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Bubun Banerjee (2015) Signpost Open Access J. Org. Biomol. Chem., 3, 28 - 33. Volume 03, 06 pages. ISSN: 2321- 4163 http://signpostejournals.com

Signpost Open Access Journal of Organic and Biomolecular Chemistry Pin-Point 1 (This feature focuses on any important chemistry and/or chemical biology related topic of current research interests, compiled exclusively by a postgraduate research scholar)

Per-6-amino- -cyclodextrin (per-6-ABCD) Compiled by Bubun Banerjee

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Bubun Banerjee was born in Mamudpur, Burdwan (West Bengal), India in 1987. He received B.Sc. (Honours) degree in Chemistry from St. Xavier’s College, Kolkata (University of Calcutta) in 2008 and M.Sc. (Organic specialization) from the University of Delhi in 2010. After qualifying CSIR-UGC NET (2010), he joined the Ph.D. programme (Synthetic Organic Chemistry) as Junior Research Fellow under the supervision of Prof. (Dr.) Goutam Brahmachari at the Chemistry Department, Visva-Bharati (a Central University), Santiniketan (West Bengal), India, and presently he is working as Senior Research Fellow. His research interest focuses on the development of novel synthetic methodologies for heterocyclic compounds with special emphasis on green chemistry. C/O Prof. (Dr.) Goutam Brahmachari, Laboratory of Natural products & Organic Synthesis, Department of Chemistry, Visva-Bharati (a Central University), Santiniketan-731-235, West Bengal, India. E-mail: [email protected]

In memory of my heavenly grandmother, Bani Banerjee

Abstract The present feature outlines the preparation and synthetic applications of per-6-amino--cyclodextrin as an ecofriendly, reusable, basic catalyst.

Keywords: Per-6-amino--cyclodextrin; Preparation; Synthetic utility Introduction Due to good binding ability toward aromatic units as well as its ready commercial availability, -cyclodextrin has drawn considerable attention to the synthetic chemists in recent times. -Cyclodextrin is a cyclic oligosaccharide that possesses hydrophobic cavities capable of binding the guest selectively and also can catalyze chemical reactions by forming reversible host-guest complexes via noncovalent interactions [1,2]. However, the major demerit of 28

using -cyclodextrin is its poor aqueous solubility. In most of situations, chemical modification of cyclodextrins is required to improve its aqueous solubility as well as the enantioselectivity [3]. Accordingly, per-6-amino-cyclodextrin (Figure 1) is a modified per-substituted homogeneous cyclodextrin derivative at the primary face electrostatic binding ability with the guest molecules better than the parent molecule [4-6]. Aminocyclodextrins and


carboxymethylated cyclodextrins are successfully used as chiral discriminating agents to separate enantiomers in capillary electrophoresis [7], biomimetic catalysts for Kemp elimination [8]; they are also successfully employed in high performance liquid chromatography [9], in chiral NMR analysis [10], deprotonation [11] and chiral recognition processes [12,13].

In the recent past, Pitchumani and his group have successfully employed per-6-amino--cyclodextrin as an efficient, reusable, basic, supramolecular host in a number of organic reactions. This spotlight highlights the preparation and the use of per-6-amino-cyclodextrin in organic reactions as a supramolecular catalyst.

Figure 1. per-6-amino--cyclodextrin (per-6-ABCD; 4)

Preparation of per-6-amino--cyclodextrin Per-6-amino--cyclodextrin (4) can be synthesized from -cyclodextrin [14] (Scheme 1). First, -cyclodextrin (1) was treated with I2 and triphenylphosphine (Ph3P) in N,N-dimethyl formamide (DMF) at 70 °C under inert atmosphere, and then NaOMe was added drop wise in the resulting mixture yielding the corresponding pure 92% per6-iodo--cyclodextrin (2). Intermediate 2 was then treated with NaN3 in DMF to give in 96% yield of the per-6azido--cyclodextrin (3) [15]. At the final step, per-6-azido--cyclodextrin (3) was reduced very efficiently by the subsequent treatment of aqueous NH3 solution to furnish per-6-amino--cyclodextrin (4) in 98% yield.

Scheme 1: Preparation of per-6-amino--cyclodextrin [14,15]

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Synthetic applications Per-6-amino-β-cyclodextrin (4) is a reusable and nontoxic supramolecular basic catalyst with enhanced hydrophobic and electrostatic binding ability towards guest molecules relative to native cylodextrins. Synthetic uses of such catalyst are documented below: 1. In 2008, Pitchumani and his group [16] demonstrated for the first time per-6-amino-β-cyclodextrin (per-6-ABCD) (4) as an excellent supramolecular host as well as an efficient ligand for CuI; the resulting catalytic system carried out N-arylation of imidazole (5) with a wide range of aryl and heteroaryl bromides (6) smoothly under refluxing in DMSO (Scheme 2).

Scheme 2. N-arylation of imidazole with aryl bromides [16] 2. The same group [17] have reported an efficient, environmentally benign and enantioselective Michael addition of nitromethane (8)/thiols (9) to trans-chalcone (10) with good yield using per-6-amino--cyclodextrin (4) as a reuseable catalyst in water at room temperature (Scheme 3).

Scheme 3. Enantioselective Michael addition of nitromethane/thiols to trans-chalcone [17] 3. A simple and one-pot, four-component protocol was developed by Kanagaraj et al. [18] for the synthesis of various dihydropyrano[2,3-c]pyrazole derivatives (17) from the reaction of hydrazine hydrate (13), ethyl acetoacetate (14), carbonyl compounds (15) and malononitrile (16) using per-6-amino--cyclodextrin (4) as a supramolecular host as well as an effective and reusable solid basic catalyst under solvent-free conditions at room temperature (Scheme 4).

Scheme 4. Syntheses of various dihydropyrano[2,3-c]pyrazole derivatives [18]

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4. In 2010, Pitchumani and co-workers [19] have developed another efficient protocol where per-6-amino-cyclodextrin (4) acts as a supramolecular chiral host, reusable promoter and basic catalyst for the enantioselective Henry (nitroaldol) reaction of nitromethane (18) with different aldehydes (15) in a syn-selective manner affording hydroxynitro compounds in good yields with 99:1 syn/anti selectivity (Scheme 5).

Scheme 5. Enantioselective Henry (nitroaldol) reaction of nitromethane with different aldehydes [19] 5. An efficient and environmentally benign cyanation of aryl halides (6) has been achieved by Azath et al. [20] using less toxic K4[Fe(CN)6] (20) as cyanating source and per-6-amino--cyclodextrin (4) as supramolecular ligands of the catalyst CuI (Scheme 6).

Scheme 6. Cyanation of aryl halides [20] 6. Recently, Azath et al. [21] have designed an efficient process for the synthesis of various substituted 2-amino-4Hbenzo[b]pyrans (23) via one-pot three-component reaction of aromatic aldehydes (15), malononitrile (16) and 1,3cyclohexanediones/dimedone (22) using per-6-amino-β-cyclodextrin (4), as an efficient, reusable basic catalyst, under solvent-free conditions at room temperature (Scheme 7).

Scheme 7. Synthesis of substituted 2-amino-4H-benzo[b]pyrans [21]

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7. Very recently, Kanagaraj and Pitchumani [22] have also developed a highly efficient, high yielding one-pot protocol for the synthesis of enantiomerically enriched (ee up to 99%) 2-aryl-2,3-dihydroquinolin-4(1H)-ones (25) from the reaction of 2-aminoacetophenone (24) and various aldehydes (15) using per-6-amino-β-cyclodextrin (4) as a supramolecular host, reusable promoter and a basic chiral catalyst (Scheme 8).

Scheme 8. One-pot synthesis of 2-aryl-2,3-dihydroquinolin-4(1H)-ones [22]

Acknowledgement BB is grateful to his honorable mentor Prof. (Dr.) Goutam Brahmachari for his constant encouragement and supports.

[10] Dignam C. F., Randall L. A., Blacken R. D., Cunningham P. R., Lester S.-K. G., Brown M. J., French S. C., Aniagyei S. E., and Wenzel T. J. (2006) Tetrahedron: Asymmetry 17, 11991208. [11] Kitae T., Nakayama T., and Kano K. (1998) J. Chem. Soc., Perkin Trans. 2, 207-212. [12] Riela S., D’Anna F., Lo Meo P., Gruttadauria M., Giacalone R., and Noto R. (2006) Tetrahedron 62, 4323-4330. [13] Brown S. E., Coats J. H., Duckworth P. A., Lincoln S. F., Easton C. J., and May B. L. (1993) J. Chem. Soc., Faraday Trans. 89, 10351040. [14] Gadelle A., and Defaye J. (1991) Angew. Chem., Int. Ed. Engl. 30, 78-80. [15] Ashton P. R., Königer R., and Stoddart J. F. J. (1996) Org. Chem. 61, 903-908. [16] Suresh P., and Pitchumani K. (2008) J. Org. Chem. 73, 9121-9124. [17] Suresh P., and Pitchumani K. (2008) Tetrahedron: Asymmetry 19, 2037-2044.

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[18] Kanagaraj K., and Pitchumani K. (2010) Tetrahedron Lett. 51, 3312-3316. [19] Kanagaraj K., Suresh P., and Pitchumani K. (2010) Org. Lett. 12, 4070-4073. [20] Azath I. A., Suresh P., and Pitchumani K. (2012) New. J. Chem. 36, 2334-2339.

[21] Azath I. A., Puthiaraj P., and Pitchumani K. (2013) ACS Sustainable Chem. Eng. 1, 174-179. [22] Kanagaraj K., and Pitchumani K. (2013) J. Org. Chem. 78, 744-751.

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