Synthesis of 2H-Indazoles via the Cadogan Reaction in Batch and Flow Mode

    • Matilda Duffy
    • Mara Di Filippo
    • Marcus Baumann
    • University College Dublin, School of Chemistry, Science Centre South, Dublin D04 N2E5, Ireland

    A continuous flow protocol is reported for the synthesis of 2H-indazoles via the thermal Cadogan reaction. Triethyl phosphite serves as a readily available deoxygenation reagent providing a facile entry to a series of drug-like 2H-indazoles in high chemical yields. Compared to an analogous batch approach, the flow process was advantageous as it provides for a contained environment where boiling of the reagent is suppressed by using a back-pressure regulator (BPR) which in turn leads to faster kinetics. Straightforward scale-up provided multi-gram quantities of 2H-indazoles whose further functionalisation via light-driven and regiospecific acetylation was demonstrated in flow mode.

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