An efficient protocol for synthesis of tetrahydrobenzo[<i>b</i>]pyrans using amino functionalized ionic liquid

P.P. Salvi; A.M. Mandhare; A.S. Sartape; D.K. Pawar; S.H. Han; S.S. Kolekar

doi:10.1016/j.crci.2011.02.007

An efficient protocol for synthesis of tetrahydrobenzo[b]pyrans using amino functionalized ionic liquid

P.P. Salvi ¹ ; A.M. Mandhare ¹ ; A.S. Sartape ¹ ; D.K. Pawar ¹ ; S.H. Han ² ; S.S. Kolekar ¹

¹ Department of Chemistry, Shivaji University, Kolhapur 416 004 (MS), India
² Inorganic Nano-Material Laboratory, Department of Chemistry, Hanyang University, Seoul 133-791, South Korea

Comptes Rendus. Chimie, Volume 14 (2011) no. 10, pp. 878-882.

Résumé

A new functionalized basic ionic liquid, 4-amino-1-(2,3-dihydroxy propyl) pyridinium hydroxide [ADPPY] [OH], has been introduced as a catalyst for the synthesis of tetrahydrobenzo[b]pyrans under mild conditions. The desired products can be separated directly from the reaction mixture with high purity. Only 10 mol% catalyst was needed with a sequestration time of 10 min. In addition, the ionic liquid used can be regenerated and recycled several times without significant loss of activity.

Métadonnées

Reçu le : 2010-11-22
Accepté le : 2011-02-09
Publié le : 2011-03-21

DOI : 10.1016/j.crci.2011.02.007

Keywords: TSIL, Regeneration of catalyst, Tetrahydrobenzo[b]pyrans, Multicomponent synthesis, Green

Affiliations des auteurs :

P.P. Salvi ¹ ; A.M. Mandhare ¹ ; A.S. Sartape ¹ ; D.K. Pawar ¹ ; S.H. Han ² ; S.S. Kolekar ¹

¹ Department of Chemistry, Shivaji University, Kolhapur 416 004 (MS), India
² Inorganic Nano-Material Laboratory, Department of Chemistry, Hanyang University, Seoul 133-791, South Korea

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     author = {P.P. Salvi and A.M. Mandhare and A.S. Sartape and D.K. Pawar and S.H. Han and S.S. Kolekar},
     title = {An efficient protocol for synthesis of tetrahydrobenzo[\protect\emph{b}]pyrans using amino functionalized ionic liquid},
     journal = {Comptes Rendus. Chimie},
     pages = {878--882},
     publisher = {Elsevier},
     volume = {14},
     number = {10},
     year = {2011},
     doi = {10.1016/j.crci.2011.02.007},
     language = {en},
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P.P. Salvi; A.M. Mandhare; A.S. Sartape; D.K. Pawar; S.H. Han; S.S. Kolekar. An efficient protocol for synthesis of tetrahydrobenzo[b]pyrans using amino functionalized ionic liquid. Comptes Rendus. Chimie, Volume 14 (2011) no. 10, pp. 878-882. doi : 10.1016/j.crci.2011.02.007. https://comptes-rendus.academie-sciences.fr/chimie/articles/10.1016/j.crci.2011.02.007/

Version originale du texte intégral

Le texte intégral ci-dessous peut contenir quelques erreurs de conversion par rapport à la version officielle de l'article publié.

1 Introduction

Ionic liquids offer the potential for ground-breaking changes to synthesis routes and unit operations in research as well as in the chemical industry. Although the ionic liquid was initially introduced as an alternative green reaction medium, today it has marched far beyond showing its significant role in controlling the reaction as catalyst. The large number of cations and anions allow a wide range of physical and chemical characteristics to be achieved, including volatile and involatile systems, and thus the terms “designer” and “task-specific” ILs have been developed [1–4]. The prospects for ionic liquid use are vast. A number of ionic liquids with unique properties have been developed and applied to catalyze many types of reactions. Recently, basic ionic liquids have aroused unprecedented interest because they showed more advantages, such as catalytic efficiency and recycling of the ionic liquid, than the combination of an inorganic base and an ionic liquid for some base-catalyzed processes [5].

Tetrahydrobenzo[b]pyrans have recently attracted attention as an important class of heterocyclic scaffolds in the field of drugs and pharmaceuticals. These compounds are widely used as anti-coagulant, diuretic, spasmolytic, anticancer and anti-anaphylactin agents [6]. In addition, they can be used as cognitive enhancers, for the treatment of neurodegenerative disease, including Alzheimer's disease, amyotrophic lateral sclerosis, Huntington's disease, Parkinson's disease, AIDS associated dementia and Down's syndrome as well as for the treatment of schizophrenia and myoclonus [7]. Some 2-amino-tetrahydrobenzo[b]pyrans can be employed as photoactive materials [8]. Synthesis of tetrahydrobenzo[b]pyrans was traditionally promoted by harsh bases, strong acids or high temperature in the presence of VOCs [9–17]. A little effort has been made in view of green synthesis of tetrahydrobenzo[b]pyrans [18–20]. Surprisingly, there have been no reports regarding the use of the amino functionalized ionic liquids in an effort to influence the outcome of multicomponent reactions. In this report we will describe the synthesis of a functionalized ionic liquid, 4-amino-1-(2,3-dihydroxy propyl) pyridinium hydroxide [ADPPY] [OH] and its application as a mild catalyst for the synthesis of tetrahydrobenzo[b]pyrans.

The basic idea here is to replace the scavenger reagents by a functionalized ionic liquid. In comparison with the other scavengers, this synthetic approach has several outstanding advantages, including a high active site of IL, time saving, easy monitoring and unlimited scale-up potential in water. It is also worth noting that the used ionic liquids could be regenerated and recycled at least five times without significant loss of activity.

2 Results and discussion

2.1 Effect of different ILs

Our efforts were focused on the synthesis of the tetrahydrobenzo[b]pyrans using amino functionalized ionic liquids in comparison with other ILs. The results are summarized in Table 1.

Entry	Ionic Liquids	Solvent^b	Mol%	Time (min.)	Yield (%)^a
1	[bmim]BF₄	H₂O	40	480	0
2	[bmim]OH	H₂O	40	50	75
3	[bmim]PF₆	H₂O + C₂H₅OH (70:30)	40	120	46
4	[bpyr]BF₄	H₂O	40	240	20
5	[bpyr]PF₆	H₂O + C₂H₅OH (70:30)	40	180	35
6	[omim]BF₄	H₂O	40	180	55
7	[omim]PF₆	H₂O + C₂H₅OH (70:30)	40	180	61
8	[ADPPY]OH	H₂O + C₂H₅OH (90:10)	10	15	90
9	[ADPPY]OH	H₂O	05	25	78
10	[ADPPY]OH	H₂O	10	10	95
11	[ADPPY]OH	H₂O	15	15	92
12	[ADPPY]OH	H₂O	30	15	91
13	[ADPPY]OH	H₂O	45	15	91

^a All reactions were carried out of benzaldehyde: malononitrile: dimedone 1:1:1 (molar ratio) at room temperature in ionic liquids; yield refer to pure isolated products.

^b Entries in bracket indicate the ratio of H₂O to C₂H₅OH on volume basis.

As shown in Table 1, the molar ratio of IL/substrate and the reaction times both have significant effects on the reaction. It could be seen that with the increase of the molar ratio of IL/substrate from 5 to 10%, the reaction yield was significantly improved and then it reduces (Table 1). Upon examination, it was found that 10 mol% of IL was sufficient to promote the reaction. In the presence of less than this amount, the yield dropped dramatically, even if longer reaction times were used (Table 1, entry 9). When the amount of IL was increased over 10 mol% equivalent, neither the yield nor the reaction time was improved (Table 1, entries 11–13). Experiments show that the catalytic performance of [ADPPY] [OH], [BMIM] [OH] could be much better than other ILs under the same reaction conditions. It is evident that the yields of the desired product in the ionic liquids containing the hydroxyl anion are higher than those in the ionic liquids containing tetrafluoroborate as well as hexaflurophosphate.

Furthermore, it can be seen that the conversion for reaction with [ADPPY] [OH] as catalyst was better than [BMIM] [OH]. These results suggest that the performance of the IL is dependent upon the character of the side chain of the cation and the N-heterocyclic ring. [ADPPY] [OH] exhibited the most excellent catalytic performance with high conversion under the same reaction conditions (Table 2, entry 10).

Entry	Ar	Product^a	Time (min.)	R1,R2	Yield (%)^b
1	4-Cl-C₆H₄	4a	15	CH₃,CH₃	94
2	4-OMe-C₆H₄	4b	20	CH₃,CH₃	90
3	C₆H₅	4c	15	CH₃,CH₃	95
4	2,5-2Cl-C₆H₃	4d	25	CH₃,CH₃	89
5	3-Pyridine	4e	15	CH₃,CH₃	84
6	3-Pyridine	4f	20	H,H	92
7	Thiophene	4g	20	CH₃,CH₃	89
8	3,4-2CH₃-C₆H₃	4h	25	CH₃,CH₃	91
9	3,4-2CH₃-C₆H₃	4i	20	H,H	94
10	4-isopropyl benzaldehyde	4j	25	CH₃,CH₃	90
11	C₆H₁₁	4k	25	CH₃,CH₃	89
12	4-CN-C₆H₄	4L	20	CH₃,CH₃	90

2.2 Reusability of [ADPPY] [OH] as catalyst

As one of the most effective functionalized IL, [ADPPY] [OH] was selected to investigate the possibility of reusability (Fig. 1). As the product is insoluble in water, it was collected by simple filtration, while the ionic liquid is completely soluble in water. So, after completion of reaction, it became very easy to isolate the product from the ionic liquid. The IL could be recovered easily by rotary evaporator and directly reused for subsequent runs. As it is shown in Fig. 1, no obvious change was observed on the recovered catalytic and excellent yield was obtained. After five times reusability, the IL remains stable and has less impact on the catalytic activity.

3 Conclusion

In summary, we have introduced a new amino functionalized ionic liquid ([ADPPY] [OH]) that can be used as an efficient catalyst for tetrahydrobenzo[b]pyrans. The heterocycles synthesized in this study were obtained in high regioselectivity, good yields and short reaction times.

4 Experimental

4.1 Synthesis of the ionic liquids (ILs)

A novel amino functionalized ionic liquid 4-amino-1-(2,3-dihydroxy propyl) pyridinium, hydroxide [ADPPY] [OH] (Fig. 2) was synthesized from commercially available 4-amino pyridine and 3-Chloro-1,2-propanediol, followed by anion exchange with Amberlite IRA 400 Cl resin (Scheme 1) [21]. The product (87% yield) was characterized by ¹H and ¹³C NMR spectroscopy.

Scheme 1
Synthesis of TSIL [ADPPY] [OH].

4.2 Synthesis of tetrahydrobenzo[b]pyran

To demonstrate the utility of [ADPPY][OH] in multicomponent reaction, we began with a mixture of aldehydes (1 mmol), malononitrile (1 mmol) and 1,3-di ketone (1 mmol) as a model. Unfortunately, as the ionic liquid is very viscous, water (3 mL) was used as sequester. The reaction was carried out at room temperature with constant stirring for just 10 minutes to get a single product in excellent yield (95%). The formation of the product was further confirmed by IR, ¹H NMR and ¹³C NMR spectroscopy. It is worthy to mention that no by-product was detected. A variety of aldehydes and 1,3-diketone also underwent to afford the corresponding tetrahydrobenzo[b]pyrans in moderate to high yields. Aromatic aldehydes with either electron donating or electron withdrawing groups underwent the reaction smoothly. Having obtained favourable results with aromatic aldehydes, we then examined heterocyclic aldehydes such as pyridine-3-carboxylaldehyde and 2-furaldehyde. The results are summarized in Table 2. A conceivable mechanism for the multicomponent synthesis of tetrahydrobenzo[b]pyran using [ADPPY] [OH] is proposed (Schemes 2 and 3).

Scheme 2
Synthesis of tetrahydrobenzo[b]pyran.

Scheme 3
A conceivable mechanism for formation of tetrahydrobenzo[b]pyran.

4.3 Spectral data

4.3.1 Spectral data for TSIL (4-amino-1-(2,3-dihydroxy propyl) pyridinium, hydroxide)

¹H NMR (300 MHz, DMSO-d⁶): δ 3.179 (m, 2H), 3.725 (s, 1H), 3.953–4.024 (m, 1H), 4.229–4.284 (m, 1H), 5.067 (t, 1H, OH), 5.425 (d, 1H, OH), 6.854 (d, J = 7.2 Hz, 2H), 8.090 (d, J = 7.2 Hz, 2H), 8.316 (bs, 2H, NH₂). ¹³C NMR (75 MHz, DMSO): δ 60.24, 63.07, 70.78, 109.24, 144.03, 159.15.

4.3.2 Spectral data for unknown tetrahydrobenzo[b]pyran derivatives

Table-2, Entry 4e: mp 184–186 ̊C; IR (KBr): 3377, 3313, 2959, 2184, 1683, 1653, 1373, 1216, 1035, 712 cm⁻¹; ¹H NMR (300 MHz, CDCl3): δ 1.019 (s, 3H, CH₃), 1.107 (s, 3H, CH₃), 2.209 (s, 2H, CH₂), 2.460 (s, 2H, CH₂), 4.433 (s, 1H, Benzylic CH), 4.992 (s, 2H, NH₂), 7.268 (s, J = 12 Hz, 1H, Ar-CH), 7.656 (s, J = 7.8 Hz, 1H, Ar-CH), 8.455 (d, 2H, Ar-CH); ¹³C NMR (75 MHz, CDCl₃): δ 27.71, 28.74, 32.21, 33.65, 40.63, 50.54, 61.57, 113.08, 118.38, 123.61, 136.06, 139.12, 147.75, 148.71, 158.08, 162.17, 195.79. M/z = 296 (M + 1); Purity by LC-MS = 98.16%.

Table-2, Entry 4f: mp 204–205 ̊C; IR (KBr): 3360, 3329, 3036, 2192, 1665, 1524, 1366, 1214, 1028, 714 cm⁻¹; ¹H NMR (300 MHz, CDCl₃): δ 1.989 (s, 2H, CH₂), 2.253 (s, 2H, CH₂), 2.516 (s, 2H, CH₂), 4.306 (s, 1H, Benzylic CH), 6.298 (s, 2H, NH₂), 7.238 (s, J = 12.6 MHz, 1H, Ar-CH), 7.577 (s, J = 14.1, 1H, Ar-CH), 8.360 (d, 2H, Ar-CH); ¹³C NMR (75 MHz, CDCl₃): δ 20.06, 27.05, 33.74, 36.64, 58.49, 113.81, 119.38, 123.85, 136.30, 140.23, 147.23, 148.25, 158.91, 164.48, 195.91. M/z = 268 (M + 1); Purity by LC-MS = 74.78%.

Table-2, Entry 4 h: mp 201–202 ̊C; IR (KBr): 3451, 3329, 2964, 2193, 1670, 1600, 1365, 1206, 1039, 773 cm⁻¹; ¹H NMR (300 MHz, DMSO-d⁶ in CDCl₃): δ 1.060 (s, 3H, CH₃), 1.113 (s, 3H, CH₃), 2.222 (s, 2H, CH₂), 2.222 (s, 3H, Ar-CH₃), 2.222(s, 3H, Ar-CH₃), 2.459 (s, 2H, CH₂), 4.325 (s, 1H, Benzylic CH), 4.572 (s, 2H, NH₂), 6.949 (d, J = 16 MHz, 2H, Ar-CH), 7.057 (s, 1H, Ar-CH); ¹³C NMR (75 MHz, DMSO in CDCl₃): δ 19.4, 19.9, 27.72, 28.29, 32.21, 35.09, 40.68, 50.69, 63.76, 114.10, 118.83, 124.76, 128.77, 129.87, 135.35, 136.61, 140.71, 157.35, 161.46, 196.00. M/z = 323 (M + 1); Purity by LC-MS = 91.00%.

Table-2, Entry 4i: mp 198–200 ̊C; IR (KBr): 3396, 3329, 2918, 2192, 1680, 1657, 1364, 1207, 1021, 795 cm⁻¹; ¹H NMR (300 MHz, DMSO-d⁶ in CDCl₃): δ 6.93 (s, 1H, Ar-CH), 6.84 (d, J = 13.8 MHz, 2H, Ar-CH), 6.57 (s, 2H, NH₂), 4.11 (s, 1H, Benzylic CH), 2.62 (s, 2H, CH₂), 2.35 (s, 2H, CH₂), 1.95 (s, 2H, CH₂), 2.12 (s, 3H, Ar-CH₃), 2.15 (s, 3H, Ar-CH₃); ¹³C NMR (75 MHz, DMSO in CDCl₃): δ 13.39, 19.96, 20.23, 27.05, 35.37, 36.84, 60.00, 114.81, 120.11, 124.94, 128.70, 129.65, 134.66, 136.12, 142.31, 158.72, 163.98, 195.90. M/z = 317 (M + Na); Purity by LC-MS = 100%.

Acknowledgment

The authors P.P.S. and S.S.K. thanks to DAE–BRNS, New Delhi (India) for the financial assistance and a research fellowship respectively.

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Vaibhav W. Godse; Umesh R. Sonwane; Parmeshwar P. Pawar; Sahebrao S. Rindhe; Pravin S. Kendrekar; Rajendra P. Pawar L-Pyrrolidine-2-Carboxylic Acid Sulfate: A New Ionic Liquid for the Synthesis of Bioactive Tetrahydrobenzo[b]pyrans, Organic Preparations and Procedures International, Volume 49 (2017) no. 4, p. 363 | DOI:10.1080/00304948.2017.1342512
Zhongqiang Zhou; Yuliang Zhang; Xiaoyun Hu Efficient One-Pot Synthesis of Tetrahydrobenzo[b]pyrans by Ethylenediamine Diacetate-Catalyzed Multicomponent Reaction Under Solvent-Free Conditions, Polycyclic Aromatic Compounds, Volume 37 (2017) no. 1, p. 39 | DOI:10.1080/10406638.2015.1088042
Ashok V. Borhade; Jyoti A. Agashe; Dipak R. Tope An efficient multicomponent synthesis of tetrahydropyrans using novel recyclable nanocrystalline Y2(CO3)3 catalyst, Russian Journal of Applied Chemistry, Volume 90 (2017) no. 6, p. 1005 | DOI:10.1134/s1070427217060234
S. Sheik Mansoor; K. Aswin; K. Logaiya; S.P.N. Sudhan; H. Ramadoss Melamine trisulfonic acid: A new, efficient and reusable catalyst for the synthesis of some fused pyranopyrrole derivatives, Journal of Saudi Chemical Society, Volume 20 (2016), p. S393 | DOI:10.1016/j.jscs.2012.12.010
Behrooz Maleki; Hossein Eshghi; Mohammad Barghamadi; Negar Nasiri; Amir Khojastehnezhad; Samaneh Sedigh Ashrafi; Omid Pourshiani Silica-coated magnetic NiFe2O4 nanoparticles-supported H3PW12O40; synthesis, preparation, and application as an efficient, magnetic, green catalyst for one-pot synthesis of tetrahydrobenzo[b]pyran and pyrano[2,3-c]pyrazole derivatives, Research on Chemical Intermediates, Volume 42 (2016) no. 4, p. 3071 | DOI:10.1007/s11164-015-2198-8
Sayyedeh Shadfar Pourpanah; Sayyed Mostafa Habibi-Khorassani; Mehdi Shahraki Fructose-catalyzed synthesis of tetrahydrobenzo[b]pyran derivatives: Investigation of kinetics and mechanism, Chinese Journal of Catalysis, Volume 36 (2015) no. 5, p. 757 | DOI:10.1016/s1872-2067(14)60302-8
Mohamed Hilmy Elnagdi; Moustafa Sherief Moustafa; Saleh Mohammed Al-Mousawi; Ramadan Ahmed Mekheimer; Kamal Usef Sadek Recent developments in utility of green multi-component reactions for the efficient synthesis of polysubstituted pyrans, thiopyrans, pyridines, and pyrazoles, Molecular Diversity, Volume 19 (2015) no. 3, p. 625 | DOI:10.1007/s11030-015-9594-2
M. Ghashang; S. Sheik Mansoor; K. Aswin Synthesis and in vitro microbiological evaluation of novel series of 8-hydroxy-2-(2-oxo-2H-chromen-3-yl)-5-phenyl-3H-chromeno [2,3-d]pyrimidin-4(5H)-one derivatives catalyzed by reusable silica-bonded N-propylpiperazine sulfamic acid, Research on Chemical Intermediates, Volume 41 (2015) no. 5, p. 3117 | DOI:10.1007/s11164-013-1419-2
Fatemeh Noori Sadeh; Malek Taher Maghsoodlou; Nourallah Hazeri; Mehrnoosh Kangani A facile and efficient synthesis of tetrahydrobenzo[b]pyrans using lactose as a green catalyst, Research on Chemical Intermediates, Volume 41 (2015) no. 8, p. 5907 | DOI:10.1007/s11164-014-1710-x
Majid Ghashang; Syed Sheik Mansoor; Krishnamoorthy Aswin Thiourea dioxide: An efficient and reusable organocatalyst for the rapid one-pot synthesis of pyrano[4,3-b]pyran derivatives in water, Chinese Journal of Catalysis, Volume 35 (2014) no. 1, p. 127 | DOI:10.1016/s1872-2067(12)60727-x
Anguo Ying; Yuxiang Ni; Songlin Xu; Shuo Liu; Jianguo Yang; Rongrong Li Novel DABCO Based Ionic Liquids: Green and Efficient Catalysts with Dual Catalytic Roles for Aqueous Knoevenagel Condensation, Industrial Engineering Chemistry Research, Volume 53 (2014) no. 14, p. 5678 | DOI:10.1021/ie500440w
Huanan Hu; Fangli Qiu; Anguo Ying; Jianguo Yang; Haiping Meng An Environmentally Benign Protocol for Aqueous Synthesis of Tetrahydrobenzo[b]Pyrans Catalyzed by Cost-Effective Ionic Liquid, International Journal of Molecular Sciences, Volume 15 (2014) no. 4, p. 6897 | DOI:10.3390/ijms15046897
Seyed Meysam Baghbanian Synthesis, characterization, and application of Cu2O and NiO nanoparticles supported on natural nanozeolite clinoptilolite as a heterogeneous catalyst for the synthesis of pyrano[3,2-b]pyrans and pyrano[3,2-c]pyridones, RSC Adv., Volume 4 (2014) no. 103, p. 59397 | DOI:10.1039/c4ra10537k
Behrooz Maleki; Samaneh Sedigh Ashrafi Nano α-Al2O3supported ammonium dihydrogen phosphate (NH4H2PO4/Al2O3): preparation, characterization and its application as a novel and heterogeneous catalyst for the one-pot synthesis of tetrahydrobenzo[b]pyran and pyrano[2,3-c]pyrazole derivatives, RSC Adv., Volume 4 (2014) no. 81, p. 42873 | DOI:10.1039/c4ra07813f
Omid Hosseinchi Qareaghaj; Sara Mashkouri; M. Reza Naimi-Jamal; Gerd Kaupp Ball milling for the quantitative and specific solvent-free Knoevenagel condensation + Michael addition cascade in the synthesis of various 2-amino-4-aryl-3-cyano-4H-chromenes without heating, RSC Adv., Volume 4 (2014) no. 89, p. 48191 | DOI:10.1039/c4ra06603k
Abdollah Fallah-Shojaei; Khalil Tabatabaeian; Farhad Shirini; Seyyedeh Zoha Hejazi Multi-walled carbon nanotube supported Fe3O4NPs: an efficient and reusable catalyst for the one-pot synthesis of 4H-pyran derivatives, RSC Advances, Volume 4 (2014) no. 19, p. 9509 | DOI:10.1039/c3ra46598e
Mohammad Abdollahi-Alibeik; Neda Sadeghi-Vasafi ${ClO}_{4}^{-}$ ClO 4 - /Al-MCM-41 as a novel nanostructured solid acid catalyst for the synthesis of 4H-benzo[b]pyrans, Reaction Kinetics, Mechanisms and Catalysis, Volume 112 (2014) no. 2, p. 511 | DOI:10.1007/s11144-014-0717-z
Mohamed H. Elnagdi; Kamal U. Sadek; Moustafa S. Moustafa Recent Advances in the Synthesis of Polysubstituted Heterocycles, Volume 109 (2013), p. 241 | DOI:10.1016/b978-0-12-407777-5.00003-8
Ying‐Lei Wang; Zhuo Li; Jun Luo; Zu‐Liang Liu One‐pot Synthesis of Tetrahydrobenzo[b]pyrans Catalyzed by PEG‐1000 Bridged Primary Amine Functionalized Dicationic Ionic Liquid in Water, Journal of the Chinese Chemical Society, Volume 60 (2013) no. 12, p. 1431 | DOI:10.1002/jccs.201300285
Yanlong Gu Multicomponent reactions in unconventional solvents: state of the art, Green Chemistry, Volume 14 (2012) no. 8, p. 2091 | DOI:10.1039/c2gc35635j
B. G. Pawar; P. P. Salvi; S. S. Kolekar Electrochemical Tailoring of Honeycomb-Structured ZnO Thin Films by Interfacial Surfactant Templating, ISRN Nanomaterials, Volume 2012 (2012), p. 1 | DOI:10.5402/2012/907340

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