Heterocyclic derivatives

A compound, or a solvate or salt thereof, of formula I: in which: R.CO- is an acyl group containing a substituted or unsubstituted carbocyclic aromatic or heterocyclic aromatic group; R1 and R2 are independently hydrogen, C1-6 alkyl, C2-6 alkenyl, C3-6 cycloalkyl or C4-12 cycloalkylalkyl groups or together form a C2-8 branched or linear polymethylene or C1-6 alkenylene group, optionally substituted with a hetero-atom; R3 is hydrogen, C1-6 alkyl, or phenyl or R3 together with R1 form a -(CH2)3- or -(CH2)4- group; R4 is C1-6 alkyl, or phenyl; R5 is hydrogen or together with R4 forms a -(CH2)n- group in which n = 1, 2 or 3; and 'Het' is an optionally substituted single or fused ring heterocyclic group, containing from 5 to 12 ring atoms and comprising up to four hetero-atoms in the or each ring selected from oxygen, nitrogen and sulphur, with the proviso that when, simultaneously, R1 and R2 together are linear butylene, R3 is hydrogen, R4 is methyl, R5 is hydrogen and R is 3,4-dichlorophenyl, 'Het' is not pyridyl, is useful for the treatment of pain.