Copper-Catalyzed Synthesis of Fused Imidazopyrazine N-Oxide Skeletons

Abstract

N-Propargyl-2-aroylimidazoles synthesized and converted into the corresponding ketoximes. Under various conditions, several mono- and diketoxime imidazole derivatives were formed by converting the carbonyl or carbonyl and propargyl groups into oxime groups. N-Propargyl monooxime imidazole derivatives were cyclized by treatment with CuI to give various imidazopyrazine N-­oxides. Several copper salts, such as CuOAc, CuSO₄, and CuOTf, formed the same cyclization product. This cyclization reaction occurred only in the presence of Cu(I) or Cu(II) salts; other transition metals such as Au, Ag, In, and Fe did not yield cyclic products. The nucleus-independent chemical shift method was used to calculate the aromaticity of the bicyclic rings.

Fused bicyclic imidazole molecules have attracted considerable attention from researchers owing to their importance in the design of drugs, for example, zolpidem and alpidem. Despite the fact that many important compounds contain an imidazole ring, for example, protein kinase inhibitors,[1] antitubercular agents,[2] and cannabinoid receptor ligands,[3] certain members of this class are virtually unexplored, including substituted imidazopyrazine N-oxides. Heterocyclic N-oxide molecules are important structural moieties, because they are present in numerous natural and pharmacological compounds that display significant biological activities (Figure [1]).[4a] [b] To access such moieties, cyclization reactions of alkynes are of particular interest.[4c,5–7]

Although some heterocyclic N-oxide derivatives, such as isoquinoline N-oxides,[8] [9] pyrrolo- and indolopyrazine N-oxides,[10] pyrazole N-oxides,[11] benzopyrazole N-oxides,[12] and benzimidazole N-oxides,[13] have been documented, this is the first study to report the synthesis of the highly substituted imidazo[1,2-a]pyrazine N-oxide skeleton. In the only example of an imidazo N-oxide reported in the literature, a carbaldehyde group attached at the C-2 position of the imidazole ring of an imidazopyrazine N-oxide was cyclized to an ethyne group by using hydroxylamine.[14] The addition of various groups to heterocyclic rings can change their pharmacological properties and the side-effects of candidate drugs; consequently, expedient strategies for synthesizing heterocyclic molecules are a welcome development. For example, the functional differences between zolpidem and alpidem derive from the different affinities of the peripheral benzodiazepine receptor subtype.[15] Efficient atom-economic synthetic routes that permit desirable diversification of such cores are proving to be of tremendous value, as they permit structure–activity relationship and quantitative structure–activity relationship studies to be performed more effectively.

Our research group has synthesized pyrrolo- and indolopyrazine N-oxide derivatives by a gold-catalyzed reaction.[10] In addition, we aimed to extend the range of heterocyclic N-oxides by replacing the heterocyclic ring in these compounds (i.e., pyrrole and indole) with imidazole.

We hypothesized that unknown imidazopyrazine N-oxides might be synthesized through a metal-catalyzed reaction of an oxime group with a propargyl group . This is the first report of synthesis of highly substituted imidazopyrazine N-oxide derivatives in which an oxime group is used as a nucleophile to attack the propargyl group in a catalytic reaction.

N-Propargyl 2,4-disubstituted imidazoles 1a–g and N-propargyl 2-substituted imidazoles 3a and 3h were synthesized by methods reported in the literature [for details of the reactions, see the Supplementary Information (SI), Scheme S3].[16] [17] [18] During the propargylation reaction to form 1a–g, 3a, and 3h, the N-1 atom was propargylated exclusively. To determine the structure of the favored N-propargylated isomer, we calculated their relative enthalpies and Gibbs free energies (SI; Figure S1).[19]

The N-propargylated derivatives 1a–g, 3a, and 3h were converted into the corresponding N-propargylated ketoximes 2a–g, 4a, and 4h in moderate yields by treatment with hydroxylamine under basic condition (Scheme [1]). However, the attempted ketoximation of 1a by using triethylamine as a base instead of pyridine did not give 2a, but instead gave a mixture of products (SI; Scheme S1).

The cyclization reaction of the oxime derivative 2a was optimized by screening various catalysts, bases, and solvents (Table [1]). Gold salts did not catalyze the reaction (Table [1], entries 1–5, and 13), although they have been used for similar reactions reported in the literature. Most Lewis acids and bases were also unreactive except for the copper salts CuI, CuSO₄, CuOTf, and CuOAc (entries 15–17, 19, and 20).[20a] Both Cu(I) and Cu(II) salts gave a cyclized product 5a, although CuCl₂ was ineffective (entry 18).[20b] Use of a hydrated copper salt decreased the yield of 5a (entry 17). Moving from CuI to other counterions such as SO₄ ^2–, OTf^–, OAc^– had little effect on the yield of reaction.[20c] The observed yields of product 5a suggest that the iodide ion might play an important role as a stabilizing ligand in the copper complex, thereby extending the lifetime of the formed intermediate.[20d] The choice of solvent was also found to be crucial for the efficiency of this process, and PrOH was found to be optimal (entry 14).

Table 1 Optimization Experiments for the Cyclization^a

Entry	Catalyst	Solvent	Yield (%)
1	AuCl + AgSbF6	DCE	NDb
2	AuCl3	acetone	ND
3	AuCl3	MeCN	ND
4	AuCl3	CHCl3	ND
5	AuBr3	CHCl3	ND
6	InCl3	acetone	ND
7	InCl3	CHCl3	ND
8	InCl3	MeCN	ND
9	InCl3	MeOH	ND
10	AgOTf	MeCN	ND
11	Yb(OTf)3	MeCN	ND
12	Cs2CO3	MeOH	ND
13	AuCl	PrOH	ND
14	CuI	PrOH	83
15	CuI	MeCN	59
16	CuSO4	PrOH	69
17	CuSO4·5H2O	PrOH	49
18	CuCl2·2H2O	PrOH	ND
19	CuOTf	PrOH	66
20	CuOAc	PrOH	63
21	FeCl3·6H2O	MeCN	ND
22	FeCl3·6H2O	PrOH	ND
23c	TiCl4	MeCN	ND
24c 25	BF3·OEt2 CuI	MeCN CHCl3	ND 48%

^a 10 mol% of the metal salt was used. All reactions were performed in the refluxing solvent.

^b ND = not detected.

^c An equivalent amount of the Lewis acid was used.

As an alternative cyclization route, we attempted to carry out the oxime formation before the propargylation reaction. All attempts at cyclization of a dipropargylated ketoxime derivative with various catalysts and bases failed to give any cyclic product (SI; Scheme S2).

Next, we examined the scope of the cyclization reaction, and various unknown imidazopyrazine N-oxide derivatives were obtained (Scheme [2]).[21] The diphenyl, bis(4-methoxyphenyl), bis(4-bromophenyl), bis(4-fluorophenyl), and di-2-naphthyl derivatives 5a, 5b, 5d, 5e, and 5g, respectively, and the monophenyl N-oxide derivatives 6a were obtained in moderate yields. Other products (not shown) were obtained only as crude products. The scope of this cyclization reaction was limited to aryl substituents, because the reaction by which we prepared the N-propargyl-2-aroylimidazole derivatives did not permit the preparation of alkyl-substituted (ketone or aldehyde) imidazole derivatives.

To examine the effect of an internal alkyne on the cyclization, we treated compound 7 with CuI under the same cyclization conditions as discussed above; however, only the starting material was obtained (Scheme [3]; Path a). When we attempted to cyclize compound 7 with AuCl₃ in PrOH or CHCl₃, as in our previous work,[10] we obtained a ­yellow precipitate. Extended stirring at room temperature led to further reduction of the Au(III) to Au(0) (Scheme [3]; Path b) and, although we checked all the reaction media, we did not see any sign of the starting material. This might be due to the formation of a coordination complex of gold(III) with a reaction intermediate, which might occur as a result of the presence of the internal alkyne unit. A similar reduction has been reported for a reaction between benzothiazole and AuCl₃.[22]

Our cyclization reaction gave pyrazine N-oxides containing two different aromatic rings. We therefore desired to investigate the aromaticity of these rings by means of the nucleus-independent chemical shift (NICS) index method. The aromaticities of bicyclic compounds 5a, 5b, 5e, 5g, and 6a were therefore calculated by using the NICS method.[19] [23] [24] NICS(1) values were chosen for this investigation. The imidazole and the pyrazine N-oxide rings were found to possess aromatic character (Table [2]). The bicyclic disubstituted N-oxide derivatives 5a, 5b, 5e, and 5g all displayed similar aromatic properties, even though they possess different functional groups with different electronic properties. The imidazole rings exhibited more-aromatic character than did the N-oxide rings. The monosubstituted imidazopyrazine N-oxide 6a exhibited the most aromatic traits for the imidazole ring; conversely, it showed the least aromatic character for the pyrazine N-oxide ring among the various cyclic derivatives (Table [2]).

It is obvious that there are marked differences between the two aromatic rings. To test the effects of this variation on a cyclization reaction, we attempted a simple [3+2] cyclization reaction of compound 5a with dimethyl acetylenedicarboxylate (DMAD) (SI; Scheme S4). However, this reaction did not proceed in refluxing xylene, and the starting material was recovered. Note that the phenyl ring adjacent to the quaternary nitrogen atom might cause steric hindrance resulting in an unfavorable geometrical position for the DMAD molecule. With this knowledge, this reaction might be fully optimized by adopting different reaction conditions.[25]

In conclusion, we investigated the oximation of N-propargyl-2 aroylimidazole derivatives. When pyridine was used as both base and solvent, the product of oximation of the carbonyl group was obtained exclusively. Monooxime derivatives were subjected to alkyne cyclization by copper salts to yield imidazopyrazine N-oxides. Both copper(I) and copper(II) salts gave the same cyclization products in different yields. However, the optimal reaction was achieved with CuI, which is a cheap and low-toxicity catalyst. Silver, gold, and iron salts were ineffective for the cyclization, possibly because of the higher catalytic activity of copper in C–X (X = C, N, O) coupling reactions. The developed cyclization protocol afforded novel bicyclic imidazole skeletons containing aryl substituents. Aromaticity values were calculated, and the aromaticities of the imidazole and N-oxide rings were found to be different. This feature of these bicyclic molecules might provide an opportunity for selective reactions.

Acknowledgment

Authors thank to TUBİTAK for funding and Van Yüzüncü Yıl University, Faculty of Pharmacy and Science Research and Applied Center for their research laboratories.

Authors also thank to graphic artist Gül Menges for designing the graphical abstract.

• References and Notes

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• 21 Imidazo[1,2-a]pyrazine N-Oxides 5a–g and 6a; General Procedure The appropriate N-propargylimidazole oxime derivative 2a–g, 4a, or 4h (1 mmol) was dissolved in PrOH (5 mL) in a 25 mL flask. A catalytic amount of CuI (10 mol%) was added, and the mixture was refluxed for the appropriate time until the reaction was completed (TLC). The mixture was then extracted with EtOAc–H₂O (3 × 40 mL). The organic layer was dried (MgSO₄) and concentrated under reduced pressure to give a crude product that was purified by column chromatography. The products were further purified by preparative TLC (hexane–EtOAc, 5:1). 6-Methyl-2,8-diphenylimidazo[1,2-a]pyrazine 7-Oxide (5a) Reaction time: 3 h; eluent for column chromatography: hexane–EtOAc (1:1). Brown solid; yield: 250 mg (83%); mp 193–195 °C. FTIR (ATR): 3140, 3038, 2969, 2925, 2163, 1732, 1660, 1604, 1576, 1496, 1489 cm^–1. ¹H NMR (400 MHz, CDCl₃): δ = 8.23 (d, J = 6.91 Hz, 2 H, Ar-H), 7.93–7.89 (m, 3 H, Ar-H), 7.78 (s, 1 H, Ar-H), 7.55–7.48 (m, 4 H, Ar-H), 7.41 (t, J = 7.36 Hz, 2 H, Ar-H), 2.51 (s, 3 H, CH₃). ¹³C NMR (100 MHz, CDCl₃): δ = 148.9, 132.6, 131.0, 130.3, 128.7, 128.6, 127.9, 127.8, 126.3, 116.9, 108.8, 15.5. LCMS: m/z [M + H]⁺ Calcd for C₁₉H₁₆N₃O: 302.12879; found: 302.12958.
• 22 Hagos TK. M.S. Dissertation. University of Stellenbosch: 2006
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• References and Notes

• 1 Dimova D, Lyer P, Vogt M, Totzke F, Kubbutat MH. G, Schachtele C, Laufer S, Bajorath J. J. Med. Chem. 2012; 55: 11067
  CrossrefPubMedSearch in Google Scholar
• 2 Thompson AM, O’Connor PD, Marshall AJ, Yardley V, Maes L, Gupta S, Launay D, Braillard S, Chatelain E, Franzblau SG, Wan B, Wang Y, Ma Z, Cooper CB, Denny WA. J. Med. Chem. 2017; 60: 4212
  CrossrefPubMedSearch in Google Scholar
• 3 Moldovan RP, Hausmann K, Deuther-Conrad W, Brust P. ACS Med. Chem. Lett. 2017; 8: 566
  CrossrefPubMedSearch in Google Scholar
• 4a O'Donnell G, Poeschl R, Zimhony O, Gunaratnam M, Moreira JB. C, Neidle S, Evangelopoulos D, Bhakta S, Malkinson JP, Boshoff HI, Lenaerts A, Gibbons S. J. Nat. Prod. 2009; 72: 360
  CrossrefPubMedSearch in Google Scholar
• 4b Nicholas GM, Blunt JW, Munro MH. G. J. Nat. Prod. 2001; 64: 341
  CrossrefPubMedSearch in Google Scholar
• 4c Wang C, Wang E, Chen W, Zhang L, Zhan H, Wu Y, Cao H. J. Org. Chem. 2017; 82: 9144
  CrossrefPubMedSearch in Google Scholar
• 5 Özer MS, Menges N, Keskin S, Şahin E, Balci M. Org. Lett. 2016; 18: 408
  CrossrefPubMedSearch in Google Scholar
• 6 Taskaya S, Menges N, Balcı M. Beilstein J. Org. Chem. 2015; 11: 897
  CrossrefPubMedSearch in Google Scholar
• 7 Boyle JW, Zhao Y, Chan PW. H. Synthesis 2018; 50: 1402
  Thieme ConnectSearch in Google Scholar
• 8 Ding Q, Wang D, Sang X, Lin Y, Peng Y. Tetrahedron 2012; 68: 8869
  CrossrefSearch in Google Scholar
• 9 Ye S, Wang H, Wu J. Eur. J. Org. Chem. 2010; 6436
• 10 Guven S, Ozer MS, Kaya S, Menges N, Balcı M. Org. Lett. 2015; 17: 2660
  CrossrefPubMedSearch in Google Scholar
• 11 Yuan B, Zhang F, Li Z, Yang S, Yan R. Org. Lett. 2016; 18: 5928
  CrossrefPubMedSearch in Google Scholar
• 12 Senadi GC, Wang J.-Q, Gore BS, Wang J.-J. Adv. Synth. Catal. 2017; 359: 2747
  CrossrefSearch in Google Scholar
• 13 Boiani M, Cerecetto H, González M, Piro OE, Castellano EE. J. Phys. Chem. 2004; 108: 11241
  CrossrefSearch in Google Scholar
• 14 Laroche C, Gilbreath B, Kerwin SM. Tetrahedron 2014; 70: 4534
  CrossrefSearch in Google Scholar
• 15 Georges GJ, Vercauteren DP, Evrard GH, Durant FV, George PG, Wick AE. Eur. J. Med. Chem. 1993; 28: 323
  CrossrefSearch in Google Scholar
• 16 Kuzu B, Genç H, Taşpınar M, Tan M, Mengeş N. Heteroatom. Chem. 2018; 29: e21412 ; DOI: 10.1002/hc.21412
  CrossrefSearch in Google Scholar
• 17 Kuzu B, Tan M, Ekmekci Z, Menges N. J. Lumin. 2017; 192: 1096
  CrossrefSearch in Google Scholar
• 18 Nagaraj M, Muthusubramanian S. J. Chem. Sci. (Amritsar, India) 2016; 128: 451
  CrossrefSearch in Google Scholar
• 19 Gaussian 09, Revision D.01 . Gaussian, Inc; Wallingford: 2009
  Search in Google Scholar
• 20a Liu Y, Wan J.-P. Org. Biomol. Chem. 2011; 9: 6873
  CrossrefPubMedSearch in Google Scholar
• 20b Ge Q, Zong J, Li B, Wang B. Org. Lett. 2017; 19: 6670
  CrossrefPubMedSearch in Google Scholar
• 20c Aradi K, Bombicz P, Novák Z. J. Org. Chem. 2016; 81: 920
  CrossrefPubMedSearch in Google Scholar
• 20d Voigtritter K, Ghorai S, Lipshutz BH. J. Org. Chem. 2011; 76: 4697
  CrossrefPubMedSearch in Google Scholar
• 21 Imidazo[1,2-a]pyrazine N-Oxides 5a–g and 6a; General Procedure The appropriate N-propargylimidazole oxime derivative 2a–g, 4a, or 4h (1 mmol) was dissolved in PrOH (5 mL) in a 25 mL flask. A catalytic amount of CuI (10 mol%) was added, and the mixture was refluxed for the appropriate time until the reaction was completed (TLC). The mixture was then extracted with EtOAc–H₂O (3 × 40 mL). The organic layer was dried (MgSO₄) and concentrated under reduced pressure to give a crude product that was purified by column chromatography. The products were further purified by preparative TLC (hexane–EtOAc, 5:1). 6-Methyl-2,8-diphenylimidazo[1,2-a]pyrazine 7-Oxide (5a) Reaction time: 3 h; eluent for column chromatography: hexane–EtOAc (1:1). Brown solid; yield: 250 mg (83%); mp 193–195 °C. FTIR (ATR): 3140, 3038, 2969, 2925, 2163, 1732, 1660, 1604, 1576, 1496, 1489 cm^–1. ¹H NMR (400 MHz, CDCl₃): δ = 8.23 (d, J = 6.91 Hz, 2 H, Ar-H), 7.93–7.89 (m, 3 H, Ar-H), 7.78 (s, 1 H, Ar-H), 7.55–7.48 (m, 4 H, Ar-H), 7.41 (t, J = 7.36 Hz, 2 H, Ar-H), 2.51 (s, 3 H, CH₃). ¹³C NMR (100 MHz, CDCl₃): δ = 148.9, 132.6, 131.0, 130.3, 128.7, 128.6, 127.9, 127.8, 126.3, 116.9, 108.8, 15.5. LCMS: m/z [M + H]⁺ Calcd for C₁₉H₁₆N₃O: 302.12879; found: 302.12958.
• 22 Hagos TK. M.S. Dissertation. University of Stellenbosch: 2006
  Search in Google Scholar
• 23 von Ragué Schleyer P, Maerker C, Dransfeld A, Jiao H, van Eikema Hommes NJ. R. J. Am. Chem. Soc. 1996; 118: 6317
  CrossrefPubMedSearch in Google Scholar
• 24 Menges N, Bildirici I. J. Chem. Sci. (Amritsar, India) 2017; 129: 741
  CrossrefSearch in Google Scholar
• 25a Sun K, Chen X.-L, Li X, Qu L.-B, Bi W.-Z, Chen X, Ma H.-L, Zhang S.-T, Han B.-W, Zhao Y.-F, Li C.-J. Chem. Commun. 2015; 51: 12111
  CrossrefPubMedSearch in Google Scholar
• 25b Yang Y, Qu C, Chen X, Sun K, Qu L, Bi W, Hu H, Li R, Jing C, Wei D, Wei S, Sun Y, Liu H, Zhao Y. Org. Lett. 2017; 19: 5864
  CrossrefPubMedSearch in Google Scholar
• 25c Li D.-Y, Huang Z.-L, Liu P.-N. Org. Lett. 2018; 20: 2028
  CrossrefPubMedSearch in Google Scholar

• Supplementary Material