Green Aspect for Multicomponent Synthesis of Spiro[4H-indeno[1,2-b]pyridine-4,3′-[3H]indoles]

Abstract An efficient, four-component reaction of isatin, 1,3-indanedione, ethyl acetoacetate, and ammonium acetate in ethanol/water (9:1) system furnished spiro[4H-indeno[1,2-b]pyridine-4,3′-[3H]indoles] at room temperature. Merits of the method are mild reaction conditions, simple workup procedure, and ambient temperature. The synthesized compounds exhibit excellent fluorescence properties. GRAPHICAL ABSTRACT


INTRODUCTION
Designing and developing an efficient, simple, and economic protocol under the umbrella of green chemistry for organic transformations is an attractive research area from both academic and industrial point of view. [1] In this vein, multicomponent reactions (MCRs) are promising tools, permitted a rapid access to large number of structurally related, druglike compounds [2][3][4] and thereby facilitating lead generation of "privileged medicinal scaffolds." [5][6][7] Thus they become a crucial part of today's arsenal of methods in combinatorial chemistry because of their valued features such as atom-economy, energy economy, straightforward reaction design, economic route, and the opportunity to construct target compounds by the introduction of several diversity elements in a single chemical event. [8] Environmental aspects triggered scientists to focus on the modification of known MCRs with green characteristics, such as development of catalyst-free and solvent-free methods, and avoidance of organic solvents and auxiliary chemicals paraphrased by the concept ''the best catalyst is no catalyst,'' reported by Maggi. [9] Indenone is ambitious nucleus of several bioactive heterocycles, for example, alkaloids like onychnine of 4-azafluorenone group involve the indenopyridine skeleton ( Fig. 1). [10] Indenopyrazoles (A) and indenopyridazines (B) have been investigated as cyclin-dependent kinase [11] and selective monoamine oxidase B (MAO-B) [12] inhibitors, respectively. Further, indenopyridines (C) exhibit cytotoxic, [13a] phosphodiesteraseinhibitory, [13b] adenosine A2a receptor antagonistic, [13c] anti-inflammatory=antiallergic, [13d] coronary dilating, [13e] and calcium modulating activities. [13f] These compounds have also been investigated for the treatment of hyperlipoproteinemia and arteriosclerosis, [13g] as well as neurodegenerative diseases. [13h] Indenopyridone NSC 314622 is serving as a lead compound for the development of anticancer agents targeting topoisomerase I. Its polycyclic planar structure allows for DNA intercalation and inhibition of DNA relegation by topoisomerase I in a manner similar to the polycyclic natural product camptothecin and its clinically useful derivative topotecan. [14] Furthermore, indole moiety is also embedded in the most well-known heterocycles, a variety of natural products and medicinal agents. [15] Furthermore, it has been reported that sharing of the indole-3-carbon atom in the formation of spiroindoline derivatives can highly enhance biological activity. [16] The spirooxindole system is the core structure of many pharmacological agents and natural alkaloids. [17]  For example, spirotryprostatin A and B, two natural alkaloids isolated from the fermentation broth of Aspergillus fumigatus, have been identified as novel inhibitors of microtubule assembly, [17d] and pteropodine and isopteropodine have been shown to modulate the function of muscarinic serotonin receptors (Fig. 1). [17a] Owing to the importance of indenones and spiroindoles, their presence in the spiro[4H-indeno [1,2-b]pyridine-4,3 0 -[3H]indoles] may confer important properties.
In continuation of our earlier experience with catalyst-free synthesis of polyhydroquinolines, [19a] we have foreseen that NH 4 OAc, one of the reactant upon hydrolysis leads to in situ formation of acetic acid, may act as a catalyst for the present transformation without need for an external catalyst. In this context, in a pilot experiment as a model reaction, a mixture of isatin, 1,3-indanedione, ethyl acetoacetate, and ammonium acetate was stirred in ethanol at ambient temperature.
good yield. The precipitated solid was filtered and washed with ethanol to furnish the desired product in high purity, thus avoiding tedious workup procedure and chromatographic separation for isolation and purification of product, respectively.
The formation of product was confirmed by spectral techniques including infrared (IR), 1 H and 13 C NMR, mass spectrometry (MS), high-resolution MS (HRMS), and elemental analysis. The IR spectrum (entry F, Table 2) exhibited bands at 3426, 3322 cm À1 for NH stretching, while band detected at 1700, 1674, 1648 cm À1 are due to stretching of >C=O. In 1 H NMR spectrum, two significant singlets at d 10.35, 10.31 ppm confirmed the presence of two -NH protons, and a multiplet depicted at d 3.76-3.80 highlighted the presence of -OCH 2 , singlet at d 2.43 ppm is due to -CH 3 protons, while another CH 3 adjacent to -OCH 2 displayed a triplet for three proton at d 0.86-0.90 ppm. In 13 C NMR three carbonyl carbons appeared at d 189.97, 179.81, 166.07 ppm while the spiro carbon and OCH 2 appeared at 60.29 and 49.98 ppm, respectively. Mass spectrum displayed peaks at 512 (Mþ), 484, 456, 439, 411 (m=z), which is also in good agreement with the proposed structure.
For optimization of reaction conditions, keeping in mind principles of green chemistry, initially we carried out model reaction in water; however, no desired product was observed even after 12 h stirring (Table 1, entry 1). With our earlier experience in a mixed solvent system, we focused our attention on screening of ethanol: water system for a model reaction (Table 1, entries 2-10). Satisfyingly, we observed formation of the desired product in excellent yield in 90% ethanol.
To obtain a library of spiroindenopyridineindoles employing the optimized reaction conditions, we used a wide diversity of substituted isatins. Electron-rich, electron-deficient, and N-substituted isatins reacted smoothly without any remarkable reactivity difference (Table 2).

Photophysical Properties of Synthesized Compounds
The absorption and fluorescent spectra of spiro[4H-indeno[1,2-b]pyridine-4,3 0 -[3H]indoles] (SIPI) and its substituted derivatives were studied. The solubility of synthesized compounds was checked in different solvents such as dichloromethane (DCM), toluene, CCl 4 , acetonitrile, methanol, ethanol, chloroform, acetone, dimethylformamide (DMF), and dimethylsulfoxide (DMSO). Excellent solubility was observed in DMSO solvent, which was used for absorption and fluorescence emission studies of synthesized compounds. Figure 2 illustrates the absorption spectra of SIPI (G) and its derivatives A to F and H to K in DMSO. The absorption spectra display no significant spectral shift in absorption maxima but an increase in absorption was observed. The absorption spectra of SIPI and its derivatives were composed of structured broad bands at 343 and 470 nm. However, the structured, broad absorption spectrum of dilute solution of compound G and its derivatives indicates that the indole-pyridine rings and substituents are coplanar and exhibit wider spectral separation in their maxima. The absorption transition S 0 →S 1 is p →p* because of conjugated indole-pyridine rings and greater number of delocalized p electrons. The values of molar extinction coefficient for synthesized compounds A to K in DMSO were estimated from absorption data, indicating that substitution by electron-donating moiety enhances the photoabsorption and substitution by electron-withdrawing moiety decreases the photoabsorption. [22] Fluorescence Study Figure 3 depicts the fluorescence emission spectra of SIPI (G) and its derivatives (A to F, H to K). The compound G in DMSO exhibits a moderate broad fluorescence band at 560 nm with shoulder peak at 597 nm attributed to most probable p* →p and p*→ n transitions, respectively. The shoulder peak appeared in almost all compounds in the region 590-600 nm assigned to the less probable p*→ n transition.

Absorption Study
The compound G has fluorescence emission maximum at 560 nm. The compounds A, B, E, J, and K are derivatives of compound G having electron-donating substituents on the indole ring and show enhanced fluorescence emission along with a remarkable red shift (∼14-18 nm) in fluorescence emission maxima. These compounds show enhanced emission and bathochromic shift of ∼ 14-18 nm with respect to the fluorescence emission spectra of compound G in DMSO (560 nm).
It is well known that electron-donating and electron-withdrawing groups and solvent polarity may change the photophysical properties of organic molecules. [22,23] The electron-donating groups that are capable to extend p conjugation in the molecule should lead to great highest occupied molecular orbital (HOMO) levels and smaller HOMO-lowest unoccupied molecular orbital (LUMO) gaps, thus resulting in a red shift. [24] The enhanced fluorescence emission and red shift in fluorescence emission maxima observed is because of the presence of an electron-donating group and polar aprotic solvent DMSO. Further compounds C, D, F, H, and I bearing electron-withdrawing groups on the indole ring of SIPI show decrease in fluorescence intensity and slight hypsochromical (blue) shift in fluorescence maxima as compared to compound G. The blue shift and observed fluorescence quenching for compounds bearing electron-withdrawing groups is due to the ability of the electron-withdrawing group to pull the electron toward itself and disturb the conjugated structure of compound in its excited state. Thus, the disruption occurs in the excited state, restricts the rotation of fluorescent compound, and favors the nonradiative pathways, resulting in a decrease in fluorescence emission and fluorescence maxima. The decrease in the absorption and fluorescence intensity owing to the disturbance in conjugated effect leads to an increase in the energy loss in the excited-state vibration. [24] A larger Stokes shift was observed for the derivatives of compound G bearing electron-donating groups while the smaller Stokes shift was observed for compounds with electron-withdrawing groups. This was also confirmed by calculating the Stokes shift of synthesized compounds. The photophysical properties of all synthesized compounds A to K are shown in Table 3.

CONCLUSION
We developed a facile method for multicomponent synthesis of spiro[4Hindeno[1,2-b]pyridine-4,3 0 -[3H]indoles] by carrying out reaction of isatin, ammonium acetate, indane-1,3-dione, and ethylacetoacetate in ethanol:water (9:1) at ambient temperature. Noticeable advantages of the present method are employment of mild conditions along with ambient temperature, operational simplicity, good yield of products in short time, easy isolation and purification of products by simple filtration followed by washing with ethanol. Significant changes in the absorption spectra and fluorescence emission of compound G and its derivatives (compounds A to F, H to K) discussed on the basis of electron-donating and electronwithdrawing power of a substituted group on indole ring of compound G. Overall, the substituent effects appear to exert a more pronounced effect on fluorescence emission and on absorption spectra. The fluorescence studies of synthesized compounds open new windows in the field of chemosensing technique.
purification. Melting points were measured by open capillary. IR spectra were recorded on a Perkin-Elmer FT-IR 783 spectrophotometer. NMR spectra were recorded on a Bruker AC-300 spectrometer in DMSO-d 6 using tetramethylsilane (TMS) as an internal standard. Mass spectra were recorded on a Shimadzu QP2010 GCMS. HRMS were performed on Thermo Scientific Q-Exactive, Accela 1250 pump, instrument. Elemental analyses were done using EURO EA3000 vectro model. UV-visible spectra and fluorescence spectra were recorded on a Shimadzu spectrophotometer and on a Jasco (FP-750) spectrofluorometer in respective solvents. The concentration of each sample was maintained in the range of ∼ 10 À5 M for absorption and fluorescence study. Isatin (1 mmol), 1,3-indanedione (1 mmol), ethyl acetoacetate (1 mmol), and NH 4 OAc (1 mmol) were placed in a 25-mL round-bottomed flask in ethanol:water (9:1) (5 mL). The resulting mixture was stirred at room temperature for time mentioned in Table 2, until completion of the reaction as monitored by thin-layer chromatography (TLC). After completion of the reaction, the mixture was filtered and washed with a small quantity of ethanol to furnish pure spiro[4H-indeno-[1,2-b]pyridine-4,3 0 -[3H]indoles]. The structure of products was confirmed by IR, 1 H and 13 C NMR, GCMS, HRMS, and elemental analysis.