Synthesis of complex dispirocyclopentanebisoxindoles via cycloaddition reactions of 4-dimethylamino-1-alkoxycarbonylmethylpyridinium bromides with 2-oxoindolin-3-ylidene derivatives

The cycloaddition reactions of 4-dimethylamino-1-alkoxycarbonylmethylpyridinium bromides with various 2-oxoindolin-3-ylidene derivatives in ethanol in the presence of triethylamine afforded the unprecedented dispirocyclopentanebisoxindole derivatives in good yields and in high diastereoselectivity. Under similar reaction conditions, the reactions of 1-alkoxycarbonylmethylpyridinium bromides with 3-phenacylideneoxindoles only resulted in 4-oxo-3-(2-oxoindolin-3-ylidene)-4-arylbutanoates. The stereochemistry of the complex spirooxindoles was established by 1H NMR data and single crystal structures. [Display omitted]