A computational study of Syn-propanethial S-oxide, as a new TAC for [3+2] cycloaddition reactions, was conducted. The study employed MEDT approach. As alkene components various nitroethenes were applied. Local reactivity descriptors N k (local nucleophilicity) and ω k (local electrophilicity) of S-oxide 1, were determined. Mechanisms of the reactions were computationally studied. The reference cycloadducts of 1 and 2a, namely 3a-6a, were also screened for bioactivity using in silico molecular docking. Gasussian 16 C.01, and AutoDock Vina implemented by SwissDock software was used perform the computations.

The regio-stereoselectivity and the molecular mechanism of the Au(III)-catalysed [3+2] cycloaddition reaction of (Z)-C,N-diphenylnitrone with nitroethene were explored in the light of the DFT calculations. We found, that the presence of the Au(III) molecular segments, in the reaction environment, dramatically has changed the cycloaddition mechanism. In particular, the observed, single step mechanism under the thermal conditions, is replaced to stepwise, zwitterionic mechanism on three from four theoretically possible paths. Its is interesting, that we identified the firstexample of the stepwise mechanism leading to the 5-nitroisoxazolidine molecular segment. Earlier, only [3+2] cycloadditions leading to the 4-nitroisoxazolidines were incidentally described as a stepwise processes.

The molecular mechanism of the [3+2] cycloaddition reactions between aryl azides and ethyl propiolate was evaluated in the framework of the Molecular Electron Density Theory. It was found that independently of the nature of the substituent within the azide molecule, the cycloaddition process is realized via a polar but single-step mechanism. All attempts of localization as postulated earlier by Abu-Orabi and coworker's zwitterionic intermediates were not successful. At the same time, the formation of zwitterions with an "extended" conformation is possible on parallel reaction paths. The ELF analysis shows that the studied cycloaddition reaction leading to the 1,4-triazole proceeds by a two-stage one-step mechanism. It also revealed that both zwitterions are created by the donation of the nitrogen atom's nonbonding electron densities to carbon atoms of ethyl propiolate.

The molecular mechanism of the reaction between 2-methoxyfuran and ethyl 13 (Z)-3-phenyl-2-nitroprop-2-enoate was investigated on the basis of wb97xd/6-311+G(d,p)(PCM) 14 quantum chemical calculations. It was found that the most probable reaction mechanism is fun- 15 damentally different from what was previously postulated. In particular, six possible zwitterionic 16 intermediates were detected on the reaction pathway. Their formation is determined by the nature 17 of local nucleophile/electrophile interactions. Additionally, the channel leading via the formation of 18 the exo-nitro Diels-Alder cycloadduct was completely ruled out. Finally, the electronic nature of the 19 five- and six-membered nitronates as potential TACs was evaluated.