HETEROCYCLES

Paper | Special issue | Vol. 84, No. 1, 2012, pp. 449-459
Received, 4th February, 2011, Accepted, 7th March, 2011, Published online, 16th March, 2011.
DOI: 10.3987/COM-11-S(P)4

■ Enamino Esters in the Synthesis of Heterocyclic Systems. Transformation of Dimethyl Acetone-1,3-dicarboxylate into Polysubstituted 1,6-Naphthyridine-8-carboxylates

Jože Šporar, Jure Bezenšek, Uroš Uršič, Amalija Golobič, Jurij Svete, and Branko Stanovnik*

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

Abstract

A simple two-step synthesis of 1,6-naphthyridine derivatives is described. Methyl 2-(2-methoxy-2-oxoethyl)-6-oxo-1,6-dihydropyridine-3-carboxylate (3), prepared from dimethyl acetone-1,3-dicarboxylate (1) and propyolic acid amide (2) according to the known procedure, was transformed with N,N-dimethylformamide dimethyl acetal (DMFDMA) first into 2-[1-(dimethylamino)-3-methoxy-3-oxoprop-1-en-2-yl]-6-oxo-1,6-dihydropyridine-3-dicarboxylate (4). This was followed by reaction with monosubstituted hydrazines 5a-h to afford substituted 1,6-naphthyridine-8-carboxylates 7a-h.

INTRODUCTION
In the context of our interest in the development of new useful strategies for the synthesis of heterocyclic systems, we report that 1,6-naphthyridine derivatives can be efficiently synthesiszed from dimethyl acetone-1,3-dicarboxylates. As an extension of our research in the field of enaminones as building blocks for the preparation of various heterocyclic systems and some natural products we have demonstrated the usefulness of dialkyl acetone-1,3-dicarboxylates and their mono- and bis(dimethylamino)methylidene derivatives as versatile building blocks in the construction of many five- and six-membered heterocyclic systems, their fused analogues, such as polysubstituted pyridines, pyrimidines, pyrazolopyridines, pyridopyrimidines, pyranopyrane-diones, pyrazolodiazepines, and others.1 Recently, we have prepared also derivatives of pyrido[4,3-d]pyrimidine-8-carboxylates,2 (pyrido[1,2-a]-pyrimidin-3-yl)thiazole-5-carboxylates,3 thiazolo[5,4-c]pyridine-7-carboxylates,4 and 2,7-naphthyridine-4-carboxylates.5
In this paper we describe the synthesis of methyl 6-substituted amino)-2,5-dioxo-1,2,5,6-tetrahydro-1,6-naphthyridine-8-carboxylates. The chemistry of naphthyridiens has been reviewed several times.6 1,6-Naphthyridines have been prepared also by palladium(0)/copper(I)-catalyzed coupling7 and by some other efficient syntheses published recently.8 Various naphthyridine derivatives have received considerable attention over the past years because of their wide range of biological activities, including antitumor, antiinflamatory and antifungal properties.9

RESULTS AND DISCUSSION
Methyl 2-(2-methoxy-2-oxoethyl)-6-oxo-1,6-dihydropyridine-3-carboxylate (3), prepared from dimethyl acetone-1,3-dicarboxylate (1) and propiolamide (2) according to the known procedure,10 was transformed with N,N-dimethylformamide dimethyl acetal (DMFDMA) first into methyl 2-[1-(dimethylamino)-3-methoxy-3-oxoprop-1-en-2-yl]-6-oxo-1,6-dihydropyridine-3-carboxylate (4). By further treatment with monosubstituted aliphatic, aromatic or hetroaromatic hydrazines 5a-h substitution of the dimethylamino group gave enehydrazine intermediates 6a-h, which could give by cyclization either substituted 1,2,5,6-tetrahydro-1,6-naphthyridine-8-carboxylates 7a-h, (1H-pyrazol-4-yl)-1,6-dihydro-pyridine-3-carboxylates (8a-h) or pyrido[3,2-d]-[1,2]diazepine-9-carboxylates 9a-h (Scheme 1).
The structures of the products were determined on the basis of elemental analyses for C, H, and N, and IR, 1H, 13C NMR, MS, and HRMS spectra. In the reaction of compound 4 with monosubstittued hydrazines 5a-h the enehydrazino intermediates 6a-h were not isolated, since the cyclization took place under relatively mild conditions. By cyclization three types of products could be formed: 1,6-naphthyridines 7a-h from (E)-6a´, pyrazolylpyridine derivatives 8a-h from (Z)-6a, and/or pyrido[3,2-d][1,2]diazepin derivatives 9a-h from (E)-6a´ (Scheme 2). First, the comparison of the 1H NMR spectral characteristics was taken into account. Appart from the signals for R groups, three signals are significant for all compounds: two doublets in the range of δ = 6.49 – 6.62 ppm and δ = 7.99 – 8.11 ppm with a coupling constant J = 9.5 – 9.7 Hz for two ortho protons of the pyridine part of the molecule, and a singlet at δ = 8.40 – 8.60 ppm, which could correspond to H7 of compounds 7a-h, H5´ of compounds 8a-h, or H8 of compounds 9a-h. Compound derived from ethyl hydrazinoacetate (5f) exhibits besides these signals also a doublet integrating for two protons at δ = 3.90 ppm with a coupling constant J = 4.1 Hz, and a triplet integrating for one proton at δ = 7.21 ppm with a coupling constant J = 4.1 Hz. This corresponds to the -NHCH2- structural element, which is compatible only with the 1,6-naphthyridines 7f. Similarly, compound derived from cyclohexylhydrazine (5g) exhibits a doublet at δ = 5.92 ppm, with a coupling constant J = 3.6 Hz. This is compatible only with 1,6-naphthyridine derivative 6g. Furthermore, the structures of compounds 3 (Figure 1) and 7d (Figure 2) were confirmed by X-ray analysis.

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.
Methyl 2-(2-methoxy-2-oxyethyl)-6-oxo-1,6-dihydropyridine-3-carboxylate (3)
This compound was prepared from dimethyl acetone-1,3-dicarboxylate (1; 17.42 g, 0.1 mol) and propynamide (2, 9.97 g, 0.115 mol) according to the literature procedure.10 Yield: 7.32 g (33%) of white solid; mp 150-152 °C, (lit.,10 mp 145-146 ˚C). 1H NMR (300 MHz, DMSO-d6): δ 3.84 (3H, s, OMe); 3.72 (3H, s, OMe); 3.99 (2H, s, CH2); 6.91 (1H, d, J = 9.7 Hz, H4); 7.84 (1H, d, J = 9.7 Hz, H5); 11.5 (1H, s, NH); 13C NMR (75.5 MHz, DMSO-d6): δ 168.8, 164.6, 162.4, 149.0, 140.7, 117.7, 107.2, 107.0, 52.0, 51.7. IR (KBr): νmax (cm-1) 3154, 3004, 2954, 2789, 1748, 1718, 1654, 1612, 1560, 1481, 1458, 1435, 1418, 1372, 1335, 1285, 1211, 1169, 1132, 1092, 1007, 971, 844, 766, 728, 638. Anal. Calcd for C10H11NO5: C, 53.33; H, 4.92; N, 6.22. Found: C, 53.51; H, 4.73; N, 6.03. ESI-HMRS: m/z = 225.0633 (M+); C10H11NO5 requires: m/z = 225.0637 (M+).
Methyl 2-[1-(dimethylamino)-3-methoxy-3-oxoprop-1-en-2-yl]-6-oxo-1,6-dihydropyridine-3- carboxylate (4)
To a solution of methyl 2-(2-methoxy-2-oxyethyl)-6-oxo-1,6-dihydropyridine-3-carboxylate (3; 135 mg, 0.6 mmol) in CH2Cl2 (2 mL), N,N-dimethylformamide dimethyl acetal (DMFDMA) (79 mg, 0.7 mmol) was added and the mixture was stirred at rt for 36 h. The volatile components were evaporated in vacuo and the crude solid residue was recrystallized from EtOAc. Yield: 127 mg (76%), mp 242-244 °C (white solid from EtOAc). 1H NMR (300 MHz, DMSO-d6): δ 2.78 (1H, s, NMe2); 3.48 (1H, s, OMe); 3.65 (1H, s, OMe); 6.23 (1H, d, J = 9.6 Hz, H4); 7.42 (1H , s, C=CH-NMe2); 7.79 (1H, d, J = 9.6 Hz, H5); 11.76 (1H, s, NH). 13C NMR (75.5 MHz, DMSO-d6): δ 166.9, 164.6, 162.3, 150.9, 148.9, 140.6, 117.0, 109.7, 90.7, 51.3, 50.6, 41.9. IR (KBr): ν (cm-1) 3118, 2953, 2830, 1720, 1698, 1651, 1607, 1554, 1438, 1401, 1312, 1285, 1265, 1218, 1192, 1135, 1115, 1082, 1041, 963, 902, 853, 817, 759, 586. Anal. Calcd for C13H16N2O5: C, 55.71; H, 5.75; N, 9.99. Found: C, 55.40; H, 5.80; N, 9.73. ESI-HRMS: m/z = 280.1063; C13H16N2O5 (M+) requires: 280.1059.
Methyl 6-(substituted-amino)-2,5-dioxo-1,2,5,6-tetrahydro-1,6-naphthyridine-8-carboxylate 7a-h. General Procedure
To compound 4 (150 mg, 0.535 mmol) dissolved in dry MeOH (2 mL) or DMF (2 mL) a monosubstittued hydrazine hydrochloride 5a-h (1.605 mmol) was added and the mixture was heated in an argon atmosphere at reflux temperature (MeOH) or at 120 °C (DMF) for 5 min. The volatile components were evaporated in vacuo. The crude product was purified by column chromatography (Fluka Silicagel 60, EtOAc/petroleum ether 2:1). The solid residue was, after evaporation of solvents recrystallized from EtOH.
The following compounds were prepared accordingly:
Methyl 6-(phenylamino)-2,5-dioxo-1,2,5,6-tetrahydro-1,6-naphthyridine-8-carboxylate (7a)
This compound was prepared from 4 (150 mg, 0.535 mmol) and phenylhydrazine hydrochloride (5a; 232 mg, 1.605 mmol). Yield: 107 mg (64%), mp 235-236 °C (white crystals from EtOH). 1H NMR (300 MHz, DMSO-d6): δ 3.88 (3H, s, OMe); 6.50 (1H, d, J = 9.7 Hz, H4); 6.69 – 7.21 (5H, m, Ph); 8.00 (1H, d, J = 9.7 Hz, H3); 8.58 (1H, s, H7); 9.50 (1H, s, NH); 11.21 (1H, s, NH). IR (KBr): ν (cm-1) 3303, 3185, 3114, 1695, 1679, 1642, 1545, 1403, 1297, 1270, 1214, 1140, 1115, 799, 755, 694. Anal. Calcd for C16H13N3O4: C, 61.73; H, 4.21; N, 13.50. Found: C, 61.72; H, 3.98; N, 13.44. ESI-HRMS: m/z = 311.0911 (M+); C16H13N2O4 requires: m/z = 311.0906 (M+).
Methyl 6-(4-methoxyphenylamino)-2,5-dioxo-1,2,5,6-tetrahydro-1,6-naphthyridine-8-carboxylate (7b)
This compound was prepared from 4 (150 mg, 0.535 mmol) and 4-methoxyphenylhydrazine hydro- chloride (5b; 280 mg, 1.605 mmol). Yield: 140 mg (75%), mp 182-184 °C (white crystals from EtOH). 1H NMR (300 MHz, DMSO-d6): δ 3.67 (3H, s, OMe); 3.88 (1H, s,OMe); 6.49 (1H, d, J = 9.7 Hz, H4); 6.68 (2H, m, J = 9.0 Hz, Ph); 6.80 (2H, m, J = 9.0 Hz, Ph); 7.99 (1H, d, J = 9.7 Hz, H3); 8.58 (1H, s, H7); 9.21 (s, 1H, NH); 11.28 (1H, s, NH). 13C NMR (75.5 MHz, DMSO-d6): δ 165.2, 160.9, 158.3, 154.6, 154.2, 145.1, 140.6, 138.9, 117.2, 115.1, 114.2, 106.9, 96.9, 55.3, 52.6. IR (KBr): ν (cm-1) 3330, 3222, 1681, 1664, 1637, 1609, 1558, 1510, 1443, 1303, 1236, 1217, 838, 808, 706, 655, 608, 503. Anal. Calcd for C17H15N3O5: C, 59.82; H, 4.43; N, 12.31. Found: C, 59.84; H, 4.24; N, 12.39. ESI-HRMS: m/z = 341.1004 (M+); C17H15N3O5 requires: m/z = 341.1012 (M+).
Methyl 6-(3-chlorophenylamino)-2,5-dioxo-1,2,5,6-tetrahydro-1,6-naphthyridine-8-carboxylate (7c)
This compound was prepared from 4 (150 mg, 0.535 mmol) and 3-chlorophenylhydrazine hydrochloride (5c; 285 mg, 1.605 mmol). Yield: 120 mg (61%), mp 245-246 °C (white crystals from EtOH). 1H NMR (300 MHz, CDCl3): δ 3.96 (3H, s, OMe); 6.62 (1H, d, J = 9.7 Hz, H4); 6.69 – 7.21 (4H, m, arom.); 7.47 (1H, s, NH); 8.09 (1H, d, J = 9.7 Hz, H3); 8.53 (1H, s, H7), 11.52 (1H, s, NH). 13C NMR (75.5 MHz, DMSO-d6): δ 165.1, 160.7, 158.1, 148.6, 148.4, 145.2, 137.7, 133.6, 130.7, 120.4, 119.7, 112.6, 111.7, 106.9, 97.5, 52.5. IR (KBr): ν (cm-1) 3261, 3066, 1685, 1646, 1598, 1555, 1437, 1353, 1299, 1279, 1211, 1144, 1042, 802, 782. Anal. Calcd for C16H12ClN3O4: C, 55.58; H, 3.50; N, 12.15. Found; C, 55.22; H, 3.25; N, 11.95. ESI-HRMS): m/z = 345.0522 (M+); C16H12ClN3O4 requires: m/z = 345.0516 (M+).
Methyl 6-(4-fluorophenylamino)-2,5-dioxo-1,2,5,6-tetrahydro-1,6-naphthyridine-8-carboxylate (7d)
This compound was prepared from 4 (150 mg, 0.535 mmol) and 4-fluorophenylhydrazine hydrochloride (5d; 261 mg, 1.605 mmol). Yield: 60 mg (35%), mp 240-242 °C (white crystals from EtOH). 1H NMR (300 MHz, CDCl3): δ 3.88 (3H, s, OMe); 6.50 (1H, d, J = 9.7 Hz, H4); 6.74 – 7.09 (4H, m, arom.); 8.00 (1H, d, J = 9.7 Hz, H3), 8.60 (1H, s, H7), 9.50 (1H, s, NH), 11.30 (1H, s, NH). IR (KBr): ν (cm-1) 3292, 3201, 3031, 1726, 1689, 1677, 1640, 1545, 1505, 1299, 1209, 1139, 1115, 835, 800. Anal. Calcd for C16H12FN3O4: C, 58.36; H, 3.67; N, 12.76. Found: C, 58.51; H, 3.46; N, 12.87. ESI-HRMS: m/z = 329.0816 (M+); C16H12FN3O4 requires: 329.0812 (M+).
Methyl 6-(tert-butylamino)-2,5-dioxo-1,2,5,6-tetrahydro-1,6-naphthyridine-8-carboxylate (7e)
This compound was prepared from 4 (150 mg, 0.535 mmol) and tert-butylhydrazine hydrochloride (5e 200 mg, 1.605 mmol). Yield: 26 mg (17%), mp 147-151 °C (white crystals from EtOH). 1H NMR (300 MHz, CDCl3): δ 1.19 (9H, s, CMe3); 3.94 (3H, s, OMe); 6,17 (1H, s, NH); 6.57 (1H, dd, J = 9.7, H4); 8.11 (1H, d, J = 9.7 Hz, H3); 8.40 (1H, s, H7); 11.44 (1H, s, NH). IR (KBr): ν (cm-1) 3297, 3265, 2971, 2930, 1696, 1683, 1657, 1557, 1438, 1366, 1344, 1293, 1267, 1242, 1206, 1180, 1038, 811, 800. Anal. Calcd for C14H17N3O4: C, 57.72; H, 5.88; N, 14.42. Found: C, 57.77; H, 5.69; N, 14.25. ESI-HRMS: m/z = 291.1222 (M+); C14H17N3O4 requires: 291,1219 (M+).
Methyl 6-(2-ethoxy-2-oxoethylamino)-2,5-dioxo-1,2,5,6-tetrahydro-1,6-naphthyridine-8-carboxylate (7f)
This compound was prepared from 4 (313 mg, 1.116 mmol) and ethyl hydrazinoacetate hydrochloride (5f; 106 mg, 0.898 mmol). Yield: 134 mg (61%), mp 139-140 °C (white crystals from EtOH). 1H NMR (300 MHz, DMSO-d6): δ 1.18 (3H, t, J = 7.0 Hz, CH2Me); 3.89 (6H, s, 2 x OMe); 3.90 (2H, d, J = 4.1 Hz, NHCH2C=O); 4.09 (2H, q, J = 7.0 Hz, CH2Me); 6.47 (1H, dd, J = 9.7; 1.1 Hz , H4); 7.21 (1H, t, J = 4.0 Hz, N-NHCH2); 8.03 (1H, d, J = 9.7 Hz, H3); 8.69 (1H, s, H7), 11.21 (1H, d, J = 1.1 Hz, NH). 13C NMR (75.5 MHz, DMSO-d6): δ 169.9, 165.1, 160.7, 158.2, 147.6, 144.7, 137.6, 119.4, 106.0, 95.4, 60.4, 52.4, 50.3, 13.8. IR (KBr): ν (cm-1) 3246, 2992, 2959, 2931, 1746, 1676, 1696, 1555, 1497, 1435, 1336, 1296, 1250, 1211, 1031, 798. Anal. Calcd for C14H15N3O6: C, 52.34; H, 4.71; N, 13.08. Found: C, 52.12; H, 4.51. N, 12.92. ESI-HRMS: m/z = 322.0688 (MH+); C14H16N3O6 requires: 322.0961 (MH+).
Methyl 6-(cyclohexylamino)-2,5-dioxo-1,2,5,6-tetrahydro-1,6-naphthyridine-8-carboxylate (7g)
This compound was prepared from 4 (150 mg, 0.535 mmol) and cyclohexylhydrazine hydrochloride (5g, 242 mg, 1.605 mmol). Yield: 27 mg (16%), mp 158-161 °C (white crystals from EtOH). 1H NMR (300 MHz, CDCl3): δ 1.24 – 3.93 (11H, m, cyclohexyl); 3.94 (3H, s, OMe); 5.92 (1H, d, J = 3.6 Hz, N-NH), 6.57 (1H, dd, J = 9.7, 1.7 Hz, H4); 8.11 (1H, d, J = 9.7 Hz, H3); 8.40 (1H, s, H7); 11.45 (1H, d, NH). IR (KBr): ν (cm-1) 3290, 3229, 2925, 2853, 1721, 1689, 1668, 1640, 1540, 1429, 1296, 1259, 1217, 1132, 841, 802, 718. Anal. Calcd for C16H19N3O4: C, 60.56; H, 6.03; N, 13.24. Found: C, 60.91, H, 5.74; N, 12.81. (ESI-HRMS: m/z = 317.1383 (M+); C16H19N3O4 requires: 317.1376 (M+).
Methyl 6-(1-phthalazinylamino)-2,5-dioxo-1,2,5,6-tetrahydro-1,6-naphthyridine-8-carboxylate (7h)
This compound was prepared from 4 (207 mg, 0.74 mmol) and 1-hydrazinophthalazine hydrochloride (5h, 131 mg, 0.666 mmol). Yield: 190 mg (79%), mp 288-289 °C (white crystals from EtOH). 1H NMR (300 MHz, DMSO-d6): δ 3.86 (3H, s, OMe); 6.50 (1H, dd, J = 9.7, 2.0 Hz, H4); ), 8.07 (1H, d, J = 9.7 Hz, H3); 8.48 (1H, s, H7); 7.80 – 8.40 (5H, m, phthalazinyl); 11.30 (1H, d, J = 1.9 Hz, NH), 12.15 (1H, s, NH). IR (KBr): ν (cm-1) 3290, 3254, 3167, 3079, 3006, 1692, 1683, 1658, 1632, 1616, 1598, 1557, 1539, 1481, 1430, 1298, 1271, 1206, 1028, 794, 757, 705, 684, 661. Anal. Calcd for C18H13N5O4: C, 59.50; H, 3.61; N, 19.28. Found: C, 59.47; H, 3.36; N, 19.51. ESI-HRMS: m/z = 363.0971 (M+); C18H13N5O4 requires: m/z = 363.0968 (M+).

X-Ray structure analysis
Diffraction data for compounds 3 and 7d were collected on a Nonius Kappa CCD diffractometer with graphite monochromated MoKα radiation. The data were processed using DENZO11 program. Structures were solved by direct methods using SIR971.12 We employed full-matrix least-squares refinements on F magnitudes with anisotropic displacement factors for all non-hydrogen atoms using Xtal3.6.13 H-atoms parameters were not refined. The resulting crystal data and details concerning data collection and refinement for both compounds are quoted in Table 1. Selected bond lengths and bond angles are listed in Table 2. ORTEP14 drawings of the content of asymmetric units of both compounds showing the atom-labeling scheme are presented in Figures 1 and 2. The final atomic and geometrical parameters, crystal data and details concerning data collection and refinement for both compounds have also been deposited with the Cambridge Crystallographic Data Center as supplementary material with the deposition numbers: CCDC 810326 & 810327, respectively. These data can be obtained, free of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html.

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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