Base‐Catalyzed Sequential 5‐exo‐dig/7‐endo‐dig Cyclization of (2‐Alkynylphenyl) Benzyl Ethers: Scope, Mechanism Studies, and Photophysical Properties

Heterocycles fused to seven‐membered rings are synthesized in a highly selective fashion from (2‐alkynylphenyl) benzyl ethers by a consecutive potassium tert‐butoxide‐catalyzed one‐pot cyclization. Systematic study to determine the ideal conditions revealed that a catalytic amount of potassium tert‐butoxide, dimethyl sulfoxide, and the tert‐BuOK/tert‐BuOH equilibrium can play a crucial role in the formation of products. The optimized reaction conditions were applied to 2‐alkynylphenyl benzyl ethers containing substituted aromatic systems directly bonded to the triple bond; however, they were ineffective for alkynes having alkyl groups. The protocol afforded the heterocycles fused to seven‐membered rings in good yields by an exclusive 5‐exo‐dig/7‐endo‐dig one‐pot sequence. The photophysical properties of compounds were investigated using absorption and emission fluorescence analyses. The emission analysis showed a blue to cyan emission. Large Stokes shift values were found for all compounds, which were attributed to the ICT state and the electronic properties of the substituents. Heterocycles fused to seven‐membered rings are synthesized in a highly selective fashion from (2‐alkynylphenyl) benzyl ethers by a consecutive potassium tert‐butoxide‐catalyzed one‐pot cyclization.