Compounds for the treatment of SARS

A compound of the formula (I):

• or a pharmaceutically acceptable salt thereof, wherein:
• R1 —O-1 to 4 carbon alkylene-6 to 10 membered aryl 6 to 10-membered heterocyclyl with one heteroatom or —O-1 to 4 carbon alkylene-6 to 10-membered heterocyclyl with one heteroatom, wherein the 6 to 10-membered heterocyclyl with one heteroatom is connected via a carbon atom on the 6 to 10-membered heterocyclyl group;
• R2 is alkyl, aryl, substituted aryl;
• R3 is 5-or 6-membered heterocyclyl with one to four heteroatoms, wherein the 5- or 6-membered heterocyclyl with one to four heteroatoms is connected to the —CH2- group via a carbon atom on the 5- or 6-membered heterocyclyl group with one to four heteroatoms;
• R4 is H, alkyl or alkenyl;
• and
• Ra is H or alkyl,
• or a pharmaceutically acceptable salt thereof.