Scientific paper
Synthesis and NMR Spectroscopic Data of Pyrazolo[4',3':5,6]pyrano[2,3-^]pyrazin -4(1fl^)-ones: Derivatives of a Novel Tricyclic
Ring System
Gernot A. Eller,1* Qian Zhang,1,2 Daniela Habicht,1 Barbara Datterl1 and Wolfgang Holzer1*
1 Department of Drug and Natural Product Synthesis, Faculty of Life Sciences, University of Vienna, A-1090 Vienna, Austria
' Department of Medicinal Chemistry, School of Pharmacy, Fudan University, 200032 Shanghai, PR China
* Corresponding author: E-mail: gernot.eller@univie.ac.at; wolfgang.holzer@univie.ac.at
Received: 21-07-2008 Dedicated to Professor Blanko Stanovnik on the occasion of his 70"' birthday
Abstract
Derivatives of a novel tricyclic ring system, pyrazolo[4',3':5,6]pyrano[2,3-fe]pyrazin-4(1fl)-one, were prepared by reaction of either 1-substituted or 1,3-disubstituted 2-pyrazolin-5-ones and 3-chloro-2-pyrazinecarbonyl chloride in the presence of Ca(OH)2 in refluxing 1,4-dioxane. In some cases the corresponding title compounds thus were obtained directly due to a spontaneous intramolecular nucleophilic substitution reaction of the intermediate 4-pyrazinoylpyrazol-5-ols. In other cases mixtures of the latter intermediates and the target compounds were obtained, which were completely converted into the desired tricycles upon treatment with HCl in a chloroform/methanol mixture. The parent system carrying no substituent in positions 1 and 3 was prepared by treatment of the 1-PMB (p-methoxybenzyl) protected congener with trifluoroacetic acid. Detailed NMR spectroscopic data (1H, 13C, 15N) are presented for the title compounds.
Keywords: Pyrazolones, cyclizations, NMR spectroscopy, fused heterocyclic systems.
1. Introduction
The xanthone core is a partial structure of several biologically active compounds (Figure 1).1 Hence, also heterocyclic analogues, in which one or both benzene rings of the parent tricyclic system is/are replaced by hete-roaromatic moieties, are of considerable interest for medicinal chemists - as an example the anti-ulcer drug amlexa-nox (AphthasolTM, Figure 1) may serve.2 In this respect, we recently presented a simple and generally applicable synthesis of various fused pyrano[2,3-c]pyrazol-4(1^)-ones of type 4, which can be considered as heterocyclic analogues of xanthone (Figure 1).3-8 In these compounds, one benzene system of the parent xanthone is replaced by a pyrazole system and the other one by a variable hetero-aromatic moiety.
The synthetic approach to compounds 4 is based on the reaction of 1-substituted or 1,3-disubstituted 2-pyra-zolin-5-ones (1) with o-haloheteroarenecarbonyl chlorides 2 under the conditions described by Jensen for the C-4 acylation of pyrazolones (calcium hydroxide, dioxane, reflux).9 The so formed 4-acylpyrazol-5-ols 3 can be smoothly cyclized into the target compounds 4. If the halogen atom in intermediates 3 is located in an 'activated' position (o- or p-position to a pyridine-type nitrogen atom) cyclization into 4 typically occurred under the conditions of the acylation reaction and hence the intermediate 3 was not isolated.3 5-8 Following this approach, we have obtained compounds of type 4 carrying - amongst others - a pyridine (all positional isomers),3 pyridazine,5 pyrimidine,5 quinoline,3 quinoxaline,6 phenanthroline,7 8 thiophene,4,5 and benzothiophene system4 as the variable heteroaromatic moiety condensed to the central y-pyrano-
Figure 1
ne ring. In continuation of these investigations we here want to present the synthesis and spectroscopic data of novel, related congeners 4a-d containing a pyrazine moiety (Scheme 1).
2. Results and Discussion
The synthetic approach to the target compounds 4a-e is presented in Scheme 1, with 3-chloro-2-pyrazine-carbonyl chloride (2a) representing a key intermediate in this sequence. Compound 2a - prepared by treatment of the corresponding 3-chloro-2-pyrazinecarboxylic acid with SOCl2 - was immediately reacted with pyrazolones 1a-1e under '/ensen'-conditions9 without any further purification. Whereas in this way reaction of educts 1b and 1c with 2a directly gave the target structures 4b and 4c, application of pyrazolones 1a, 1d and 1e led to mixtures of 4-aroylpyrazol-5-ols of type 3 and the corresponding tricycles of type 4. Only in series d the 'open' product (3d) was predominating and thus was isolated from the reaction mixture (Scheme 1). Generally, the mixtures obtained upon the acylation reaction could be exhaustively converted into the corresponding target tricycles 4 upon treatment with HCl in CHCl /MeOH. Conversion of 3d
into 4d was also carried out via treatment of 3d with K2C03 in refluxing DMF, however, the above mentioned cyclization method in acidic medium turned out to be more convenient and led to better yields.
Finally, the synthesis of the 'parent' compound 4f -carrying no substituents at N-1 and C-3 - was accomplished by treatment of the N1-PMB ^-methoxybenzyl) protected10 congener 4e with trifluoroacetic acid at 70 °C (Scheme 2).
The title compounds 4a-f were subjected to extensive multinuclear NMR spectroscopic studies. Our goal was to provide reliable data as reference material for databases used in NMR prediction programs such as CSEARCH/ NMRPREDICT11 or ACD/C+H Predictor.12 In the last years these programs became more and more popular, especially for the prediction of 13C NMR chemical shifts. However, the quality of such predictions is considerably dependent on the availability of reliable reference data of related structures. This criterion is frequently not fulfilled with less prevalent condensed heteroaromatic systems. In consideration of this fact, full and unambiguous assignments for all 1H, 13C and 15N NMR resonances for all investigated compounds were accomplished by the combined application of standard NMR techniques.13 Moreover, two-dimensional (5, J) long-range INEPT spectra with se-
R
R'
CIO +
1a-e
O
er
Ca(0H)2 dioxane
2a
OH CI N 3
only 3d isolated
JHCI
CHCI3 MeOH
R'
O
N
"O N 4a-e

Scheme 1
a: R1 = Me, R3 = Me b: Ri = Ph, R3 = Me c: Ri = Ph, R3 = Ph d: Ri = Ph, R3= H e: W = PMB, R3 = H
Scheme 2
lective excitation14 permitted the unequivocal mapping of 13C,1H spin coupling constants. Detailed data for each compound are given in the experimental part. As a representative example, the 1H, 13C and 15N NMR chemicals shifts for target compound 4b are pictured in Figure 2.
Figure 2. 'H (in italics), 13C and 15N NMR chemical shifts (5, ppm) of 4b in CDCU
The parent compound 4f (which is characterized by a low solubility, even in DMSO-rf6) is capable of prototro-pic tautomerism and shows a dynamic behavior, what is reflected by broad signal lines. Characteristic differences in the 13C chemical shifts of C-3 (129.9 ppm) and C-9a (159.8 ppm) in 4f compared to those of the corresponding N-1 PMB congener 4e (C-3: 136.4 ppm, C-9a: 152.0 ppm) or the 1-phenyl derivative 4d (C-3: 137.1 ppm, C-9a: 151.5 ppm) hint to a substantial contribution of the 2H-form to the overall tautomeric composition (Scheme 2). The presence of the third possible tautomeric form (OH-form) is improbable as the 13C chemical shift of C-4 (172.9 ppm) closely resembles those of the 'fixed' ketones 4a-d (170.7-171.4 ppm). For the enol C-4 in the OH-form one would expect a somewhat smaller chemical shift. The dynamic behavior of 4f may also be jointly responsible for the fact that only one single signal (due to N-8) was found in the 15N-NMR spectra.
3. Conclusion
In conclusion, we have presented a short and convenient method for the synthesis of a variety of hitherto unknown pyrazolo[4',3':5,6]pyrano[2,3-&]pyrazin-4(1H)-ones starting from the corresponding pyrazolones and 3-chloro-2-pyrazinecarbonyl chloride. Moreover, we have provided valuable NMR data of the investigated fused he-terocycles.
4. Experimental
Melting points were determined on a Reichert-Kofler hot-stage microscope and are uncorrected. Mass spectra were obtained on a Shimadzu QP 1000 instrument (El, 70 eV) and on a Finnigan MAT 8230 instrument (El, 70 eV, HRMS). IR spectra were recorded on a Perkin-Elmer FTlR 1605 spectrophotometer or on a Perkin-Elmer FTlR spectrum 1000 spectrometer. Elemental analyses (C, H, N) were performed at the Microanalytical Laboratory, University of Vienna, and were in good agreement (±0.4%) with the calculated values. 1H and 13C NMR spectra were recorded on a Varian UnityPlus 300 spectrometer at 28 °C (299.95 MHz for 1H, 75.43 MHz for 13C) or on a Bruker Avance 500 spectrometer at 293 K (500.13 MHz for 1H, 125.77 MHz for 13C). The centre of the signal was used as an internal standard which was related to TMS with 5 7.26 ppm (1H in CDa3), 5 2.49 ppm ^H in DMSO-rf6), 5 77.0 ppm (13C in CDCl3), and 5 39.5 ppm (13C in DMSO-d6). 15N NMR spectra (gradient-selected HMBC and/or refocused INEPT) were obtained on a Bruker Avance 500 instrument with a 'directly' detecting broadband observe probe and were referenced against external nitromethane (coaxial capillary). Digital resolutions were 0.25 Hz/data point in the 1H and 0.4 Hz/data point in the 1H coupled 13C NMR spectra (gated decoupling). Systematic names according to lUPAC recommendations were generated with ACD/Name15 and subsequently proved manually to ensure correct nomenclature within this publication.16 Pyrazolones 1 were commercially available and/or prepared similarly to literature procedures: 1c,17 1d,18 1e.10 3-Chloro-2-pyrazinecarboxylic acid was purchased from Tyger Scientific lnc., Ewing, NJ, USA. Yields of products 4a-e were not optimized.
3-Chloro-2-pyrazinecarbonyl chloride (2a): To a mixture of 3-chloro-2-pyrazinecarboxylic acid (0.50 g, 3.15 mmol), toluene (35 mL) and 3 drops of DMF was added SOQ2 (2.3 mL, 31.6 mmol) and the solution was heated to reflux for 3 h. Then the solvents were removed under reduced pressure using a water aspirator, another 10 mL of toluene were added and again distilled off under reduced pressure. The remaining crude acid chloride, a brownish oil, was used in the next reaction step without further purification. 1H NMR (500 MHz, CDa3): 5 8.62 (d, J = 2.2 Hz, H-5), 8.68 (d, J = 2.2 Hz, H-6); 13C NMR (125 MHz, CDa3): 5 142.0 (C-6), 143.4 (C-2), 147.2 (C-5), 147.3 (C-3), 165.7 (CO); 15N NMR (50 MHz, CDa3): 5 -47.8 (N-4), -39.5 (N-1).
General procedure for the reaction of pyrazolones 1a-e with acid chloride 2a
To a suspension of pyrazolone 1a-e (3.15 mmol) and Ca(OH)2 (467 mg, 6.30 mmol) in 5 mL of dry 1,4-dioxane was added a solution of crude 3-chloro-2-pyrazi-necarbonyl chloride (2a) (prepared from 3.15 mmol of 2 of the corresponding carboxylic acid as described above) in 8 mL of dry 1,4-dioxane and the mixture was heated to reflux for 3 h. After cooling to room temperature, 2N HCl was added and the pH was adjusted to ~2. The mixture was then stirred for 15 min, subsequently poured onto water (50 mL) and further treated as described below.
1,3-Dimethylpyrazolo[4',3':5,6]pyrano[2,3-^]pyrazin-4(1Ä)-one (4a). The mixture was extracted with CH2Cl2 (4 X 30 mL), the combined organic layers were washed with water, dried (Na2SO4) and evaporated under reduced pressure to afford a mixture of 3a (minor component) and 4a (major component). This mixture was dissolved in CHa3-CH3OH (7:3) (50 mL), then 2N HCl (5 mL) was added and the whole was stirred overnight at 50 °C. After removing the solvents under reduced pressure, water (50 mL) was added, the precipitated solid was filtered off, washed with water and dried to afford 0.45 g (66%) of 4a (pure according to 1H NMR). Recrystallization from EtOH gave 0.35 g (51%) of pale yellow needles; mp 264-268 °C (EtOH); 1H NMR (300 MHz, CDa3): 5 2.57 (s, 3H, 3-CH3), 3.91 (s, 3H, NCH3), 8.62 (d, 3J(H3 7,H6) = 2.2 Hz, 1H, 11-7), 8.87 (d, 3J(H6,117) = 2.2 Hz, 1H, H-6); 13C NMR (75 MHz, CDa3): 5 13.9 (3-CH3, 1J = 129.3 Hz), 34.1 (NCH3, 1J = 141.9 Hz), 105.4 (C-3a, 3J(C3a,3-CH3) = 2.8 Hz), 135.0 (C-4a, 3J(C4a,H6) = 10.5 Hz, 4J(C4a,H7) = 1.5 Hz), 143.4 (C-6, 1J = 188.1 Hz, 2J(C6,H7) = 10.4 Hz), 145.3 (C-7, 1J = 186.8 Hz, 2J(C7,H6) = 12.5 Hz), 147.5 (C-3, 2J(C3,3-CH3) = 7.1 Hz), 152.5 (C-9a, 3J(C9a,NCH3) = 2.3 Hz), 155.8 (C-8a, 3J(C8a,H7) = 11.8 Hz, 4J(C8a,H6) = 1.6 Hz), 171.1 (C-4); 15N NMR (50 MHz, CDCl3): 5 -209.4 (N-1), -89.3 (N-2), -81.7 (N-8), -38.9 (N-5); IR (KBr): v 1677 cm-1 (C=O); MS m/z (relative intensity): 216 (M+, 100), 67 (78), 43 (53); Anal. Calcd for C10H8N4O2 ■ 0.1 H2O: C 55.10, H 3.79, N 25.70. Found: C 55.33, H 3.74, N 25.33.
3-Methyl-1-phenylpyrazolo[4',3':5,6]pyrano[2,3-Ä] pyrazin-4(1Ä)-one (4b). The precipitated solid was filtered with suction, washed with water and dried to afford 0.67 g (76%) of 4b (pure according to 1H NMR). Recrystallization from EtOH gave 0.60 g (68%) of brownish crystals; mp 230-232 °C (EtOH); 1H NMR (500 MHz, CDCl3): 5 2.70 (s, 3H, 3-CH3), 7.39 (m, 1H, Ph H-4), 7.53 (m, 2H, Ph H-3,5), 7.89 (m, 2H, Ph H-2,6), 8.66 (d, 3J(H7,H6) = 2.1 Hz, 1H, H-7), 8.91 (d, 3J(H6,H7) = 2.1 Hz, 1H, H-6); 13C NMR (125 MHz, CDCl3): 5 14.1 (3-CH3, 1J = 129.6 Hz), 107.0 (C-3a, 3J(C3a,3-CH3) = 2.8 Hz), 121.4 (Ph C-2,6), 127.9 (Ph C-4), 129.5 (Ph C-3,5), 135.0 (C-4a, 3J(C4a,H6) = 10.5 Hz, 4J(C4a,H7) = 1.5 Hz), 136.4 (Ph C-1), 143.7 (C-6, 1J = 188.0 Hz, 2J(C6,H7) = 10.6 Hz), 145.7 (C-7, 1J = 186.8 Hz, 2J(C7,H6) = 12.6 Hz), 148.6 (C-3, 2J(C3,3-CH3) = 7.2 Hz), 151.6 (C-9a), 156.0 (C-8a, 3J(C8a,H7) = 11.8 Hz, 4J(C8a,H6) = 1.6 Hz), 171.4 (C-4); 15N NMR (50 MHz, CDCl3): 5 -191.3 (N-1), -91.5 (N-2), -80.5 (N-8), -39.2 (N-5); IR (KBr): v 1681 cm-1 (C=O); MS m/z (relative intensity): 278 (M+, 100), 91 (47), 77 (59), 51 (38); HRMS-El (m/z): M+ calcd for C15H10N4O2, 278.0804; found, 278.0800; Anal. Calcd for C15H10N4O2: C 64.74, H 3.62, N 20.13. Found: C 64.84, H 3.70, N 19.88.
1,3-Diphenylpyrazolo[4',3':5,6]pyrano[2,3-Ä]pyrazin-4(1Ä)-one (4c). The precipitated solid was filtered with suction, washed with water and dried to afford 0.83 g (77%) of crude 4c. Recrystallization from EtOAc gave 0.63 g (59%) of yellow crystals; mp 272-275 °C (EtOAc); 1H NMR (500 MHz, CDCl3): 5 7.48 (m, 2H, NPh H-4, CPh H-4), 7.52 (m, 2H, CPh H-3,5), 7.61 (m, 2H, NPh H-
3.5),	8.03 (m, 2H, NPh H-2,6), 8.47 (m, 2H, CPh H-2,6), 8.70 (d, 3J(H7,H6) = 2.1 Hz, 1H, H-7), 8.97 (d, 3J(H6,H7) = 2.1 Hz, 1H, H-6); 13C NMR (125 MHz, CDCl3): 5 105.9 (C-3a), 122.0 (NPh C-2,6), 128.4 (NPh C-4), 128.5 (CPh C-3,5), 128.6 (CPh C-2,6), 129.6 (NPh C-3,5), 129.9 (CP-h C-4), 130.8 (CPh C-1), 134.9 (C-4a, 3J(C4a,H6) = 10.5 Hz, 4J(C4a,H7) = 1.4 Hz), 136.5 (NPh C-1), 144.0 (C-6, 1J = 188.4 Hz, 2J(C6,H7) = 10.4 Hz), 145.9 (C-7, 1J = 187.0 Hz, 2J(C7,H6) = 12.4 Hz), 150.2 (C-3), 152.4 (C-9a), 155.3 (C-8a, 3J(C8a,H7) = 11.8 Hz, 4J(C8a,H6) = 1.6 Hz), 170.7 (C-4); 15N NMR (50 MHz, CDCl3): 5 -189.5 (N-1), -81.3 (N-8), -38.0 (N-5), (N-2 was not found); IR (KBr): v 1675 cm-1 (C=O); MS m/z (relative intensity): 340 (M+, 100), 218 (23), 77 (77), 51 (45); HRMS-El (m/z): M+ calcd for C20H12N4O2, 340.0960; found, 340.0953; Anal. Calcd for C20H12N4O2: C 70.58, H 3.55, N 16.46. Found: C 70.38, H ^.32, N 1^.17.
(3-Chloropyrazin-2-yl)(5-hydroxy-1-phenyl-1H-pyra-zol-4-yl)methanone (3d) and 1-phenylpyrazolo[4',3':
5.6]pyrano[2,3-^]pyrazin-4(1H)-one	(4d). The precipitated solid was filtered with suction, washed with water and dried to afford 0.69 g of a mixture of 3d and 4d in a ~ 5:1 ratio (according to 1H NMR).
3d: In a first run this mixture was repeatedly crystallized from aqueous EtOH to give 0.49 g (52%) of 3d as yellow-brownish powder; mp 222-225 °C; 1H NMR (300 MHz, CDCl3): 5 7.35 (m, 1H, Ph H-4), 7.49 (m, 2H, Ph H-
3.5),	7.84 (n3, 2H, Ph H-2,6), 8.04 (s, 1H, pyrazole H-3), 8.59 (d, 3/(H5,H6) = 2.4 Hz, 1H, pyrazine H-5), 8.65 (d, 3/(H6, H5) = 2.4 Hz, 1H, pyrazine H-6), 10.40 (s, 1H, OH); 13C NMR (75 MHz, CDCl3): 5 104.1 (pyrazole C-4), 121.4 (Ph C-2,6), 127.4 (Ph C-4), 129.2 (Ph C-3,5), 137.0 (Ph C-1), 141.2 (pyrazole C-3 and pyrazine C-6), 145.7 (pyrazine C-5), 146.9 (pyrazine C-2), 148.0 (pyrazine C-
3),	159.0 (br, pyrazole C-5), 185.3 (br, CO); IR (KBr): v 1634 cm-1 (C=O); MS m/z (relative intensity): 302 (M+, 5), 300 (M+,19), 264 (28), 141 (37), 113 (30), 91 (28), 77 (100), 53 (31), 52 (29), 51 (78); HRMS-EI (m/z): M+ cal-cd for C14H9ClN4O2, 300.0414; found, 300.0412.
4d: In a second run the 5:1 mixture of 3d and 4d (0.69 g) was dissolved in CHCl3-CH3OH (7:3) (50 mL), then 2N HCl (5 mL) was added and the whole was stirred overnight at 50 °C. After removing the solvents under reduced pressure the residue was recrystallized from EtOH to give 0.43 g (52%) of a brownish powder; mp 226-228 °C; 1H NMR (500 MHz, CDCl3): 5 7.43 (m, 1H, Ph H-4), 7.56 (m, 2H, Ph H-3,5), 7.92 (m, 2H, Ph H-
2.6),	8.31 (s, 1H, H-3), 8.70 (br s, 1H, H-7), 8.94 (br s, 1H, H-6); 13C NMR (125 MHz, CDCl3): 5 108.6 (C-3a, 2/(C3a,H3) = 10.3 Hz), 121.5 (Ph C-2,6), 128.3 (Ph C-
4),	129.6 (Ph C-3,5), 134.5 (C-4a, 3/(C4a,H6) = 10.4 Hz), 136.4 (Ph C-1), 137.1 (C-3, 1J = 196.3 Hz), 143.9 (C-6, 1J = 188.4 Hz, 2J(C6,H7) = 10.5 Hz), 146.1 (C-7, 1J = 187.0 Hz, 2J(C7,H6) = 12.6 Hz), 151.5 (C-9a, 3J(C9a,H3) = 4.8 Hz), 155.9 (C-8a, 3J(C8a,H7) = 11.9 Hz), 170.9 (C-4); 15N NMR (50 MHz, CDCl3): 5 -185.1 (N-1), -83.7 (N-2), -80.1 (N-8), -39.1 (N-5); IR (KBr): v 1678 cm-1 (C=O); MS m/z (relative intensity): 264 (M+, 65), 91 (36), 77 (100), 51 (96); Anal. Calcd for C14H8N4O2 ■ 0.3 H2O: C 62.36, H 3.21, N 20.78. Found: C ^2.59, ÌÌ 3.20, N 20.55.
Cyclization of 3d under basic conditions. Under anhydrous conditions, compound 3d (200 mg, 0.66 mmol) was dissolved in dry DMF (4 mL) and K2CO3 (93 mg, 0.66 mmol) was added to the mixture. The reaction mixture was refluxed overnight. Then the solvent was evaporated under reduced pressure, H2O (2 mL) was added to the residue and the mixture stirred for 15 minutes. The precipitate was filtered off, washed with H2O and recrystallized from EtOH to afford 101 mg (57%) of 4d.
was added and the whole was stirred overnight at 50 °C. After removing the solvents under reduced pressure, water (50 mL) was added, the precipitated solid was filtered off, washed with water and dried to afford 0.77 g (79%) of 4e (pure according to 1H NMR). Recrystallization from EtOH gave 0.40 g (41%) of brownish crystals; mp 200-201 °C (EtOH); 1H NMR (500 MHz, CDCl3): 5 3.76 (s, 3H, OMe), 5.43 (s, 2H, NCH2), 6.87 (m, 2H3 , Ph H-
3.5),	7.36 (m, 2H, Ph H-2,6), 8.16 (s, 1H, H-3), 8.68 (d, 3J(H7,H6) = 2.1 Hz, 1H, H-7), 8.92 (d, 3J(H6,H7) = 2.1 Hz, 1H, H-6); 13C NMR (125 MHz, CDCl3): 5 55.3 (OCH3, 1J = 144.0 Hz), 51.5 (NCH2, 1J = 141.3 Hz, 3J(NCH2,Ph H-2,6) = 4.6 Hz), 107.4 (C:-3a, 2J(C3a,H3) = 10.4 Hz), 114.3 (Ph C-3,5), 126.3 (Ph C-1), 129.7 (Ph C-
2.6),	134.7 (C-4a, 3J(C4a,H6) = 10.5 Hz, 4J(C4a,H7) = 1.5 Hz), 136.4 (C-3, 1J = 195.6 Hz), 143.6 (C-6, 1J = 188.2 Hz, )J(C6,H7) = 10.5 Hz), 145.8 (C-7, 1J = 186.8 Hz, )J(C7,H6) = 12.6 Hz), 152.0 (C-9a, 3J(C9a,H3) = 5.0 Hz, 3J(C9a,NCH) = 2.5 Hz), 155.8 (C-8a, 3J(C8a,H7) = 11.8 Hz, 4J(C8a,H26) = 1.7 Hz), 159.8 (Ph C-4), 170.7 (C-4); 15N NMR (50 MHz, CDCl3): 5 -189.4 (N-1), -82.9 (N-2), -81.0 (N-8), -38.9 (N-5); I3R (KBr): v 1679 cm-1 (C=O); MS m/z (relative intensity): 308 (M+, 10), 121 (100), 78 (14), 77 (13); Anal. Calcd for C1gH1)N4O3: C 62.33, H 3.92, N 18.17. Found: C 62.08, H 3.84, Nf 18.13.
Pyrazolo[4',3':5,6]pyrano[2,3-Ä]pyrazin-4(1H)-one (4f). A solution of 4e (180 mg, 0.58 mmol) in trifluoro-acetic acid (TFA) (10 mL) was stirred for 24 h at 70 °C. After removal of excess TFA under reduced pressure the residue was dried over solid KOH for 1 h. Then it was digested with ice-cold Et)O-acetone (2:1, 5 mL) and filtered with suction. The remaining solid was washed with Et2O and dried to afford crude 4f in quantitative yield (109 mg). The unsoluble product was further purified by refluxing in CHCl3 (30 mL), the suspension was then filtered with suction and the residue dried in vacuo. Yield: 99 mg (90%) of a brownish powder; mp > 350 °C; 1H NMR (500 MHz, DMSO-rfg): 5 8.71 (br s, 1H, H-3), 8.79 (br s, H-7), 8.87 (br s, H-6), 14.0 (br s, NH); 13C NMR (125 MHz, DMSO-dg): 107.3 (C-3a), 129.9 (C-3), 133.8 (C-4a), 142.4 (C-6), 146.6 (C-7), 157.4 (C-8a), 159.8 (C-9a), 172.9 (C-4); 15N NMR (50 MHz, DMSO-d,): 5 -81.0 (N-8), N-1, N-2 and N-5 were not found; IR (ICBr): v 1670 (C=O), 3152 cm-1 (NH); MS m/z (relative intensity): 188 (M+, 100), 110 (46), 78 (18), 67 (18), 54 (15), 53 (52), 52 (16), 51 (18); HRMS-EI (m/z): M+ calcd for C8H4N4O), 188.0334; found, 188.0332.
1-(4-Methoxybenzyl)pyrazolo[4',3':5,6]pyrano[2,3-Ä] pyrazin-4(1Ä)-one (4e). The mixture was extracted with CH)Cl) (4 X 30 mL), the combined organic layers were washed with water, dried (Na)SO4) and evaporated under reduced pressure to afford a mixture of 3e (minor component) and 4e (major component). This mixture was dissolved in CHCl3-CH3OH (7:3) (50 mL), then 2N HCl (5 mL)
5. Acknowledgements
Qian Zhang gratefully acknowledges a scholarship provided by the Eurasia-Pacific Uninet program. Moreover, we thank Dr. L. Jirovetz and Ing. P. Unteregger for recording the mass spectra.
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Povzetek
Opisana je sinteza derivatov [4',3':5,6]pirano[2,3-b]pirazin-4(1H)-ona kot novega triciklicnega sistema iz 1,3-disubsti-tuiranih 2-pirazolin-5-onov in 3-kloro-2-pirazinkarbonil klorida v prisotnosti Ca(OH)2. V nekaterih primerih so bile koncne spojine dobljene direktno, ko posledica spontane intramolekularne ciklizacije 5-hidroksi-4-pirazinoilpirazolske-ga intermediate, v ostasli primerih pa je bilo potrebno ciklizacijo omenjenih intermediatov izvesti pod kislimi pogoji. Osnovni system brez substituentov na položajih 1 in 3 je bil pripreavljen v reakciji 1-(4-metoksifenil) substituiranega derivata s trifluoroocetno kislino. Predstavljeni so tudi detajlni NMR spektroskopski podatki (1H, 13C, 15N) končnih produktov.