The invention provides a compound of formula (I) or a salt thereof: wherein Ar has the sub-formula (x) or (z): and wherein R3 is optionally substituted C3-8cycloalkyl, optionally substituted C5-7cycloalkenyl, an optionally substituted heterocyclic group (aa), (bb) or (cc), or a bicyclic group (ee); and wherein R4 is H, C1-3alkyl, C1-2fluoroalkyl, cyclopropyl, —CH2OR4a, —CH(Me)OR4a, or —CH2CH2OR4a; and R5 is inter alia H, C1-8alkyl, C1-3fluoroalkyl, C3-8cycloalkyl, certain substituted alkyl groups, —(CH2)n13-Het, or optionally substituted phenyl or —CH2-Ph; or R4 and R5 taken together are —(CH2)p1— or —(CH2)p3—X5—(CH2)p4—; provided that at least one of R4 and R5 is not a hydrogen atom (H). The invention also provides the use of the compounds as inhibitors of phosphodiesterase type IV (PDE4) and/or for the treatment and/or prophylaxis of inflammatory and/or allergic diseases such as chronic obstructive pulmonary disease (COPD), asthma, rheumatoid arthritis, allergic rhinitis or atopic dermatitis.