The relative configuration of spirooxindole-fused cyclopentanes 3 was adopted on the basis of characteristic chemical shifts and J values of the cyclopentane ring (for details, please see the SI). In addition, the absolute configuration of 3a was ascertained using X-ray diffraction analysis, and the configuration of 3a was assigned as 2R, 9R, 10R (Figure , for details, see the SI). On the basis of the absolute configuration of products and the previous report, 16 the transition state was proposed to rationalize the stereochemical outcome of the cascade process. The tertiary amine moiety of catalyst C1 deprotonates an acidic proton of nitroalkane 2a, generating the complex of nitronate non-covalently bounded to the tertiary amine. Simultaneously, the thiourea part of the catalyst activates methyleneindolinone 1a prompting Si-face addition of nitronate to the electron-deficient alkene. Additionally, the sterical hindrance of tert-butyl moiety of the N-Boc group may increase the rigidity of the ternary complex, which seems crucial for high stereocontrol. That hypothesis is supported by lowered diasterocontrol, when methyleneindolinone 3b with more planar N-CBz protecting group is used.