Clean One-Pot Multicomponent Synthesis of Pyrans Using a Green and Magnetically Recyclable Heterogeneous Nanocatalyst

Dedicated to Prof. Issa Yavari

Keywords

green chemistry - multicomponent reactions - magnetic nanoparticles - pyran

In attempts to mitigate the greenhouse effect and environmental pollution, chemical and pharmaceutical companies look to environmentally friendly protocols to reduce environmental pollution using so-called green and sustainable chemistry.[1] Multicomponent reactions (MCRs), in which one-pot reactions involving more than two reactants to produce a single product, represent one of the important strategies in green chemistry.[2] These reactions produce multifunctionalized products using fewer steps compared to classical synthesis approaches.[3] Strecker reported first MCR in 1850 for the synthesis of α-amino cyanides,[4] and nowadays MCRs have been applied to the synthesis of a wide range of complex molecules.[5] [6] [7] [8] In this context, catalysts play a major role; in particular nanocatalysts provide a large surface-to-volume ratio, which increases their activity further.[9–11] However, because of their nanoscale size, separating them from the reaction mixture by conventional methods is not efficient, but use of magnetic nanoparticles (MNPs) can overcome this issue.[12,13] These particles can be synthesized in various forms such as metal nanoparticles, iron oxides, and ferrites.[14] Copper ferrite (CuFe₂O₄) is one member of the ferrite family that has been widely applied as a catalyst in organic transformations.[15] [16] [17]

2-Amino-3-cyano-4H-pyrans are important heterocyclic scaffolds considering their varied biological activities and pharmaceutical properties such as antitumor (Figure [1], I, II),[18] antibacterial (Figure [1], III, IV), antiviral, antiallergic, spasmolytic, anticoagulant, antianaphylactic,[5] [19] [20] and antioxidant (Figure [1], V–VII) activities.[21] They have also been applied to treatment of neurodegenerative disorders including Alzheimer’s disease (Figure [1], VIII, IX),[22] [23] amyotrophic lateral sclerosis, Huntington’s disease, and Parkinson’s disease.[24] Additionally, they can be found in cosmetic products.[25] Some examples of biologically active 4H-pyrans are shown in Scheme [1]. 4H-Pyrans are also components of some plant-derived natural products.[26] In addition, 4H-pyrans can be efficiently applied as precursors to produce different classes of heterocycles.[27] Many examples of 4H-pyran synthesis using different catalyst systems have been reported in the literature, including potassium phthalimide-N-oxyl,[28] baker’s yeast,[5] MgO,[19] Mg/La,[20] SiO₂,[24] SnCl₂/ nano SiO₂,[29] ionic liquids such as [2-aemim][PF₆];[30] and catalyst-free conditions have also been disclosed.[31]

Figure 1 The structures of some biologically active molecules with 4H-pyran cores

Scheme 1 Synthesis of 2-amino-3-cyano-4H-pyrans

In continuation of our interest in the design, discovery, and application of new catalysts in organic syntheses via MCRs to develop green procedures,[7] we present herein an environmentally friendly synthesis of 4H-pyrans 4 via a green one-pot three-component reaction of an aldehyde 1, malononitrile 2, and methyl/ethyl acetoacetate 3 using CuFe₂O₄ magnetic nanoparticles as an efficient and green catalyst under mild reaction conditions in good to excellent yields (Scheme [1]).To the best of our knowledge, this is the first time that copper ferrite magnetic nanoparticles have been applied as catalyst for the synthesis of this class of heterocycles.

The CuFe₂O₄ nanoparticles were prepared by thermal decomposition of copper(II) nitrate and iron(III) nitrate by a published method[16] [32] and characterized by FT-IR spectroscopy (Figure S11), EDX analysis (Figure S12), SEM analysis (Figure S13), and TGA/DTG analysis (Figure S15). To optimize conditions, the three-component reaction of 3-nitrobenzaldehyde (0.5 mmol), malononitrile (0.5 mmol), ethyl acetoacetate (0.5 mmol), and CuFe₂O₄ was run in various solvents at room temperature, as the model reaction for pyran derivative synthesis. Initially, the amount of CuFe₂O₄ catalyst was optimized. Best results were obtained with 20 mol% of the catalyst. No further increase in yield was observed with additional amounts of catalyst. Next, the role of the solvent was reconsidered with the best yield being obtained in ethanol[33] (Table [1]). Following the optimization efforts, a range of reactions was run under optimized conditions, and the desired products were obtained in good to excellent yields (Table [2]). Known compounds were identified by comparison of their physical data (melting points) with those of authentic samples. In addition, ¹HNMR and IR analyses were carried out. These data are provided in the Supporting Information.

Table 1 Optimization of Reaction Conditions in the Synthesis of 5-Ethoxycarbonyl-2-amino-4-(3-nitrophenyl)-3-cyano-6-methyl-4H-pyran (4e)^a

Entry	Catalyst (mol%)	Solvent	Time (h)b	Yield (%)c
1d	–	EtOH	6	53
2	–	EtOH	3	trace
3e	–	EtOH	3	70
4f	–	EtOH	2	75
5	1	EtOH	3	10
6	5	EtOH	3	59
7	10	EtOH	3	59
8	20	EtOH	3	86
9	20	EtOH	2	86
10f	20	EtOH	0.75	85
11	20	–	10	23
12	20	H2O	20	12
13	20	MeCN	4	25

^a Room temperature unless otherwise temperature is mentioned.

^b Reactions were followed by TLC.

^c Isolated yields.

^d Optimization studies of this entry are omitted, and just highest yield in the shortest time is noted.

^e This reaction was run at 40 °C.

^f This reaction was run at 60 °C.

Table 2 Synthesis of 2-Amino-4-aryl-3-cyano-6-methyl-4H-pyran Derivatives 4a–t ^a

Entry	R1	R2	Product	Time (h)	Yield (%)b	mp (°C)
						Found	Reported
1	H	Et	4a	3	77	189–193	189–191[29]
2	2-NO2	Et	4b	3	73	178–179	176–178[24]
3	2-Cl	Et	4c	3.5	79	190–192	191–193[34]
4	3-OH	Et	4d	3	83	168–171	162–164[29]
5	3-NO2	Et	4e	2	86	187–188	181–183[29]
6	4-OH	Et	4f	3	75	196–198	192–193[35]
7	4-NO2	Et	4g	2	78	179–181	180–182[29]
8	4-Cl	Et	4h	2	73	174–176	174–176[24]
9	4-Br	Et	4i	3.5	70	176–177	176–178[35]
10	4-Me	Et	4j	3	79	139–140	158[36]
11	4-OMe	Et	4k	2	72	133–136	138–140[35]
12	2-NO2	Me	4l	4	70	187–189	181[37]
13	2-Cl	Me	4m	3	75	151–153	148–150[38]
14	3-OH	Me	4n	3.5	70	136–139c	–
15	3-NO2	Me	4o	3	84	210–212	212–213[38]
16	4-OH	Me	4p	4	65	163–165	160–162[38]
17	4-NO2	Me	4q	3	70	155–157	165[37]
18	4-Cl	Me	4r	3	79	171–173	172–173[39]
19	4-Me	Me	4s	3	71	165–167	164–165[38]
20	4-OMe	Me	4t	3	76	141–143	138–140[38]

^a Reaction conditions: aldehyde (1 mmol), malononitrile (1 mmol), ethyl/methylacetoacetate (1 mmol), catalyst (20 mol%), ethanol (5 mL), room temperature.

^b Isolated yield.

^c The products were characterized by ¹H NMR and IR spectroscopy.

To investigate the catalyst reusability, the catalyst was recovered and washed with distilled water and ethanol, and the model reaction was run again in the presence of recycled catalyst. The results shown in Figure [2] indicate that very slight decreases in yields were observed after 3 cycles and after the 5^th cycle, catalyst activity was still satisfying.

Figure 2 Catalyst recyclability for model reaction (4e)

In order to demonstrate the advantages of this methodology, some other methods for the synthesis of 4H-pyran (4e) were compared with the present protocol. Some of the methods need an external source of energy such as heating or ultrasonic radiation. In some cases, the catalysts are expensive or may not be recyclable. Typical results are gathered in Table [3].

Table 3 Comparison of the Present Work with other Methods for the Synthesis of Pyran 4e

Entry	Catalyst	Solvent/conditions	Temp (°C)	Time (min)	Yield (%)
1	MgO	water/grinding/two steps	r.t.	25	92[19]
2	Mg/La	MeOH/reflux	65	60	86[20]
3	SiO2	EtOH	r.t.	120	86[24]
5	SnCl2/SiO2	EtOH/reflux	reflux	30	93[29]
6	CuFe2O4	EtOH	r.t.	120	86a

^a This work.

In summary, we have represented clean, efficient, one-pot methodology for the synthesis of highly functionalized 4H-pyrans using CuFe₂O₄ magnetic nanoparticles as a reusable and green nanocatalyst. Reactions are run at room temperature in ethanol providing a green synthesis of 4H-pyran heterocycles. Short reaction times, nontoxic catalyst, ease of catalyst separation by using an external magnet, catalyst recyclability, no need for heating, good to excellent yields, and mild conditions are advantages of the reported protocol. Moreover, the high tolerance of this procedure towards various functional groups, easy and simple work-up procedure, exceptionally high yields of the desired products, and scalability are the added advantages.

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