Stereoselective Synthesis of a Novel Series of Dispiro-oxindolopyrrolizidines Embodying Thiazolo[3,2-a]benzimidazole Motif: A Molecular Electron Density Theory Study of the Mechanism of the [3 + 2] Cycloaddition Reaction

A one-pot multi-component reaction was employed for the stereoselective synthesis of a novel set of dispiro-oxindolopyrrolizidines analogs incorporating a thiazolo[3,2-a]benzimidazole scaffold based on the [3 + 2] cycloaddition (32CA) reaction approach. The desired novel dispiro-oxindolopyrrolizidines 9a–d were achieved using the 32CA reaction of new ethylene derivatives based on thiazolo[3,2-a]benzimidazole moiety seven with thiazolidine derivatives eight and different substituted isatin compounds 5a–d (R = H, Cl, NO2, and Br). The final dispiro-oxindolopyrrolizidines cycloadducts were separated, purified, and fully characterized by means of a set of spectroscopic tools including IR, HNMR, CNMR, and MS. The Molecular Electron Density Theory (MEDT) was applied to explain the mechanism and stereoselectivity in the of the key 32CA reaction step. The reactive pseudo(mono)radical electronic structure of the in situ generated azomethine ylides and the high polar character of the corresponding 32CA reactions account for the low computed activation Gibbs free energies and total endo stereoselectivity of this kinetically controlled exergonic reaction. The computed relative Gibbs free activation energies of competitive reaction paths and regioisomers ratio distribution of 80:20 justify the major formation of 9a via the most favorable ortho/endo reaction path.