The present invention provides a compound of the general formula (I) or a pharmaceutical salt thereof, wherein, when R1 and R2 are the same they are represented by =O, =N, -OR9 (where R9 is hydrogen, lower alkyl or benzyl), when R1 and R2 are different and one of R1 and R2 is represented by hydrogen, the other is represented by -R5-R6-, (where R5 is -O-, -NH-, -NHCO-, or -NHSO2-, and where R6 is hydrogen, lower alkyl, -Ph-R7, -(CH2)n-O-Ph-R7, (where n=1-6, R7 is hydrogen, hydroxy, lower alkyl, lower alkoxy, halogen, nitro, or pyridyloxy), indolyl, N-oxidepyridyl, phthalimide, thienyl, or pyridyl); R3 represents -COOH, -COOEt, -COOMe, -CH2N(OH)CHO, or -CONHOH; R4 represents lower alkyl, thienyl, -Ph-R8 (where R9 represents hydroxy, lower alkyl, lower alkoxy, nitro, halogen, pyridyloxy, or phenyl group substituted hydrogen, lower alkyl, lower alkoxy, hydroxy, and halogen), and methods of making compounds within the class of the general formula (I).