Abstract: Lactam or lactone derivatives of formula Ia: or a pharmaceutically acceptable salt or solvate thereof, wherein X1 is NH or O; X2 is —OH, —O-alkyl, —NH2, an amino acid group or a peptide group, wherein the amino acid group or peptide group is linked to the carbonyl group by its N-terminal residue; Z1 is S, SO, SO2, O, CR3 or NR4; and R1, R2, R3 and R4 each independently are H, alkyl or aryl. Also, the use of lactam or lactone derivatives of formula Ia in treating a disease caused by a viral infection, especially a coronavirus infection and/or a viral respiratory infection, such as COVID-19 caused by SARS-CoV-2.