Synthesis of Tris-Heterocycles via a Cascade IMCR/Aza Diels-Alder + CuAAC Strategy.

6-Triazolylmethyl-pyrrolo[3,4-b]pyridin-5-one tris-heterocycles were synthesized in 43-57% overall yields. The two-stage synthesis involved a cascade process (Ugi-3CR/aza Diels-Alder/N-acylation/aromatization) followed by a copper-assisted alkyne-azide [3+2] cycloaddition (CuAAC). This efficient and convergent strategy proceeded via complex terminal alkynes functionalized with a fused bis-heterocycle at the α-position. The final products are ideal candidates for SAR studies as they possess two privileged scaffolds in medicinal chemistry: 4-substituted or 1,4-substituted 1H-1,2,3-triazoles and pyrrolo[3,4-b]pyridin-5-ones.

Introduction

class="Chemical">Polyheterocycles are organic mole<class="Chemical">span class="Chemical">cules containing three or more heterocyclic moieties, which may be joined by one or more different kinds of connectivity (Ibarra et al., 2018). Nitrogen-containing polyheterocycles are of particular interest in the synthesis of bioactive molecules (Dener et al., 2006; Dolle et al., 2008; Atobe et al., 2013). Tris-heterocyclic molecules have been reported in optics and in coordination chemistry (Stibrany et al., 2003; Burling et al., 2007; Tahara et al., 2009).

The class="Chemical">4-substituted <class="Chemical">span class="Chemical">1H-1,2,3-triazole is a heterocycle of high interest in medicinal chemistry, and it is well-documented that incorporation of this moiety into several bioactive compounds has resulted in advantages such as reduced toxicity or increased antibacterial or antimalarial activity (Shchepin et al., 2008; Zhou et al., 2008) [e.g., Figure 1, compound A (Dixit et al., 2012)]. Triazoles may also increase the stability and polarity of compounds by coordination of the N1 and N2 triazole nitrogens to active-site metal atoms in metalloproteases. The N3 nitrogen appears not to be directly involved in binding metals but can form hydrogen bonds with amino acid residues (Kallander et al., 2005; Huang et al., 2011; Röhri et al., 2012; Borkin et al., 2016). Additionally, 4-substituted 1H-1,2,3-triazoles are precursors of bioactive disubstituted 1,2,3-triazoles (Duan et al., 2009; Oh et al., 2010; Yan et al., 2010; Hsu et al., 2013; Bakulev and Beryozkina, 2016) and of pyridyl-1H-1,2,3-triazolate complexes that have applications in optics and coordination chemistry (Sinn et al., 2014; Prabhath et al., 2015). On the other hand, 1,4-disubstituted 1H-1,2,3-triazoles display structural and electronic similarities with the trans-amide bond. Their overall dipolar moment and hydrogen-bonding properties are greater than those of an amide bond, making these heterocycles effective peptidomimetics (Tron et al., 2008). These triazoles also function as flat bivalent elements, imitating the rigid conformational constraints of double bonds in alkyl chains. 1,4-Disubstituted 1H-1,2,3-triazoles are also capable substitutes for other five-membered nitrogen-containing heterocycles such as imidazoles, pyrazoles, 1,2,4-triazoles, oxazoles, isoxazoles, and oxazolidinones. Finally, these rings can act as more stable isosteres of phosphate linkers (Bonandi et al., 2017).

Figure 1

Bioactive bis- and tris-heterocycles and our synthetic targets.

Bioactive <span class="Chemical">bis- and tris-heterocycles and our synthetic targets.

Common synthetic approaches to class="Chemical">4-substituted 1H-1,2,3-triazoles involve a [3+2] cycloaddition between <class="Chemical">span class="Chemical">sodium azide (NaN3) or hydrazoic acid (HN3) and terminal alkynes activated with electron-withdrawing groups (EWGs) (Hartzel and Benson, 1954; Balle et al., 2003; Blass et al., 2006; Koszytkowska-Stawińska et al., 2012). A variation of this classical methodology using trimethylsilyl azide (TMSN3) provides a much safer procedure. The use of a copper(I) catalyst allows the use of non-activated terminal alkynes in shorter reaction times and under milder conditions (Jin et al., 2004). For preparing 1,4-disubstituted 1-H-1,2,3-triazoles, the classic synthetic approach is the regiospecific copper-assisted [3+2] cycloaddition between terminal alkynes with organic azides (Rostovtsev et al., 2002; Tornøe et al., 2002). The most common technique is in situ reduction of copper(II) salts, such as CuSO4•5H2O or Cu(OAc)2, forming copper(I) salts using sodium ascorbate as the reducing agent. A second option is to use a copper(I) salt such as CuCl, CuBr, CuI, [Cu(CH3CN)4]OTf, or Cu(CH3CN)4PF6 in a deoxygenated environment and in organic solvent, typically with an amine such as TEA, DIEA, DIPEA, or PMDETA (Bock et al., 2006; Hein and Fokin, 2010; Lauria et al., 2014).

On the other hand, the fused heterocycle class="Chemical">pyrrolo[3,4-b]pyridin-5-one is an <class="Chemical">span class="Chemical">aza-analog of isoindolin-1-one natural products and is present in several bioactive molecules; for example: hypoglycemic [Figure 1, compound B (Devasthale et al., 2013)], analgesic, anticancer, and therapeutic agents for CNS-related diseases like Alzheimer's, epilepsy, and schizophrenia (Unverferth et al., 2002; Chang et al., 2008; Pajouhesh et al., 2012; Lindsley et al., 2013). The synthesis of analogs with a brain-selective radioligand has also been reported [Figure 1, compound C (Wager et al., 2017)].

There are no previous reports of moleclass="Chemical">cules containing <class="Chemical">span class="Chemical">4-substituted or 1,4-disubstituted 1H-1,2,3-triazoles and pyrrolo[3,4-b]pyridin-5-ones, though there are a few examples of each of these ring systems connected to other heterocycles using multistep approaches [Figure 1, compounds A-C) (Dixit et al., 2012; Devasthale et al., 2013; Mallemula et al., 2015; Maračić et al., 2015; Wager et al., 2017)].

class="Chemical">Isocyanide-based multicomponent reactions (IMCRs) are the most efficient strategies to synthesize <class="Chemical">span class="Chemical">pyrrolo[3,4-b]pyridin-5-ones, and Zhu first reported a one-pot synthesis in 2001 (Sun et al., 2001). We synthesized various annulated polyheterocycles containing this fused bis-heterocycle via IMCR strategies (Islas-Jácome et al., 2011, 2012). However, there are no published reports of pyrrolo[3,4-b]pyridin-5-ones linked to other heterocycles in a non-annulated fashion using IMCRs. Recently, Van der Eycken reported a one-pot synthesis of disubstituted pyrrolo[3,4-b]pyridin-5-ones by an Ugi four-component reaction (Ugi-4CR)/carbocyclization/deacylation sequence (Scheme 1; Trang et al., 2015). While that work can generate oxidized 7-hydroxy derivatives, we sought a route that would feature improved atom economy, shorter reaction times, and milder conditions in addition to greater structural complexity with more functionalized products (Scheme 1).

Scheme 1

Previous works.

Besson reported the two-stage synthesis of class="Chemical">tris-heterocycles with <class="Chemical">span class="Chemical">pyrido[2,3-d]pyrimidin-4(3H)-one linked to 1,2,3-triazoles via a one-pot DMFDMA-mediated cyclization followed by a CuAAC (Scheme 1; Hédou et al., 2013). However, only two heterocycles are constructed in this process and the scope of the alkynes is limited.

Our ongoing research program foclass="Chemical">cuses on the design of rapid, convergent and efficient IMCR/post-transformation strategies toward novel mole<class="Chemical">span class="Chemical">cules containing privileged heterocycles: azepino[4,5-b]indol-4-ones (Rentería-Gómez et al., 2016b), 2-tetrazolylmethyl-isoindolin-1-ones (Rentería-Gómez et al., 2016a), tetrazolo[1,5-a]quinolines (Unnamatla et al., 2016), 3-tetrazolylmethyl-azepino[4,5-b]indol-4-ones (Gordillo-Cruz et al., 2013), 2,3,4,9-tetrahydro-1H-β-carbolines (Cárdenas-Galindo et al., 2014), 4-(pyridine-3-yl)pyrimidines (Cortes-García et al., 2016), and spiro[pyrrolidine-3,3′-oxindoles] (Alvárez-Rodríguez et al., 2018). Herein we describe the first cascade IMCR process/post-transformation strategy toward the synthesis of 4-substituted 1H-1,2,3-triazoles linked to a fused, bis-heterocyclic peptidomimetic. A key aspect of this work is that the cascade IMCR process rapidly generates a complex alkyne for the subsequent [3+2] cycloaddition (Scheme 2).

Scheme 2

Cascade IMCR /[3+2] cycloaddition strategy.

Results and Discussion

In this work, we report the two-step synthesis of compounds 9a-m and 19a-d, which contain three different heterocycles: class="Chemical">pyridine, <class="Chemical">span class="Chemical">pyrrolidin-2-one, and 1H-1,2,3-triazole (4-substituted and 1,4-disubstituted) (Scheme 3). The use of orthogonal, bifunctional reagents plays a central role in the IMCR/post-transformation strategy, leading to the rapid generation of molecular complexity in both bis-heterocycles 6a-m and final products 9a-l. In the first step, the synthesis of 6a-m occurs by a cascade process combining an Ugi-3CR with aza Diels-Alder, N-acylation, and aromatization reactions to give a complex terminal alkyne functionalized at the α-position with a fused bis-heterocycle. Two fused rings were created in the process, resulting in pyrrolo[3,4-b]pyridin-5-ones 6a-m.

Scheme 3

Synthesis of 4-substituted 1H-1,2,3-triazoles 9a-l and 1,4-disubstituted 1H-1,2,3-triazoles 19a-l via an IMCR/aza Diels-Alder/N-acylation/aromatization/[3+2] cycloaddition sequence.

Synthesis of class="Chemical">4-substituted 1H-1,2,3-triazoles 9a<class="Chemical">span class="Chemical">-l and 1,4-disubstituted 1H-1,2,3-triazoles 19a-l via an IMCR/aza Diels-Alder/N-acylation/aromatization/[3+2] cycloaddition sequence.

To optimize the cascade process, we attempted to synthesize complex class="Chemical">alkyne 6a by sequentially reacting <class="Chemical">span class="Chemical">propargylamine (1), benzaldehyde (2a), isocyanide 3a, and maleic anhydride (4) using toluene (Table 1). Performing the reaction without catalyst at 60–80 °C under conventional conditions produced 6a in 13% yield (entry 1, Table 1). When the reaction was carried out using catalytic amounts of NH4Cl at 60–80 °C (Janvier et al., 2002), the product was isolated in 43% yield (entry 2, Table 1). Under microwave conditions, the yield increased to 57% (entry 3, Table 1). In previous reports, we used TsOH for the aromatization process after a Diels-Alder cycloaddition to construct the isoindolin-1-one moiety (Rentería-Gómez et al., 2016a). Unfortunately, bis-heterocycle 6a was not detected when TsOH was used; only decomposition was observed by TLC.

Table 1

IMCR based cascade strategy screening conditions.

Entrya	Solventb	Additive	T (°C)	t (h)	Yield (%)g
1	PhMe	——-	65–80	12	13
2	PhMe	NH4Clc	65–80	12	43
3	PhMe	NH4Clc	65–80d	1.5	57
4	PhMe	TsOHe	65-80d	1.5	—
5	PhMe	Sc(OTf)3f	65–80	12	42
6	PhMe	Sc(OTf)3f	65–80d	1.5	69
7	PhMe	Yb(OTf)3f	65-80d	1.5	62

1.0 equiv. .

1.0 mL solvent.

1.4 equiv.

MW (100 W).

1.5 equiv.

3.0 mol%.

isolated products.

Bold values indicate the best conditions.

Using catalytic Sc(OTf)3 under conventional heating, 6a was obtained in 42% yield (entry 5, Table 1). Microwave heating with Sc(OTf)3 or Yb(OTf)3 (Islas-Jácome et al., 2011, 2012) raised the yield to 69 and 62%, respectively (entries 6-7, Table 1). We therefore used the optimal conditions (entry 6: 3 mol% Sc(OTf)3, microwave heating 60–80 °C, 1.5 h) to synthesize the series of fused bis-heterocycles 6a<span class="Chemical">-l (Table 2).

Table 2

Synthesis of the 6-propargyl-pyrrolo[3,4-b]pyridin-5-ones 5a-l.

Producta	R1	R2	Yield (%)b
6a	Ph	piperidine	64
6b	3,4-diOMePh	piperidine	72
6c	4-FPh	piperidine	62
6d	n-hexyl	piperidine	64
6e	Ph	morpholine	69
6f	3,4-diOMePh	morpholine	66
6g	4-FPh	morpholine	66
6h	n-hexyl	morpholine	67
6i	Ph	diethylamine	59
6j	3,4-diOMePh	diethylamine	63
6k	4-FPh	diethylamine	64
6l	n-hexyl	diethylamine	66

Reactions performed with 1.0 equiv. propargylamine (.

isolated products.

Synthesis of the <span class="Chemical">6-propargyl-pyrrolo[3,4-b]pyridin-5-ones 5a<class="Chemical">span class="Chemical">-l.

Reactions performed with 1.0 equiv. <span class="Chemical">propargylamine (.

The scope of this cascade process was explored using class="Chemical">alkyl and aryl aldehydes 2a-d and <class="Chemical">span class="Chemical">amide-containing isocyanides 3a-c (Table 2). The role of fluorine atoms in improving bioavailability, lipophilicity and metabolic resistance in bioactive molecules is well-documented (Purser et al., 2008). The products 6c, 6g, and 6k containing fluorine atom was synthesized. Piperidine, morpholine, and diethylamine were incorporated as substituents of isocyanides 3a-c. These fragments can act as structural bioisosteres, preferably interacting with some amino acids allowing in some cases improve biological activity (Kalinsky and Weinstein, 1954; Sander et al., 2008; Meng et al., 2011; El-Nassan, 2015; Yu et al., 2015; Sato et al., 2017).

The highest yield (72%) was obtained for product 6b, which contains class="Chemical">2,3-dimethoxyphenyl and piperidine as substituents at R1 and R2, reclass="Chemical">spectively. Contrarily, bis-heterocycle 6i, with <class="Chemical">span class="Chemical">phenyl and diethylamine substituents, was obtained in the lowest yield (59%). In fact, among all products, the diethylamine-containing analogs 6i-l were synthesized in lower yields, which can be attributed to the lower stability of this isocyanide in acidic media. In all cases, the primary byproducts were the corresponding 5-aminooxazoles resulting from Lewis-acid-catalyzed chain-ring tautomerization of the isocyanides 3a-c (Gao et al., 2016). Consistent with reports by Zhu (Cuny et al., 2004; Wang et al., 2007), we also observed, as minor byproducts, the alcohols resulting from isocyanide addition to the aldehydes prior to oxazole formation (7) (Scheme 4). The plausible reaction mechanism for the formation of pyrrolo[3,4-b]pyridin-5-ones 6a-l is supported by computational calculations performed previously using DFT methods (Scheme 4) (Islas-Jácome et al., 2016).

Scheme 4

Plausible reaction mechanism.

Conditions were screened for the [3+2] cycloaddition using terminal class="Chemical">alkyne 6a (Table 3). Heating at 100 °C for 12 h with 1.0 equiv. <class="Chemical">span class="Chemical">TMSN3 and 3 mol% CuI provided tris-heterocycle 9a in a modest 48% yield (entry 1, Table 2). Increasing the equivalents of the volatile TMSN3 to 1.5 or 2.0 raised the yield of 9a to 69 and 77%, respectively (entries 2-3, Table 2). Using additional CuI (5% mol) and increasing the reaction time to 18 h did not improve the yield (entry 4, Table 2). Microwave heating reduced both the reaction time and the yield to 52%, as high amounts of byproducts were detected (entry 5, Table 2).

Table 3

Screening conditions for the [3+2] cycloaddition.

Entry	TMSN3 (equiv.)	CuI (mol%)	T (°C)	t (h)	Yield (%)a
1	1.0	3	100	12	48
2	1.5	3	100	12	69
3	2.0	3	100	12	77
4	2.0	5	100	18	75
5	2.0	3	100b	0.5	52

isolated products.

MW (100 W).

Bold values indicate the best conditions.

Using the optimized conditions, a series of class="Chemical">tris-heterocycles (9a<class="Chemical">span class="Chemical">-l) was synthesized from complex alkynes 6a-I via the [3+2] cycloaddition in good yields (70–80%, Table 4). The highest yields were obtained for the 4-fluorophenyl analogs. Alkynes 6a-l and triazole products 9a-l were fully characterized by IR, 1H and 13C NMR, and HRMS (see the Supplementary Material for further details). Several attempts to obtain adequate crystals for X-ray analysis were performed without success.

Table 4

Synthesis of 4-substituted 1H-1,2,3-triazoles 9a-l.

Producta	R1	R2	Yield (%)b
9a	Ph	piperidine	77
9b	3,4-diOMePh	piperidine	75
9c	4-FPh	piperidine	80
9d	n-hexyl	piperidine	70
9e	Ph	morpholine	63
9f	3,4-diOMePh	morpholine	73
9g	4-FPh	morpholine	80
9h	n-hexyl	morpholine	75
9i	Ph	diethylamine	73
9j	3,4-diOMePh	diethylamine	70
9k	4-FPh	diethylamine	78
9l	n-hexyl	diethylamine	75

1.0 equiv. alkyne .

isolated products.

Synthesis of <span class="Chemical">4-substituted 1H-1,2,3-triazoles 9a<class="Chemical">span class="Chemical">-l.

1.0 equiv. <span class="Chemical">alkyne .

In Scheme 5, we show a plausible reaction mechanism for the class="Chemical">alkyne-azide [3+2] cycloaddition to produce <class="Chemical">span class="Chemical">4-substituted 1H-1,2,3-triazoles. The reaction likely proceeds through the formation of copper acetylide species 13a-l from terminal alkynes 6a-l with CuI (12) and in situ generation of HN3 (17) from the reaction of TMSN3 (15) and MeOH (14). Copper-assisted cycloaddition between 13a-l and HN3 (17) takes place to form intermediates 18a-l. Protonolysis of the C-Cu bond of 18a-l by terminal alkynes 6a-l, HI, or MeOH affords 4-substituted 1H-1,2,3-triazoles 9a-l (Jin et al., 2004).

Scheme 5

Plausible CuAAC reaction mechanism.

Plausible <span class="Chemical">CuAAC reaction mechanism.

Having the methodology for the synthesis of compounds 9aclass="Chemical">-l, we next explored the scope of use de terminal <class="Chemical">span class="Chemical">alkynes linked to bis-heterocycles using organic azides to obtain the 1,4-disubstituted 1,2,3-triazoles (19a-d) via CuAAC. Compound 6m was synthetized (60% yield) and selected as model. Phenyl azides with different stereo-electronic natures (20a-d) were prepared from aromatic amines via diazotization with sodium nitrite in water in the presence of p-TsOH followed by reaction with sodium azide at room temperature (Kutonova et al., 2013).

First, the reaction was carried out under constant stirring, at room temperature, using 6m and class="Chemical">azide 20a obtaining an 85% yield of <class="Chemical">span class="Chemical">1,4-disubstituted 1,2,3-triazole 19a after 5 h. When the reaction was carried out using ultrasound-assisted irradiation (USI) at room temperature, the product 19a was obtained in 1.5 h, with a yield of 83%. For this reason, we decided to use the USI protocol for the synthesis of 1,4-disubstituted 1-H-1,2,3-triazoles (19a-d). The reactions under USI resulted in reduced reaction times (30–90 min) and good yields (83–93%) in the CuAAC for the synthesis of 19a-e (Table 5).

Table 5

Synthesis of 1,4-disubstituted 1H-1,2,3-triazoles 19a-e.

Producta	R3	t (min)	Yield (%)b
19a		90	83
19b		30	89
19c		45	93
19d		45	90

1.0 equiv. azide .

isolated products.

Synthesis of <span class="Chemical">1,4-disubstituted 1H-1,2,3-triazoles 19a-e.

1.0 equiv. <span class="Chemical">azide .

Conclusions

We have developed a new and efficient strategy to synthesize class="Chemical">4-substituted and <class="Chemical">span class="Chemical">1,4-disubstituted 1H-1,2,3-triazoles linked to pyrrolo[3,4-b]pyridin-5-ones. Molecules containing these heterocycles together are novel. The molecules synthesized contain privileged tris-heterocycles which could have applications in medicinal chemistry and coordination chemistry.

The IMCR based cascade process coupled with <span class="Chemical">CuAAC strategy, as convergent and powerful tool toward the synthesis of <class="Chemical">span class="Chemical">bis and tris heterocycles is unreported.

Data Availability

All datasets generated for this study are included in the manuscript and/or the Supplementary Files.

Author Contributions

MR-G, RG-M, and DV have made a substantial, direct and intellectual contribution to the work. <span class="Chemical">SP was reclass="Chemical">sponsible for performing the initial experiments. AI-J was reclass="Chemical">sponsible for designing and analyzing the results. All authors dis<class="Chemical">span class="Chemical">cussed the whole project, wrote the publication, and approved it for publication.

Conflict of Interest Statement

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.