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Direct C–H amination and C–H chloroamination of 7-deazapurines

Nazarii Sabatab, Martin Klečkaab, Lenka Slavětínskáb, Blanka Klepetářováb and Michal Hocek*ab
aDepartment of Organic Chemistry, Faculty of Science, Charles University in Prague, Hlavova 8, CZ-12843 Prague 2, Czech Republic
bInstitute of Organic Chemistry and Biochemistry, Academy of Sciences of the Czech Republic, Gilead & IOCB Research Center, Flemingovo nam. 2, CZ-16610 Prague 6, Czech Republic. E-mail: hocek@uochb.cas.cz

Received 25th October 2014 , Accepted 12th November 2014

First published on 12th November 2014


Abstract

Protocols for selective Pd–Cu-catalyzed direct C–H amination or C–H chloroamination of 7-deazapurines with N-chloro-N-alkyl-arylsulfonamides have been developed leading either to 8-(arylsulfonyl)methylamino-7-deazapurines or to 7-chloro-8-(arylsulfonyl)methylamino-7-deazapurines. The scope and limitations of the methods, as well as synthesis of a small series of 6,8,9-tri- and 6,7,8,9-tetrasubstituted 7-deazapurines and deprotection of the sulfonamide are presented.


Introduction

Modified pyrrolo[2,3-d]pyrimidine (7-deazapurine)1 bases and nucleosides display a variety of biological effects. Substituted 7-deazapurine bases induct neurogenesis2 or exert antitumor3 or antiinflammatory4 activity, whereas 6- or 7-aryl-7-deazapurine nucleosides are potent cytostatics.5,6 Therefore, development of regioselective synthesis of 7-deazapurines bearing multiple substituents is a worthwhile goal. Some regioselective cross-couplings of di- or trihalogenated pyrrolo[2,3-d]pyrimidines were recently reported for the synthesis of di- or triaryl derivatives.7,8 We have developed regioselective C–H borylation9 and C–H sulfenylation10 leading to 8-B- or 8-S-substituted 7-deazapurines. Here we report on complementary C–H aminations.

Metal-catalyzed direct C–H aminations are increasingly popular reactions for modifications of arenes and heterocycles.11–14 One of the most efficient reagents are N-chloro-sulfonamides under Pd/Cu catalysis.13 Inspired by related C–H aminations of indoles,13 we decided to study the C–H aminations of pyrrolo[2,3-d]pyrimidines (7-deazapurines). The only previously reported C–H amination of 7-deazapurine was performed through hypervalent iodanes and proceeded at position 7.14

Results and discussion

For our initial study, we selected easily accessible 6-phenyl-9-benzyl-7-deazapurine 1a.9 We started testing its reaction with N-chloro-N-methyl-tosylamide 2 under literature13 conditions in presence of Pd(OAc)2, Cu(acac)2, 2,2′-bipyridine (bpy), Na2CO3 in dioxane (Scheme 1 and Table 1). The reaction with 2 equiv. of 2 in presence of 2 equiv. of Na2CO3 gave the desired 8-tosylamino product 5a in 13% only (entry 1). The use of a larger excesses of the base (5–7 equiv.) and of reagent 2 (3 equiv.) led to only low increase of yields (18–29%). Only the use of large excess (5 equiv.) of 2 gave product 5a in acceptable preparative yields of 68%.
image file: c4ra13143f-s1.tif
Scheme 1 C–H aminations of 1a.
Table 1 Optimization of C–H aminations of 7-deazapurine 1a with N-chloro-N-methyl-arylsulfoneamides (2–4)a
Entry Ar 2–4 (equiv.) Na2CO3 (equiv.) Product(s) (yield)
a Reagents and conditions: Pd(OAc)2 (5%), Cu(acac)2 (10%), bpy (10%), Na2CO3, 1,4-dioxane, Ar, rt, 24 h.b Reaction time 72 h.
1 4-MePh 2 (2) 2 5a (13%)
2 4-MePh 2 (2) 5 5a (18%)
3 4-MePh 2 (3) 5 5a (25%)
4 4-MePh 2 (3) 7 5a (29%)
5b 4-MePh 2 (5) 7 5a (68%)
6 4-NO2Ph 3 (3) 7 6a (47%)
7 2-NO2Ph 4 (5) 5 7a (28%) + 8a (33%) + 9a (25%)
8 2-NO2Ph 4 (3) 7 7a (60%)


In order to have a choice of some more easily cleavable N-protecting groups,15 we also tested 4-nitrophenylsulfonyl(p-nosyl, pNs) and 2-nitrophenylsulfonyl (o-nosyl, oNs) chloroamides 3 and 4. The reaction of 1a with pNs reagent 3 (3 equiv.) gave the 8-p-nosylamino product 6a in acceptable 48% yield (entry 6). The reactions of 1a with oNs chloroamide 4 (2–3 equiv.) gave very low conversions (see Table S1 in ESI), whereas the reaction with 5 equiv. of 4 gave a mixture of the desired product of 8-amination 7a (28%) accompanied by 7-chloro-8-amino- 8a and 7-chloro-7-deazapurine 9a as side-products. Apparently, the chloroamide 4 in larger excess can act as an electrophilic chlorination reagent which halogenates the deazapurine at position 7 (similarly as it was shown in indoles13). Therefore, we performed a detailed optimization of this reaction using different ratios of reagents, catalysts and additives and different conditions (see Table S1 in ESI). The optimum protocol for aminations used 3 equiv. of 4 in presence of large excess of Na2CO3 (7 equiv.) to give the desired product 7a in 60% yield (Table 1, entry 8).

The detailed optimization also revealed some ratios of reagents and conditions under which the chloroamination proceeds. Also inspired by the related work on indoles,13 we employed CuCl as copper source, Ag2CO3 as base and LiCl as additive (Table S1 in ESI) to find an optimum protocol leading exclusively to chloroamination,13,16 employing 4 (3 equiv.) in presence of Pd(OAc)2, CuCl (10 mol%), LiCl (2 equiv.) and Ag2CO3 (2 equiv.).

The next step was the study of the scope and limitation of the methods. A series of five 9-benzyl-7-deazapurine derivatives 1a–1e bearing phenyl, methoxy, methyl, chloro or amino group at position 6 was tested in the amination and chloroamination protocols (Scheme 2 and Table 2). The preparative aminations were performed with choroamide 4 (3 equiv.) in presence of Pd(OAc)2, Cu(acac)3, bpy and 7 equiv. of Na2CO3. The reactions of 6-phenyl, -methoxy and -methyl deazapurines proceeded smoothly to give desired 8-(o-nosyl)methylamino-7-deazapurines 7a–7c in acceptable yields of 41–62%. Conversely, analogous reaction of 6-chloro- and 6-amino-derivatives 1d,e led to very complex inseparable mixtures.


image file: c4ra13143f-s2.tif
Scheme 2 C–H aminations and chloroaminations of 7-deazapurines.
Table 2 Preparative C–H aminations chloroaminations of 7-deazapurines
Entry Starting compd R Product (yield)
1 1a Ph 7a (62%)
2 1b OMe 7b (60%)
3 1c Me 7c (41%)
4 1d Cl Complex mixture
5 1e NH2 Complex mixture
6 1a Ph 8a (51%)
7 1b OMe 8b (42%)
8 1c Me Low conversion, complex mixture
9 1d Cl Complex mixture
10 1e NH2 Complex mixture
11 9a Ph 8a (41%)


Then we tested the chloroamination protocol on the same series of deazapurines 1a–1e. The reactions with 4 (3 equiv.) were performed in presence of Pd(OAc)2, CuCl, LiCl and Ag2CO3. The reactions of 6-phenyl and 6-methoxy derivatives 1a,b proceeded well to get desired 7-chloro-8-(oNs)MeNH-7-deazapurines 8a,b in acceptable yields of 51 and 42%, whereas the reaction of 6-methyl derivative 1c gave low conversion to inseparable mixture containing products of chlorination and chloroamination. Similarly, the reactions of 6-chloro- and 6-aminodeazapurines 1d,e gave complex inseparable mixtures. Finally, 6-phenyl-7-chloro-7-deazapurine 9a was also converted to 7-chloro-8-aminated derivative 8a in 41% yield showing that the chlorine at position 7 is better tolerated (as it is less reactive toward nucleophiles) than the chlorine at position 6.

The last goal in this study was to test a deprotection of the sulfonamides and the stability of the corresponding 8-amino-7-deazapurines (2-aminoindoles are prone to tautomerization17 and oxidation18). Any attempt to cleave the Ts- or pNs-groups in compounds 5a or 6a according to literature15 either did not work or led to decomposition of the heterocycles. Therefore, major part of this study was performed with oNs-group which is more easily cleavable.15 The deprotection of compound 7a was successfully performed using thiophenol and cesium carbonate15d to afford 8-methylamino-7-deazapurine 10a in 75% yield (Scheme 3). We performed also one-pot C–H amination deprotection sequence to furnish the desired compound 10a directly in 35% for two steps. The 8-(methylamino)-7-deazapurine 10a was reasonably stable under neutral conditions but quickly decomposed when exposed to even traces of acid (e.g. in chlorinated solvents).


image file: c4ra13143f-s3.tif
Scheme 3 Deprotection of 7a.

All new compounds were fully characterized by NMR spectroscopy including assignment of all signals. In addition, to confirm the regioselectivity of the reactions, single-crystal X-ray diffraction was performed with compounds 5a, 7a, 7b and 8a. Fig. 1 shows the crystal structures of compounds 7a and 8a (for structures of 5a and 7b, see ESI).


image file: c4ra13143f-f1.tif
Fig. 1 An ORTEP view of compounds 7a (CCDC 1014820) and 8a (1014817) shown with 50% probability displacement ellipsoids. Structures 5a (1014819) and 7b (1014818) are shown in ESI.

Conclusions

In conclusion, we have developed selective protocols for palladium/copper-catalyzed direct C–H amination and C–H chloroamination of 7-deazapurines with N-chloro-N-alkyl-arylsulfoneamides. Reactions proceed under mild conditions regioselectively at position 8 of 7-deazapurines (in analogy to aminations at position 2 of indoles13) and are applicable to 6-aryl, -alkyl and -alkoxy 7-deazapurine derivatives. On the other hand, they are not compatible with 6-amino- and 6-chloro derivatives. Apart from the potential for the synthesis of series of 8-(arylsulfonyl)methylamino-7-deazapurines (and their 7-chloro-derivatives), when using oNs sulfonamides, the deprotection is possible to 8-methylamino-7-deazapurines. These protocols nicely complement the current toolbox of reactions for modifications of these privileged heterocycles and will be used for generation of libraries of compounds for biological activity screening.

Experimental

General procedure for C–H amination of 7-deazapurines

7-Deazapurine 1a–1e (0.5 mmol), Pd(OAc)2 (0.025 mmol), Cu(acac)2 (0.05 mmol), bpy (0.05 mmol), Na2CO3 (3.5 mmol) and chlorosulfonamide (1–1.75 mmol) were placed in a vial which was purged with argon. Then 1,4-dioxane (2 mL) was added and the reaction mixture was then stirred for 24 h at rt, then quenched with H2O (2 mL), extracted with ethyl acetate (3 × 20 mL) and washed with brine (2 mL). The organic phases were combined and dried over sodium sulphate, filtered, and evaporated under vacuum. The crude product was purified by column chromatography on silica gel, eluting with hexanes/EtOAc (5[thin space (1/6-em)]:[thin space (1/6-em)]1 to 1[thin space (1/6-em)]:[thin space (1/6-em)]2) to afford the corresponding product.

N-(7-benzyl-4-phenyl-7H-pyrrolo[2,3-d]pyrimidin-6-yl)-N-methyl-2-nitrobenzenesulfonamide (7a)

1a (285 mg, 1 mmol) and N-chloro-N-methyl-2-nitrobenzenesulfonamide 4 (877 mg, 3.5 mmol) were used as starting compounds to give product 7a (310 mg, 62%) as colourless crystals after chromatography with hexanes/EtOAc 5[thin space (1/6-em)]:[thin space (1/6-em)]1 to 1[thin space (1/6-em)]:[thin space (1/6-em)]1 and crystallization from EtOAc/hexane. M.p. 102–103 °C. 1H NMR (500.0 MHz, CDCl3): 2.94 (s, 3H, CH3N); 5.67 (bs, 2H, CH2Ph); 6.45 (s, 1H, H-5); 7.22 (m, 2H, H-o-Bn); 7.27 (m, 1H, H-p-Bn); 7.30 (m, 2H, H-m-Bn); 7.50–7.53 (m, 3H, H-m,p-Ph); 7.61 (ddd, 1H, J5,6 = 8.1, J5,4 = 7.5, J5,3 = 1.3, H-5-C6H4NO2); 7.67 (ddd, 1H, J3,4 = 8.0, J3,5 = 1.3, J3,6 = 0.5, H-3-C6H4NO2); 7.76 (ddd, 1H, J6,5 = 8.1, J6,4 = 1.4, J6,3 = 0.5, H-6-C6H4NO2); 7.77 (ddd, 1H, J4,3 = 8.0, J4,5 = 7.5, J4,6 = 1.4, H-4-C6H4NO2); 7.97 (m, 2H, H-o-Ph); 9.10 (s, 1H, H-2). 13C NMR (125.7 MHz, CDCl3): 40.53 (CH3N); 45.29 (CH2Ph); 96.40 (CH-5); 113.91 (C-4a); 124.12 (CH-3-C6H4NO2); 127.65 (CH-o-Bn); 127.89 (CH-p-Bn); 128.85 (CH-m-Ph, CH-o-Bn); 128.94 (CH-m-Ph); 130.16 (C-1-C6H4NO2); 130.64 (CH-p-Ph); 131.26 (CH-5-C6H4NO2); 132.23 (CH-6-C6H4NO2); 134.62 (CH-4-C6H4NO2); 136.54 (C-i-Bn); 136.86 (C-i-Ph); 137.04 (C-6); 148.55 (C-2-C6H4NO2); 150.56 (C-7a); 152.40 (CH-2); 157.48 (C-4). IR(KBr): 2821, 1545, 1376, 1368, 1360, 1343, 1318, 1309, 1180, 1163, 924. HRMS (ESI) calculated for C26H22N5O4S: 500.1387; found 500.1387.

General procedure for C–H chloroamination of 7-deazapurines

7-Deazapurine 1a–1e (0.5 mmol), Pd(OAc)2 (0.0125 mmol), CuCl (0.05 mmol), LiCl (1.0 mmol), Ag2CO3 (1.0 mmol) and chlorosulfonamide (1.5–1.75 mmol) were placed in a vial which was purged with argon. Then 1,4-dioxane (2 mL) was added and the reaction mixture was stirred for 24 h at room temperature, then quenched with H2O (2 mL), extracted with ethyl acetate (3 × 20 mL) and washed with brine (2 mL). The organic phases were combined and dried over sodium sulphate, filtered, and evaporated under vacuum. The crude product was purified by column chromatography on silica gel, eluting with hexanes/EtOAc (5[thin space (1/6-em)]:[thin space (1/6-em)]1 to 1[thin space (1/6-em)]:[thin space (1/6-em)]2) to afford the corresponding product.

N-(7-benzyl-5-chloro-4-phenyl-7H-pyrrolo[2,3-d]pyrimidin-6-yl)-N-methyl-2-nitrobenzenesulfonamide (8a)

1a (285 mg, 1 mmol) and 4 (877 mg, 3.5 mmol) were used as starting compounds to give product 8a (273 mg, 51%) as white crystals after chromatography with hexanes/EtOAc 5[thin space (1/6-em)]:[thin space (1/6-em)]1 to 1[thin space (1/6-em)]:[thin space (1/6-em)]1 and crystallization from EtOAc/hexane. M.p. 215–216 °C. 1H NMR (500 MHz, CDCl3): 2.91 (s, 3H, CH3N); 5.42 (d, 1H, Jgem = 15.3 Hz, CH2a-Ph); 6.16 (d, 1H, Jgem = 15.3 Hz, CH2b-Ph); 7.26–7.31 (m, 3H, H-o,p-Bn); 7.33 (m, 2H, H-m-Bn); 7.42–7.50 (m, 3H, H-m,p-Ph); 7.58–7.63 (m, 2H, H-3,5-C6H4NO2); 7.71 (m, 2H, H-o-Ph); 7.73 (ddd, 1H, J4,3 = J4,5 = 7.7 Hz, J4,6 = 1.4 Hz, H-4-C6H4NO2); 7.84 (m, 1H, H-6-C6H4NO2); 9.09 (s, 1H, H-2). 13C NMR (125.7 MHz, CDCl3): 38.28 (CH3–N); 45.68 (CH2-Ph); 103.08 (C-5); 112.02 (C-4a); 124.00 (CH-3-C6H4NO2); 127.84 (CH-m-Ph); 128.09 (CH-p-Bn); 128.11 (CH-o-Bn); 128.95 (CH-m-Bn); 129.86 (CH-p-Ph); 130.20 (CH-o-Ph); 131.45 (CH-5-C6H4NO2); 131.62 (C-1-C6H4NO2); 131.65 (CH-6-C6H4NO2); 131.89 (C-6); 134.49 (CH-4-C6H4NO2); 136.48 (C-i-Bn); 136.62 (C-i-Ph); 148.56 (C-2-C6H4NO2); 148.78 (C-7a); 153.18 (CH-2); 160.20 (C-4). IR(KBr): 3050, 1583, 1545, 1374, 1345, 1165, 826, 558. HRMS (ESI) calculated for C26H21N5O4SCl: 534.0998; found: 534.0997.

Acknowledgements

This work was supported by the institutional support of the Charles University and Academy of Sciences of the Czech Republic (RVO: 61388963), by the Czech Science Foundation (P207/12/0205) and by Gilead Sciences, Inc.

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Footnote

Electronic supplementary information (ESI) available: Complete experimental part and characterization of all compounds, additional figures of X-ray structures, copies of NMR spectra. CCDC 1014817–1014820. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c4ra13143f

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