Compounds of Formulae (IA), (IB) or pharmaceutically acceptable salts thereof; wherein Het, X<SUP>1</SUP>, R<SUP>1</SUP>, R<SUP>2</SUP>, R<SUP>3</SUP>, R<SUP>3</SUP><SUB>a </SUB>and R<SUP>4 </SUP>are as defined in the specification as well as salts and pharmaceutical compositions including the compounds are prepared. They are useful in therapy, in particular in the management of pain.