HETEROCYCLES

Paper | Special issue | Vol. 79, No. 1, 2009, pp. 669-680
Received, 26th September, 2008, Accepted, 20th November, 2008, Published online, 21st November, 2008.
DOI: 10.3987/COM-08-S(D)34

■ Iodobenzene Diacetate–Promoted N–N and N–O Bond Formation for Pyrazolo- and Isoxazolopyrimidine Syntheses

Yasunari Monguchi, Kazuyuki Hattori, Tomohiro Maegawa, Kosaku Hirota, and Hironao Sajiki*

Laboratory of Medicinal Chemistry, Gifu Pharmaceutical University, 6-1 Mitahora-higashi 5-chome, Gifu 502-8585, Japan

Abstract

Pyrazolo[3,4-d]pyrimidine-4,6-dione derivatives were efficiently synthesized via the intramolecular N–N bond coupling of 5-iminomethyl-6-aminouracil derivatives using iodobenzene diacetate. The oxidative coupling was also applied to the analogous N–O bond formation producing isoxazolo[3,4-d]primidine-4,6-dione derivatives.

This paper is dedicated to the memory of Dr. John Daly.

INTRODUCTION
Since allopurinol, pyrazolo[3,4-d]pyrimidin-4-one, was developed as a clinical medicine for the treatment of hyperuricemia and gouty arthritic diseases based on the inhibition of xanthine oxidase,1 much attention has been focused on the biological evaluation of the pyrazolo[3,4-d]pyrimidine derivatives.2,3 Recent biological studies of such pyrazolopyrimidine derivatives have targeted a wide range of biomolecules, such as Src protein kinases,4,5 EGF receptor tyrosine kinases,6 Abelson kinase,7 p38 MAP kinase,8 cyclic cyclooxygenase-2,9 bacterial DNA polymerase III,10 mycobacterial lumazine synthase,11 adenosine A1 receptor,12 cannabinoid receptor type 1,13 etc. The synthetic methods for the construction of the pyrazolo[3,4-d]pyrimidine ring system have involved the cyclization of the pyrazole5,7,8,12,14,15 or pyrimidine intermediates,11,16 but the nitrogen–nitrogen (N–N) bond forming reaction was never used for the cyclization of a pyrazole ring until our published communication related to the iodobenzene diacetate-promoted N–N bond formation.17 Although the isoxazolo[3,4-d]pyrimidine derivatives were structurally very similar to the pyrazolo[3,4-d]pyrimidine derivatives, they have been basically synthesized by the isoxazole ring formation starting from the 5,6-disubstituted pyrimidine derivatives,18,19 and the ring closure reaction via the nitrogen–oxygen (N–O) bond formation appeared during the thermolysis of the 5-acyl-6-azidouracils20,21 and photolysis of the 5-acyl-6-sulfiliminouracils.22
Hypervalent iodine reagents,23 such as iodobenzene diacetate and phenyliodine bis(trifluoroacetate), have been used for the oxidative N–N17,24,25 and N–O26 bond formations to build the pyrazolo- and isoxazoloarene motifs in past decade. We now describe the details of the iodobenzene diacetate-promoted cyclization method for the preparation of pyrazolo[3,4-d]pyrimidine-4,6-diones and isoxazolo[3,4-d]pyrimidine-4,6-diones via the N–N and N–O bond creation, respectively.

RESULTS AND DISCUSSION
During the course of our study on the fused pyrimidine synthesis, we found that 5-[(dimethylamino)methylene]dihydro-6-imino-1,3-dimethyl-2,4(1H,3H)-pyrimidinedione hydrochloride (1), which was readily prepared by the reaction of the commercially available 6-amino-1,3-dimethyluracil with phosphoryl chloride in dimethylformamide (DMF), underwent a nucleophilic attack on the dimethylaminomethylene carbon at the 5-position of the pyrimidine ring by a nitrogen atom from a series of amides, and subsequent cyclization afforded the pyrimido[4,5-d]pyrimidine derivatives (Scheme 1).27,28

When we used excess amounts of primary amines in place of the amides as nucleophiles, the 6-amino-5-(substituted iminomethyl)-1,3-dimethyluracil derivatives (2)29 was stably obtained, which were used as starting materials for the intramolecular N–N bond formation (Table 1). The addition of a base, such as triethylamine (Et3N) or lithium hydride (LiH), increased the reaction efficiency due to the enhanced nucleophilicity of the amine nitrogen atoms (Entries 1, 10–12); e.g., the use of n-butylamine could be reduced to 1.1 equiv by the addition of 1.1 equiv of Et3N, while the reaction with 2.1 equiv of n-butylamine gave 2j in only 31% yield (Entry 10).
Our attempts of the intramolecular N–N bond formation were initiated by the investigation of some oxidants using 2a as a substrate. While the use of 4 equiv of N-bromosuccinimide resulted in the formation of a complex mixture containing unreacted 2a, the oxidation with 4 equiv of lead tetraacetate in DMF at rt gave the desired 5,7-dimethyl-2-phenylpyrazolo[3,4-d]pyrimidine-4,6(5H,7H)-dione (3a) in 47% yield. Furthermore, the employment of iodobenzene diacetate significantly improved the yield up to 87% (Entry 1). Therefore, we chose iodobenezene diacetate as the oxidant for the present cyclization. As shown in Entries 1–7, the 5-aryl substituted iminomethyluracil derivatives (R = aryl: 2a–2g) were smoothly oxidized to afford the desired pyrazolo[3,4-d]pyrimidine derivatives (3a–3g) in good to excellent yields. Although the 5-alkyl substituted iminomethyluracil derivatives (R = alkyl: 2h–2m) were less reactive under the same reaction conditions, the addition of 2 equiv of LiH improved the N–N bond formation efficiency (Entries 9–13). For example, the yield of the t-butyl substituted pyrazolo[3,4-d]pyrimidine 3i was improved from 35% to 56% (Entry 9).

We next explored the intramolecular N–O bond forming reaction using the 5-acyl-substituted 6-amino-1,3-dimethyl-2,4-pyrimidinedione derivatives (4a–4c) by iodobenzene diacetate (Table 2). The oxidative cyclization in the presence of 2 equiv of LiH was successfully achieved to give the corresponding 3-substituted 5,7-dimethylisoxazolo[3,4-d]pyrimidine-4,6(5H,7H)-dione derivatives (5a–5c) in good to excellent yields.

A plausible mechanism for the intramolecular N–N bond formation is depicted in Scheme 2. The oxidation would be initiated by the reaction of the iodobenzene diacetate with the amine nitrogen of 2 or/and its tautomer, 6-aminomethylidene-6-iminouracil (6). Iodobenzene would be dissociated from the resulting intermediates A and B to give the corresponding nitrenium acetate ion pairs C and D, respectively. Nitrenium ions are highly electrophilic and stabilized by the neighboring electron-donating group such as aryl, alkoxy, and amino groups.25,30 In this study, the 5-arylimino-substituted uracils were rather reactive compared with the 5-alkylimino-substituted uracils, and the reaction was promoted in the presence of LiH (Table 1). Therefore, the more stable nitrenium ion D could be favorably formed and attacked by the lone pair of the 6-imino nitrogen atom, the nucleophilicity of which would be enhanced under the basic conditions.

CONCLUSION
We have developed a facile synthetic method for the construction of a variety of pyrazolo[2,3-d]pyrimidine and isoxazolo[2,3-d]pyrimidine derivatives by the iodobenzene diacetate-promoted oxidative N–N or N–O bond formation. The oxidant is relatively less toxic and produces no hazardous inorganic wastes. Since such fused pyrimidine analogs have recently attracted attention as target molecules for drug development, the present study would offer a facile and safe synthetic approach to potential bioactive compounds.

EXPERIMENTAL
General Methods. Unless otherwise stated, the commercially obtained materials were used without further purification. The 1H NMR spectra were recorded by a JEOL JNM EX-400 spectrometer. Chemical shifts (δ) are expressed in ppm and internally referenced (0.00 ppm for tetramethylsilane-CDCl3 and 2.49 ppm for DMSO-d6). The EI mass spectra were obtained using a JEOL JMS-SX102A instrument. The elemental analyses were performed by a YANACO MT-5 instrument. The flash column chromatography was performed using Kanto Chemical Co., Inc., silica gel 60N, spherical neutral (63-210 µm).

Synthesis of substrate:
General preparation method of 6-amino-5-[(substituted)-iminomethyl]-1,3-dimethyluracil derivatives (2a–2m). To a suspension of 5-[(dimethylamino)methylene]dihydro-6-imino-1,3-dimethylpyrimidine-2,4(1H,3H)-dione hydrochloride (1) (2.47 g, 10.0 mmol) in dry DMF (70 mL) was dropwise added an amine (21.0 mmol). After stirring at rt for 12 h, the mixture was concentrated in vacuo. The residue was triturated with Et2O (20 mL), and the resulting precipitate was collected on a Kiriyama funnel and then recrystallized from EtOH.
6-Amino-1,3-dimethyl-5-[(phenylimino)methyl]uracil (2a)29: Aniline (1.0 mL, 11.0 mmol) and Et3N (1.5 mL, 10.8 mmol) were added to a suspension of 1 (2.47 g, 10.0 mmol) in DMF (20 mL). mp 259–160 °C. 1H NMR (DMSO-d6) δ 3.22 (3H, s), 3.39 (3H, s), 7.27 (5H, m), 8.34 (1H, br s), 8.76 (1H, s), 11.11 (1H, br s). MS (EI) m/z: 258 (M+). Anal. Calcd for C13H14N4O2: C, 60.45; H, 5.46; N, 21.70. Found: C, 60.64; H, 5.55; N, 21.67.
6-Amino-5-[{(4-fluorophenyl)imino}methyl]-1,3-dimethyluracil (2b)17: mp 288–290 °C (recrystallized from EtOH). 1H NMR (DMSO-d6) δ 3.17 (3H, s), 3.35 (3H, s), 7.16 (2H, m), 7.17 (2H, m), 8.31 (1H, br s), 8.68 (1H, s), 10.99 (1H, br s). MS (EI) m/z: 276 (M+). Anal. Calcd for C13H13FN4O2: C, 56.50; H, 4.75; N, 20.29. Found: C, 56.33; H, 4.72; N, 20.30.
6-Amino-5-[{(4-chlorophenyl)imino}methyl]-1,3-dimethyluracil (2c)17: mp 258–259 °C (recrystallized from EtOH). 1H NMR (CDCl3) δ 3.39 (3H, s), 3.49 (3H, s), 5.50 (1H, br s), 7.10 (2H, d, J = 8.8 Hz), 7.31 (2H, d, J = 8.8 Hz), 8.80 (1H, s), 11.95 (1H, br s). MS (EI) m/z: 292 (M+). Anal. Calcd for C13H13ClN4O2: C, 53.34; H, 4.48; N, 19.14. Found: C, 53.48; H, 4.59; N, 19.22.
6-Amino-5-[{(3-chlorophenyl)imino}methyl]-1,3-dimethyluracil (2d)17: mp 265–267 °C (recrystallized from EtOH). 1H NMR (DMSO-d6) δ 3.18 (3H, s), 3.36 (3H, s), 7.10 (1H, d, J = 8.3 Hz), 7.15–7.22 (2H, m), 7.38 (1H, t, J = 8.3 Hz), 8.38 (1H, br s), 8.70 (1H, s), 10.90 (1H, br s). MS (EI) m/z: 292 (M+). Anal. Calcd for C13H13ClN4O2: C, 53.34; H, 4.48; N, 19.14. Found: C, 53.15; H, 4.46; N, 19.21.
6-Amino-1,3-dimethyl-5-[{(3-nitrophenyl)imino}methyl]uracil (2e)17: mp >300 °C. 1H NMR (DMSO-d6) δ 3.19 (3H, s), 3.37 (3H, s), 7.59–7.66 (2H, m), 7.92 (1H, s), 7.96–8.01 (1H, m), 8.46 (1H, br s), 8.79 (1H, s), 10.87 (1H, br s). MS (EI) m/z: 303 (M+). Anal. Calcd for C13H13N5O4: C, 51.48; H, 4.32; N, 23.09. Found: C, 51.25; H, 4.33; N, 23.08.
6-Amino-5-[{(4-methoxyphenyl)imino}methyl]-1,3-dimethyluracil (2f)17: mp 212–214 °C (recrystallized from EtOH). 1H NMR (CDCl3) δ 3.38 (3H, s), 3.48 (3H, s), 3.82 (3H, s), 5.54 (1H, br s), 6.91 (2H, d, J = 8.8 Hz), 7.14 (2H, d, J = 8.8 Hz), 8.86 (1H, s), 12.23 (1H, br s). MS (EI) m/z: 288 (M+). Anal. Calcd for C14H16N4O3: C, 58.32; H, 5.59; N, 19.44. Found: C, 58.17; H, 5.56; N, 19.38.
6-Amino-1,3-dimethyl-5-[(1-naphthalenylimino)methyl]uracil (2g)17: mp 275–278 °C. 1H NMR (CDCl3) δ 3.41 (3H, s), 3.54 (3H, s), 5.64 (1H, br s), 7.12 (1H, d, J = 7.8 Hz), 7.43–7.55 (3H, m), 7.69 (1H, d, J = 8.3 Hz), 7.86 (1H, d, J = 8.3 Hz), 8.17 (1H, d, J = 7.8 Hz), 8.99 (1H, s), 12.25 (1H, br s). MS (EI) m/z: 308 (M+). Anal. Calcd for C17H16N4O2: C, 66.22; H, 5.23; N, 18.17. Found: C, 66.12; H, 5.27; N, 18.13.
6-Amino-1,3-dimethyl-5-[(methylimino)methyl]uracil (2h)29: mp 232–233 °C (recrystallized from MeOH). 1H NMR (CDCl3) δ 3.28 (3H, s), 3.32 (3H, s), 3.36 (3H, s), 8.33 (1H, s). MS (EI) m/z: 196 (M+). HRMS (EI) Calcd for C8H12N4O2 (M+) 196.0960. Found 196.0972.
6-Amino-5-[(t-butylimino)methyl]-1,3-dimethyluracil (2i): mp 186 °C. 1H NMR (CDCl3) δ 1.39 (9H, s), 3.32 (3H, s), 3.33 (3H, s), 8.39 (1H, s). MS (EI) m/z: 238 (M+). Anal. Calcd for C11H18N4O2: C, 55.45; H, 7.61; N, 23.51. Found: C, 55.22; H, 7.52; N, 23.25.
6-Amino-5-[(butylimino)methyl]-1,3-dimethyluracil (2j)31: n-Butylamine hydrochloride (1.22 g, 11.0 mmol) and Et3N (1.5 mL, 10.8 mmol) were added to a suspension of 1 (2.47 g, 10.0 mmol) in DMF (50 mL). mp 171–172 °C. 1H NMR (CDCl3) δ 0.95 (3H, t, J = 7.0 Hz), 1.40 (2H, m), 1.62 (2H, m), 3.32 (3H, s), 3.35 (3H, s), 3.46 (2H, t, J = 7.0 Hz), 6.30 (1H, br s), 8.30 (1H, s), 11.90 (1H, br s). MS (EI) m/z: 238 (M+). Anal. Calcd for C11H18N4O2: C, 55.45; H, 7.61; N, 23.51. Found: C, 55.20; H, 7.52; N, 23.41.
6-Amino-5-[(cyclopentylimino)methyl]-1,3-dimethyl uracil (2k): Cyclopentylamine (0.12 mL, 2.4 mmol) and LiH (11.9 mg, 3.0 mmol) were added to a suspension of 1 (493 mg, 2 mmol) in DMF (10 mL). After 3 h, the mixture was concentrated in vacuo and the residue was extracted using H2O (10 mL) and CH2Cl2 (10 mL × 2). The combined organic layers were successively washed with H2O (20 mL) and brine (20 mL), dried over MgSO4, and concentrated in vacuo. The residue was triturated with Et2O (10 mL) and the resulting precipitate was collected on a Kiriyama funnel. mp 212 °C. 1H NMR (CDCl3) δ 1.60–1.79 (6H, m), 1.96–2.02 (2H, m), 3.32 (3H, s), 3.35 (3H, s), 3.84 (1H, m), 6.21 (1H, br s), 8.36 (1H, s), 11.95 (1H, br s). MS (EI) m/z: 250 (M+). Anal. Calcd for C12H18N4O2: C, 57.58; H, 7.25; N, 22.38. Found: C, 57.39; H, 7.30; N, 22.30.
6-Amino-5-[(cyclohexylimino)methyl]-1,3-dimethyluracil (2l): Cyclohexylamine hydrochloride (298 mg, 2.2 mmol) and Et3N (0.6 mg, 4.3 mmol) were added to a suspension of 1 (493 mg, 2 mmol) in DMF (10 mL). After 1.5 h, the mixture was poured into H2O (10 mL). The resulting precipitate was collected on a Kiriyama funnel. mp 178–179 °C. 1H NMR (CDCl3) δ 1.34–1.49 (4H, m), 1.58–1.64 (2H, m), 1.76–1.79 (2H, m), 1.91–1.94 (2H, m), 3.29 (1H, m), 3.32 (3H, s), 3.34 (3H, s), 6.33 (1H, br s), 8.34 (1H, s), 12.03 (1H, br s). MS (EI) m/z: 264 (M+). Anal. Calcd for C13H20N4O2: C, 59.07; H, 7.63; N, 21.20. Found: C, 59.13; H, 7.61; N, 21.21.
6-Amino-5-[(cyclohexylmethylimino)methyl]-1,3-dimethyluracil (2m): mp 186 °C. 1H NMR (CDCl3) δ 0.97 (2H, m), 1.14–1.30 (3H, m), 1.54–1.75 (6H, m), 3.29 (2H, d, J = 6.3 Hz), 3.32 (3H, s), 3.35 (3H, s), 6.36 (1H, br s), 8.24 (1H, s), 11.95 (1H, br s). MS (EI) m/z: 278 (M+). Anal. Calcd for C14H22N4O2: C, 60.41; H, 7.97; N, 20.13. Found: C, 60.34; H, 7.87; N, 19.92.

General procedure for the intramolecular N–N bond formation (3a–3m). To a suspension of 6-amino-5-[(substituted)-iminomethyl]-1,3-dimethyluracil (1.94 mmol) in dry DMF (20 mL) was dropwise added iodobenzene diacetate (2.49 g, 7.74 mmol). The mixture was stirred at 80 °C for a given time and concentrated in vacuo. To the residue were added H2O (15 mL) and CHCl3 (15 mL) and the layers were separated. The aqueous layer was extracted with CHCl3 (15 mL × 2). The combined organic layers were successively washed with H2O (20 mL) and brine (20 mL), dried over MgSO4, and concentrated in vacuo. The residue was triturated with Et2O (10 mL) and the resulting precipitate was collected on a Kiriyama funnel and then recrystallized from MeOH.
5,7-Dimethyl-2-phenyl-2H-pyrazolo[3,4-d]pyrimidine-4,6(5H,7H)-dione (3a)17: mp 288–291 °C (recrystallized from MeOH). 1H NMR (CDCl3) δ 3.43 (3H, s), 3.61 (3H, s), 7.39 (1H, t, J = 7.8 Hz), 7.51 (2H, t, J = 7.8 Hz), 7.72 (2H, d, J = 7.8 Hz), 8.43 (1H, s). MS (EI) m/z: 256 (M+). Anal. Calcd for C13H12N4O2: C, 60.93; H, 4.72; N, 21.87. Found: C, 60.88; H, 4.86; N, 22.08.
2-(4-Fluorophenyl)-5,7-dimethyl-2H-pyrazolo[3,4-d]pyrimidine-4,6(5H,7H)-dione (3b)17: mp 270–271 °C (recrystallized from EtOH). 1H NMR (CDCl3) δ 3.43 (3H, s), 3.60 (3H, s), 7.21 (2H, dd, J = 9.0, 8.5 Hz), 7.70 (2H, m), 8.36 (1H, s). MS (EI) m/z: 274 (M+). Anal. Calcd for C13H11FN4O2: C, 56.93; H, 4.04; N, 20.43. Found: C, 56.80; H, 4.12; N, 20.21.
2-(4-Chlorophenyl)-5,7-dimethyl-2H-pyrazolo[3,4-d]pyrimidine-4,6(5H,7H)-dione (3c)17: mp 290–293 °C (recrystallized from EtOH). 1H NMR (CDCl3) δ 3.43 (3H, s), 3.60 (3H, s), 7.48 (2H, d, J = 9.0 Hz), 7.67 (2H, d, J = 9.0 Hz), 8.40 (1H, s). MS (EI) m/z: 290 (M+). Anal. Calcd for C13H11ClN4O2: C, 53.71; H, 3.81; N, 19.27. Found: C, 53.71; H, 3.81; N, 19.09.
2-(3-Chlorophenyl)-5,7-dimethyl-2H-pyrazolo[3,4-d]pyrimidine-4,6(5H,7H)-dione (3d)17: mp 286–288 °C. 1H NMR (CDCl3) δ 3.43 (3H, s), 3.61 (3H, s), 7.36 (1H, d, J = 8.1 Hz), 7.44 (1H, t, J = 8.1 Hz), 7.59 (1H, d, J = 8.1 Hz), 7.80 (1H, s), 8.43 (1H, s). MS (EI) m/z: 290 (M+). Anal. Calcd for C13H11ClN4O2·3/5H2O: C, 51.79; H, 4.08; N, 18.58. Found: C, 51.72; H, 3.84; N, 18.67.
5,7-Dimethyl-2-(3-nitrophenyl)-2H-pyrazolo[3,4-d]pyrimidine-4,6(5H,7H)-dione (3e)17: mp >300 °C. 1H NMR (CDCl3) δ 3.44 (3H, s), 3.63 (3H, s), 7.72 (1H, t, J = 8.3 Hz), 8.07 (1H, dd, J = 8.3, 2.2 Hz), 8.24 (1H, dd, J = 8.3, 2.2 Hz), 8.54 (1H, s), 8.66 (1H, dd, J = 2.2, 2.2 Hz). MS (EI) m/z: 301 (M+). HRMS (EI) Calcd for C13H11N5O4 (M+) 301.0811. Found 301.0795.
2-(4-Methoxyphenyl)-5,7-dimethyl-2H-pyrazolo[3,4-d]pyrimidine-4,6(5H,7H)-dione (3f)17: mp 260–262 °C. 1H NMR (CDCl3) δ 3.43 (3H, s), 3.60 (3H, s), 3.87 (3H, s), 7.01 (2H, d, J = 9.0 Hz), 7.61 (2H, d, J = 9.0 Hz), 8.31 (1H, s). MS (EI) m/z: 286 (M+). Anal. Calcd for C14H14N4O3: C, 58.74; H, 4.93; N, 19.57. Found: C, 58.66; H, 4.97; N, 19.52.
5,7-Dimethyl-2-(1-naphthalenyl)-2H-pyrazolo[3,4-d]pyrimidine-4,6(5H,7H)-dione (3g)17: mp 233–235 °C (recrystallized from MeOH). 1H NMR (CDCl3) δ 3.47 (3H, s), 3.62 (3H, s), 7.53–7.66 (4H, m), 7.79 (1H, d, J = 8.1 Hz), 7.93–8.06 (2H, m), 8.30 (1H, s). MS (EI) m/z: 306 (M+). Anal. Calcd for C17H14N4O2: C, 66.65; H, 4.61; N, 18.29. Found: C, 66.80; H, 4.62; N, 18.32.
2,5,7-Trimethyl-2H-pyrazolo[3,4-d]pyrimidine-4,6(5H,7H)-dione (3h)14: mp 206–207 °C. 1H NMR (CDCl3) δ 3.39 (3H, s), 3.52 (3H, s), 3.94 (3H, s), 7.87 (1H, s). MS (EI) m/z: 194 (M+). HRMS (EI) Calcd for C8H10N4O2 (M+) 194.0804. Found 194.0813.
2-t-Butyl-5,7-dimethyl-2H-pyrazolo[3,4-d]pyrimidine-4,6(5H,7H)-dione (3i): 2i (238 mg, 1 mmol), iodobenzene diacetate (1.29 g, 4 mmol), and LiH (15.9 mg, 2 mmol) were used. The residue from the extracts was purified by column chromatography on silica gel (hexane/EtOAc, 3 : 1) to give 3i (131 mg, 56%). mp 171–172 °C. 1H NMR (CDCl3) δ 1.61 (s, 9H), 3.39 (3H, s), 3.54 (3H, s), 8.02 (1H, s). MS (EI) m/z: 236 (M+). Anal. Calcd for C11H16N4O2: C, 55.92; H, 6.83; N, 23.71. Found: C, 55.86; H, 6.78; N, 23.61.
2-n-Butyl-5,7-dimethyl-2H-pyrazolo[3,4-d]pyrimidine-4,6(5H,7H)-dione (3j): 2j (477 mg, 2 mmol), iodobenzene diacetate (2.58 g, 8 mmol), and LiH (31.8 mg, 4 mmol) were used. The residue from the extracts was purified by column chromatography on silica gel (hexane/EtOAc, 3 : 1) to give 3j (399 mg, 85%). mp 127–128 °C. 1H NMR (CDCl3) δ 0.96 (3H, t, J = 7.3 Hz), 1.34 (2H, m), 1.88 (2H, quint, J = 7.3 Hz), 3.39 (3H, s), 3.53 (3H, s), 4.12 (2H, t, J = 7.3 Hz), 7.89 (1H, s). MS (EI) m/z: 236 (M+). Anal. Calcd for C11H16N4O2: C, 55.92; H, 6.83; N, 23.71. Found: C, 55.85; H, 6.76; N, 23.74.
2-Cyclopentyl-5,7-dimethyl-2H-pyrazolo[3,4-d]pyrimidine-4,6(5H,7H)-dione (3k): 2k (125 mg, 0.5 mmol), iodobenzene diacetate (644 mg, 2 mmol), and LiH (8.0 mg, 1 mmol) were used. The residue from the extracts was purified by column chromatography on silica gel (hexane/EtOAc, 3 : 1) to give 3k (89.3 mg, 72%). mp 137 °C. 1H NMR (CDCl3) δ 1.74 (2H, m), 1.89 (2H, m), 2.04 (2H, m), 2.18 (2H, m), 3.39 (3H, s), 3.52 (3H, s), 4.64 (1H, m), 7.93 (1H, s). MS (EI) m/z: 248 (M+). Anal. Calcd for C12H16N4O2: C, 58.05; H, 6.50; N, 22.57. Found: C, 58.01; H, 6.46; N, 22.60.
2-Cyclohexyl-5,7-dimethyl-2H-pyrazolo[3,4-d]pyrimidine-4,6(5H,7H)-dione (3l): 2l (132 mg, 0.5 mmol), iodobenzene diacetate (403 g, 1.25 mmol), and LiH (8.0 mg, 2 mmol) were used. The residue from the extracts was purified by column chromatography on silica gel (CHCl3) to give 3l (89.2 mg, 68%). mp 172 °C. 1H NMR (CDCl3) δ 1.29 (1H, m), 1.44 (2H, m), 1.65–1.78 (3H, m), 1.93 (2H, m), 2.20 (2H, m), 3.39 (3H, s), 3.53 (3H, s), 4.09 (1H, m), 7.93 (1H, s). MS (EI) m/z: 262 (M+). Anal. Calcd for C13H18N4O2: C, 59.53; H, 6.92; N, 21.35. Found: C, 59.50; H, 7.01; N, 21.38.
2-Cyclohexylmethyl-5,7-dimethyl-2H-pyrazolo[3,4-d]pyrimidine-4,6(5H,7H)-dione (3m): 2m (139 mg, 0.5 mmol), iodobenzene diacetate (644 mg, 2 mmol), and LiH (8.0 mg, 1 mmol) were used. The residue from the extracts was purified by column chromatography on silica gel (hexane/EtOAc, 3 : 1) to give 3m (92.1 mg, 67%). mp 161–162 °C. 1H NMR (CDCl3) δ 0.97 (2H, m), 1.20–1.26 (4H, m), 1.72–1.75 (4H, m), 1.93 (1H, m), 3.39 (3H, s), 3.53 (3H, s), 3.93 (2H, d, J = 7.0 Hz), 7.84 (1H, s). MS (EI) m/z: 276 (M+). Anal. Calcd for C14H20N4O2: C, 60.85; H, 7.29; N, 20.27. Found: C, 60.86; H, 7.35; N, 20.32.
General procedure for the intramolecular N–O bond formation (5a–5c). To a suspension of 5-acyl-6-amino-1,3-dimethyluracil (4a–4c) (1 mmol) in dry DMF (5 mL) were dropwise added iodobenzene diacetate (805 g, 2.5 mmol) and LiH (15.9 mg, 2 mmol). The mixture was stirred at 80 °C for a given time and concentrated in vacuo. To the residue were added H2O (15 mL) and CHCl3 (15 mL) and the layers were separated. The aqueous layer was extracted with CHCl3 (15 mL × 2). The combined organic layers were successively washed with H2O (20 mL) and brine (20 mL), dried over MgSO4, and concentrated in vacuo. The residue was then purified by column chromatography on silica gel (CHCl3).
3,5,7-Trimethylisoxazolo[3,4-d]pyrimidine-4,6(5H,7H)-dione (5a)18: mp 201 °C. 1H NMR (CDCl3) δ 2.75 (3H, s), 3.37 (3H, s), 3.49 (3H, s). MS (EI) m/z: 195 (M+). HRMS (EI) Calcd for C8H9N3O3 (M+) 195.0644. Found 195.0635.
3-Ethyl-5,7-dimethylisoxazolo[3,4-d]pyrimidine-4,6(5H,7H)-dione (5b)18: mp 91 °C. 1H NMR (CDCl3) δ 1.38 (3H, t, J = 7.8 Hz), 3.13 (2H, q, J = 7.8 Hz), 3.36 (3H, s), 3.48 (3H, s). MS (EI) m/z: 209 (M+). Anal. Calcd for C9H11N3O3: C, 51.67; H, 5.30; N, 20.09. Found: C, 51.55; H, 5.24; N, 19.83.
5,7-Dimethyl-3-propylisoxazolo[3,4-d]pyrimidine-4,6(5H,7H)-dione (5c)18: mp 43 °C. 1H NMR (CDCl3) δ 1.00 (3H, t, J = 7.6 Hz), 1.83 (2H, m), 3.07 (2H, t, J = 7.6 Hz), 3.34 (3H, s), 3.47 (3H, s). MS (EI) m/z: 223 (M+). Anal. Calcd for C10H13N3O3: C, 53.81; H, 5.87; N, 18.82. Found: C, 53.89; H, 5.76; N, 18.57.

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