Peptidyl Acyloxymethyl Ketones as Activity‐Based Probes for the Main Protease of SARS‐CoV‐2 **

The global pandemic caused by SARS‐CoV‐2 calls for the fast development of antiviral drugs against this particular coronavirus. Chemical tools to facilitate inhibitor discovery as well as detection of target engagement by hit or lead compounds from high‐throughput screens are therefore in urgent need. We here report novel, selective activity‐based probes that enable detection of the SARS‐CoV‐2 main protease. The probes are based on acyloxymethyl ketone reactive electrophiles combined with a peptide sequence including unnatural amino acids that targets the nonprimed site of the main protease substrate binding cleft. They are the first activity‐based probes for the main protease of coronaviruses and display target labeling within a human proteome without background. We expect that these reagents will be useful in the drug‐development pipeline, not only for the current SARS‐CoV‐2, but also for other coronaviruses.