Controllable synthesis of N–H or N-Me C7-substituted indazoles via a cascade reaction of α-diazo esters with aryne precursors

Abstract

A metal-free, highly regioselective [3 + 2] cycloaddition of cyclic diaryl λ³-bromanes and α-diazo esters has been developed for the controllable synthesis of valuable C7-aryl 3-substituted 1H-indazoles. This protocol involves cascade cycloaddition/decarboxylation to afford N–H indazoles or cycloaddition/decarboxylation/methylation to deliver N-Me indazoles. Mechanistic studies revealed that the [3 + 2] cycloaddition proceeded in a highly regioselective manner, followed by decarboxylation instead of acyl group migration, likely due to steric hindrance. Both α-diazo esters and the [3 + 2] adduct can serve as methylation reagents.