Synthesis of sulfonyl‐containing spiro[5,5]trienones from sodium metabisulfite

An efficient three‐component reaction of biaryl ynones, sodium metabisulfite and aryldiazonium tetrafluoroborates is developed. The reaction occurs at room temperature without any additives, thus offering a sustainable approach to access sulfonyl‐containing spiro[5,5]trienones. This protocol features mild conditions, broad substrate scope and high functional group compatibility, furnishing the target products in moderate to good yields. Mechanistic studies indicate that the reaction undergoes a 6‐ exo ‐ trig dearomative spirocyclization process which is initated by an aryl radical with the insertion of sulfur dioxide.