Compound 2, a derivative that contains an indole capping group at P2, displayed modest levels of irreversible inhibition (kobs/I = 586 ± 11 M–1 s–1) that enabled cocrystallization in complex with SARS CoV-1 3CLpro (PDB code 6XHO).29 Comparison of this structure with the complex of 1 in HRV 3Cpro helped to rationalize the poor performance of the warhead, which had been successful in HRV.