Metal-Free Sulfonylative Spirocyclization of Indolyl-ynones via Insertion of Sulfur Dioxide: Access to Sulfonated Spiro[cyclopentenone-1,3′-indoles]

A three-component sulfonylative spirocyclization of indolyl ynones with aryldiazonium salts and a sulfur dioxide surrogate of DABCO·(SO2)2 has been developed, providing a range of sulfonated spiro­[cyclopentenone-1,3′-indoles] in moderate to good yields. This transformation was initiated by an in situ generated arylsulfonyl radical and proceeded efficiently under metal-free conditions, involving a radical-induced dearomative cascade cyclization accompanied by the insertion of sulfur dioxide. This protocol provides an efficient and convenient method to access sulfonated spiroindolenines, and tolerant various functional groups.