Asymmetric Synthesis of Spirooxindoles via Nucleophilic Epoxidation Promoted by Bifunctional Organocatalysts

摘要

Taking into account the postulated reaction mechanism for the organocatalytic epoxidation of electron-poor olefins developed by our laboratory, we have investigated the key factors able to positively influence the H-bond network installed inside the substrate/catalyst/oxidizing agent. With this aim, we have: (i) tested a few catalysts displaying various effects that noticeably differ in terms of steric hindrance and electron demand; (ii) employed α-alkylidene oxindoles decorated with different substituents on the aromatic ring (>11a–>g), the exocylic double bond (>11h–>l), and the amide moiety (>11m–>v). The observed results suggest that the modification of the electron-withdrawing group (EWG) weakly conditions the overall outcomes, and conversely a strong influence is unambiguously ascribable to either the N-protected or N-unprotected lactam framework. Specifically, when the NH free substrates (>11m–>u) are employed, an inversion of the stereochemical control is observed, while the introduction of a Boc protecting group affords the desired product >12v in excellent enantioselectivity (97:3 er).