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

Article history: Received October 21, 2015 Received in revised form December 20, 2015 Accepted 1 Februray 2016 Available online 2 February 2016 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.


Introduction
6][7][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][10][11][12][13][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 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

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.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][16][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 adducts 11 were determined on the basis of electronic values of chemical potentials ( and chemical hardness. whereas the global nucleophilicity (N) 18 of nitrone can be expressed in terms of equation: 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 : The local nucleophilicity (k) 21 condensed to atom k was calculated using global nucleophilicity N and Paar function P -k 20 according to the formula: Such obtained reactivity indexes are gathered in Table 1.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.

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 recently [23][24][25][26] .Due to this, mechanistic aspects of nitrone / 1-halo-1-nitroethene cycloadditions would be subject of further, comprehensive studies.

Instruments
Melting points were determined on a 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. 1 H-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 100-10-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.

Experimental procedure and physical data
A mixture of a suitable nitroalkene (0.02mol) and nitrone (0.01mol) in 5cm 3 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.

Table 2 .
The

Table 3 .
The