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
Polyheterocycles are organic molecules 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).
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).

RESULTS AND DISCUSSION
In this work, we report the two-step synthesis of compounds 9a-m and 19a-d, which contain three different heterocycles: pyridine, 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/posttransformation 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.
The scope of this cascade process was explored using alkyl and aryl aldehydes 2a-d and 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 2,3-dimethoxyphenyl and piperidine as substituents at R 1 and R 2 , respectively. Contrarily, bis-heterocycle 6i, with phenyl and diethylamine substituents, was obtained in the lowest yield (59%). In fact, among all products, the diethylaminecontaining 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-acidcatalyzed 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[3,4b]pyridin-5-ones 6a-l is supported by computational calculations performed previously using DFT methods (Scheme 4) .
Conditions were screened for the [3+2] cycloaddition using terminal alkyne 6a (Table 3). Heating at 100 • C for 12 h with 1.0 equiv. TMSN 3 and 3 mol% CuI provided tris-heterocycle 9a in a modest 48% yield (entry 1, Table 2). Increasing the equivalents of the volatile TMSN 3 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). Using the optimized conditions, a series of tris-heterocycles (9a-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 6al and triazole products 9a-l were fully characterized by IR, 1 H and 13 C NMR, and HRMS (see the Supplementary Material for further details). Several attempts to obtain adequate crystals for X-ray analysis were performed without success.
Having the methodology for the synthesis of compounds 9a-l, we next explored the scope of use de terminal alkynes linked to bis-heterocycles using organic azides to obtain the 1,4disubstituted 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 azide 20a obtaining an 85% yield of 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-1,2,3triazoles (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).

CONCLUSIONS
We have developed a new and efficient strategy to synthesize 4substituted and 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 CuAAC strategy, as convergent and powerful tool toward the synthesis of 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. SP was responsible for performing the initial experiments. AI-J was responsible for designing and analyzing the results. All authors discussed the whole project, wrote the publication, and approved it for publication.