Unprecedented Rearrangement of 2-(2-Aminoethyl)-1-aryl-3,4-dihydropyrazino[1,2-b]indazole-2-ium 6-oxides to 2,3-Dihydro-1H-imidazo[1,2-b]indazoles

Abstract

Easily accessible 2-(2-aminoethyl)-1-aryl-3,4-dihydropyrazino[1,2-b]indazole-2-ium 6-oxides rearranged to 2,3-dihydro-1H-imidazo[1,2-b]indazoles under mild conditions. The rearrangement appeared to be general, tolerated a wide range of functional groups, and provided access to an as yet unexplored class of heterocycles. Herein we report the characterization of this heterocycles.

In continuation of our search for novel and efficient routes to pharmacologically relevant heterocyclic compounds we discovered a process for tandem carbon-carbon followed by nitrogen-nitrogen bond formation yielding indazole oxides (Scheme 1) of excellent purity.¹

Scheme 1.

Synthesis of indazole oxides¹

Synthetic compounds comprising indazole core have recently become an increasingly frequent subject of biological studies. A review article by Cerecetto and colleagues² portrayed the diversity of biological activities exhibited by indazoles, recent advances in the chemistry of indazoles were reviewed by Schmidt and colleagues.³ Since then, numerous new studies identified indazole-based compounds as potent agents with anti-inflammatory, anticancer,^4-6 antimicrobial,^7,8 antifungal,^9,10 and cytotoxic¹¹ activities.

Indazoles were found to be potent inhibitors of nitric oxide synthetase,^12-14 factor Xa,¹⁵ protein kinases,^16,17 tubulin,¹⁸ reverse transcriptase,¹⁹ vascular endothelial growth factor receptor,²⁰ and TRPV1.^21,22 Indazoles were active as male contraceptives^23,24 and 5-HT2C receptor agonists.²⁵

Wide range of biological activities prompted us to extend our indazole chemistry for traceless solid-phase synthesis of pyrazino[1,2-b]indazoles²⁶ and 2-(2-aminoethyl)-1-aryl-3,4-dihydropyrazino[1,2-b]indazole-2-ium 6-oxides.²⁷ Heterocycles were synthesized in a very efficient three-step procedure on solid phase under mild conditions using commercially available building blocks: amines, 2-nitrobenzenesulfonyl chlorides and bromoketones.

Here, we report an unprecedented rearrangement of 2-(2-aminoethyl)-1-aryl-3,4-dihydropyrazino[1,2-b]indazole-2-ium 6-oxides to 2,3-dihydro-1H-imidazo[1,2-b]indazoles, formally involving concomitant 5-membered ring opening, 6- to 5-membered ring contraction, amide formation, and deoxygenation. The rearrangement proceeded quantitatively under mild conditions and provided a route to a very efficient synthesis of this class of thus far unexplored heterocycles. 2,3-Dihydro-1H-imidazo[1,2-b]indazoles have not been reported in the literature and we found only one report related to our fused heterocycles that described synthesis of benzimidazoindazoles²⁸ and oxazolo[3,2-b]indazoles.²⁹

During the isolation of 2-(2-aminoethyl)-1-aryl-3,4-dihydropyrazino[1,2-b]indazole-2-ium 6-oxides, prepared following our recently published procedure,²⁷ we observed a quantitative rearrangement of the targeted compounds. Indazole oxide derivative 1(1,1) (R¹ = H, and R² = Me, refer to Table 2 for notation) prepared using 2-nitrobenzenesulfonyl chloride and 2-bromo-1-p-tolylethanone, was purified on semi preparative HPLC using aqueous ammonium acetate buffer and acetonitrile. The purified compound was isolated after solvent evaporation at elevated temperature (50 °C) and freeze drying. The LCMS analysis of supposedly purified compound revealed that the target compound had transformed to a new product that exhibited an identical mass spectrum. However, the retention time had changed; the new compound was more hydrophobic. Its UV spectrum was nearly identical to compound 1(1,1). We isolated and fully characterized the unexpected product by 1D and 2D NMR spectroscopy and high resolution MS. In addition, we were able to crystallize the product from acetonitrile solution and its structure was determined by a single crystal X-ray diffraction study (Figure in the Supporting Information section). The structure of the rearranged product was 2,3-dihydro-1H-imidazo[1,2-b]indazole derivative 2(1,1) (Scheme 2)

Table 2.

The effect of R¹ and R² on formation of 2,3-dihydro-1H-imidazo[1,2-b]indazoles 2^a

Entry	R1	R2	T (°C)	Rt of 1a	Rt of 2a	Purityb	Yield
2(1,1)	H	4-Me	50 °C	5.13	5.70	86%	53%
2(1,2)	H	4-OMe	50 °C	4.52	5.25	97%	65%
2(1,4)	H	4-CN	80 °C	4.58	5.13	58%	26%
2(1,5)	H	c	80 °C	5.23	5.75	83%	50%
2(2,2)	4-CF3	4-OMe	50 °C	6.23	6.58	94%	71%
2(2,3)	4-CF3	4-Cl	50 °C	7.30	7.52	76%	37%
2(2,4)	4-CF3	4-CN	80 °C	6.15	6.47	79%	23%
2(2,5)	4-CF3	c	80 °C	6.87	6.97	85%	67%
2(3,2)	4-NO2	4-OMe	50 °C	5.47	5.80	94%	53%
2(3,3)	4-NO2	4-Cl	50 °C	6.57	6.77	87%	41%
2(3,4)	4-NO2	4-CN	80 °C	5.37	5.75	72%	41%
2(3,5)	4-NO2	c	80 °C	6.15	6.18	89%	52%

^aRetention time (min) on analytical C18 column (for conditions, c.f., supporting information)

^bcalculated from UV response on LC traces at 200 – 400 nm

^cR² = 3,5-diCl-4-NH₂

Scheme 2.

Rearrangement of indazoles 1

Note: (i) 50% acetonitrile in 10 mM aqueous ammonium acetate, 16h, for temperature, c.f., Table 2

This unexpected and very clean (and thus potentially very useful from the preparative point of view) rearrangement prompted a focused study of this transformation reaction. The model experiments were carried out with indazole oxide derivative 1(1,1).

Solutions of 1(1,1) in 50% aqueous acetonitrile were subjected to seven reaction conditions and the conversion of 1(1,1) to 2(1,1) was monitored by LCMS analysis. A solution containing 10 mM ammonium acetate was heated to 50 °C for 16 h and a quantitative conversion to 2(1,1) was observed (Table 1). A solution containing 0.1% TFA, typically used for HPLC purification, was completely stable under identical conditions. The data indicated that solutions at pH above 7 caused a quantitative conversion to 2(1,1). Crude preparations of 1, obtained after TFA/DCM cleavage from resin and evaporation of TFA and DCM, still contained residual TFA. Therefore, for practical syntheses of 2, pH of the solution was adjusted by saturated solution of sodium carbonate and exposed to elevated temperature.

Table 1.

The effect of a base on conversion to the rearranged product 2^a

Entry	Aqueous solution	21 °C, 15 min	50°C, 16 h
1	10 mM NH4OAc	NT	100%
2	0.1% TFA	NT	0%
3	10 mM TFA, 10 mM NH4OAc	1%	5%
4	10 mM TEA, 10 mM HOAc	50%	100%
5	10 mM NaOAc	10%	100%
6	10 mM NH2OH.HCl	1%	5%
7	entry 3 plus saturated solution Na2CO3	1%	100%

^aNote: The compound 1(1,1), ~0.6 mg, was dissolved in 200 μL acetonitrile and 200 μL of aqueous solution was added.

To assess the scope and limitation of the rearrangement, we prepared a set of compounds with building blocks containing both electron-donating and electron-withdrawing groups. The reaction conditions, temperature and time, for individual compounds are listed Table 2. The course of the reaction and formation of the rearranged product was monitored by both LC and ¹H NMR. A few of the products, 2(2,3), 2(3,3), 2(2,5) and 2(3,5), eluted very close to their corresponding precursors (Table 2) and monitoring of completion of the rearrangement by LC was problematic. ¹H NMR spectra were collected to reliably determine completion of the reaction. ¹H NMR spectra exhibited typical signals for two methylene groups of the constituent aliphatic chain (two triplets in the area 4.5-4.0 ppm) and a triplet at 8.5 assigned to the NH proton. The ¹³C NMR spectrum revealed the typical C=O resonance at δ 164 - 166 ppm and it was used as the indicator for the formation of the products.

All combinations of building blocks afforded the corresponding rearranged products. The analytical data indicated that the rates of the conversions were not significantly influenced by the R¹ substituent. The effect of the R² substituent was more pronounced. The rearrangement was complete to give compounds 2 with electron donating groups (R² = 4-Me, 4-OMe), and 4-Cl (R¹ = CF₃, and NO₂) at 50 °C. To observe complete transformation of derivatives 2 with R² = 4-Cl (R¹ = H), 4-CN, the temperature was elevated to 80 °C. The LC results showed that, although the products 2(1,1) and 2(1,5) were complete after 3.5 and 2h respectively, the conversion of compounds 1(R¹,3) was only 40 to 70% after 3h, hence they were allowed to react overnight.

Starting materials and products of compounds 2(2,5) and 2(3,5) eluted close to each other and ¹H NMR spectra, recorded after overnight reaction, confirmed quantitative rearrangement. Analyses of LC traces and ¹H NMR spectra confirmed that the rearrangements were very clean. The only minor impurities we detected were present in 1 – 7% with respect to the products and gave MS spectra with ions of m/z = product −15, which may supposedly belong to deoxygenated products formed during indazole cyclizations and subsequent rearrangements.

We propose the following mechanistic explanation of the rearrangement (Scheme 3). In neutral pH, the iminium 1 closed a five-membered ring and formed the imidazolidine derivative 3.²⁷ This rearrangement was triggered by an initial formation of carbon-oxygen bond by nucleophilic attack at the quaternary carbon by water with concurrent ring contraction and formation of imidazolidine 4. The formation of this new carbon-nitrogen bond was facilitated by the presence of the N-oxide, responsible for electronic activation of the electron-deficient nitrogen. Formation of N-hydroxy derivative 4 was the critical step towards water elimination that stabilized the intermediate structure by re-aromatization of indazole core and formation of carboxamide 2.

Scheme 3.

Proposed mechanism of 2,3-dihydro-1H-imidazo[1,2-b]indazoles formation

An alternative reaction mechanism, suggested by one of the reviewers, included formation of an intermediate 3′, followed by the water attack.

To confirm the essential role of the N-oxide, we prepared the deoxygenated analog of 1 using methanesulfonyl chloride and triethylamine.³⁰ After two days of heating in ammonium acetate solution at 50 °C no change of the deoxygenated material was observed.

To conclude, we report an unprecedented formation of 2,3-dihydro-1H-imidazo[1,2-b]indazole by rearrangement formally involving 5-membered ring opening, 6- to 5-membered ring contraction, amide formation and deoxygenation. In several examples, the rearrangements proceeded cleanly under mild conditions and tolerated a wide range of substitution pattern on both aromatic rings. 2,3-Dihydro-1H-imidazo[1,2-b]indazoles can be regarded as 2,3-substituted-2H-indazoles with an additional ring closed between substituents at positions 2 and 3. Synthetic compounds comprising indazole core have recently become an increasingly frequent subject of biological studies: indazole derivatives possessed significant potency in a wide range of diverse biological targets.²

Experimental Section

2-(2-Aminoethyl)-1-aryl-3,4-dihydropyrazino[1,2-b]indazole-2-ium 6-oxides 1

Synthesis of 1 was carried out on solid phase as described previously.²⁷ After finishing the solid-phase synthesis, products were cleaved from the resin with 50 % TFA / DCM for 1 h. The TFA solution was collected. The resin was washed with 50 % TFA / DCM (3 ×) and combined extracts were evaporated by a stream of nitrogen.

2,3-Dihydro-1H-imidazo[1,2-b]indazoles 2

After cleavage from the resin, the crude oily residue, typically ~100 mg, was dissolved in 3 mL of acetonitrile, diluted with 3 mL of 10 mM aqueous ammonium acetate and the pH was adjusted by addition of saturated solution of Na₂CO₃ to pH = 8. The resulting solution was heated to 50 – 80 °C for 16 h. Solvents were evaporated under reduced pressure. The oily residue was dissolved in a minimum volume of acetonitrile and purified by semi-preparative reversed phase HPLC in a gradient formed from acetonitrile and 10 mM aqueous ammonium acetate. Products were collected after freeze-drying.

N-(2-(2,3-dihydro-1H-imidazo[1,2-b]indazol-1-yl)ethyl)-4-methoxybenzamide 2(1,2)

Yield (HPLC purified) 27.3 mg (65%). ESI-MS m/z = 337 [M+H]⁺. ¹H NMR (500MHz, DMSO-d₆) δ: 8.53 (t, J = 5.6 Hz, 1 H), 7.75 - 7.81 (m, 2 H), 7.61 (d, J = 8.4 Hz, 1 H), 7.27 (d, J = 8.8 Hz, 1 H), 7.06 (ddd, J = 1.1, 6.6, 8.9 Hz, 1 H), 6.94 - 6.99 (m, 2 H), 6.72 (ddd, J = 0.9, 6.6, 8.5 Hz, 1 H), 4.38 (t, J = 8.4 Hz, 2 H), 4.00 (t, J = 8.4 Hz, 2 H), 3.79 (s, 3 H), 3.52 - 3.59 (m, 4 H). ¹³C NMR (126MHz, DMSO-d₆) δ: 166.0, 161.5, 152.5, 147.4, 128.9, 126.6, 125.6, 119.6, 117.1, 116.7, 113.4, 103.8, 55.3, 54.5, 49.4, 46.5, 37.7 HRMS (FAB) m/z calcd for C₁₉H₂₁N₄O₂ [M + H]⁺ 337.1665, found 337.1666.