Multicomponent Synthesis of 2-(2,4-Diamino-3-cyano-5H-chromeno[2,3-b]pyridin-5-yl)malonic Acids in DMSO

Dimethyl sulfoxide is a widely used solvent in organic synthesis and in the pharmaceutical industry because of its low cost, stability, and low toxicity. Multicomponent reactions are an advanced approach that has become an efficient, economical, and eco-friendly substitute for the conventional sequential multi-step synthesis of various biologically active compounds. This approach was adopted for the synthesis of previously unknown 2-(2,4-diamino-3-cyano-5H-chromeno[2,3-b]pyridin-5-yl)malonic acids via transformation of salicylaldehydes, malononitrile dimer, and malonic acid. It was shown that the use of DMSO at room temperature makes it possible to synthesize previously unavailable compounds. The investigation of the reaction mechanism using 1H-NMR monitoring made it possible to confirm the proposed mechanism of the transformation. The structure of synthesized 5H-chromeno[2,3-b]pyridines was confirmed by 2D-NMR spectroscopy.


Introduction
Multicomponent reactions (MCRs) are an important methodological arsenal in synthetic and medicinal chemistry [1]. A large number of publications that have appeared in this area over the past 5 years can confirm the significance of MCRs. This advanced approach has emerged as an efficient, economical, and eco-friendly substitute for the conventional sequential multi-step synthesis of various biologically active compounds [2]. MCRs exhibit a very high bond-forming index (BFI) as several non-hydrogen atom bonds are formed in one synthetic transformation [3]. Therefore, MCRs are the best strategy for the synthesis of complex heterocyclic structures.
Chromeno [2,3-b]pyridines are one of the important classes of condensed heterocyclic compounds from the point of view of medicinal chemistry. Depending on the substituents, they can exhibit different types of biological activity, such as antimicrobial [4], anticancer [5], antirheumatic [6], antimyopic [7], neuroprotective [8], and hypotensive [9] activities. Thus, the synthesis of a new type of chromeno [2,3-b]pyridines is an important goal for researchers.
Dimethyl sulfoxide (DMSO) is widely used as a solvent in organic synthesis and in the pharmaceutical industry because of its low cost, stability, and low toxicity [10]. Some of the characteristics of this polar solvent, such as its ability to stabilize charged intermediates and its high boiling point, are similar to those of dimethylacetamide, dimethylformamide (DMF), and N-methyl-2-pyrrolidone (NMP). However, DMSO is less toxic than other polar solvents and is extensively used as a solvent or an effective oxidant.

Multicomponent Synthesis of 2-(2,4-Diamino-3-cyano-5H-chromeno[2,3-b]pyridin-5-yl)malonic Acids 4a-h
Previously we reported the pot, atom, and step economy (PASE) synthesis of hydroxyquinolinone substituted chromeno [2,3-b]pyridines [21]. In this article, we presented 1 H NMR real-time monitoring of the reaction in an NMR sample tube in DMSO-d 6 to confirm one of the proposed pathways of the transformation. The reaction in an NMR spectrometer proceeded efficiently and quickly. This fact gave us a reason to study the obtainment of chromeno [2,3-b]pyridines in DMSO already in a flask.

Multicomponent Synthesis of 2-(2,4-Diamino-3-cyano-5H-chromeno[2,3-b]pyridin-5yl)malonic Acids 4a-h
Previously we reported the pot, atom, and step economy (PASE) synthesis of hydroxyquinolinone substituted chromeno [2,3-b]pyridines [21]. In this article, we presented 1 H NMR real-time monitoring of the reaction in an NMR sample tube in DMSO-d6 to confirm one of the proposed pathways of the transformation. The reaction in an NMR spectrometer proceeded efficiently and quickly. This fact gave us a reason to study the obtainment of chromeno [2,3-b]pyridines in DMSO already in a flask.
When studying the reaction in DMSO, DMF, and NMP without heating for 24 h, the final compound 4a was isolated in good yields of 70-92% (Table 1, Entries 1-3). For isolating 4a, 15 mL of water was added to the reaction mixture. After that, we tried to carry out the transformation of the salicylaldehyde 1a, malononitrile dimer 2, and malonic acid 3 in our best previously found reaction systems ( When studying the reaction in DMSO, DMF, and NMP without heating for 24 h, the final compound 4a was isolated in good yields of 70-92% (Table 1, Entries 1-3). For isolating 4a, 15 mL of water was added to the reaction mixture.
Since the highest yield of chromeno[2,3-b]pyridine 4a was achieved in DMSO, the reaction was further investigated in this solvent (Table 1, Entries 4-6). The reaction time ( Table 1, Entry 4), the amount of water added to isolate compound 4a (Table 1, Entry 5), and the reaction temperature (Table 1, Entry 6) were varied. However, in all these cases, it was not possible to increase the yield 4a.
The substituent affects the yields of chromeno[2,3-b]pyridines 4. Electron-donating methyl-, methoxy-, and ethoxy-groups decrease the yields of 4. In the case of halogen substituents, the yield of compound 4 is increased. In the presence of both types of substituents (methoxy-and bromine, 4g), the yield is average. This is due to the fact that halogens promote the delocalization of the negative charge, stabilize the intermediate anion, and thereby increase its acidity. Table 2. Multicomponent reaction of salicylaldehydes 1a-h, malononitrile dimer 2 and malonic acid 3 1 .
The substituent affects the yields of chromeno[2,3-b]pyridines 4. Electron-donating methyl-, methoxy-, and ethoxy-groups decrease the yields of 4. In the case of halogen substituents, the yield of compound 4 is increased. In the presence of both types of substituents (methoxy-and bromine, 4g), the yield is average. This is due to the fact that halogens promote the delocalization of the negative charge, stabilize the intermediate anion, and thereby increase its acidity. Table 2. Multicomponent reaction of salicylaldehydes 1a-h, malononitrile dimer 2 and malonic acid 3 1 .
The substituent affects the yields of chromeno[2,3-b]pyridines 4. Electron-donating methyl-, methoxy-, and ethoxy-groups decrease the yields of 4. In the case of halogen substituents, the yield of compound 4 is increased. In the presence of both types of substituents (methoxy-and bromine, 4g), the yield is average. This is due to the fact that halogens promote the delocalization of the negative charge, stabilize the intermediate anion, and thereby increase its acidity.

2D-NMR Study of the Structure of Compound 4f
The structure of compound 4f was confirmed by NMR spectroscopy. The proton spectrum contained signals from all groups, including carboxyl fragments (broad signal at 12.8 ppm). The benzene fragment is substituted at position 7, as evidenced by the characteristic set of signals in the proton spectrum (two doublets at 7.65 and 7.01 ppm and one doublet of doublets at 7.43 ppm). The signals of the amino groups of the pyridine ring (δ 6.60 and 6.48 ppm) could be distinguished due to the detected correlation in the 1 H-13 C-HMBC spectrum of 4-NH 2 with C 4a (δ H/C 6.60/87.7 ppm) (Figure 1). The carbon signal of the third position has a very upfield chemical shift (δ 71.1 ppm). This chemical shift is due to the substitution of the nitrile group, and the shielding of the nucleus by electrons of the triple bond. In position 5, there is a malonic acid residue. This is confirmed by the spin interaction from the HMBC spectrum of H a with the carbons of the benzene and pyridine rings (Figure 1).
The proton spectrum contained signals from all groups, including carboxyl fragments (broad signal at 12.8 ppm). The benzene fragment is substituted at position 7, as evidenced by the characteristic set of signals in the proton spectrum (two doublets at 7.65 and 7.01 ppm and one doublet of doublets at 7.43 ppm). The signals of the amino groups of the pyridine ring (δ 6.60 and 6.48 ppm) could be distinguished due to the detected correlation in the 1 H-13 C-HMBC spectrum of 4-NH2 with C 4a (δH/C 6.60/87.7 ppm) (Figure 1). The carbon signal of the third position has a very upfield chemical shift (δ 71.1 ppm). This chemical shift is due to the substitution of the nitrile group, and the shielding of the nucleus by electrons of the triple bond. In position 5, there is a malonic acid residue. This is confirmed by the spin interaction from the HMBC spectrum of H a with the carbons of the benzene and pyridine rings (Figure 1 Two-dimensional (2D) NMR spectra of the compound 4f are presented in Supplementary Materials (Figures S17 and S18).

1 H-NMR Reaction Monitoring
We assumed that the reaction proceeds according to the standard mechanism, which we proved earlier [21]. However, to prove our assumption, the reaction was monitored using 1 H-NMR spectroscopy (Figure 2).
To reduce the influence of sample preparation, the transformation of starting materials into chromeno[2,3-b]pyridine 4a was carried out and monitored directly in an NMR sample tube into a spectrometer without catalyst in DMSO-d6 to slow down the reaction.
As shown in Figure 2, malononitrile dimer 2 is consumed quickly, and the Knoevenagel adduct 5 is formed. Compound 5 cyclizes to intermediate 6. In this spectrum, we also found intermediate 7, which is the final compound of the Michael reaction. Also in Figure 2, the target chromeno[2,3-b]pyridine 4a is recorded. Two-dimensional (2D) NMR spectra of the compound 4f are presented in Supplementary Materials ( Figure S17 and Figure S18).

1 H-NMR Reaction Monitoring
We assumed that the reaction proceeds according to the standard mechanism, which we proved earlier [21]. However, to prove our assumption, the reaction was monitored using 1 H-NMR spectroscopy (Figure 2).  To reduce the influence of sample preparation, the transformation of starting materials into chromeno [2,3-b]pyridine 4a was carried out and monitored directly in an NMR sample tube into a spectrometer without catalyst in DMSO-d 6 to slow down the reaction.
As shown in Figure 2, malononitrile dimer 2 is consumed quickly, and the Knoevenagel adduct 5 is formed. Compound 5 cyclizes to intermediate 6. In this spectrum, we also found intermediate 7, which is the final compound of the Michael reaction. Also in Figure 2, the target chromeno[2,3-b]pyridine 4a is recorded.
Based on the above data and taking into consideration earlier published results [19,21], we suggest that the first stage was a rapid formation of

General Information
The solvents and reagents were purchased from commercial sources and used as received. 2-Aminoprop-1-ene-1,1,3-tricarbonitrile 2 was obtained from malononitrile according to the literature [24].
During the investigation of the reaction mechanism using 1 H-NMR monitoring, it was determined that the multicomponent process proceeds according to the usual mechanism confirmed by us earlier. Two-dimensional (2D) NMR spectroscopy confirmed the proposed structure of synthesized 5H-chromeno [2,3-b]pyridines.