Compounds of formula (I), wherein A is a hydrogen atom or a hydroxyl group; B is a hydrogen atom or a C.sub.1-8 alkyl, C.sub.1-8 alkenyl, C.sub.1-8 fluoroalkyl or C.sub.1-8 perfluoroalkyl group; each of R.sub.1, R.sub.2 and R.sub.5, which are the same or different, is a hydrogen atom, a halogen such as chlorine, bromine or fluorine, a C.sub.1-8 alkyl, C.sub.1-8 alkenyl, C.sub.3-8 cycloalkyl, C.sub.3-8 cycloalkenyl, C.sub.6, C.sub.10 or C.sub.14 aryl, C.sub.1-6 alkoxycarbonyl, C.sub.1-6 hydroxyalkyl, C.sub.1-6 alkoxyalkyl, C.sub.1-6 alkoxy, C.sub.1-8 fluoroalkyl or C.sub.1-8 perfluoroalkyl group, or R.sub.1 and R.sub.2 taken together form a C.sub.3-8 cycloalkyl, C.sub.3-8 cycloalkenyl or C.sub.6, C.sub.10 or C.sub.14 aryl ring, except for compounds in which R.sub.1 and R.sub.2 are both hydrogen; each of R.sub.3 and R.sub.4, which are the same or different, is a hydrogen atom or a C.sub.1-8 alkyl, C.sub.1-8 alkenyl, C.sub.3-8 cycloalkyl or C.sub.3-8 cycloalkenyl group, or R.sub.3 and R.sub.4 taken together form a C.sub.2-6 cycloalkyl or C.sub.3-6 cycloalkenyl ring, e.g., piperidyl, azetidinyl or pyrrolidyl; in the form of enantiomers, diastereoisomers or mixtures thereof, including racemic mixtures;, as well as pharmaceutically acceptable acid addition salts thereof. The compounds may be used in therapy.