Contrasting Carboannulation Involving δ-Acetoxy Allenoate as a Four-Carbon Synthon Using DABCO and DMAP: Access to Spiro-carbocyclic and <i>m</i>-Teraryl Scaffolds

Abstract

Spiro-annulation involving δ-acetoxy allenoate and alkyl benzoisothiazole dioxide (<i>N</i>-sulfonyl ketimine) triggered by DABCO/MeCO₂H combination leads to an <i>essentially single diastereomer</i> via chemo- and regiospecific [4 + 2]-carboannulation and a new <i>hydroxyl</i> group is introduced. In contrast, DMAP-catalyzed benzannulation using the same reactants affords unsymmetrical <i>m</i>-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.