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Current Chemistry Letters 5 (2016) 123–128 Contents lists available at GrowingScience Current Chemistry Letters homepage: www.GrowingScience.com [3+2] Cycloadditions of 1-halo-1-nitroethenes with (Z)-C-(3,4,5-trimethoxyphenyl)-N-methyl-nitrone as regio- and stereocontrolled source of novel bioactive compounds: preliminary studies Radomir Jasińskia*, Ewa Dreslerb, Maria Mikulskaa and Daniel Polewskia a Institute of Organic Chemistry and Technology, Cracow University of Technology, Warszawska Str. 24, 31155 Cracow, Poland Institute of Heavy Organic Synthesis „Blachownia", Energetyków Str. 9, 47-225 Kędzierzyn-Koźle, Poland b CHRONICLE Article history: Received October 21, 2015 Received in revised form December 20, 2015 Accepted 1 Februray 2016 Available online 2 February 2016 Keywords: Cycloaddition Nitroalkenes Nitrones Reactivity Regioselectivity ABSTRACT Preliminary experiments shows, that [3+2] cycloadditions reactions proceeds with full regioselectivity and high stereoselectivity. In consequence, 3,4-trans-2-methyl-3-(3,4,5trimethoxyphenyl)-4-halo-4-nitroisoxazolidines are forming as predominantly (or sole) products. Additionally, prognosis for the synthesized compounds to be potential ingredients of drugs is good. © 2016 Growing Science Ltd. All rights reserved. 1. Introduction Isoxazolidines are an important class of bioactive compounds.1-3 The presence of the nitro group in the molecule of these compounds stimulate extra forms of biological activity 4 and allows for a wide range of further functionalizations.5-8 This justifies undertaken by us a few years ago a comprehensive study on the synthesis of nitro substituted isoxazolidines by [3+2] cycloaddition of nitroalkenes to nitrones. So far we have dealt mainly with reactions where 2-substituted nitroethenes 9-14 were participated. The accumulated research material makes it possible to formulate some general conclusions about regioselectivity, stereoselectivity and reaction mechanisms of this group. A little is currently known about similar reactions involving nitroethenes functionalized from a position 1 of the nitrovinyl fragment. This paper provides a summary of our recent, preliminary studies in this area. In particular, in the context of the whole study, we present a diagnosis for a course of two [3+2] cycloadditions of 1-chloro-1-nitroethene (1a) and 2-(trichloromethyl)-1-bromo-1-nitroethene (1b) with * Corresponding author. E-mail address: radomir@chemia.pk.edu.pl (R. Jasiński) © 2015 Growing Science Ltd. All rights reserved. doi: 10.5267/j.ccl.2016.2.001 124 a strong nucleophile (Z)-C-(3,4,5-trimethoxyphenyl)-N-methyl-nitrone (2), which was previously tested in the reactions with 2-substituted nitroethenes.12 2. Results and Discussion Assuming that the reaction takes place through a one-step mechanism, the cycloadditions of the test components could in theory be accomplished on four regio- and stereoisomeric paths, leading finally to nitroisoxazolidines 3-6a, b. NO2 Ar A X Me N R O 3a,b Ar X B X R C C H NO2 H 1a,b + C Me N Ar + Me N O 2 X = Cl (a), Br (b) R = H (a), CCl3 (b) Ar = (3,4,5-MeO)3-C6H2 NO2 R O 4a,b Ar R C Me N NO2 X O 5a,b Ar R D X Me N O 6a,b NO2 Scheme 1. Theoretically possible paths of [3+2] cycloaddition between 1-halo-1-nitroethenes 1a,b with (Z)-C-(3,4,5-trimethoxyphenyl)-N-methyl-nitrone 2 In the first step of our studies we decided to determine which of the paths are actually proceed in reality. For this purpose, we performed a series of tests of varying the reaction time, temperature and solvent. We have found that both cycloadditions in methylene chloride (DCM) are easily progressing at room temperatures, at twice molar excess of nitroalkene. Under these conditions, the conversion of the nitrone is finished in 2 hours. Both reaction mixtures were analyzed by HPLC. It has been found that the reaction of 1-chloro-1nitroethene (1a) from (Z)-C-(3,4,5-trimethoxyphenyl)-N-methyl-nitrone (2) leads toward two products, which could be isolated with semi-preparative HPLC. Based on elemental analysis and spectral data we have established that these are stereoisomeric 3,4-trans- (3a) and 3,4-cis-2-methyl-3- (3,4,5trimethoxyphenyl)-4-chloro-4-nitroisoxazolidines (4a). In turn, a similar reaction involving 2(trichloromethyl)-1-bromo-1-nitroethene (1b) has produced only the product, which by elemental analysis and spectral data was assigned as 3,4-cis-2-methyl-3-(3,4,5-trimethoxyphenyl)-4-bromo-4nitro-5-(trichloromethyl)-isoxazolidine (3b). We have decided to explain the observed regioselectivity on the basis of a recently intensively developed,11,15-17 conceptual DFT study based on the analysis of the electrophilicity  and nucleophilicity N indices. The necessary descriptors havebeen obtained from quantum-chemical calculations. So, the global electrophilicity () of adducts11 were determined on the basis of electronic values of chemical potentials ( and chemical hardness. (1) (η)  η, (2) LUMO  ηLUHOMO, whereas the global nucleophilicity (N)18 of nitrone can be expressed in terms of equation: nitroneHOMO (tetracyanoethene) (3) The local electrophilicity (k)19 condensed to atom k was calculated by projecting the index  onto any reaction center k in the molecule by using Parr function P+k 20: R. Jasiński et al. / Current Chemistry Letters 5 (2016) 125 k = P+k·  The local nucleophilicity (k)21 condensed to atom k was calculated using global nucleophilicity N and Paar function P-k20 according to the formula: k = P-k·  Such obtained reactivity indexes are gathered in Table 1. Table 1. Global and local electronic properties for 1-halo-1-nitroethenes 1a,b and (Z)-C-(3,4,5trimethoxyphenyl)-N-methyl-nitrone 2 1a 1b 2 Global properties η  (au) (au) -0.2019 0.1927 -0.2010 0.1879 -0.1191 0.1499  (eV) 2.88 2.93 1.29 N (eV) Local properties NO NC (eV) (eV) 3.84 0.17  (eV) 0.02 0.00  (eV) 1.29 0.17 1.47 From the collected data it can be shown that a stronger nucleophilic center for this reaction in the nitrone molecule is a carbon atom on the CNO fragment. In turn, a stronger electrophilic reaction center in the nitroalkene molecules is always -carbon atom on the nitrovinyl fragment. An interaction of these centers controls flow of the reaction, directing the reaction along the pathways leading to adducts with a nitro group at C4 position of the isoxazolidine ring. Finally, we have decided to diagnose a potentially interesting bioactivity properties of the synthesized compounds. For this purpose we have used PASS simulation program, which has recently been successfully applied to a design of a number of medicines 22. It has been found (Table 2), that the compound 1a shows in five areas a high probability of an action in a manner similar to known drugs (Pa = 0.7 or more), at a relatively low coefficient of inactivity (Pi <0.05). Slightly less attractive from the application point of view seems to be the product 1b (Table 3). In the near future the issue of bioactivity will be a subject of our detailed laboratory analysis. Table 2. The PASS simulations for 2-methyl-3-(3,4,5-trimethoxyphenyl)-4-chloro-4nitroisoxazolidines Activity Ubiquinol-cytochrome-c reductase inhibitor Membrane permeability inhibitor Acrocylindropepsin inhibitor Chymosin inhibitor Saccharopepsin inhibitor Antileukemic Aspulvinone dimethylallyltransferase inhibitor Polarisation stimulant Phobic disorders treatment The probability of activity (Pa) 0.767 0.706 0.700 0.700 0.700 0.603 0.636 0.549 0.620 The probability of non-occurrence of activity (Pi) 0.044 0.036 0.044 0.044 0.044 0.009 0.085 0.021 0.113 Table 3. The PASS simulations for 2-methyl-3-(3,4,5-trimethoxyphenyl)-4-bromo-4-nitro-5(trichloromethyl)-isoxazolidine Activity Antileukemic Antifungal Ubiquinol-cytochrome-c reductase inhibitor CYP2H substrate Calcium regulator Acrocylindropepsin inhibitor Chymosin inhibitor Saccharopepsin inhibitor Antineoplastic (lung cancer) The probability of activity (Pa) 0.572 0.545 0.618 0.544 0.419 0.476 0.476 0.476 0.378 The probability of non-occurrence of activity (Pi) 0.010 0.024 0.100 0.096 0.035 0.123 0.123 0.123 0.026 126 3. Conclusions Cycloaddition reactions involving 1-halo-substituted nitroalkenes are proceeding under mild conditions. There is an evident preference for one of the four theoretically possible configurations of the transition state. This makes the synthesis to be accomplished with regiospecificity and in a highly stereoselective manner. A prognosis for the synthesized compounds to be potential ingredients of drugs is good. Finally it should be noted, that due to unequivocal degree of substitution of reactions centers of addents, [3+2] cycloadditions involving 1-halo-1-nitroethenes may proceed via stepwise mechanisms. Some examples of these type mechanisms were described recently23-26. Due to this, mechanistic aspects of nitrone / 1-halo-1-nitroethene cycloadditions would be subject of further, comprehensive studies. 4. Experimental 4.1. Instruments Melting points were determined on a Boetius apparatus and are uncorrected. Elemental analyses were determined on a Perkin-Elmer PE-2400 CHN apparatus. Mass spectra (EI, 70eV) were obtained using a Hewlett-Packards 5989B spectrometer. IR spectra were recorded on a Bio-Rad spectrophotometer. 1H-NMR spectra were taken on a Bruker (500 MHz) spectrometer, using TMS as an internal standard, and CDCl3 as a solvent. Liquid chromatography (HPLC) was done using a Knauer apparatus equipped with a UV-VIS detector. For monitoring of the reaction progress, LiChrospher 10010-RP column (4x240 mm) and 75 % methanol as the eluent at flow rate 1.2 ml/min were used. The separation of the post-reaction mixtures was performed on the same Knauer apparatus, using a semipreparative column (LiChrospher 100-10-RP, 16x240 mm) and 70 % methanol as the eluent at flow rate 10 ml/min. 4.2. Reagents 3,3,3-trichloro-1-bromo-1-nitroethene 1b and (Z)-C-(3,4,5-trimethoxyphenyl)-N-methyl-nitrone 2 was synthesized according to the procedures described in the literature: 1b,27 2.28 For 1-chloro-1nitroethene 1a a new efficient synthetic procedure has been developed, which was a modification of a synthetic route developed in the early twentieth century.29 A Claisen flask was charged with P2O5 (42 g). The flask was heated to 150 °C, after which the pressure in the distillation assembly was reduced to 110 mmHg. Then it has been slowly added dropwise to the flask of 2-chloro-2-nitroethanol (38 g), which had been synthesized from 2-nitroethanol by a procedure described in detail in 30. The resulting 1-chloronitroethene immediately distilled out and condensed in a receiver cooled to 0 °C. The crude product was dried with Na2SO4 and then distilled under a reduced pressure in an atmosphere of argon. On this way, 16g (52%) of 1-chloronitroethene 1a was obtained. Its purity was confirmed by GC. B.p. 55-56 °C / 90 mmHg (Ref.25) 54 °C. / 14 mmHg); 1 H NMR: 6.85 ppm (1H, d, J = 3.92Hz), 6.04 ppm (1H, d, J = 3.92Hz); IR cm-1: 1548, 1333 (NO2), 1620 (C=C). 4.3 Experimental procedure and physical data A mixture of a suitable nitroalkene (0.02mol) and nitrone (0.01mol) in 5cm3 of DCM was stirred at room temperature for 2 hours. The solvent was evaporated in vacuo to dryness and the semiliquid residue was separated by semipreparative HPLC. Evaporation of the eluent from the obtained fractions gave the diarylnitroisoxazolidines 3 and 4. 3,4-trans-2-methyl-3-(3,4,5-trimethoxyphenyl)-4-chloro-4-nitroisoxazolidine (3a) Yield 75 %, colorless crystals, m.p. 134 (cyclohexane). RT (min): 3.7. IR, , cm-1: 1567, 1352 (NO2), 1182 (-C-N-), 1235, 1051 (CH3-O-Ar), 858 (Ar), 807 (C-Cl). NMR 1H, ppm (J, Hz): 6.54 (2H, s, C6H2), 5.25 (1H, d, J=10.24, H5), 4.45 (1H, d, J=10.24, H5’), 4.00 (1H, s, H3), 3.88 (3H, s, p-OCH3), R. Jasiński et al. / Current Chemistry Letters 5 (2016) 127 3.85 (6H, s, m-OCH3), 2.74 (3H, s, N-CH3). NMR 13C, ppm: 139.2 138.4, 137.5, 106.2, 96.1, 78.3, 76.8, 62.6, 58.1, 45.3. Found, %: C 46.67; H 5,13; N 8,32. C13H17N2O6Cl. Calculated, %: C 46.90; H 5,11, N 8.42. 3,4-cis-2-methyl-3-(3,4,5-trimethoxyphenyl)-4-chloro-4-nitroisoxazolidine (4a) Yield 18 %, oil. RT (min): 5.4. IR, , cm-1: 1574, 1349 (NO2), 1182 (-C-N-), 1236, 1057 (CH3-OAr), 867 (Ar), 797 (C-Cl). NMR 1H, ppm (J, Hz): 6.62 (2H, s, C6H2), 5.13 (1H, d, J=10.96, H5), 4.30 (1H, d, J=10.96, H5’), 4.21 (1H, s, H3), 3.91 (3H, s, p-OCH3), 3.88 (6H, s, m-OCH3), 2.76 (3H, s, NCH3). NMR 13C, ppm: 138.9 138.2, 137.6, 105.9, 96.0, 78.3, 77.1, 62.4, 59.3, 46.2. Found, %: C 46.92; H 5.10; N 8.37. C13H17N2O6Cl. Calculated, %: Calculated, %: C 46.90; H 5,11, N 8.42. 3,4-trans-2-methyl-3-(3,4,5-trimethoxyphenyl)-4-bromo-4-nitro-5-(trichloromethyl)-isoxazolidine (3b) Yield 92 %, colorless crystals, m.p. 152 (ethanol-hexane 9:1 v/v). RT (min): 5.4. IR, , cm-1: 1573, 1352 (NO2), 1186 (-C-N-), 1240, 1047 (CH3-O-Ar), 859 (Ar), 788 (C-Cl), 698 (C-Br). NMR 1H, ppm (J, Hz): 6.53 (2H, s, C6H2), 5.36 (1H, s, H5), 4.41 (1H, s, H3), 3.87 (3H, s, p-OCH3), 3.85 (6H, s, mOCH3), 2.76 (3H, s, N-CH3). NMR 13C, ppm: 139.8, 138.5, 137.0, 106.7, 99.0, 79.6, 78.5, 62.3, 58.9, 47.2. 19.2. Found, %: C 34.05.92; H 3.25; N 5.70. C14H16N2O6Cl3Br. Calculated, %: Calculated, %: C 34.00; H 3,26, N 5.66. 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