Diastereoselective Syntheses of Spiro[indoline-3,4′-pyridin]-2-yl Carbamates via AgOTf/Ph3P‑Catalyzed Tandem Cyclizations of Tryptamine-Ynesulfonamides

Spiro­[indoline-3,4′-piperidine] is a significant structural scaffold in numerous polycyclic indole alkaloids with a variety of bioactivities. In this study, a synthetic strategy was developed to access spiro­[indoline-3,4′-pyridin]-2-yl carbamate via an AgOTf/PPh3-catalyzed tandem cyclization of tryptamine-ynesulfonamides. The unique feature of this strategy is the efficient intermolecular capturing of the in situ generated spiroindoleninium intermediates with carbamates, leading to the diastereoselective syntheses of spiro­[indoline-3,4′-pyridin]-2-yl carbamate derivatives. A broad scope of this cyclization was demonstrated by a variety of tryptamine-ynesulfonamide substrates and several carbamates. A plausible mechanism of this reaction was proposed.