Base-catalyzed synthesis of substituted indazoles under mild, transition-metal-free conditions

Back to basics: A transition-metal-free method developed for the synthesis of indazoles involves an inexpensive catalytic system composed of a diamine and K₂CO₃. Various (Z)-2-bromoacetophenone tosylhydrazones were converted into indazoles at room temperature in excellent yields (see example; Ts=p-toluenesulfonyl). The yield was improved by photoisomerization with UV light when E/Z isomeric mixtures of the starting material were used.