A Novel One-Pot Green Synthesis of Dispirooxindolo-pyrrolidines via1,3-Dipolar Cycloaddition Reactions of Azomethine Ylides

A facile synthesis of dispirooxindolopyrrolidines has been accomplished via a one-pot three component 1,3-dipolar cycloaddition reaction. The reaction of azomethine ylides generated in situ from L-phenylalanine and substituted isatins with a series of unusual (E)-2-oxoindolino-3-ylidene acetophenone dipolarophiles in the ionic liquid 1-butyl-3-methylimidazolium bromide [bmim]BF4, furnished the cycloadducts in good yields, with the regioisomers 5a–f being obtained with high selectivity. Furthermore, the recyclability of [bmim]BF4, up to five times, was also investigated.


Introduction
Ionic liquids are widely recognized as "green" solvents in organic synthesis because of their unique properties, such as low vapor pressure, high chemical and thermal stability, good solvating OPEN ACCESS ability, non-flammability, behavior as acidic or basic catalysts and recyclability [1][2][3][4][5]. In this context, ionic liquids have emerged as new green solvents to replace volatile organic compounds and they are suitable for executing many diverse organic reactions [6][7][8]. One-pot three component reactions, widely useful in both combinatorial and medicinal chemistry arenas, with their powerful bond forming and atom efficiencies, represent another eco-friendly synthetic approach for the expedient construction of molecules of structural diversity and complexity [9][10][11][12]. These reactions also assume importance from the viewpoint of minimized waste generation due to the avoidance of intermediate isolation and purification steps. For these reasons, the development of multi-component reactions in ionic liquids, although relatively unexplored [13], is of great interest.
The spirooxindole core is a privileged heterocyclic system that is featured in a large number of bioactive naturally occurring alkaloids (Figure 1) that display a wide spectrum of biological activities such as antimicrobial [14], inhibition of human NK-1 receptor [15] and potent non-peptide inhibition of the p53-MDM2 interaction [16]. Particularly, the spiropyrrolidinyloxindole nucleus is often found in the molecular framework of many natural products, viz., horsfiline [17], coerulescine [18], and elacomine [19] (Figure 1) which possess a myriad of biological activities such as inhibition of the mammalian cell cycle at G2/Mphase [20,21], inhibition of microtubule assembly [22], modulation of the function of muscarinic serotonin receptors [23], antitumor activity against human brain cancer cell lines, neuroblastoma SKN-BE (2), and malignant glioma GAMG [24].  The biological significance of the above heterocycles prompted us to explore the synthesis of novel hybrid spiroheterocycles comprising pyrrolidine and oxindole units via a one-pot three component 1,3-dipolar cycloaddition reaction. Recently, we embarked on a program for the synthesis and/or biological screening of structurally diverse novel spiroheterocycles, which has brought to light various antimicrobial leads [25][26][27][28]. In continuation of our research in the area of 1,3-dipolar cycloaddition reactions [29,30], herein we wish to report an expeditious and facile protocol for the synthesis of dispirooxindolopyrrolidines in ionic liquid medium; highly desirable from the viewpoint of green chemistry. Furthermore, to the best of our knowledge, this is the first report on the generation of azomethine ylide from L-phenylalanine and isatin in ionic liquid medium.

Results and Discussion
In the present investigation, the one-pot three component reaction of (E)-2-oxoindolino-3-ylidene acetophenones 4a-b with non-stabilized azomethine ylides 3, generated in situ by the decarboxylative condensation of isatin 1a-c and L-phenylalanine (2) in [bmim]BF4 afforded the dispiropyrrolidines 5a-f in good yields (70%-77%) with selectivity, along with trace amounts of the regioisomers 6a-f (5%-7%) (Scheme 1). Solvent-optimization for this cycloaddition reaction was investigated by the reaction of an equimolar ratio of (E)-2-oxoindolino-3-ylidene acetophenone, isatin and L-phenylalanine in organic solvents viz. methanol, ethanol, dioxane, dioxane/methanol (1:1) mixture under heating in an oil-bath ( Table 1). The cycloadducts 5a and 6a were obtained only in 28%and 12% yield in methanol ( Table 1, entry 1), whilst the reactions in ethanol, dioxane and dioxane/methanol (1:1) mixture furnished the products 5a and 6a in 30% and 15%, 34% and 16%, 38% and 18% yields, respectively, which indicated that the solvents had little effect on the selectivity and yield of the reaction ( Table 1). The same reaction was also investigated in ionic liquids, such as [bmim]Br, [bmim]BF4 and also with combination of a catalyst, [bmim][BF4]/CuI (10 mol %), [bmim][BF4]/Zn(OTf)2 (10 mol %). Like the reactions in organic solvents, these reactions also furnished 5a and 6a, but with good yield and high selectivity, the isomer 5 being obtained predominantly in all cases. The ionic liquids [bmim]BF4 and [bmim]Br were found to be the appropriate reaction medium for these cycloaddition reactions in terms of yield and selectivity (Table 1, entry 6). Hence, all the subsequent reactions were performed by heating an equimolar mixture of the reactants in [bmim]BF4 (3 mL) in an oil-bath at 100 °C for 2 h ( Table 2). After completion of the reaction (by TLC), the product was isolated and purified by flash column chromatography, while the [bmim]BF4 was recovered by vacuum distillation then dried under vacuum at 40 °C overnight for recycling (Table 3). Notes: Optimized reaction condition is in bold (entry 6); Ionic liquids were subjected to high vacuum before use (entry 5-8).  The structure of regioisomers was elucidated using IR, 1 H-, 13 C-NMR spectroscopic and mass spectrometry studies. For instance, the IR spectrum of the cycloadduct 5a displayed characteristic bands at 1628, 1706, 1710 cm −1 corresponding to the two oxindoles and benzoyl carbonyls, respectively. In the 1 H-NMR spectrum of 5a, the benzylic and Ha protons appeared as multiplets in the region δH 3.08-3.12 and 5.57-5.61 ppm, respectively. The Hb proton attached to the benzoyl group appeared as doublet at δH 4.29 ppm and the trans stereochemistry was confirmed through coupling constant of Hb proton (J = 8.8 Hz). The aromatic protons appear as multiplets around 6.43-7.78 ppm. In the 13 C-NMR spectrum, the signals at δC 177.2, 180.5 and 195.4 ppm were attributed to two oxindole and benzoyl ring carbonyls. The two spiro carbons resonated at δC 63.9 and 83.3 ppm. Furthermore, the presence of the molecular ion peak at m/z 533 (M + ) in the mass spectrum of 5a supported the formation of the cycloadduct. The stereochemistry of compound 5a has been assigned based on the fact that the carbonyls of the two isatin rings are in trans relationship as to minimize the repulsion between the two isatin carbonyl as evidenced in the literature [31][32][33]. The structure of other regioisomer 6a was also elucidated by similar straight forward considerations. The singlet at δH 4.90 ppm in the 1 H-NMR spectrum, confirms the formation of regioisomer 6a. A pictorial representation of the 1 H-and 13 C-NMR chemical shifts of 5a and 6a is shown in Figure 2.  A rational mechanism for the formation of the cycloadducts 5 and 6 in the presence of [bmim]BF4 is described in Scheme 2. It is known that ionic liquid play dual roles as solvent and catalyst. For instance, the hydrogen atom of [bmim] + , being electron-deficient can form hydrogen bonds with heteroatoms thereby catalyzing reactions. Thus, in the present transformation, a hydrogen bond between the imidazole ring hydrogen atom of [bmim] + and the carbonyl group of 4 furnishes 4', which readily reacts with azomethine ylide to afford 5 and 6. The azomethine ylide may also be catalyzed by [ The regio-and stereo-chemistry of cycloadduct 6f was further supported by single crystal X-ray diffraction studies [34] (Figure 3). The molecule 6f is composed of a central pyrrolidine ring with benzoyl group at C9, benzyl group at C11, spiro-carbons fused with oxindoles at C7 and C10 (Figure 3). The dihedral angle between the pyrrolidine ring A (N2/C7/C9/C10/C11), indolin-2-one ring B (C1-C8/N1) and 2,3-dihydro-1H-inden-1-one ring C (C19-C26) are 77.96 (2)

General Methods
Melting points were taken using open capillary tubes and are uncorrected. Unless stated otherwise, solvents and chemicals were obtained from commercial sources and used without further purification. IR Spectra were measured as KBr pellets on a Nicolet 6700 FT-IR spectrophotometer (Madison, WI, USA). 1 H-and 13 C-NMR spectra were recorded on a Varian Mercury JEOL-400 NMR spectrometer (Tokyo, Japan) and Bruker 500 MHz NMR spectrometers (Faellanden, Switzerland) operating at 400, 500, 100 and 125 MHz, respectively, and chemical shifts are reported as δ values (ppm) relative to tetramethylsilane. Elemental analyses were performed on a Perkin-Elmer 2400 series II elemental CHNS analyser (Waltham, MA, USA). Flash column chromatography was performed on silica gel (230-400 mesh) using petroleum ether (60-80 °C)/EtOAc as eluent.

X-ray Crystallography
A clear intense yellow block-like specimen of C37H36N3O7S2, approximate dimensions 0.39 × 0.457 × 0.674 mm, was used for the X-ray crystallographic analysis. A total of 684 frames were collected. The total exposure time was 2.85 h. The frames were integrated with the Bruker SAINT software package [35] using a narrow-frame algorithm. The integration of the data using a monoclinic unit cell, space group P21/n, yielded a total of 35419 reflections to a maximum θ angle of 28.33° (0.75 Å resolution), of which 8504 were independent (average redundancy 5.743, completeness = 99.6%, Rint = 3.18%, Rsig = 2.93%) and 6264 (76.53%) were greater than 2σ(F 2 ). The final cell constants of a = 13.5273(5) Å, b = 14.8582(6) Å, c = 17.1768(7) Å, β = 95.0630(10)°, volume = 3437.3(2) Å 3 , are based upon the refinement of the XYZ-centroids of 9790 reflections above 20 σ(I) with 4.588° < 2θ < 56.36°. Data were corrected for absorption effects using the multi-scan method (SADABS). The ratio of minimum to maximum apparent transmission was 0.907. The calculated minimum and maximum transmission coefficients (based on crystal size) are 0.9490 and 0.9690.
The final anisotropic full-matrix least-squares refinement on F 2 with 388 variables converged at R1 = 6.41%, for the observed data and wR2 = 20.09% for all data. The goodness-of-fit was 1.06. The largest peak in the final difference electron density synthesis was 8.210 e-/Å 3 and the largest hole