A compound having the structure of Formula (III) or Formula (IV):or a pharmaceutically acceptable salt thereof, wherein R<;sup>;1<;/sup>;is -COOR<;sup>;5<;/sup>;, R<;sup>;2<;/sup>;is H, R<;sup>;3<;/sup>;and R<;sup>;4<;/sup>;are independently H or substituted or unsubstituted aryl or heteroaryl, R<;sup>;5<;/sup>;is an alkyl group having 1-6 carbon atoms, and B is O or S.