Compounds of general formula I: wherein: R1 and R2 are, independently of each other, selected from hydrogen, optionally substituted C1-10alkyl, optionally substituted -CO-(C1-10alkyl), optionally substituted C3-10cycloalkyl, optionally substituted -CO-(C3-10cycloalkyl), optionally substituted C2-10alkenyl, optionally substituted -CO-(C2-10alkenyl), optionally substituted aryl, and optionally substituted -CO-aryl, or R1 and R2 together represent an optionally substituted saturated or unsaturated C1-10alkylidene group, or an optionally substituted saturated or unsaturated C3-10cycloalkylidene group, or R1 and R2 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； R3, which may be the same as, or different from, either of R1 and R2, is selected from optionally substituted C1-10alkyl, optionally substituted C3-10cycloalkyl, optionally substituted C2-10alkenyl, and optionally substituted aryl； R4, R5 and R6 are, independently of each other, selected from hydrogen, optionally substituted C1-10alkyl, OH, optionally substituted C1-10 alkoxy, halo, optionally substituted aryloxy, optionally substituted (C1-10alkyl)-S(O)n- where n=0, 1 or 2, optionally substituted aryl-S(O)n- where n=0, 1 or 2, or R4 is hydrogen and R5 and R6 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 R1-R2=hydrogen, then any optionally substituted C1-10alkyl or optionally substituted C2-10alkenyl for R3 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 R3 is attached； or a physiologically ac
