New [4 + 2] Cycloadditions of in situ Generated Indolyl Enol Ethers and Their Anions with Dimethyl Acetylenedicarboxylate: A One-Pot Access to 4-Alkoxy-Substituted Carbazoles

Abstract

The 3-indolyl(methyl)methoxycarbenium tetrafluoroborates 1a-d were deprotonated with NaH to furnish in situ the highly reactive 3-indolyl enol ethers 2a or their corresponding 3-indolyl enol ether anions 2b. Subsequent trapping of the enol ethers 2 with dimethyl acetylenedicarboxylate gave rise to 4-methoxy-functionalised carbazole derivatives in a HOMO(diene)-LUMO(dienophile)-controlled [4 + 2] cycloaddition. After variation of the 3-indolyl(alkoxy)carbenium tetrafluoroborates 1, Michael-type adducts and a ring-opened, biaryl product 6 were formed additionally.