The usefulness of various quantum chemical algorithms (semiempirical, HF, DFT) for predicting the energy and geometry of transition states of polar pseudocyclic processes was analyzed using the example of a model cycloaddition process between (Z)-C,N-diphenylnitrone and (E)-2-phenyl-1-nitroethene. These studies clearly recommend the ωB97XD functional in 6-311+G(d) basis set as the relatively most precise tool for studying the mechanisms of polar pseudocyclic processes.

Application of the Molecular Electron Density Theory (MEDT) for the exploration of the [3+2] cycloaddition processes between methyl propynoate 1 and difluoromethyldiazomethane T-1, have been implemented using the DFT/B3LYP/6-311(d,p) level of theory. According to an examination of conceptual DFT indices, difluoromethyldiazomethane (T-1) participates in this reaction as a nucleophile, while methyl propynoate (1) should be considered as an electrophile. This cyclization is regiospecific, as evidenced by the activation and reaction energies, this agrees with the experiment's findings. It was discovered throughout ELF analysis that analyzed [3+2] cycloaddition is realised by a two-step mechanism. In addition, study of interactions of the products studied in this paper with the protein protease Covid-19 (PDB ID: 7R98) were carried out, by means of molecular docking study). The results indicate that the occurrence of the transfer of the proton to the nitrogen atom, increases the affinity of these products to the protein (CA32-F1 and CA32-F2).

DFT calculations indicate that the decomposition reaction of nitroethyl benzoates in the presence of 1,3-dimethylimidazolium cation takes place much faster than in the case of the non-catalyzed process. Additionally, our calculations suggest one-step polar mechanism of title reactions.

Preliminary experiments shows, that [3+2] cycloadditions reactions proceeds with full regioselectivity and high stereoselectivity. In consequence, 3,4-trans-2-methyl-3-(3,4,5-trimethoxyphenyl)-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.

Analysis of addent interactions with respect to the theory of electrophilicity and nucleophilicity indexes and kinetic studies shows the polar nature of [2+3] cycloaddition of (Z)-C-(m,m,p-trimethoxyphenyl)-N-(p-methylphenyl)-nitrone to (E)-3,3,3-trichloro-1-nitroprop-1-ene. However, PES exploration results do not confirm any zwitterion on the reaction paths. On the other hand, the nature of the solvent effect and the experimentally determined activation parameter values do not exclude a scenario in which one-step and stepwise processes occur simultaneously.

Analysis of global electrophilicity and nucleophilicity power of the addends indicate polar character of [2+3] cycloaddition reactions between arylnitrones and trans-substituted nitroethenes. The regioselectivity of these reactions is determined by nucleophilic attack of oxygen atom from nitrone on activated ?-position of nitroalkene. Interaction of this type leads to 4-nitroisoxazolidines, which are the only reaction products.