Catalytic Asymmetric [3+2] Annulation of Allylsilanes with I satins: Synthesis of Spirooxindoles

摘要

Allylsilanes are readily available, nontoxic, and versatile reagents for organic synthesis. Although fairly weak nucleo-philes, allylsilanes exhibit a dynamic reactivity pattern that is dependent upon the electronic and steric properties of the silyl group, the electrophilic partner, and the reaction conditions. Two primary pathways are known for the addition of an allylsilane to a G=X n electrophile in the presence of a Lewis acid: 1) an elimination pathway to afford allylation products (Hosomi-Sakurai reaction), and 2) a pathway wherein the allylsilane acts as a three-carbon unit in a [3+2] annulation reaction to afford cyclized products (Scheme 1). The silyl group enhances the nucleophilicity of the alkene and stabilizes the β-carbocation, which is formed upon initial attack, through σ-p hyperconjugation (referred to as the β-silyl effect). The [3+2] allylsilane annulation reaction represents a powerful method for efficient stereoselective synthesis of complex heterocycles and carbocycles; however, catalytic asymmetric variants have remained elusive. In contrast, numerous methods have been reported that describe enantioselective allylation reactions with allyl- silanes. This enantioselective annulation represents a particular challenge because Lewis acid catalysts often favor the competing allylation pathway, and a balance of reactivity and product selectivity is required. Herein we report the first example of a catalytic asymmetric [3+2] annulation of allylsilanes and develop a method to access silyl- and hydroxy-substituted spirooxindoles with superb enantioselec-tivity.