Contrasting Carboannulation Involving δ-Acetoxy Allenoate as a Four-Carbon Synthon Using DABCO and DMAP: Access to Spiro-carbocyclic and -Teraryl Scaffolds.

Spiro-annulation involving δ-acetoxy allenoate and alkyl benzoisothiazole dioxide (-sulfonyl ketimine) triggered by DABCO/MeCOH combination leads to an via chemo- and regiospecific [4 + 2]-carboannulation and a new group is introduced. In contrast, DMAP-catalyzed benzannulation using the same reactants affords unsymmetrical -teraryls via Mannich coupling, sequential proton transfers, and C-N bond cleavage. Here, δ-acetoxy allenoate serves as a 4C-synthon and the carboannulation is completely base dependent and mutually exclusive.