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The [3 + 2] cycloaddition (32CA) reactions of 1-pyrroline-1-oxide with N -vinyl nucleobases leading to bicyclic N,O nucleoside analogues have been studied within the molecular electron density theory (MEDT) at the MPWB1K/6-311G(d,p) computational level. These non-polar zwitterionic type 32CA reactions take place through a one-step mechanism with minimal global electron density transfer (GEDT) at the TSs and the exo / ortho approach mode as the energetically favoured reaction path. The 32CA reactions of N- vinyl nucleobases with thymine and cytosine substituents respectively show the activation enthalpies of 15.2 and 12.5 kcal mol -1 in toluene. The reactions are irreversible due to strong exothermic character of - 35.4-- 26.4 kcal mol -1 in toluene. The bonding evolution theory (BET) study suggests that these 32CA reactions take place through the coupling of pseudoradical centres with earlier C-C bond formation and the formation of new C-C and C-O covalent bonds has not been started in the TSs. Non-covalent interactions (NCI) are predicted at the TSs from the visualization of NCI gradient isosurfaces.

Unravelling the regio- and stereoselective synthesis of bicyclic N,O-nucleoside analogues within the molecular electron density theory perspective