HETEROCYCLES

Note | Special issue | Vol. 82, No. 1, 2010, pp. 895-907
Received, 14th June, 2010, Accepted, 12th July, 2010, Published online, 13th July, 2010.
DOI: 10.3987/COM-10-S(E)56

■ [2+2+2]-Cocyclotrimerization of 6-Alkynyl-7-benzylpurines with α,ω-Diynes

Stanislav Opekar, Pavel Turek, Radek Pohl, Blanka Klepetářová, Ivan Votruba, Michal Hocek,* and Martin Kotora*

Department of Organic and Nuclear Chemistry, Faculty of Science, Charles University, Hlavova 8, 128 43 Praha 2, Czech Republic

Abstract

6-Alkynyl-7-benzylpurines were prepared by the Sonogashira reaction from 6-chloro-7-benzylpurines and terminal alkynes. The prepared alkynylpurines were cyclotrimerized with various 1,6-diynes in the presence of Ni(cod)2/2PPh3 catalytic system into the corresponding 6-aryl-7-benzylpurines under ambient conditions. The prepared 6-alkynyl- and 6-aryl-7-benzylpurines were tested for cytostatic activity.

INTRODUCTION
Purine bases and nucleosides bearing aryl moiety in the position 6 display diverse types of biological activity: some substituted 6-arylpurine bases are antagonists of corticotropin-releasing hormone,1 or adenosine receptors,2 or possess antimycobacterial and antibacterial activity,3 while 6-arylpurine ribonucleosides show significant cytostatics4 and anti-HCV5 effect. Also 6-alkynylpurines are potent cytostatics6 and inhibit 15-lipoxygenase.7 All the above mentioned facts provided necessary impetus for synthesis of various derivatives and study of their properties. In our previous reports we studied catalytic cyclotrimerization of 6-alkynylpurines with α,ω-diynes in the presence of various transition metal complexes (CoBr(PPh3)3), RhCl(PPh3)3), [IrCl(cod)]2, Cp2Ni, NiBr2(PPh3)2/Zn, NiI2(PPh3)2/Zn, NiBr2(dppe)/Zn, Ni(cod)2/PPh3, etc.).8-10 The highest yields of the desired 9-Bn- or 9-THP-protected 6-arylpurines were obtained when the Ni(0)-catalyst was generated “in situ” from NiBr2(dppe)/Zn.8,9
Later we showed further that increase in yields of the 6-arylpurines could be achieved by using the more effective catalytic system based on the use of Ni(cod)2/2PPh3.10 Herein we would like to report on the use of the Ni(cod)2/2PPh3 catalytic system for the synthesis of 6-aryl-7-benzylpurines. In addition to the primary interest in biological properties of 6-aryl-7-benzylpurines, steric effect of benzyl group in the near vicinity of the triple bond on the course of the cyclotrimerization reaction was worthy of exploring.

RESULTS AND DISCUSSION
We envisioned that the same synthetic strategy used for the synthesis of 9-protected 6-arylpurines, i.e. Ni-catalyzed cyclotrimerization, could be also applied for preparation of 7-protected 6-arylpurines. The first step concerned a preparation of protected 7-benzyl-6-chloro-7-H-purine 3. The starting material, commercially available 6-chloro-9-H-purine 1, was dissolved with potassium carbonate in DMF and alkylated with benzyl chloride. Workup of the reaction mixture resulted in the isolation of two compounds: 9-benzyl-6-chloropurine 2 (58%) as the major product and 7-benzyl-6-chloropurine 3 (24%) as the minor product (Scheme 1). Then the Sonogashira reaction under standard reaction conditions of 7-benzyl-6-chloropurine 3 with three different alkynes (trimethylsilylethyne, hex-1-yne, and phenylethyne) gave rise to the alkynyl purines 4a-4c in good yields (83-94%, isolated) (Scheme 2).

Recently, we have found, that 6-arylpurines bearing various groups in the position 9 (benzyl, tetrahydropyranyl, and ribosyl) could be achieved by [2+2+2]-cocyclotrimerization reactions.8-10 These cocyclotrimerizations were carried out in the presence of various catalysis such as CoBr(PPh3)3, RhCl(PPh3)3, [IrCl(cod)]2, Cp2Ni, NiBr2(PPh3)2/Zn, NiI2(PPh3)2/Zn, NiBr2(dppe)/Zn, Ni(cod)2/PPh3, etc in various solvents. The best yields of the corresponding products were obtained in cyclotrimerizations catalyzed by the catalytic system composed of Ni(cod)2/PPh3 in MeCN and 20 °C. Thus we decided to apply the same Ni(0)-based protocol for the cyclotrimerization reactions of 6-alkynyl-7-benzylpurines 4 with α,ω-diynes 5 (Scheme 3). It is necessary to emphasize, that although the combination of Ni(cod)2 with PPh3 give rise to a very reactive catalyst, it is also moisture and air sensitive and requires careful storing and handling.
In the first set of experiments 6-(phenylethynyl)-7-benzylpurine 4a was reacted with various α,ω−diynes 5, concretely diester 5a, diketone 5b, and ketoester 5c for 24 h (Table 1). All reactions proceeded uneventfully and the corresponding 6-aryl-7-benzylpurines 6aa, 6ab, and 6ac were isolated in very good 81, 80, and 62% yields. The structure of 6-aryl-7-benzylpurine derivative 6aa was unequivocally confirmed by a single-crystal X-ray analysis (Figure 1). The second set of reactions was based on the cyclotrimerizations of 6-(hexyn-1-yl)-7-benzylpurine 4b with α,ω−diynes 5a-5c. These reactions resulted in the formation of expected products 6ba, 6bb, and 6bc that were isolated in very good yields of 73, 73, and 84%. Analogically, the cyclotrimerization reactions of 6-(trimethylsilylethynyl)-7-benzylpurine 4c were carried out. Although in all cases were obtained the desired products 6ca-6cc, their yields were considerable lower than in previous cases: 6ca 32%, 6cb 26%, and 6cc 27%. The low yields of the isolated products were caused by two reasons: firstly, by difficult separation of 6ca-6cc from the remaining starting 7-trimethylsilyl-6-alkynylpurine 4c and secondly, the lower reactivity of the TMS-derivative (probably caused by steric hindrace of the the triple bond by the TMS group) resulted also in lower conversions. The comparison of these results with those obtained in cyclotrimerization reactions of 9-benzyl-6-alkynylpurines8,9 clearly shows that the yields were within the region. This indicates that the presence of benzyl group in the near vicinty of the triple bond does not exert any steric effects on its reactivity, i.e. ability to participate in the cyclotrimerization reaction.

Herein some remarks regarding difficulties pertaining to the isolation of products from reaction mixtures should be mentioned. We observed two problems during work-up procedure: 1) the residual starting 6-alkynylpurines had a similar retention factor like the formed 6-arylpurines and 2) the presence of triphenylphosphinoxide formed by the oxidation of the catalyst’s ligand. This made the product separation a rather tedious process and in some cases the products had to be purified several times. Logically, it decreased the final reaction yields.

Preliminary in vitro cytostatic activity tests of the starting 6-alkynyl-7-benzylpurines 4a-4c and some of the products 6 were performed using the following cell cultures: mouse leukemia L1210 cells (ATCC CCL 219); human promyelocytic leukemia HL60 cells (ATCC CCL 240); human cervix carcinoma HeLa S3 cells (ATCC CCL 2.2); and human T lymphoblastoid CCRF-CEM cell line (ATCC CCL 119). The testing showed that the prepared 6-aryl-7-benzylpurines 6 were entirely inactive (IC50 >250 µmol.l-1). On the other hand, the testing of the starting 6-alkynyl-7-benzylpurines 4a-4c revealed them to be active against all cell cultures: the compound 4a (IC50 = 5.07, 9.40, 1.81, and 2.04 µM against L1210, HL60, HeLa S3, and CCRF-CEM cell-lines, respectively), the compound 4b (IC50 = 2.78, 3.10, 3.24, and 2.63 µM against L1210, HL60, HeLa S3, and CCRF-CEM cell-lines, respectively), and the compound 4c (IC50 = 7.52, 11.81, 4.92, and 1.53 µM against L1210, HL60, HeLa S3, and CCRF-CEM cell-lines, respectively).

In conclusion, the cyclotrimerization of 6-alkynyl-7-benzylpurines with various α,ω−diynes can be conveniently carried out in the presence of Ni(cod)2/PPh3 catalytic system with good yields of the corresponding 6-aryl-7-benzylpurines. Although, the cytostatic activity tests showed that the obtained arylpurine derivatives were inactive, the starting alkynylpurines exhibited interesting levels of activity. These results thus might indicate the next direction of further research.

EXPERIMENTAL
All solvents were used as obtained unless otherwise noted. DMF and acetonitrile were used as commercially available dry solvents stored under septum and above sieves. All other reagents were obtained from commercial sources. 1H and 13C NMR spectra were recorded on a Bruker AVANCE 400 (1H at 400 MHz, 13C at 100.6 MHz), a Bruker AVANCE 500 (1H at 500 MHz, 13C at 125.8 MHz) or a Bruker AVANCE 600 (1H at 600 MHz, 13C at 151 MHz) as solutions in CDCl3 with Me4Si as an internal standard. Chemical shifts are given in δ-scale, coupling constants J are given in Hz. Melting points (uncorrected) were determined using a Kofler apparatus. Mass spectra were recorded on a ZAB-SEQ (VG-Analytical) instrument. Infrared spectra were recorded on a Bruker IFS 55 spectrometer as CHCl3 solutions and are reported in wave numbers (cm-1). Fluka 60 silica gel was used for flash chromatography. TLC was performed on silica gel 60 F254-coated aluminum sheets (Merck). All cross-coupling and cyclotrimerization reactions were carried out under an argon atmosphere.

General procedure for cross-coupling reactions of 6-chloropurine with acetylenes. DMF (10 mL), acetylene (1.2 eq., 14.7 mmol) and triethylamine (2 mL) were added to an argon purged mixture of 7-benzyl-6-chloropurine (3 g, 12.3 mmol), CuI (140 mg, 6 mol%, 0.736 mmol), and PdCl2(PPh3)2 (176 mg, 0.245 mmol) and the reaction mixture was stirred at 70 °C for 24 h. The course of the reaction was monitored by TLC. Then volatiles were evaporated under reduced pressure and the residue was chromatographed on silica gel column to give the corresponding products 4a-4c.

6-[(Phenyl)ethynyl]-7-benzyl-7H-purine (4a). Column chromatography on silica gel (EtOAc) afforded 3,318 g (87%) of a reddish brown solid. Recrystallization from CH2Cl2/hexane gave 2.88 g (76%) of yellowish crystals: mp 135–136.5 °C; 1H NMR (600 MHz, CDCl3) δ 5.83 (s, 2H, CH2Ph), 7.24 (m, 2H, H-o-Bn), 7.34–7.40 (m, 5H, H-m -Ph a H-m,p-Bn), 7.44 (m, 1H, H-p-Ph), 7.45 (m, 2H, H-o-Ph), 8.24 (s, 1H, H-8), 9.14 (s, 1H, H-2); 13C NMR (151 MHz, CDCl3) δ 49.88 (CH2Ph), 83.75 (Pur-C≡C-Ph), 98.09 (Ph-C≡C-Pur), 120.59 (C-i-Ph), 124.73 (C-5), 127.01 (CH-o-Bn), 128.65 (CH-m-Ph), 128.77 (CH-p-Bn), 129.35 (CH-m-Bn), 130.28 (CH-p-Ph), 132.02 (CH-o-Ph), 134.31 (C-6), 135.01 (C-i-Bn), 148.74 (CH-8), 153.46 (CH-2), 161.17 (C-4); IR (CHCl3) ν 3000, 2212, 1600, 1589, 1556, 1486, 1444, 1396, 1375 cm-1; MS (FAB, m/z (rel. %)) 311 (M+ + H, 100), 221 (11), 91 (36); HR-MS (FAB) calcd for C20H15N4 [M++H] 311.1296, found 311.1300. Anal. Calcd for C20H14N4: C 77.40, H 4.55, N 18.05. Found: C 77.34, H 4.39, N 17.79. Rf(5/1 EtOAc/MeOH) =0.66.

6-(Hex-1-yn-1-yl)-7-benzyl-7H-purine (4b). Column chromatography on silica gel (EtOAc) afforded 3,337 g (94%) of a deep red oil. Crystallization from CH2Cl2/hexane gave 3.07 g (86%) of brownish crystals: mp 91–92 °C; 1H NMR (500 MHz, CDCl3) δ 0.88 (t, Jvic = 7.3 Hz, 3H, CH3CH2CH2CH2-), 1.40 (m, 2H, CH3CH2CH2CH2-), 1.53 (m, 2H, CH3CH2CH2CH2-), 2.47 (t, Jvic = 7.1 Hz, 2H, CH3CH2CH2CH2-), 5.76 (s, 2H, CH2Ph), 7.18 (m, 2H, H-o-Ph), 7.34–7.41 (m, 3H, H-m,p-Ph), 8.21 (s, 1H, H-8), 9.07 (s, 1H, H-2); 13C NMR (125.8 MHz, CDCl3) δ 13.50 (CH3CH2CH2CH2-), 19.35 (CH3CH2CH2CH2-), 22.04 (CH3CH2CH2CH2-), 29.74 (CH3CH2CH2CH2-), 49.68 (CH2Ph), 75.86 (Pur-C≡C-Bu), 101.32 (Bu-C≡C-Pur), 124.65 (C-5), 126.79 (CH-o-Ph), 128.60 (CH-p-Ph), 129.18 (CH-m-Ph), 134.86 (C-6), 135.17 (C-i-Ph), 148.50 (CH-8), 153.31 (CH-2), 160.87 (C-4); IR (CHCl3) ν 2999, 2234, 1593, 1558, 1490, 1448, 1396, 1376 cm-1; MS (FAB, m/z (rel. %)) 291 (M+ + H, 100), 91 (56); HR-MS (FAB) calcd for C18H19N4 [M++H] 291.1610, found 291.1616. Rf(5/1 EtOAc/MeOH) = 0.60.

6-[(Trimethylsilyl)ethynyl]-7-benzyl-7H-purine (4c). Column chromatography on silica gel (hexane/EtOAc 1/3) afforded 3.14 g (83%) of a deep brown solid. Recrystallization from CH2Cl2/heptane gave 2.49 g (66%) of brownish crystals: mp 119–121 °C; 1H NMR (500 MHz, CDCl3) δ 0.23 (s, 9H, CH3Si), 5.77 (s, 2H, CH2Ph), 7.21 (m, 2H, H-o-Ph), 7.34–7.41 (m, 3H, H-m,p-Ph), 8.22 (s, 1H, H-8), 9.10 (s, 1H, H-2); 13C NMR (125.8 MHz, CDCl3) δ 0.80 (CH3Si), 49.57 (CH2Ph), 98.36 (Pur-C≡C-SiMe3), 105.93 (Me3Si-C≡C-Pur), 124.77 (C-5), 127.01 (CH-o-Ph), 128.72 (CH-p-Ph), 129.28 (CH-m-Ph), 133.72 (C-6), 135.10 (C-i-Ph), 148.84 (CH-8), 153.33 (CH-2), 161.20 (C-4); IR (CHCl3) ν 3001, 1591, 1558, 1397, 1375 cm-1; MS (FAB, m/z (rel. %)) 307 (M+ + H, 100), 91 (52); HR-MS (FAB) calcd for C17H19N4Si [M++H] 307.1379, found 307.1390. Anal. Calcd for C17H18N4Si: C 66.63, H 5.92, N 18.28. Found: C 66.55, H 5.80, N 18.21. Rf(5/1 EtOAc/MeOH) = 0.65.

General procedure for preparation of 6-aryl-7-benzylpurines 6xy by cyclotrimerization reactions. Ni(cod)2 (0.022 g, 0.08 mmol) was placed into the Schlenk flask with a mixture of 6-alkynyl-7-benzylpurine (0.4 mmol), diyne (0.44 mmol), and PPh3 (0.052 g, 0.2 mmol) in a glove-box and the final mixture of components was dissolved in MeCN (8 mL). The reaction mixture was stirred at 20 °C for 24 h. The course of the reaction was monitored by TLC. Then volatiles were evaporated under reduced pressure and the residue was chromatographed on silica gel or aluminium oxide column to give the corresponding products 6xy.

6-[6-Phenyl-2,2-di(ethoxycarbonyl)indan-5-yl]-7-benzyl-7H-purine (6aa). Column chromatography on aluminum oxide (toluene, 10/1 to 9/1 toluene/acetone) yielded 177 mg (81%) of a white solid: mp 160–162.5 °C; 1H NMR (600 MHz, CDCl3) δ 1.28, 1.32 (2 × t, Jvic = 7.1 Hz, 2 × 3H, CH3CH2), 3.52, 3.61 (2 × dd, Jgem = 16.7 Hz, J3´,4´ = 0.6 Hz, 2H, H-3´), 3.69, 3.76 (2 × dd, Jgem = 17.5 Hz, J1´,7´ = 0.6 Hz, 2H, H-1´), 4.23, 4.24 (2 × dq, Jgem = 12.7 Hz, Jvic = 7.1 Hz, 2H, CH2CH3), 4.29 (q, Jvic = 7.1 Hz, 2H, CH2CH3), 4.83, 4.97 (2 × d, Jgem = 15.4 Hz, 2H, CH2Ph), 6.47 (m, 2H, H-o-Bn), 6.95 (m, 2H, H-o-Ph), 6.99 (q, J4´,3´ = J4´,7´ = 0.6 Hz, 1H, H-4´), 7.07 (m, 2H, H-m-Ph), 7.10 (m, 1H, H-p-Ph), 7.14 (m, 2H, H-m-Bn), 7.20 (m, 1H, H-p-Bn), 7.37 (q, J7´,1´ = J7´,4´ = 0.6 Hz, 1H, H-7´), 8.00 (s, 1H, H-8), 9.12 (s, 1H, H-2); 13C NMR (151 MHz, CDCl3) δ 14.02, 14.07 (CH3CH2) 40.12 (CH2-3´), 40.44 (CH2-1´), 50.36 (CH2-Ph), 60.30 (C-2´), 61.91, 61.95 (CH2CH3), 123.03 (C-5), 125.70 (CH-7´), 125.96 (CH-4´), 126.54 (CH-o-Bn), 127.30 (CH-p-Ph), 128.32 (CH-m-Ph, CH-p-Bn), 128.74 (CH-m-Bn), 128.94 (CH-o-Ph), 133.42 (C-5´), 134.47 (C-i-Bn), 139.28 (C-i-Ph), 139.71 (C-3´a), 140.07 (C-6´), 142.59 (C-7´a), 148.67 (CH-8), 152.91 (CH-2), 153.08 (C-6), 161.32 (C-4), 171.09, 171.64 (CO); IR (CHCl3) ν 2986, 1730, 1592, 1497, 1456, 1446, 1369, 1265, 1248, 1191 cm-1; MS (FAB, m/z (rel. %)) 547 (M+ + H, 100), 473 (8), 417 (7), 398 (10), 91 (30); HR-MS (FAB) calcd for C33H31N4O4 [M+ + H] 547.2345, found 547.2355. Rf(5/1 EtOAc/MeOH) = 0.68.

6-[6-Phenyl-2,2-diacetylindan-5-yl]-7-benzyl-7H-purine (6ab). Column chromatography on silica gel (10/1, 8/1 to 6/1 Et2O/acetone) yielded 155 mg (80%) of a white foam: 1H NMR (500 MHz, CDCl3) δ 2.20, 2.25 (2 × s, 2 × 3H, CH3CO), 3.42, 3.48 (2 × dd, Jgem = 16.7 Hz, J3´,4´ = 0.8 Hz, 2H, H-3´), 3.61, 3.67 (2 × dd, Jgem = 16.9 Hz, J1´,7´ = 0.8 Hz, 2H, H-1´), 4.80, 4.95 (2 × d, Jgem = 15.5 Hz, 2H, CH2Ph), 6.45 (m, 2H, H-o-Bn), 6.96-7.00 (m, 3H, H-4´, H-o-Ph), 7.06–7.15 (m, 5H, H-m,p-Ph, H-m-Bn), 7.20 (m, 1H, H-p-Bn), 7.37 (td, J7´,1´ = 0.8 Hz, J7´,4´ = 0.4 Hz, 1H, H-7´), 7.98 (s, 1H, H-8), 9.13 (s, 1H, H-2); 13C NMR (125.8 MHz, CDCl3) δ 26.43, 26.64 (CH3CO), 37.07 (CH2-3´), 37.35 (CH2-1´), 50.24 (CH2-Ph), 74.81 (C-2´), 123.00 (C-5), 125.85 (CH-7´), 126.15 (CH-4´), 126.37 (CH-o-Bn), 127.43 (CH-p-Ph), 128.29 (CH-p-Bn), 128.39 (CH-m-Ph), 128.74 (CH-m-Bn), 128.92 (CH-o-Ph), 133.62 (C-5´), 134.49 (C-i-Bn), 139.13 (C-i-Ph), 139.29 (C-3´a), 140.20 (C-6´), 142.25 (C-7´a), 148.71 (CH-8), 152.90 (C-6), 152.92 (CH-2), 161.30 (C-4), 203.89, 204.17 (CO); IR (CHCl3) ν 3006, 1702, 1592, 1491, 1370, 1358, 1262 cm-1; MS (FAB, m/z (rel. %)) 487 (M+ + H, 18), 147 (25), 91 (18), 73 (100); HR-MS (ESI) calcd for C31H27N4O2 [M+ + H] 487.2134, found 487.2129. Rf(5/1 EtOAc/MeOH) = 0.63.

6-[6-Phenyl-2-acetyl-2-(ethoxycarbonyl)indan-5-yl]-7-benzyl-7H-purine (6ac). Column chromatography on aluminum oxide (toluene, 10/1 to 8/1 toluene/acetone) followed by column chromatography on silica gel (10/1, 8/1 to 6/1 Et2O/acetone) yielded 128 mg (62%) of a yellowish foam: 1H NMR (500 MHz, CDCl3) δ 1.29, 1.33 (2 × t, Jvic = 7.1 Hz, 2 × 3H, CH3CH2), 2.26, 2.31 (2 × s, 2 × 3H, CH3CO), 3.40, 3.43, 3.52, 3.55 (4 × dd, Jgem = 16.8 Hz, J3´,4´ = 0.7 Hz, 4 × 1H, H-3´), 3.63, 3.64, 3.67, 3.69 (4 × dd, Jgem = 16.9 Hz, J1´,7´ = 0.7 Hz, 4 × 1H, H-1´), 4.25, 4.30 (2 × q, Jvic = 7.1 Hz, 2 × 2H, CH2CH3), 4.80, 4.82, 4.95, 4.96 (4 × d, Jgem = 15.5 Hz, 4 × 1H, CH2Ph), 6.45, 6.47 (2 × m, 2 × 2H, H-o-Bn), 6.94–7.00 (m, 6H, H-4´, H-o-Ph), 7.05–7.14 (m, 10H, H-m,p-Ph, H-m-Bn), 7.20, 7.21 (2 × m, 2 × 1H, H-p-Bn), 7.36, 7.37 (2 × q, J7´,1´ = J7´,4´ = 0.7 Hz, 2 × 1H, H-7´), 7.99 (s, 2H, H-8), 9.116, 9.121 (s, 1H, H-2); 13C NMR (125.8 MHz, CDCl3) δ 14.01, 14.08 (CH3CH2), 26.07, 26.20 (CH3CO), 38.57, 38.67 (CH2-3´), 38.87, 38.89 (CH2-1´), 50.29, 50.35 (CH2-Ph), 62.03, 62.12 (CH2CH3), 66.67, 66.80 (C-2´), 123.03 (C-5), 125.73, 125.79 (CH-7´), 125.99, 126.06 (CH-4´), 126.44, 126.54 (CH-o-Bn), 127.34 (CH-p-Ph), 128.29, 128.34 (CH-m-Ph, CH-p-Bn), 128.74, 128.76 (CH-m-Bn), 128.93 (CH-o-Ph), 133.46, 133.53 (C-5´); 134.48 (C-i-Bn), 139.22, 139.25 (C-i-Ph), 139.45, 139.51 (C-3´a), 140.09, 140.18 (C-6´), 142.44, 142.49 (C-7´a), 148.66, 148.70 (CH-8), 152.90 (CH-2), 153.01, 153.04 (C-6), 161.30 (C-4), 171.95, 172.27 (COOEt), 201.89, 202.20 (CO); IR (CHCl3) ν 3032, 2987, 1714, 1592, 1560, 1491, 1442, 1370, 1240, 1172 cm-1; MS (FAB, m/z (rel. %)) 517 (M++H, 30), 91 (55), 57 (100); HR-MS (ESI) calcd for C32H29N4O3 [M++H] 517.2240, found 517.2249. Rf(5/1 EtOAc/MeOH) = 0.67.

6-[6-Butyl-2,2-di(ethoxycarbonyl)indan-5-yl]-7-benzyl-7H-purine (6ba). Column chromatography on aluminum oxide (toluene, 10/1 to 8/1 toluene/acetone) yielded 154 mg (73%) of a yellowish oil. Crystallization from CH2Cl2/heptane gave white crystals: mp 80–82 °C; 1H NMR (400 MHz, CDCl3) δ 0.68 (t, Jvic = 7.8 Hz, 3H, CH3CH2CH2CH2), 1.05 (m, 2H, CH3CH2CH2CH2), 1.16, 1.26 (2 × m, 2H, CH3CH2CH2CH2), 1.27, 1.30 (2 × t, Jvic = 7.1 Hz, 2 × 3H, CH3CH2), 2.11 (m, 2H, CH3CH2CH2CH2), 3.48 (s, 2H, H-3´), 3.59, 3.69 (2 × d, Jgem = 17.1 Hz, 2H, H-1´), 4.21, 4.23 (2 × dq, Jgem = 12.6 Hz, Jvic = 7.1 Hz, 2H, CH2CH3), 4.26 (q, Jvic = 7.1 Hz, 2H, CH2CH3), 4.94, 5.03 (2 × d, Jgem = 15.7 Hz, 2H, CH2Ph), 6.49 (m, 2H, H-o-Bn), 6.82 (s, 1H, H-4´), 7.10 (s, 1H, H-7´), 7.15 (m, 2H, H-m-Bn), 7.22 (m, 1H, H-p-Bn), 8.27 (s, 1H, H-8); 9.15 (s, 1H, H-2); 13C NMR (100.6 MHz, CDCl3) δ 13.66 (CH3CH2CH2CH2), 14.01, 14.07 (CH3CH2), 22.35 (CH3CH2CH2CH2), 32.54, 32.84 (CH3CH2CH2CH2), 40.05 (CH2-3´), 40.47 (CH2-1´), 50.60 (CH2-Ph), 60.41 (C-2´), 61.80, 61.83 (CH2CH3), 123.29 (C-5), 124.74 (CH-4´), 125.04 (CH-7´), 126.23 (CH-o-Bn), 128.25 (CH-p-Bn), 128.74 (CH-m-Bn), 133.41 (C-5´), 134.60 (C-i-Bn), 137.56 (C-3´a), 140.17 (C-6´), 142.03 (C-7´a), 149.07 (CH-8), 153.01 (CH-2), 153.06 (C-6), 161.52 (C-4), 171.17, 171.79 (CO); IR (CHCl3) ν 2985, 1729, 1591, 1456, 1369, 1250, 1191 cm-1; MS (FAB, m/z (rel. %)) 527 (M+ + H, 100), 453 (7), 437 (10), 91 (78); HR-MS (FAB) calcd for C31H35N4O4 [M++H] 527.2658, found 527.2669. Anal. Calcd for C31H34N4O4: C 70.70, H 6.51, N 10.64. Found: C 70.71, H 6.56, N 10.67. Rf(5/1 EtOAc/MeOH) = 0.80.

6-[6-Butyl-2,2-diacetylindan-5-yl]-7-benzyl-7H-purine (6bb). Column chromatography on silica gel (10/1 to 7/1 Et2O/acetone) yielded 137 mg (73%) of a yellowish oil. Crystallization from CH2Cl2/heptane gave white crystals: mp 133–134.5 °C; 1H NMR (500 MHz, CDCl3) δ 0.69 (t, Jvic = 7.3 Hz, 3H, CH3CH2CH2CH2), 1.05, 1.07 (2 × dt, Jgem = 14.8 Hz, Jvic = 7.3 Hz, 2H, CH3CH2CH2CH2), 1.17, 1.32 (2 × m, 2H, CH3CH2CH2CH2), 2.14 (m, 2H, CH3CH2CH2CH2), 2.18, 2.22 (2 × s, 2 × 3H, CH3CO), 3.36 (s, 2H, H-3´), 3.55 (s, 2H, H-1´), 4.90, 5.02 (2 × d, Jgem = 14.8 Hz, 2H, CH2Ph), 6.50 (m, 2H, H-o-Bn), 6.81 (s, 1H, H-4´), 7.11 (s, 1H, H-7´), 7.16 (m, 2H, H-m-Bn), 7.23 (m, 1H, H-p-Bn), 8.25 (s, 1H, H-8), 9.16 (s, 1H, H-2); 13C NMR (125.8 MHz, CDCl3) δ 13.66 (CH3CH2CH2CH2), 22.34 (CH3CH2CH2CH2), 26.11, 26.24 (CH3CO), 32.61 (CH3CH2CH2CH2), 32.89 (CH3CH2CH2CH2), 37.09 (CH2-3´), 37.84 (CH2-1´), 50.49 (CH2-Ph), 74.88 (C-2´), 123.32 (C-5), 124.97 (CH-4´), 125.24 (CH-7´), 126.13 (CH-o-Bn), 128.30 (CH-p-Bn), 128.81 (CH-m-Bn), 133.52 (C-5´), 134.62 (C-i-Bn), 137.25 (C-3´a), 140.35 (C-6´), 141.74 (C-7´a), 149.15 (CH-8), 152.79 (C-6), 153.04 (CH-2), 161.46 (C-4), 204.04, 204.41 (CO); IR (CHCl3) ν 2998, 2962, 2933, 1701, 1592, 1498, 1456, 1372, 1358, 1233 cm-1; MS (FAB, m/z (rel. %)) 467 (M+ + H, 100), 91 (63); HR-MS (FAB) calcd for C29H31N4O2 [M++H] 467.2447, found 467.2424. Rf(5/1 EtOAc/MeOH) = 0.67.

6-[6-Butyl-2-acetyl-2-(ethoxycarbonyl)indan-5-yl]-7-benzyl-7H-purine (6bc). Column chromatography on silica gel (12/1, 10/1 to 8/1 Et2O/acetone) yielded 167 mg (84%) of a yellowish oil. Recrystallization from CH2Cl2/heptane gave white crystals: mp 131–134 °C; 1H NMR (400 MHz, CDCl3) δ 0.68 (t, Jvic = 7.3 Hz, 6H, CH3CH2CH2CH2), 1.05 (m, 4H, CH3CH2CH2CH2), 1.16, 1.28 (2 × m, 4H, CH3CH2CH2CH2), 1.29, 1.31 (2 × t, Jvic = 7.1 Hz, 2 × 3H, CH3CH2), 2.12 (m, 4H, CH3CH2CH2CH2), 2.24, 2.28 (2 × s, 2 × 3H, CH3CO), 3.33, 3.38, 3.43, 3.44 (4 × d, Jgem = 16.7 Hz, 4 × 1H, H-3´), 3.53, 3.55, 3.60, 3.62 (4 × d, Jgem = 17.3 Hz, 4 × 1H, H-1´), 4.24, 4.27 (2 × q, Jvic = 7.1 Hz, 2 × 2H, CH2CH3), 4.91, 4.93, 5.03 (3 × d, Jgem = 15.6 Hz, 4H, CH2Ph), 6.48, 6.50 (2 × m, 2 × 2H, H-o-Bn), 6.79, 6.82 (2 × s, 2 × 1H, H-4´), 7.09, 7.10 (2 × s, 2 × 1H, H-7´), 7.16 (m, 4H, H-m-Bn), 7.22, 7.23 (2 × m, 2 × 1H, H-p-Bn), 8.26 (s, 2H, H-8), 9.15 (s, 2H, H-2); 13C NMR (100.6 MHz, CDCl3) δ 13.66 (CH3CH2CH2CH2), 14.02, 14.08 (CH3CH2), 22.34 (CH3CH2CH2CH2), 26.12, 26.16 (CH3CO), 32.55, 32.84, 32.87 (CH3CH2CH2CH2), 38.48, 38.63 (CH2-3´), 38.92 (CH2-1´), 50.50, 50.59 (CH2-Ph), 61.93, 62.01 (CH2CH3), 66.79, 66.84 (C-2´), 123.28 (C-5), 124.76, 124.84 (CH-4´), 125.06, 125.13 (CH-7´), 126.12, 126.24 (CH-o-Bn), 128.21, 128.27 (CH-p-Bn), 128.75 (CH-m-Bn), 133.45, 133.52 (C-5´), 134.59, 134.63 (C-i-Bn), 137.31, 137.40 (C-3´a), 140.14, 140.28 (C-6´), 141.83, 141.94 (C-7´a), 149.05, 149.10 (CH-8), 152.96, 152.99 (C-6), 153.03 (CH-2), 161.47, 161.50 (C-4), 172.04, 172.41 (COOEt), 202.18, 202.38 (CO); IR (CHCl3) ν 3020, 2988, 2962, 1714, 1591, 1456, 1370, 1240, 1220 cm-1; MS (FAB, m/z (rel. %)) 497 (M+ + H, 83), 91 (100); HR-MS (FAB) calcd for C30H33N4O3 [M++H] 497.2553, found 497.2569. Rf(5/1 EtOAc/MeOH) = 0.74.

6-[6-Trimethylsilyl-2,2-di(ethoxycarbonyl)indan-5-yl]-7-benzyl-7H-purine (6ca). Column chromatography on silica gel (Et2O/acetone 10/1 to 8/1) followed by column chromatography on aluminum oxide (10/1 Et2O/acetone) yielded 70 mg (32%) of a yellowish oil. Crystallization from CH2Cl2/heptane gave white crystals: mp 177.5–179 °C; 1H NMR (400 MHz, CDCl3) δ -0.26 (s, 9H, (CH3)3Si), 1.29 (t, Jvic = 7.1 Hz, 6H, CH3CH2), 3.53 (s, 2H, H-3´), 3.69 (s, 2H, H-1´), 4.24 (q, Jvic = 7.1 Hz, 4H, CH2CH3), 5.03 (s, 2H, CH2Ph), 6.48 (m, 2H, H-o-Bn), 6.95 (bd, J4´,7´ = 0.6 Hz, 1H, H-4´), 7.14 (m, 2H, H-m-Bn), 7.20 (m, 1H, H-p-Bn), 7.48 (bd, J7´,4´ = 0.6 Hz, 1H, H-7´), 8.30 (s, 1H, H-8), 9.11 (s, 1H, H-2); 13C NMR (100.6 MHz, CDCl3) δ -0.16 ((CH3)3Si), 14.04 (CH3CH2), 40.37 (CH2-3´), 40.48 (CH2-1´), 50.96 (CH2-Ph), 60.27 (C-2´), 61.87 (CH2CH3), 123.29 (C-5), 124.97 (CH-4´), 126.60 (CH-o-Bn), 128.34 (CH-p-Bn), 128.76 (CH-m-Bn), 131.03 (CH-7´), 134.56 (C-i-Bn), 138.79 (C-6´), 140.02 (C-5´), 140.58 (C-7´a), 140.90 (C-3´a), 148.99 (CH-8), 152.50 (CH-2), 154.63 (C-6), 161.54 (C-4), 171.42 (CO); IR (CHCl3) ν 2986, 2906, 1729, 1589, 1492, 1457, 1441, 1369, 1273, 1249 cm-1; MS (FAB, m/z (rel. %)) 543 (M+ + H, 76), 91 (100), 73 (58); HR-MS (FAB) calcd for C30H35N4O4Si [M++H] 543.2428, found 543.2434. Anal. Calcd for C30H34N4O4Si: C 66.39, H 6.31, N 10.32. Found: C 66.16, H 6.19, N 10.13. Rf(EtOAc/MeOH 5/1) = 0.71.

6-[6-Trimethylsilyl-2,2-diacetylindan-5-yl]-7-benzyl-7H-purine (6cb). Column chromatography (10/1, 8/1 to 6/1 Et2O/acetone) yielded 51 mg (26%) of a yellowish oil. Crystallization from CH2Cl2/heptane gave white crystals: mp 98–99 °C; 1H NMR (500 MHz, CDCl3) δ -0.24 (s, 9H, (CH3)3Si), 2.21 (s, 6H, CH3CO), 3.40 (s, 2H, H-3´), 3.60 (s, 2H, H-1´), 5.00 (s, 2H, CH2Ph), 6.49 (m, 2H, H-o-Bn), 6.93 (bd, J4´,7´ = 0.6 Hz, 1H, H-4´), 7.16 (m, 2H, H-m-Bn), 7.22 (m, 1H, H-p-Bn), 7.49 (bd, J7´,4´ = 0.6 Hz, 1H, H-7´), 8.29 (s, 1H, H-8), 9.12 (s, 1H, H-2); 13C NMR (125.8 MHz, CDCl3) δ -0.17 ((CH3)3Si), 26.54 (CH3CO), 37.31 (CH2-3´), 37.44 (CH2-1´), 50.80 (CH2-Ph), 74.69 (C-2´), 123.33 (C-5), 125.08 (CH-4´), 126.41 (CH-o-Bn), 128.35 (CH-p-Bn), 128.81 (CH-m-Bn), 131.20 (CH-7´), 134.57 (C-i-Bn), 138.93 (C-6´), 140.13 (C-5´), 140.20 (C-7´a), 140.50 (C-3´a), 149.02 (CH-8), 152.52 (CH-2), 154.42 (C-6), 161.45 (C-4), 204.12 (CO); IR (CHCl3) ν 3004, 2960, 2930, 1702, 1589, 1491, 1456, 1370, 1359, 1249 cm-1; MS (FAB, m/z (rel. %)) 483 (M++H, 60), 91 (100), 73 (75); HR-MS (ESI) calcd for C28H31N4O2Si [M++H] 483.2216, found 483.2214. Rf(5/1 EtOAc/MeOH) = 0.56.

6-[6-Trimethylsilyl-2-acetyl-2-(ethoxycarbonyl)indan-5-yl]-7-benzyl-7H-purine (6cc). Column chromatography on silica gel (0/1, 8/1 to 6/1 Et2O/acetone 1) followed by column chromatography on aluminum oxide (10/1 toluene/acetone) afforded 56 mg (27%) of a yellowish oil. Crystallization from CH2Cl2/heptane gave white crystals: mp 136–137 °C; 1H NMR (400 MHz, CDCl3) δ -0.25 (s, 9H, (CH3)3Si), 1.30 (t, Jvic = 7.1 Hz, 3H, CH3CH2), 2.27 (s, 3H, CH3CO), 3.41, 3.47 (2 × d, Jgem = 17.0 Hz, 2H, H-3´), 3.59, 3.65 (2 × d, Jgem = 16.6 Hz, 2H, H-1´), 4.26 (q, Jvic = 7.1 Hz, 2H, CH2CH3), 5.00, 5.04 (2 × d, Jgem = 15.5 Hz, 2H, CH2Ph), 6.48 (m, 2H, H-o-Bn), 6.93 (s, 1H, H-4´), 7.15 (m, 2H, H-m-Bn), 7.19 (m, 1H, H-p-Bn), 7.48 (s, 1H, H-7´), 8.30 (s, 1H, H-8), 9.11 (s, 1H, H-2); 13C NMR (100.6 MHz, CDCl3) δ -0.15 ((CH3)3Si), 14.05 (CH3CH2), 26.16 (CH3CO), 38.83 (CH2-3´), 38.99 (CH2-1´), 50.89 (CH2-Ph), 62.03 (CH2CH3), 66.65 (C-2´), 123.31 (C-5), 125.01 (CH-4´), 126.13 (CH-o-Bn), 128.30 (CH-p-Bn), 128.81 (CH-m-Bn), 131.09 (CH-7´), 134.59 (C-i-Bn), 138.81 (C-6´), 140.06 (C-5´), 140.41 (C-3´a), 140.69 (C-7´a), 149.00 (CH-8), 152.52 (CH-2), 154.56 (C-6), 161.51 (C-4), 172.17 (COOEt), 202.17 (CO); IR (CHCl3) ν 2996, 1713, 1589, 1491, 1456, 1441, 1369, 1247 cm-1; MS (FAB, m/z (rel. %)) 513 (M++H, 100), 91 (82), 73 (35); HR-MS (FAB) calcd for C29H33N4O3Si [M++H] 513.2322, found 513.2311. Rf(5/1 EtOAc/MeOH) = 0.68.

Single crystal X-ray structure analysis. The diffraction data of single crystals of 6aa (colourless, 0.17 × 0.25 × 0.43 mm) were collected on Xcalibur X-ray diffractometer with CuKα (λ=1.54180 Å) at 295 K. The structure was solved by direct methods with SIR9211 and refined by full-matrix, least-squares methods based on F with CRYSTALS.12 Non-hydrogen atoms were refined with anisotropic thermal displacement parameters; hydrogen atoms were treated as riding atoms. Crystal data for 6aa: C36H33N4O4, monoclinic, space group C2/c, a = 34.1117(4) Å, b = 14.9903(6) Å, c = 12.2830(5) Å, β = 103.7390(9)°, V = 6101.1(4) Å3, Z = 8, M = 585.68, 92294 reflections measured, 6319 independent reflections. Final R = 0.049, wR = 0.059, GoF = 1.316 for 2464 reflections with I > 2σ(I) and 398 parameters. CCDC 779450 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.

ACKNOWLEDGEMENTS
This work was supported by Project No. 1M0508 to the Centre for New Antivirals and Antineoplastics and Project No. MSM0021620857 from the Ministry of Education of the Czech Republic.

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