Diastereoselective Synthesis of Dispiro[Imidazothiazolotriazine-Pyrrolidin-Oxindoles] and Their Isomerization Pathways in Basic Medium

Highly diastereoselective methods for the synthesis of two series of regioisomeric polynuclear dispyroheterocyclic compounds with five or six chiral centers, comprising moieties of pyrrolidinyloxindole and imidazo[4,5-e]thiazolo[3,2-b]-1,2,4-triazine of linear structure or imidazo[4,5-e]thiazolo[2,3-c]-1,2,4-triazine of angular structure, have been developed on the basis of a [3+2] cycloaddition of azomethine ylides to functionalized imidazothiazolotriazines. Depending on the structure of the ethylenic component, cycloaddition proceeds as an anti-exo process for linear derivatives, while cycloaddition to angular ones resulted in a syn-endo diastereomer. Novel pathways of isomerization for the synthesized anti-exo products upon treatment with sodium alkoxides have been found, which resulted in two more series of diastereomeric dispiro[imidazothiazolotriazine-pyrrolidin-oxindoles] inaccessible with the direct cycloaddition reaction. For the first series, the inversion of the configuration of one stereocenter, i.e., C-4′ atom of the pyrrolidine cycle, (epimerization) was established. For the second one, configuration of the obtained diastereomer formally corresponded to the syn-endo approach of the azomethine ylide in the case of cycloaddition to the ethylenic component.