HETEROCYCLES

Paper | Special issue | Vol. 82, No. 2, 2011, pp. 1435-1445
Received, 16th July, 2010, Accepted, 14th September, 2010, Published online, 15th September, 2010.
DOI: 10.3987/COM-10-S(E)97

■ Transformations of Dimethyl (2E,3E)-2-[(Dimethylamino)methylene]-3-(1-methyl-2,5-dioxoimidazolidin-4-ylidene)succinate with C-Nucleophiles

Uroš Uršič, Jurij Svete, and Branko Stanovnik*

Faculty of Chemistry and Chemical Technology, University of Ljubljana, Askerceva 5, 1000 Ljubljana, Slovenia

Abstract

(2E,3E)-2-[(Dimethylamino)methylene]-3-(1-methyl-2,5-dioxoimidazolidin-4-ylidene)succinate (1) was transformed with 1,3-dicarbonyl compounds 2a,b via substituted dimethyl (2H-imidazol-4-yl)-2-butenedioates 3a,b and dimethyl (2,5-dioxo-4-imidazolidinylidene)succinates 4a,b into 2H-pyrano[2,3-d]pyrimidines 5a,b. Compound 1 was cyclized by heating sin glacial acetic acid into dimethyl 1H-pyrrolo[1,2-c]imidazole-6,7-dicarboxylate (8).

INTRODUCTION
Enaminones, masked α-formyl carbonyl compounds, are versatile intermediates extensively utilized in organic synthesis.1 The 3-(dimethylamino)propenoates and related compounds have been demonstrated to be versatile building blocks in the synthesis of many heterocyclic systems,2 including some natural products, especially indole alkaloids and their analogs.3 Recently, a thermal [4+2] cycloaddition of enaminones to dimethyl acetylenedicarboxylate followed by 1,3-H shift4 and microwave assisted regiospecific [2+2] cycloadditions of electron-poor acetylenes to 2-(acylamino)-3-(dimethylamino)- prop-2-enoates5 and some transformations of the cycloadducts6 including the formation of a new triazafulvalene system7 have been reported. In this paper we describe some transformations of (2E,3E)-2- [(dimethylamino)methylene]-3-(1-methyl-2,5-dioxoimidazolidin-4-ylidene)succinate (1), prepared by [2+2] cycloaddition of (5Z)-5-[(dimethylamino)methylene]-3-methylimidazolidine-2,4-dione and dimethyl acetylenedicarboxylate,5c and N-methylated compound 7 with 1,3-dicarbonyl compounds, such as barbituric acid derivatives 2a,b and 5,5-dimethyl-1,3-cyclohexanedione (9), into pyrano[2,3-d]pyrimidine and benzopyran derivatives, respectively.

RESULTS AND DISCUSSION
In the reaction of succinate 1 with 1,3-dimethylbarbituric acid (2a) and barbituric acid (2b) at room temperature addition reaction took place to give the corresponding dimethyl 2-butenedioates 3a,b. Elimination of dimethylamine from 3a,b was achieved by addition of hydrochloric acid to the water suspension of 3a,b at room temperature to afford the precipitates of dimethyl succinates 4a,b. These compounds, when heated in acetic acid, cyclized into pyrano[2,3-d]pyrimidine derivatives 5a,b. The (Z)-isomer 5a is the major, while the (E)-isomer 5´a is the minor isomer (Scheme 1, Table 1).

5a and 5´a can be distinguished on the basis of chemical shifts for NH protons in 1H NMR spectrum. The NH signal in 5a appears at lower field at δ = 9.51 ppm due to the hydrogen bond formation, while in 5´a the NH signal appears at higher field at δ = 8.26 ppm (Figure 1).

By cyclisation of 4b two products can be formed, either fused 2H-pyranone derivative 5b or oxepino[2,3-d]pyrimidine derivative 6b. Between these two possibilities one can differentiate on the basis of the HMBC spectrum. Since there is no heterocoupling between the proton and the 13C of the carbonyl group attached to the double bond, it means that pyrano[2,3-d]pyrimidine derivative 5b is formed by cyclisation of 4b and not a fused oxepine derivative 6b (Figures 2 and 3).

Methylation of 1 with methyl iodide in DMF at room temperature afforded the N-methylated product 7 in 88% yield. On the other hand, by heating 1 in glacial acetic acid cyclisation occurred to give derivative of 1H-pyrrolo[1,2-c]imidazole-6,7-dicarboxylate 8 in 99% yield (Scheme 2).
By heating compound 7 with 5,5-dimethyl-1,3-cyclohexanedione (9) in acetic acid N,N-dimethylamino group was exchanged with a C-nucleophile followed by cyclisation with the ester group to give a mixture of two isomeric 7,7-dimethyl-2,5-dioxo-5,6,7,8-tetrahydro-2H-chromenes 10 and 10´ (Scheme 3, Table 2).

The structures of isomeric compounds 10 and 10´ were determined by NOESY experiments. The (E)-isomer 10 shows the n.O.e. with the N(3˝)-Me group by irradiation of the proton H(4´) and vice versa the irradiation of the N(3˝)-Me shows the n.O.e. with the H(4´) proton, indicating the proximity of the H(4´) and N(3˝)-Me groups in space, confirming thus the (E)-orientation around the double bond (Figure 4). The observed n.O.e. in (E)-isomer 10 confirms also that in cyclisation the adjacent ester group is involved to give the corresponding 2H-pyranone system.

When compound 7 reacted with 1,3-dimethylbarbituric acid (2a) at room temperature the first addition of 1,3-dimethylbarbituric acid followed by elimination of dimethylamine, is followed by the Michael addition of the second molecule of 1,3-dimethylbarbituric acid to give 11 in 25% yield (Scheme 4).

In summary, the above mentioned experiments can be conveniently employed for the preparation of 2H-pyrano[2,3-d]pyrimidine derivatives and pyrrolo[1,2-c]imidazole-6,7-carboxylate, since the starting materials are readily available by [2+2] cycloaddition of [(dimethylamino)methylene]imidazolidine-2,4- diones to acetylenedicarboxylate.

EXPERIMENTAL
Melting points were taken on a Kofler micro hot stage. The NMR spectra were obtained on a Bruker Advance DPX 300 (300 MHz) spectrometer in DMSO-d6 or CDCl3 with TMS as the internal standard, MS spectra on a Q-Tof Premier spectrometer, IR spectra on a Perkin-Elmer 1310 infrared spectrophotometer and elemental analyses for C, H and N on a Perkin-Elmer CHN Analyser 2400.

Dimethyl 2-[(dimethylamino)(6-hydroxy-1,3-dimethyl-2,4-dioxo-1,2,3,4-tetrahydropyrimidin-5-yl)methyl]-3-(5-hydroxy-1-methyl-2-oxo-2,3-dihydro-1H-imidazol-4-yl)but-2-enedioate (3a)
A mixture of dimethyl (2E,3E)-2-[(dimethylamino)methylene]-3-(1-methyl-2,5-dioxoimidazolidin-4-ylidene)succinate (1; 0.623 g, 2 mmol) and 1,3-dimethylbarbituric acid (2a; 0.370 g, 2.4 mmol) in anhydrous acetic acid (5 mL) was stirred at rt for 48 h. After the reaction was completed volatile components were evaporated in vacuo and the residue was disolved in EtOH (4 mL). The product was left to precipitate for 5 days at 4 °C and then filtered under reduced pressure. Yield: 0.560 g (60%); mp 154–156 °C (toluene/DMF). 1H NMR (DMSO-d6): δ 2.55 (6H, s, NMe2); 2.84 (3H, s, NMe); 3.01 (6H, s, 2×NMe); 3.49 (3H, s, COOMe); 3.57 (3H, s, COOMe); 8.10 (1H, s, CH); 8.14 (2H, br s, NH in OH); 9.93 (1H, br s, OH). Anal. Calcd for C19H25N5O9: C, 48.82; H, 5.39; N, 14.98. Found: C, 48.53; H, 5.65; N, 15.17. νmax (KBr) 3502, 3410, 3265, 2958, 1768, 1722, 1704, 1691, 1626, 1609, 1428, 1393, 1271, 1251, 1235, 1187, 1162, 777 cm-1.

Dimethyl 2-[(dimethylamino)(6-hydroxy-2,4-dioxo-1,2,3,4-tetrahydropyrimidin-5-yl)methyl]-3-(5-hydroxy-1-methyl-2-oxo-2,3-dihydro-1H-imidazol-4-yl)but-2-enedioate (3b)
A mixture of dimethyl (2E,3E)-2-[(dimethylamino)methylene]-3-(1-methyl-2,5-dioxoimidazolidin-4-yl-idene)succinate (1; 0.623 g, 2 mmol) and barbituric acid (2b; 0.256 g, 2 mmol) in anhydrous acetic acid (8 mL) was stirred at rt for 72 h. After the reaction was completed the precipitated product was filtered under reduced pressure. Yield: 0.777 g (88%); mp decomposition above 172 °C. 1H NMR (DMSO-d6): δ 2.55 (6H, s, NMe2); 2.84 (3H, s, NMe); 3.48 (3H, s, COOMe); 3.58 (3H, s, COOMe); 7.99 (1H, s, CH); 8.15 (2H, br s, NH and OH); 9.34 (2H, s, 2×NH); 9.80 (1H, br s, OH). Anal. Calcd for C17H21N5O9: C, 46.47; H, 4.82; N, 15.94. Found: C, 46.17; H, 4.77; N, 16.22. νmax (KBr) 3433, 3178, 1771, 1726, 1688, 1645, 1591, 1452, 1434, 1403, 1339, 1243, 1156, 1064, 779, 531 cm-1.

Synthesis of dimethyl 2-[(6-hydroxy-2,4-dioxo-1,2,3,4-tetrahydropyrimidin-5-yl)methylene]-3-(1-methyl-2,5-dioxoimidazolidin-4-ylidene)succinates. General procedure for the preparation of 4:
To a stirred suspension of dimethyl-2-[(dimethylamino)(6-hydroxy-2,4-dioxo-1,2,3,4-tetrahydropyrimidin-5-yl)methyl]-3-(5-hydroxy-1-methyl-2-oxo-2,3-dihydro-1H-imidazol-4-yl)but-2-endioate 3a,b in water (2 mL) 37% HCl (0.5 mL) was added dropwise. The suspension was stirred at room temperature. After the reaction was completed, the product was filtered under reduced pressure and washed with water.

Dimethyl 2-[(6-hydroxy-1,3-dimethyl-2,4-dioxo-1,2,3,4-tetrahydropyrimidin-5-yl)methylene]-3-(1-methyl-2,5-dioxoimidazolidin-4-yilidene)succinate (4a)
Prepared from dimethyl 2-[(dimethylamino)(6-hydroxy-1,3-dimethyl-2,4-dioxo-1,2,3,4-tetrahydropyrimidin-5-yl)methyl]-3-(5-hydroxy-1-methyl-2-oxo-2,3-dihydro-1H-imidazol-4-yl)but-2-endioate (3a; 0.206 g, 0.44 mmol) and 37% HCl (6 drops) in water (2 mL), 20 minutes. Yield: 0.169 g (91%); mp 116–118 °C (washed with H2O). Ratio of isomers: 90:10. ESI-MS: m/z = 421.1 ((M–H)–). 1H NMR (DMSO-d6): major isomer: δ 2.84 (3H, s, NMe); 3.02 (6H, s, 2×NMe); 3.49 (3H, s, COOMe); 3.57 (3H, s, COOMe); 8.07 (1H, s, CH); 9.95 (1H, s, NH); OH exchanged; minor isomer: δ 2.82 (3H, s, NMe); 3.00 (6H, s, 2×NMe); 3.56 (3H, s, COOMe); 8.16 (1H, s, CH); 8.69 (1H, s, NH). Anal. Calcd for C17H18N4O9: C, 48.34; H, 4.30; N, 13.27. Found: C, 48.18; H, 4.53; N, 13.07. ESI-HRMS: m/z = 421.0981 ((M–H)–); C17H17N4O9 requires: m/z = 421.0996 ((M–H)–). νmax (KBr) 3353, 2955, 1780, 1732, 1703, 1689, 1637, 1606, 1458, 1440, 1395, 1283, 1257, 1246, 1216, 1156, 770, 753 cm-1.

Dimethyl 2-[(6-hydroxy-2,4-dioxo-1,2,3,4-tetrahydropyrimidin-5-yl)methylene]-3-(1-methyl-2,5-dioxoimidazolidin-4-ylidene)succinate (4b)
Prepared from dimethyl 2-[(dimethylamino)(6-hydroxy-2,4-dioxo-1,2,3,4-tetrahydropyrimidin-5-yl)methyl]-3-(5-hydroxy-1-methyl-2-oxo-2,3-dihydro-1H-imidazol-4-yl)but-2-endioate (3b; 0.659 g, 1.5 mmol) and 37% HCl (1 mL) in water (3 mL), 20 minutes. Yield: 0.445 g (75%); mp decomp. above 195 °C (washed with H2O). Ratio of isomers: 92:8. ESI-MS: m/z = 393.1 ((M–H)–). 1H NMR (DMSO-d6): major isomer: δ 2.85 (3H, s, NMe); 3.53 (3H, s, COOMe); 3.60 (3H, s, COOMe); 7.81 (1H, s, CH); 9.94 (2H, br s, 2×NH); 10.02 (1H, s, NH); OH exchanged; minor isomer: δ 2.86 (3H, s, NMe); 3.56 (3H, s, COOMe); 8.21 (1H, s, CH); 9.73 (1H, s, NH); 9.94 (2H, br s, 2×NH). Anal. Calcd for C15H14N4O9: C, 45.69; H, 3.58; N, 14.21. Found: C, 45.42; H, 3.83; N, 13.99. ESI-HRMS: m/z = 393.0670 ((M–H)–); C15H13N4O9 requires: m/z = 393.0683 ((M–H)–). νmax (KBr) 3376, 3280, 3236, 2959, 1781, 1740, 1721, 1703, 1647, 1604, 1438, 1391, 1343, 1260, 1143, 1063, 770 cm-1.

(Z)-Methyl 2-(1,3-dimethyl-2,4,7-trioxo-2,3,4,7-tetrahydro-1H-pyrano[2,3-d]pyrimidin-6-yl)-2-(1-methyl-2,5-dioxoimidazolidin-4-ylidene)acetate (5a) and (E)-isomer (5´a)
A solution of dimethyl 2-[(6-hydroxy-1,3-dimethyl-2,4-dioxo-1,2,3,4-tetrahydropyrimidin-5-yl)methylene]-3-(1-methyl-2,5-dioxoimidazolidin-4-ylidene)succinate (4a; 170 g, 0.4 mmol) in anhydrous acetic acid (3 mL) was refluxed for 8 h. Volatile components were evaporated in vacuo and the residue was purified by column chromatography on silica gel (ethyl acetat/petroleum ether = 2/1). Fractions containing the product were combined and volatile components were evaporated in vacuo. Yield: 0.044 g (28%); mp 234–237 °C (EtOH). Ratio of isomers: 5a:5´a = 95:5. 1H NMR (CDCl3): (Z)-isomer: δ 3.07 (3H, s, NMe); 3.42 (3H, s, NMe); 3.61 (3H, s, NMe); 3.81 (3H, s, COOMe); 7.87 (1H, s, CH); 9.51 (1H, br s, NH); (E)-isomer: δ 3.10 (3H, s, NMe); 3.43 (3H, s, NMe); 3.62 (3H, s NMe); 8.15 (1H, s, CH); 8.26 (1H, br s, NH). Anal. Calcd for C16H14N4O8: C, 49.24; H, 3.62; N, 14.35. Found: C, 49.17; H, 3.64; N, 14.17. νmax (KBr) 3303, 2954, 1787, 1755, 1736, 1720, 1698, 1670, 1660, 1581, 1500, 1444, 1390, 1277, 1244, 1143, 771 cm-1.

(Z)-Methyl 2-(1-methyl-2,5-dioxoimidazolidin-4-ylidene)-2-(2,4,7-trioxo-2,3,4,7-tetrahydro-1H-pyrano[2,3-d]pyrimidin-6-yl)acetate (5b)
A solution of dimethyl 2-[(6-hydroxy-2,4-dioxo-1,2,3,4-tetrahydropyrimidin-5-yl)methylene]-3-(1-methyl-2,5-dioxoimidazolidin-4-ylidene)succinate (4b; 0.197 g, 0.5 mmol) in anhydrous acetic acid (2 mL) was refluxed for 7 h. The precipitated product was filtered under reduced pressure. Yield: 0.127 g (70%); mp decomp. above 193°C (EtOH). ESI-MS: m/z = 361.0 ((M–H)–). 1H NMR (DMSO-d6): δ 2.90 (3H, s, NMe); 3.74 (3H, s, COOMe); 7.80 (1H, s, CH); 10.76 (1H, s, NH); 11.52 (1H, s, NH); 12.92 (1H, br s, NH). 13C NMR (DMSO-d6): δ 24.4, 52.5, 92.0, 104.4, 111.6, 136.7, 142.1, 148.9, 153.9, 157.6, 159.8, 160.4, 162.4, 165.5. Anal. Calcd for C14H10N4O8: C, 46.42; H, 2.78; N, 15.47. Found: C, 46.65; H, 2.89; N, 15.29. ESI-HRMS: m/z = 361.0430 ((M–H)–); C14H9N4O8 requires: m/z = 361.0420 ((M–H)–). νmax (KBr) 3282, 3217, 3053, 1784, 1756, 1727, 1708, 1681, 1645, 1584, 1535, 1458, 1436, 1394, 1360, 1336, 1229, 1144, 815, 785, 772 cm-1.

(2E,3E)-Dimethyl 2-(1,3-dimethyl-2,5-dioxoimidazolidin-4-ylidene)-3-[(dimethylamino)methylene]succinate (7)
Iodomethane (0.124 mL, 2 mmol) was added dropwise to a stirred mixture of dimethyl (2E,3E)-2-[(dimethylamino)methylene]-3-(1-methyl-2,5-dioxoimidazolidin-4-ylidene)succinate (1; 0.311 g, 1 mmol) and K2CO3 (0.276 g, 2 mmol) in DMF (5 mL) at rt. The mixture was stirred for 2.5 h. After the reaction was completed water (1 mL) was added to the reaction mixture and volatile components were evaporated in vacuo. The residue was suspended in water (5 mL) and the product was extracted with CHCl3 (2×5 mL). The organic phase was dried with anhydrous Na2SO4. Na2SO4 was removed by filtration and chloroform was evaporated in vacuo. The residue was purified by column chromatography on silica gel (ethyl acetate/petroleum ether = 2/1). Fractions containing the product were combined and volatile components were evaporated in vacuo. Yield: 0.286 g (88%) of oily product. ESI-MS: m/z = 326.1 (MH+). 1H NMR (CDCl3): δ 3.03 (6H, s, NMe2); 3.08 (3H, s, NMe); 3.12 (3H, s, NMe); 3.69 (3H, s, COOMe); 3.84 (3H, s, COOMe); 7.63 (1H, s, CH). 13C NMR (DMSO-d6): δ 25.0, 28.8, 51.6, 53.0, 88.2, 115.1, 131.1, 152.4, 155.2, 161.7, 168.3, 168.4. Anal. Calcd for C14H19N3O6×½H2O: C, 50.33; H, 6.03; N, 12.57. Found: C, 50.56; H, 5.86; N, 12.69. ESI-HRMS: m/z = 326.1358 (MH+); C14H20N3O6 requires: m/z = 326.1352 (MH+). νmax (KBr) 2995, 2951, 1771, 1719, 1686, 1603, 1455, 1430, 1392, 1286, 1218, 1123, 1095, 1043, 1009, 784, 767 cm-1.

Dimethyl 2-methyl-1,3-dioxo-2,3-dihydro-1H-pyrrolo[1,2-c]imidazole-6,7-dicarboxylate (8)
A solution of dimethyl (2E,3E)-2-[(dimethylamino)methylene]-3-(1-methyl-2,5-dioxoimidazolidin-4-ylidene)succinate (1; 0.104 g, 0.33 mmol) in anhydrous acetic acid (1.5 mL) was heated to 100 °C for 45 minutes. Volatile components were evaporated in vacuo and the residue was purified by column chromatography on silica gel (ethyl acetate/petroleum ether = 1/2). Fractions containing the product were combined and and volatile components were evaporated in vacuo. Yield: 0.088 g (99%); mp 143–144 °C (EtOH). 1H NMR (CDCl3): δ 3.19 (3H, s, NMe); 3.87 (3H, s, COOMe); 3.96 (3H, s, COOMe); 7.76 (1H, s, CH). Anal. Calcd for C11H10N2O6: C, 49.63; H, 3.79; N, 10.52. Found: C, 49.89; H, 3.81; N, 10.53. νmax (KBr) 3122, 2958, 1809, 1751, 1742, 1720, 1509, 1435, 1382, 1361, 1298, 1227, 1206, 1184, 1074, 768, 744 cm-1.

Preparation of compounds 10 and 10': A mixture of (2E,3E)-dimethyl 2-(1,3-dimethyl-2,5- dioxoimidazolidin-4-ylidene)-3-[(dimethylamino)methylene]succinate (7; 0.325 g, 1 mmol) and 5,5-dimethylcyclohexane-1,3-dione (9; 0.168 g, 1.2 mmol) in anhydrous acetic acid (3 mL) was heated to 100 °C for 7 h. Volatile components were evaporated in vacuo and the residue was purified by column chromatography on silica gel (ethyl acetate/petroleum ether = 2/3 and ethyl acetate). Fractions containing the product were combined and and volatile components were evaporated in vacuo.

(E)-Methyl 2-(7,7-dimethyl-2,5-dioxo-5,6,7,8-tetrahydro-2H-chromen-3-yl)-2-(1,3-dimethyl-2,5-dioxoimidazolidin-4-ylidene)acetate (10)
Yield: 0.163 g (42%); mp 208–210 °C (EtOH). 1H NMR (CDCl3): δ 1.18 (6H, s, C(CH3)2); 2.46 (2H, s, CH2); 2.79 (2H, s, CH2); 2.97 (3H, s, N(3')-Me); 3.12 (3H, s, N(1')-Me); 3.87 (3H, s, COOMe); 7.75 (1H, s, CH). Anal. Calcd for C19H20N2O7: C, 58.72; H, 5.19; N, 7.21. Found: C, 58.91; H, 5.02; N, 7.25. νmax (KBr) 2958, 1770, 1746, 1714, 1687, 1622, 1573, 1472, 1431, 1397, 1283, 1200, 1127, 1019, 748 cm-1.

(Z)-Methyl 2-(7,7-dimethyl-2,5-dioxo-5,6,7,8-tetrahydro-2H-chromen-3-yl)-2-(1,3-dimethyl-2,5-dioxoimidazolidin-4-ylidene)acetate (10´)
Yield: 0.121 g (31 %); mp 169–172 °C (EtOH). 1H NMR (CDCl3): δ 1.17 (6H, s, C(CH3)2); 2.44 (2H, s, CH2); 2.77 (2H, s, CH2); 3.06 (3H, s, N(3')-Me); 3.22 (3H, s, N(1')-Me); 3.82 (3H, s, COOMe); 7.76 (1H, s, CH). Anal. Calcd for C19H20N2O7: C, 58.72; H, 5.19; N, 7.21. Found: C, 58.65; H, 5.07; N, 7.14. νmax (KBr) 3054, 2962, 1785, 1739, 1716, 1677, 1621, 1562, 1461, 1443, 1395, 1276, 1208, 1111, 1062, 749 cm-1.

Dimethyl 2-[bis(1,3-dimethyl-2,4,6-trioxohexahydropyrimidin-5-yl)methyl]-3-(1,3-dimethyl-2,5-dioxoimidazolidin-4-ylidene)succinate (11)
A mixture of (2E,3E)-dimethyl 2-(1,3-dimethyl-2,5-dioxoimidazolidin-4-ylidene)-3-[(dimethylamino)methylene]succinate (7; 0.325 g, 1 mmol) and 1,3-dimethylbarbituric acid (2a; 0.187 g, 1.2 mmol) in anhydrous acetic acid (2 mL) was stirred at rt for 4 days. The precipitated product was filtered under reduced pressure. Yield: 0.150 g (25%); mp 254–264 °C (decomp). ESI-MS: m/z = 593.1 (MH+). 1H NMR (DMSO-d6): δ 2.53 (3H, s, NMe); 2.78 (3H, s, NMe); 2.93 (3H, s, NMe); 3.05 (3H, s, NMe); 3.13 (3H, s, NMe); 3.31 (3H, s, NMe); 3.59 (3H, s, COOMe); 3.61 (3H, s, COOMe); 3.73 (1H, d, J = 12.3 Hz, CH); 3.93 (1H, dd, J = 7.0, 11.6 Hz, CH); 4.19 (1H, d, J = 11.6 Hz, CH); 4.20 (1H, dd, J = 6.9, 12.3 Hz, CH). Anal. Calcd for C24H28N6O12. Found: C, 48.82; H, 4.66; N, 14.29, C, 48.65; H, 4.76; N, 14.18.) νmax (KBr) 2957, 1790, 1743, 1734, 1696, 1684, 1671, 1455, 1428, 1382, 1339, 1289, 1224, 1181, 755 cm-1.

ACKNOWLEDGEMENTS
The financial support from the Slovenian Research Agency, Slovenia through grants P0-0502-0103, P1-0179, and J1-6689-0103 is gratefully acknowledged. Financial support by the pharmaceutical companies LEK-SANDOZ, Ljubljana, and KRKA, Novo mesto, is fully appreciated.

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