Synthesis of 3-Acyloxyindolenines by TiCl-Mediated Reductive Cyclization of 2-(-Nitroaryl)-Substituted Enol Esters.

In the presence of TiCl, the reductive cyclization of tetrasubstituted enol esters bearing a 2-(-nitroaryl) substituent affords 3-acyloxy-2,3-disubstituted indolenines in good yields. A domino process involving the partial reduction of nitro to a nitroso group followed by 5-center-6π-electrocyclization, 1,2-acyloxy migration, and the further reduction of the resulting nitrone intermediate accounts for the reaction outcome. The so-obtained indolenines are converted smoothly to 2,2-disubstituted oxindoles via a sequence of saponification and semipinacol rearrangement.