Compounds of general formula (I) wherein R.sup.1 is selected from any one of phenyl, pyridinyl, thienyl, furanyl, imidazolyl, pyrrolyl and triazolyl; where each R.sup.1 phenyl ring and R.sup.1 heteroaromatic ring may optionally and independently be further substituted by 1, 2 or 3 substituents selected from straight and branched C.sub.1 C.sub.6 alkyl, NO.sub.2, CF.sub.3, C.sub.1 C.sub.6 alkoxy, chloro, fluoro, bromo, and iodo. The substitutions on thephenyl ring and on the heteroaromatic ring may take place in any position on said ring systems; are disclosed and claimed in the present application, as well as their pharmaceutically acceptable salts and pharmaceutical compositions comprising the novel compounds and their use in therapy, inparticular in the management of pain.