R2 is selected from: H; unsubstituted or substituted C1-4 alkyl, unsubstituted or substituted C3-6 cycloalkyl or unsubstituted or substituted aryl, wherein the substituent is selected from: ##STR6## R3, R4 and R5 are independently selected from: H, C1-4 alkyl, and halogen; provided that R2 is H when R3 is other than H;