Direct Vinylogous Michael Addition of Prochiral 3-Alkylideneoxindoles to Nitroolefins

3-Alkylidene-2-oxindoles represent a simple, yet enabling subfamily of indole alkaloids, and their ability to react as electron-poor acceptors has largely been investigated. In contrast, their utility as pronucleophilic synthons remains elusive. In this context, the present describes the successful execution of the direct, organocatalytic asymmetric Michael addition of prochiral 3-alkylideneoxindoles to nitroolefins. A variety of ?-substituted alkylideneoxindoles carrying two stereocenters at both the ?- and ?-carbon sites was assembled with excellent stereoselectivity and without olefin isomerization or stereochemical ablation. In summary, to complement our previous results with methyl-substituted alkylideneoxindoles (1), we have herein provided a limpid testimony to the unique capability of enolizable 3-alkylideneoxindoles to serve as pronucleophiles for the direct, asymmetric vinylogous Michael addition to nitroolefins (2). (1) Curti, C.; Rassu, G.; Zambrano, V.; Pinna, L.; Pelosi, G.; Sartori, A.; Battistini, L.; Zanardi, F.; Casiraghi, G. Angew. Chem. 2012, 124, 6304-6308; Angew. Chem. Int. Ed. 2012, 51, 6200-6204. (2) Rassu, G.; Zambrano, V.; Pinna, L.; Curti, C.; Battistini, L.; Sartori, A.; Pelosi, G.; Zanardi, F.; Casiraghi, G. Adv. Synth. Catal. 2013, 355, 1881-1886.