy) or electron-withdrawing groups (e.g., fluoro, chloro, and nitro groups), but also to heterocyclic aldehydes. Therefore, we concluded that the electronic nature of the substituents of aldehydes has no significant effect on this reaction.

On the basis of these results, a plausible mechanism for the construction of fused thiazolo[3,2-a]- pyrimidinones is proposed (Scheme 2). The formation of ether A occurs through S-alkylation of 7-chloromethyl-5H-thiazolo[3,2-a]pyrimidin-5-one 1 and potassium N-phenyl-N'-cyanoimidothio- carbonate (2). Then, the ether A occurred via Thorpe-Ziegler isomerization reaction to generate 7-(3-amino-5-phenylaminothiazol-2-yl)-5H-thiazolo[3,2-a]pyrimidin-5-one (3). Next, the intermediate amine 3 underwent a cationic π-cyclization with aldehyde (4) under Pictet-Spengler cyclization to form D, which effects aromatisation to give tetracyclic product 5.

In summary, we have developed an efficient and versatile method for the preparation of thiazolopyrido-fused thiazolo[3,2-a]pyrimidine derivatives based on 5,6,6,5-tetracyclic systems using the modified Pictet-Spengler reaction with good yields. This method has the advantages of readily available starting materials, mild reaction conditions, and operational simplicity. Further study is underway to the scope of this methodology for some new fused heterocyclic systems.

EXPERIMENTAL
Melting points were determined in open capillaries and are uncorrected. The NMR spectra were recorded with a Bruker Avance 400 spectrometer (400 MHz for 1H and 100 MHz for 13C) using TMS an internal reference. IR spectra were measured on Shimadzu FTIR-8300 spectrophotometer. C, H and N analyses were performed by a HP-MOD 1106 microanalyzer. The preparation of 7-chloromethyl-5H-thiazolo- [3,2-a]pyrimidin-5-one (1)15 and potassium N-phenyl-N'-cyanoimidothiocarbonate (2)16 were according to the literature procedure. All other chemicals used in this study were commercially available.
Preparation of 7-(3-Amino-5-phenylaminothiazol-2-yl)-5H-thiazolo[3,2-a]pyrimidin-5-one (3): To a solution of 7-chloromethyl-5H-thiazolo[3,2-a]pyrimidin-5-one 1 (2.01 g, 10.0 mmol) in DMF (25 mL) was added potassium N-phenyl-N'-cyanoimidothiocarbonate 2 (2.15 g, 10.0 mmol) and anhydrous potassium carbonate (2.76 g, 20.0 mmol). The mixture was heated at 100 °C for 6 h (monitored by TLC). After cooling to rt, then water (50 mL) was added and stirred for 20 min. The solid was filtered and recrystallized from HOAc to give 3 (2.90 g, 85%). Yellow crystals. mp > 300 °C; IR (KBr): ν 3426, 3356 (NH), 1680 cm-1 (C=O). 1H NMR (400 MHz, DMSO-d6): δ 5.45 (s, 1H), 7.02 (d, J = 4.8 Hz, 1H), 7.01-7.06 (m, 5H), 7.67 (s, 2H), 7.89 (d, J = 4.8 Hz, 1H), 10.62 (s, 1H). Anal. Calcd for C15H11N5OS2: C 52.77, H 3.25, N 20.51. Found: C 52.84, H 3.32, N 20.58
Typical Procedure for the Preparation of 6-Aryl-9-phenylamino-5H-thiazolo[3’,2’:2,3]pyrido[4,5-d]-thiazolo[3,2-a]pyrimidin-5-ones. To a stirred solution of 7-(3-amino-5-phenylaminothiazol-2-yl)- 5H-thiazolo[3,2-a]pyrimidin-5-one (3) (283 mg, 1.0 mmol), aromatic aldehyde (1.0 mmol), and NH2SO3H (10 mg, 0.1 mmol) in DMF (20 mL) was heated at 120 °C (monitored by TLC). At the end of the reaction, the reaction mixture was cooled to rt, and then water (20 mL) was added to the mixture. The solid was filtered and recrystallized from DMF to afford the corresponding products 5a-m.
6-Phenyl-9-phenylamino-5H-thiazolo[3’,2’:2,3]pyrido[4,5-d]thiazolo[3,2-a]pyrimidin-5-one (5a): Yellow crystals. mp > 300 °C; IR (KBr): ν 3347, 1682 cm-1 (C=O); 1H NMR (400 MHz, CF3CO2D): δ 7.16 (d, J = 8.8 Hz , 2H), 7.29 (d, J = 4.8 Hz , 1H), 7.45-7.51 (m, 4H), 7.54-7.58 (m, 5H), 8.08 (d, J = 4.8 Hz , 1H). 13C NMR (100 MHz, CF3CO2D): δ 175.5, 168.6, 158.1, 158.0, 155.8, 150.4, 135.6, 130.9, 130.4, 130.3, 129.4, 123.2, 123.0, 122.9, 122.5, 115.9, 113.4, 104.8. Anal. Calcd for C22H13N5OS2: C 61.81, H 3.07, N 16.38. Found: C 61.89, H 3.13, N 16.44.
6-(4-Methylphenyl)-9-phenylamino-5H-thiazolo[3’,2’:2,3]pyrido[4,5-d]thiazolo[3,2-a]pyrimidin-5-one (5b): Yellow crystals. mp > 300 oC; IR (KBr): ν 3351 (NH), 1685 cm-1 (C=O). 1H NMR (400 MHz, CF3CO2D): δ 2.47 (s, 3H), 7.18-7.20 (m, 1H), 7.30 (d, J = 4.8 Hz , 1H), 7.41-7.55 (m, 8H,), 7.63-7.64 (m, 1H), 8.09 (d, J = 4.8 Hz , 1H). 13C NMR (100 MHz, CF3CO2D): δ175.1, 168.3, 159.0, 155.4, 144.7, 144.0, 136.5, 135.5, 130.3,129.6, 129.1, 128.2,126.8, 125.4, 122.8, 122.7, 113.6, 104 .9, 19.6. Anal. Calcd for C23H15N5OS2: C 62.57, H 3.42, N 15.86. Found: C 62.65, H 3.49, N 15.95.
6-(2-Methoxyphenyl)-9-phenylamino-5H-thiazolo[3’,2’:2,3]pyrido[4,5-d]thiazolo[3,2-a]pyrimidin- 5-one (5c): Yellow crystals. mp > 300 oC; IR (KBr): ν 3343 (NH), 1681 cm-1 (C=O). 1H NMR (400 MHz, CF3CO2D): δ 3.78 (s, 3H), 7.14-7.18 (m, 2H), 7.29 (d, J = 4.8 Hz , 1H), 7.36 (d, J = 8.0 Hz , 1H), 7.47-7.57 (m, 6H), 7.63-7.67 (m, 1H), 8.06 (d, J = 4.8 Hz , 1H). 13C NMR (100 MHz, CF3CO2D): δ 177.6, 168.5, 156.6, 156.2, 155.8, 155.3, 149.9, 135.6, 134.3, 130.3, 129.4, 129.1, 122.9, 122.4, 121.2, 118.9, 115.2, 113.4, 111.4, 106.2, 54.7. Anal. Calcd for C23H15N5O2S2: C 60.38, H 3.30, N 15.31. Found: C 60.44, H 3.39, N 15.37.
6-(3-Methoxyphenyl)-9-phenylamino-5H-thiazolo[3’,2’:2,3]pyrido[4,5-d]thiazolo[3,2-a]pyrimidin-5-one (5d): Yellow crystals. mp > 300 oC; IR (KBr): ν 3346 (NH), 1678 cm-1 (C=O). 1H NMR (400 MHz, CF3CO2D): δ 3.90 (s, 3H), 7.21 (d, J = 8.0 Hz , 1H), 7.28 (d, J = 8.0 Hz , 2H), 7.43-7.58 (m, 7H), 8.04 (d, J = 4.8 Hz , 1H). 13C NMR (100 MHz, CF3CO2D): δ 175.7, 168.7, 158.6, 157.4, 155.6, 152.3, 150.3, 135.6, 131.6, 130.6, 130.3, 129.4, 123.0, 122.4, 121.6, 117.3, 115.0, 113.3, 112.5, 105.1, 55.2. Anal. Calcd for C23H15N5O2S2: C 60.38, H 3.30, N 15.31. Found: C 60.46, H 3.37, N 15.36.
6-(4-Methoxyphenyl)-9-phenylamino-5H-thiazolo[3’,2’:2,3]pyrido[4,5-d]thiazolo[3,2-a]pyrimidin-5-one (5e): Yellow crystals. mp > 300 oC; IR (KBr): ν 3340 (NH), 1675 cm-1 (C=O). 1H NMR (400 MHz, CF3CO2D): δ 4.00 (s, 3H), 7.19 (d, J = 8.0 Hz , 2H), 7.29 (d, J = 4.8 Hz , 1H), 7.44-7.62 (m, 8H), 8.07 (d, J = 4.8 Hz , 1H); 13C NMR (100 MHz, CF3CO2D): δ 175.3, 168.5, 156.1, 155.8, 150.4, 150.3, 135.6, 130.7, 130.3, 129.9, 129.4, 123.0, 122.8, 122.5, 122.1, 114.6, 113.4, 104.7, 54.8. Anal. Calcd for C23H15N5O2S2: C 60.38, H 3.30, N 15.31. Found: C 60.46, H 3.37, N 15.39.
6-(3,4-Dimethoxyphenyl)-9-phenylamino-5H-thiazolo[3’,2’:2,3]pyrido[4,5-d]thiazolo[3,2-a]pyrimi-din-5-one (5f): Yellow crystals. mp > 300 oC; IR (KBr): ν 3343 (NH), 1683 cm-1 (C=O). 1H NMR (400 MHz, CF3CO2D): δ 3.96 (s, 3H), 4.03 (s, 3H), 7.14-7.22 (m, 2H), 7.30 (d, J = 4.8 Hz , 1H), 7.37-7.38 (m, 2H), 7.64-7.66 (m, 1H), 8.13 (d, J = 4.8 Hz , 1H). 13C NMR (100 MHz, CF3CO2D): δ175.1, 167.9, 157.8, 154.9, 152.4, 148.4, 144.1, 136.6, 135.5, 130.4, 129.2, 125.5, 123.9, 123.1, 122.9, 122.6, 113.9, 112.2, 113.4, 104.8, 55.4, 55.0. Anal. Calcd for C24H15N5O3S2: C 59.12, H 3.51, N 14.36. Found: C 59.19, H 3.60, N 14.46.
6-(4-Hydroxyphenyl)-9-phenylamino-5H-thiazolo[3’,2’:2,3]pyrido[4,5-d]thiazolo[3,2-a]pyrimidin-5-one (5g): Yellow crystals. mp > 300 oC; IR (KBr): ν 3348 (NH), 3336 (OH), 1682 cm-1 (C=O). 1H NMR (400 MHz, CF3CO2D): δ 7.22 (d, J = 4.8 Hz , 1H), 7.43-7.44 (m, 4H), 7.49-7.55 (m, 6H), 8.01 (d, J = 4.8 Hz , 1H). 13C NMR (100 MHz, CF3CO2D): δ175.6, 168.7, 158.5, 155.6, 152.3, 150.3, 135.6, 132.3, 130.2, 130.0, 129.4, 128.8, 128.0, 122.9, 122.4, 113.3, 110.0, 104.9. Anal. Calcd for C22H13N5O2S2: C 59.58, H 2.95, N 15.79. Found: C 59.65, H 3.04, N 15.86.
6-(2-Fluorophenyl)-9-phenylamino-5H-thiazolo[3’,2’:2,3]pyrido[4,5-d]thiazolo[3,2-a]pyrimidin-5-one (5h): Yellow crystals. mp > 300 oC; IR (KBr): ν 3342 (NH), 1685 cm-1 (C=O). 1H NMR (400 MHz, CF3CO2D):δ 7.24-7.28 (m, 2H), 7.37-7.41(m, 2H), 7.44-7.55 (m, 6H), 7.68-7.70 (m, 1H), 8.07 (d, J = 4.8 Hz , 1H). 13C NMR (100 MHz, CF3CO2D): δ168.7, 158.2, 155.2, 152.2, 149.9, 135.6, 134.7, 134.6, 130.3, 129.4, 129.0, 124.8, 124.7, 122.4, 118.7, 118.5, 116.0, 115.8, 113.3, 106.3. Anal. Calcd for C22H12FN5OS2: C 59.31, H 2.72, N 15.72. Found: C 59.38, H 2.77, N 15.79.
6-(4-Fluorophenyl)-9-phenylamino-5H-thiazolo[3’,2’:2,3]pyrido[4,5-d]thiazolo[3,2-a]pyrimidin-5-one (5i): Yellow crystals. mp > 300 oC; IR (KBr): ν 3346 (NH), 1681 cm-1 (C=O). 1H NMR (400 MHz, CF3CO2D): δ 7.25-7.28 (m, 3H), 7.44-7.48 (m, 3H), 7.53-7.59 (m, 5H), 8.05 (d, J = 4.8 Hz , 1H). 13C NMR (100 MHz, CF3CO2D): δ 168.8, 166.8, 164.2, 157.3, 155.6, 150.3, 135.6, 130.8, 130.7, 129.4, 126.0, 125.9, 122.9, 122.5, 116.3, 116.0, 113.3, 105.0. Anal. Calcd for C22H12FN5OS2: C 59.31, H 2.72, N 15.72. Found: C 59.40, H 2.76, N 15.76.
6-(4-Chlorophenyl)-9-phenylamino-5H-thiazolo[3’,2’:2,3]pyrido[4,5-d]thiazolo[3,2-a]pyrimidin-5-one (5j): Yellow crystals. mp > 300 oC; IR (KBr): ν 3349 (NH), 1684 cm-1 (C=O). 1H NMR (400 MHz, CF3CO2D): δ 7.28 (d, J = 4.8 Hz , 1H), 7.46-7.48 (m, 3H), 7.51-7.58 (m, 7H), 8.05 (d, J = 4.8 Hz , 1H); 13C NMR (100 MHz, CF3CO2D): δ 175.6, 168.7, 157.1, 155.5, 152.5, 150.2, 139.7, 135.5, 130.2, 129.4, 129.2, 128.3, 122.9, 122.4, 113.3, 113.2, 112.2, 105.0. Anal. Calcd for C22H12ClN5OS2: C 57.20, H 2.62, N 15.16. Found: C 57.27, H 2.68, N 15.23.
6-(4-Nitrophenyl)-9-phenylamino-5H-thiazolo[3’,2’:2,3]pyrido[4,5-d]thiazolo[3,2-a]pyrimidin-5-one (5k): Yellow crystals. mp > 300 oC; IR (KBr): ν 3353 (NH), 1686 cm-1 (C=O). 1H NMR (400 MHz, CF3CO2D): δ 7.27 (d, J = 4.8 Hz , 1H), 7.48-7.52 (m, 4H), 7.55-7.57 (m, 2H), 7.80 (d, J = 8.8 Hz , 2H), 8.02 (d, J = 4.8 Hz , 1H), 8.46 (d, J = 8.8 Hz , 2H). 13C NMR (100 MHz, CF3CO2D): δ 168.7, 155.9, 155.3, 152.6, 150.0, 149.7, 149.1, 138.0, 135.4, 130.3, 129.7, 129.6, 123.8, 123.0, 122.3, 113.3, 110.0, 106.7. Anal. Calcd for C22H12N6O3S2: C 55.92, H 2.56, N 17.79. Found: C 55.98, H 2.63, N 17.86.
6-(2-Furyl)-9-phenylamino-5H-thiazolo[3’,2’:2,3]pyrido[4,5-d]thiazolo[3,2-a]pyrimidin-5-one (5l): Yellow crystals. mp > 300 oC; IR (KBr): ν 3341 (NH), 1678 cm-1 (C=O). 1H NMR (400 MHz, CF3CO2D): δ 6.80-6.81 (m, 1H), 7.26 (d, J = 4.8 Hz , 1H), 7.39-7.43 (m, 4H), 7.45-7.50 (m, 2H), 7.84-7.85 (m, 1H), 8.14 (d, J = 4.8 Hz , 1H), 8.46 (d, J = 4.2 Hz , 2H). 13C NMR (100 MHz, CF3CO2D): δ 180.9, 175.2,168.0, 154.5, 151.3, 150.7, 149.5, 143.9, 142.3, 135.5, 130.2, 129.5, 127.5, 123.1, 122.8, 114.5, 113.2, 102.5. Anal. Calcd for C20H11N5O2S2: C 57.54, H 2.66, N 16.78. Found: C 57.63, H 2.74, N 16.85.
6-(2-Thienyl)-9-phenylamino-5H-thiazolo[3’,2’:2,3]pyrido[4,5-d]thiazolo[3,2-a]pyrimidin-5-one (5m): Yellow crystals. mp > 300 oC; IR (KBr): ν 3343 (NH), 1681 cm-1 (C=O). 1H NMR (400 MHz, CF3CO2D): δ 7.24-7.28 (m, 2H), 7.41-7.43 (m, 4H), 7.47-7.52 (m, 2H), 7.64-7.65 (m, 1H), 7.81-7.82 (m, 1H), 8.08 (d, J = 4.8 Hz , 1H). 13C NMR (100 MHz, CF3CO2D): δ175.4, 168.5, 164.5, 155.5, 152.3, 151.6, 150.3, 135.6, 133.6, 133.3, 130.3, 129.5, 129.2, 127.9, 123.0, 122.6, 113.4, 105.3. Anal. Calcd for C20H11N5OS3: C 55.41, H 2.56, N 16.15. Found: C 55.49, H 2.64, N 16.24.

ACKNOWLEDGEMENTS
This work was partially supported by innovation team project of Liaoning Province Education Department (Grant No. 2015001).

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