Compounds of general formula I: wherein: R.sub.1 and R.sub.2 are, independently of each other, selected from hydrogen, optionally substituted C.sub.1-10alkyl, optionally substituted --CO--(C.sub.1-10alkyl), optionally substituted C.sub.3-10cycloalkyl, optionally substituted --CO--(C.sub.3-10cycloalkyl), optionally substituted C.sub.2-10alkenyl, optionally substituted --CO--(C.sub.2-10alkenyl), optionally substituted aryl, and optionally substituted --CO-aryl, or R.sub.1 and R.sub.2 together represent an optionally substituted saturated or unsaturated C.sub.1-10alkylidene group, or an optionally substituted saturated or unsaturated C.sub.3-10cycloalkylidene group, or R.sub.1 and R.sub.2 together with the carbon atom to which they are attached represent an optionally substituted saturated or unsaturated organic ring containing 3, 4, 5, 6, 7 or 8 ring carbon atoms and optionally 1, 2 or 3 ring heteroatoms selected from O, N and S; R.sub.3, which may be the same as, or different from, either of R.sub.1 and R.sub.2, is selected from optionally substituted C.sub.1-10alkyl, optionally substituted C.sub.3-10cycloalkyl, optionally substituted C.sub.2-10alkenyl, and optionally substituted aryl; R.sub.4, R.sub.5 and R.sub.6 are, independently of each other, selected from hydrogen, optionally substituted C.sub.1-10alkyl, OH, optionally substituted C.sub.1-10 alkoxy, halo, optionally substituted aryloxy, optionally substituted (C.sub.1-10alkyl)-S(O).sub.n-- where n=0, 1 or 2, optionally substituted aryl-S(O).sub.n-- where n=0, 1 or 2, or R.sub.4 is hydrogen and R.sub.5 and R.sub.6 together represent an optionally substituted saturated or unsaturated organic chain containing 1, 2, 3, 4, 5, 6 or 7 chain carbon atoms and optionally 1, 2 or 3 chain heteroatoms selected from O, N and S, provided that the chain is at least 3 atoms long; with the proviso that, when R.sub.1.dbd.R.sub.2=hydrogen, then any optionally substituted C.sub.1-10alkyl or optionally substituted C.sub.2-10alkenyl for R.sub.3 must have a branch point at one or more of the .alpha. and .beta. positions counted from the carbonyl group (or tautomeric form thereof) to which R.sub.3 is attached; or a physiologically acceptable salt, complex or prodrug thereof; are disclosed per se and for use in the treatment or prophylaxis of hypersensitivity, smooth muscle disorders, spasmodic conditions, allergic conditions, inflammatory conditions and/or pain.