Diastereomers of Cyclopropa[a]pyrrolizine Spiro-fused with a Benzo[4,5]imidazo[1,2-a]indole Fragment: Structure Determinations Using NMR Methods

NMR spectroscopy methods have been used to prove structures of two diastereomers of cyclopropa[a]pyrrolizine spiro-fused with a benzo[4,5]imidazo[1,2-a]indole fragment which were obtained through a three-component 1,3-dipolar cycloaddition reaction and isolated in a ratio of 6:1. Complete signal assignment in H-1 and C-13 spectra of each compound was made by using some homonuclear (COSY, NOESY) and heteronuclear (HMQC, HMBC) NMR experiments. The spatial structure of the studied diastereomers was proved on the basis of data on interproton through-space interactions obtained from the NOESY spectra at a mixing time of 0.5 s. The comparison of the experimental and calculated values of the vicinal constants and interproton distances indicates that each of the studied diastereomers in solution is characterized by a fast (in the NMR time scale) conformational equilibrium due to the inversion of the nitrogen atom in the cyclopropa[a]pyrrolizine fragment.