1. INTRODUCTION:

Multicomponent reactions (MCRs) allow the creation of several bonds in a single operation. For an efficient synthesis of a wide diversity of organic molecules, multicomponent reactions are valuable and powerful tools. Some advantages like operational simplicity, reduction in the number of workup steps and minimization of waste generation makes considerable interest for the development of new MCRs and improving the known MCRs in current organic synthesis. Therefore, development of MCRs has attracted great attention, especially in the areas of drug discovery and organic synthesis^1-4.

Indenopyridine (azafluorene) compounds possess a broad spectrum of biological activity, such as cytotoxic⁵, potential antidepressant activity⁶ and antispermatogenic effects⁷. Organic compounds are very good pharamacophores performing wide range of biological activities^8-10. Therefore, the synthesis of indenopyridines has attracted considerable attention.

According to the literature, several synthetic methods have been developed. Indeno[1,2-b]pyridine derivatives are synthesized by using tribromomelamine (TBM) as catalyst¹¹, grinding¹² and using L-proline (15 mol%) as catalyst¹³. Khaksar et al., synthesized indeno[1,2-b]pyridine derivatives using 2,2,2-trifluoroethanol as solvent¹⁴. However, the requirement of high temperatures, occurrence of side reactions, low yields, expensive catalysts and long reaction time limits the use of these methods. Therefore, the search for a better method for the synthesis of indenopyridines is of prime importance. Organic compounds are very good pharamacophores performing wide range of biological activities^15-19.

Chitin is ranked as the second most abundant resource after cellulose with an annual production estimated to be of several tons. It is routinely extracted from cell fungi; the exoskeletons of arthropods, such as crustaceans and insects; the radulas of mollusks; and the beaks of cephalopods. Chitosan, a derivative of chitin, contains repeat units of β-(1 → 4)-2-amino-2-deoxy-β-D-glucose and features a primary amine functionality (Figure 1). This functionality is typically afforded through deacetylation of chitin in basic media, resulting in hydrolysis of the acetamide group to form chitosan and acetic acid^20,21. A known unit of measurement to quantify this procedure is the degree of deacetylation (DDA), where pure chitin has a DDA of 0–15%, primarily containing acetamide groups at the C2 position, while chitosan has a DDA of 75–80%, with mostly amine functionalities on the C2 position. Due to this chemical modification via deacetylation, chitosan has a noted advantage in solubility in acidic, aqueous media compared to chitin, making chitosan highly applicable to current challenges^22,23.

In addition, some intrinsic properties of chitosan (e.g., self-assembly, hydrogen bonding, gel formation, and filmogenic ability) provide handles that allow one to tune catalyst topology on the macroscale and to engineer the polymer texture on the micro- and nanoscales. This has allowed for the preparation of chitosan-based materials as fine particles, membranes, monoliths, and packed-bed microreactors^24–27.

Figure 1 Chemical structures of chitin and chitosan via deacetylation

In view of the earlier points and as a continuation of our ongoing work on the development of MCRs and environmentally friendly methodologies^28–32, attempts were made to construct an efficient one-pot four-component synthesis of 4-aryl-4,5-dihydro-1H-indeno[1,2-b]pyridines using chitosan as an efficient novel organocatalyst via the cyclocondensation reaction of aromatic aldehydes, 1,3-indanedione, β-ketoesters and ammonium acetate under neat conditions at 80 ^oC (Scheme 1).

2. EXPERIMENTAL:

2.1 Chemicals and analysis:

Chemicals were purchased from Merck, Fluka and Aldrich Chemical Companies. All yields refer to isolated products unless otherwise stated.¹H NMR (500 MHz) and¹³C NMR (125 MHz) spectra were obtained using Bruker DRX- 500 Avance at ambient temperature, using TMS as internal standard. FT-IR spectra were obtained as KBr discs on Shimadzu spectrometer. Mass spectra were determined on a Varion-Saturn 2000 GC-MS instrument. Elemental analysis were measured by means of Perkin Elmer 2400 CHN elemental analyzer flowchart.

2.2 General procedure for the synthesis of 4-aryl-4,5-dihydro-1H-indeno[1,2-b]pyridine derivatives (4a-r)

A mixture of aromatic aldehyde 1 (1 mmol), 1,3-indanedione 2 (1mmol), ethylacetoacetate 3 (1 mmol), ammonium acetate (1.5 mmol) and chitosan (0.04 g) were heated at 80 °C for 10–18 min. The reaction process was monitored by TLC. The resultant solid material was washed thoroughly with water to remove any unreacted ammonium acetate and was air-dried overnight, which was purified by silica gel column chromatography using ethyl acetate and n-hexane as eluent to give compounds 4a-j in high yields (Scheme 1). The IR, ¹H-NMR, ¹³C-NMR, mass and elemental analysis data of the synthesized compounds are given below.

Scheme 1 Synthesis of 4-aryl-4,5-dihydro-1H-indeno[1,2-b]pyridine derivatives

2.3 Spectral data for the synthesized compounds (4a-j)

2-Methyl-4-phenyl-5-oxo-4,5-dihydro-1H-indeno-[1,2-b]pyridine-3-carboxylic acid ethyl ester (4a)

IR (KBr, cm^-1): 3244, 2955, 1711, 1652, 1548; ¹H NMR (500 MHz, DMSO-d₆) δ: 1.04 (t, J = 7.2 Hz, 3H, CH₃), 2.39 (s, 3H, CH₃), 4.12 (q, J = 7.2 Hz, 2H), 4.86 (s, 1H), 6.94 (s, 1H, NH), 7.20-7.34 (m, 5H, Ar-H), 7.50–7.76 (m, 4H, Ar-H) ppm; ¹³C NMR (125 MHz, DMSO-d₆) δ: 13.8, 17.1, 36.2, 52.0, 103.9, 107.0, 117.5, 122.1, 128.4, 129.0, 131.3, 131.9, 133.8, 134.6, 138.2, 146.7, 147.6, 153.0, 170.2, 193.0 ppm.

2-Methyl-4-(4-chlorophenyl)-5-oxo-4,5-dihydro-1H-indeno-[1,2-b]pyridine-3-carboxylic acid ethyl ester (4b)

IR (KBr, cm^-1): 3253, 2977, 1710, 1665, 1572; ¹H NMR (500 MHz, DMSO-d₆) δ: 1.11 (t, J = 7.2 Hz, 3H, CH₃), 2.60 (s, 3H, CH₃), 4.11 (q, J = 7.2 Hz, 2H), 4.95 (s, 1H), 6.94 (s, 1H, NH), 7.17 (d, J = 8.0 Hz, 2H, Ar-H), 7.44 (d, J = 8.0 Hz, 2H, Ar-H), 7.56–7.70 (m, 4H, Ar-H) ppm; ¹³C NMR (125 MHz, DMSO-d₆) δ: 14.9, 18.9, 36.8, 52.8, 103.6, 106.1, 117.9, 123.2, 127.9, 128.4, 130.5, 131.9, 132.9, 133.8, 137.3, 145.6, 147.4, 156.4, 169.0, 192.0 ppm.

2-Methyl-4-(4-nitrophenyl)-5-oxo-4,5-dihydro-1H-indeno-[1,2-b]pyridine-3-carboxylic acid ethyl ester (4c)

IR (KBr, cm^-1): 3225, 2964, 1708, 1671, 1562; ¹H NMR (500 MHz, DMSO-d₆) δ: 1.07 (t, J = 7.2 Hz, 3H, CH₃), 2.52 (s, 3H, CH₃), 4.12 (q, J = 7.2 Hz, 2H), 4.84 (s, 1H), 6.88 (s, 1H, NH), 7.08 (d, J = 8.2 Hz, 2H, Ar-H), 7.44 (d, J = 8.2 Hz, 2H, Ar-H), 7.58–7.69 (m, 4H, Ar-H) ppm; ¹³C NMR (125 MHz, DMSO-d₆) δ: 14.7, 18.7, 36.4, 53.0, 105.5, 109.8, 116.0, 121.7, 126.1, 128.9, 130.2, 131.7, 132.4, 134.0, 136.7, 143.1, 144.0, 155.0, 166.2, 194.3 ppm.

2-Methyl-4-(3-nitrophenyl)-5-oxo-4,5-dihydro-1H-indeno-[1,2-b]pyridine-3-carboxylic acid ethyl ester (4d)

IR (KBr, cm^-1): 3263, 2966, 1700, 1672, 1557; ¹H NMR (500 MHz, DMSO-d₆) δ: 1.01 (t, J = 7.2 Hz, 3H, CH₃), 2.33 (s, 3H, CH₃), 4.12 (q, J = 7.2 Hz, 2H), 5.11 (s, 1H), 6.88 (s, 1H, NH), 7.22-7.33 (m, 4H, Ar-H), 7.48–7.66 (m, 4H, Ar-H) ppm; ¹³C NMR (125 MHz, DMSO-d₆) δ: 14.0, 18.0, 35.1, 49.9, 102.9, 107.7, 116.4, 119.4, 126.2, 129.0, 132.4, 132.9, 133.1, 134.7, 136.0, 145.9, 146.7, 155.7, 166.7, 191.8 ppm.

2-Methyl-4-(4-methylphenyl)-5-oxo-4,5-dihydro-1H-indeno-[1,2-b]pyridine-3-carboxylic acid ethyl ester (4e)

IR (KBr, cm^-1): 3274, 2966, 1700, 1644, 1555; ¹H NMR (500 MHz, DMSO-d₆) δ: 1.12 (t, J = 7.2 Hz, 3H, CH₃), 2.04 (s, 3H, CH₃), 2.44 (s, 3H, CH₃), 4.13 (q, J = 7.2 Hz, 2H), 5.15 (s, 1H), 6.91 (s, 1H, NH), 7.23 (d, J = 8.2 Hz, 2H, Ar-H), 7.41 (d, J = 8.2 Hz, 2H, Ar-H), 7.56–7.69 (m, 4H, Ar-H) ppm; ¹³C NMR (125 MHz, DMSO-d₆) δ: 13.9, 17.4, 19.0, 35.4, 52.3, 105.8, 109.0, 117.9, 121.1, 127.8, 129.3, 131.3, 132.4, 132.9, 134.0, 137.1, 145.1, 147.0, 156.2, 169.5, 192.5 ppm.

2-Methyl-4-(4-methoxyphenyl)-5-oxo-4,5-dihydro-1H-indeno-[1,2-b]pyridine-3-carboxylic acid ethyl ester (4f)

IR (KBr, cm^-1): 3243, 2960, 1701, 1657, 1570; ¹H NMR (500 MHz, DMSO-d₆) δ: 1.12 (t, J = 7.2 Hz, 3H, CH₃), 2.39 (s, 3H, CH₃), 3.47 (s, 3H, OCH₃), 4.13 (q, J = 7.2 Hz, 2H), 4.88 (s, 1H), 6.86 (s, 1H, NH), 7.09 (d, J = 8.2 Hz, 2H, Ar-H), 7.29 (d, J = 8.2 Hz, 2H, Ar-H), 7.39–7.59 (m, 4H, Ar-H) ppm; ¹³C NMR (125 MHz, DMSO-d₆) δ: 15.0, 17.9, 33.7, 52.0, 54.2, 107.3, 109.6, 117.0, 122.0, 127.7, 128.9, 130.7, 131.9, 132.8, 134.4, 135.9, 143.9, 146.6, 154.0, 168.4, 194.4 ppm.

2-Methyl-4-(2-chlorophenyl)-5-oxo-4,5-dihydro-1H-indeno-[1,2-b]pyridine-3-carboxylic acid ethyl ester (4g)

IR (KBr, cm^-1): 3239, 2970, 1703, 1661, 1570; ¹H NMR (500 MHz, DMSO-d₆) δ: 1.09 (t, J = 7.2 Hz, 3H, CH₃), 2.45 (s, 3H, CH₃), 4.14 (q, J = 7.2 Hz, 2H), 4.92 (s, 1H), 6.78 (s, 1H, NH), 7.15-7.31 (m, 4H, Ar-H), 7.52–7.69 (m, 4H, Ar-H) ppm; ¹³C NMR (125 MHz, DMSO-d₆) δ: 13.9, 19.0, 36.7, 51.0, 104.9, 107.8, 117.9, 122.6, 127.3, 128.9, 131.2, 132.0, 132.9, 134.0, 137.2, 145.0, 146.5, 155.0, 169.0, 193.0 ppm.

2-Methyl-4-(4-fluorophenyl)-5-oxo-4,5-dihydro-1H-indeno-[1,2-b]pyridine-3-carboxylic acid ethyl ester (4h):

IR (KBr, cm^-1): 3266, 2944, 1700, 1647, 1538; ¹H NMR (500 MHz, DMSO-d₆) δ: 1.11 (t, J = 7.2 Hz, 3H, CH₃), 2.33 (s, 3H, CH₃), 4.10 (q, J = 7.2 Hz, 2H), 4.91 (s, 1H), 6.93 (s, 1H, NH), 7.09 (d, J = 8.1 Hz, 2H, Ar-H), 7.29 (d, J = 8.1 Hz, 2H, Ar-H), 7.39–7.69 (m, 4H, Ar-H) ppm; ¹³C NMR (125 MHz, DMSO-d₆) δ: 15.0, 18.1, 35.7, 52.4, 106.1, 108.0, 117.9, 122.2, 128.6, 129.0, 131.0, 133.0, 132.6, 134.5, 135.8, 144.2, 145.9, 153.4, 168.8, 192.0 ppm.

2-Methyl-4-(3-chlorophenyl)-5-oxo-4,5-dihydro-1H-indeno-[1,2-b]pyridine-3-carboxylic acid ethyl ester (4i)

IR (KBr, cm^-1): 3237, 2949, 1709, 1657, 1577; ¹H NMR (500 MHz, DMSO-d₆) δ: 1.00 (t, J = 7.2 Hz, 3H, CH₃), 2.44 (s, 3H, CH₃), 4.00 (q, J = 7.2 Hz, 2H), 5.12 (s, 1H), 6.75 (s, 1H, NH), 7.09-7.29 (m, 4H, Ar-H), 7.47–7.71 (m, 4H, Ar-H) ppm; ¹³C NMR (125 MHz, DMSO-d₆) δ: 13.9, 17.7, 35.0, 52.0, 105.0, 107.1, 117.6, 122.0, 126.8, 128.6, 130.3, 131.8, 132.1, 134.1, 136.0, 145.1, 146.0, 152.9, 169.1, 190.9 ppm.

2-Methyl-4-(4-bromophenyl)-5-oxo-4,5-dihydro-1H-indeno-[1,2-b]pyridine-3-carboxylic acid ethyl ester (4j):

IR (KBr, cm^-1): 3233, 2960, 1717, 1641, 1570; ¹H NMR (500 MHz, DMSO-d₆) δ: 1.02 (t, J = 7.2 Hz, 3H, CH₃), 2.47 (s, 3H, CH₃), 3.99 (q, J = 7.2 Hz, 2H), 4.97 (s, 1H), 6.89 (s, 1H, NH), 7.15 (d, J = 8.0 Hz, 2H, Ar-H), 7.41 (d, J = 8.0 Hz, 2H, Ar-H), 7.50–7.70 (m, 4H, Ar-H) ppm; ¹³C NMR (125 MHz, DMSO-d₆) δ: 14.7, 18.0, 35.8, 52.1, 105.2, 107.7, 117.9, 123.0, 126.1, 128.6, 129.8, 131.3, 132.8, 134.1, 135.7, 146.0, 147.0, 155.0, 168.5, 193.1ppm.

3. RESULTS AND DISCUSSION:

In pursuit of continued interest in the development of solvent-free and green synthetic procedures, it was decided to explore the use of chitosan as catalyst for synthesis of 4-aryl-4,5-dihydro-1H-indeno[1,2-b]pyridine derivatives from four components coupling of aromatic aldehydes, alkyl acetoacetate, β-keto compounds and ammonium acetate in high to excellent yields at 80°C under solvent-free conditions.

Initially the reaction between 4-chlorobenzaldehydes (1 mmol), 1,3-indanedione (1mmol), ethyl acetoacetate (1 mmol) and ammonium acetate (1.5mmol) as the model reaction was examined in the presence of varying amount of chitosan as a catalyst at 80°C under solvent-free conditions and the results are presented in Table 1. The best result was achieved by performing the reaction with 0.04g of catalyst (Table 1, entry 6). The use of a higher amount of catalyst did not improve the yield, while a decrease in the amount of catalyst decreases the yield (Table 1). In the absence of catalyst, the reaction did not proceed even after a long reaction time (Table 1, entry 1). In addition, the effect of temperature was studied by performing the model reaction at different temperatures under solvent-free conditions (room temperature (RT), 40, 50, 60, 70 and 90°C) and the best results were obtained at 80°C (Table 1, entry 6).

To evaluate the scope of this catalytic transformation, the optimized reaction conditions were subsequently applied to the reaction of 1,3-indanedione, ethylaceto- acetate and ammonium acetate with a variety of different aromatic aldehydes (Table 2, entries 1-10). A wide range of aromatic aldehydes bearing either electron-donating or electron-withdrawing substituents reacted successfully with 1,3-indanedione, ethylacetoacetate and ammonium acetate to give the corresponding 4-aryl-4,5-dihydro-1H-indeno[1,2-b]pyridine derivatives in high yields over short reaction times. The electronic nature and the position of the substituent on the aromatic aldehydes have little influence on the yields of 4-aryl-4,5-dihydro-1H-indeno[1,2-b]pyridines. Aldehydes with electron-withdrawing substituents provided the desired 4-aryl-4,5-dihydro-1H-indeno[1,2-b]pyridine derivatives in slightly higher yields, whereas the yields were slightly lower for aldehydes with electron‐donating substituents.

Table 1: Optimization of reaction conditions for the synthesis of 4b^a

Entry	Amount of catalyst (g)	Temperature (^oC)	Time (min)	Yield (%)^b
1	0.00	80	70	0
2	0.01	80	50	39
3	0.02	80	30	43
4	0.03	80	20	77
5	0.04	80	10	94
6	0.04	80	10	94
7	0.05	RT	65	0
8	0.04	40	55	34
9	0.04	50	40	48
10	0.04	60	25	72
11	0.04	70	15	84
12	0.04	90	10	94

^aReaction conditions: 4-chlorobenzaldehyde (1 mmol), 1,3-indanedione (1 mmol), ethyl acetoacetate (1 mmol) and ammonium acetate (1.5 mmol)

^bIsolated yields

Table 2: Synthesis of various 4-aryl-4,5-dihydro-1H-indeno[1,2-b]pyridine derivatives in the presence of Chitosan^a

Entry	R	Product	Time (min)	Yield (%)^b
1	4-H	4a	12	90
2	4-Cl	4b	10	94
3	4-NO₂	4c	10	90
4	3-NO₂	4d	14	87
5	4-CH₃	4e	20	85
6	4-OCH₃	4f	20	85
7	2-Cl	4g	16	84
8	4-F	4h	10	91
9	3-Cl	4i	12	89
10	4-Br	4j	10	91

^aReaction conditions: benzaldehyde (1 mmol), 1,3-indanedione (1 mmol), ethylacetoacetate (1 mmol) and ammonium acetate (1.5 mmol) under solvent-free conditions at 80 °C.

^bIsolated yields

A mechanistic rationale portraying the probable sequence of events for the formation of 4-aryl-4,5-dihydro-1H-indeno[1,2-b]pyridines is given in Scheme 2. From a mechanistic point of view, the first step is the formation of Knoevenagel product A. The second key intermediate is ester enamine B, produced by the condensation of β-ketoester with ammonia. Condensation of these two fragments gives the acyclic Michael adduct intermediate C, which undergoes intramolecular cyclization with participation of the amino function and one of the indanedione carbonyl group to form the dihydroindenopyridine 4.

Scheme 2 Mechanism for the synthesis of 4-aryl-5-oxo-4,5-dihydro-1H-indeno[1,2-b]pyridines.

4. CONCLUSIONS:

In conclusion, we have developed a facile and efficient one-pot synthesis of 4-aryl-4,5-dihydro-1H-indeno[1,2-b]pyridine in the presence of chitosan (0.04 g) under solvent-free conditions at 80ºC in high yields. The milder conditions, shorter reaction times, low costs, easy workup and high yields make this process attractive over the other available methods.

5. CONFLICT OF INTEREST:

The authors have no conflicts of interest regarding this investigation.

6. ACKNOWLEDGEMENTS:

All the authors are grateful to C. Abdul Hakeem College Management for the facilities and support.

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Received on 10.05.2023 Modified on 04.08.2023

Asian J. Research Chem. 2023; 16(6):412-416.

DOI: 10.52711/0974-4150.2023.00067