The invention relates to a compound of formula (I) or a salt thereof: wherein: R1 is C1-4alkyl, C1-3fluoroalkyl or —(CH2)2OH; R2 is a hydrogen atom (H), methyl or C1fluoroalkyl; R3a is a hydrogen atom (H) or C1-3alkyl; R3 is optionally substituted branched C3-6alkyl, optionally substituted C3-8cycloalkyl, optionally substituted mono-unsaturated-C5-7cycloalkenyl, optionally substituted phenyl, or an optionally substituted heterocyclic group of sub-formula (aa), (bb) or (cc): in which n1 and n2 independently are 1 or 2; and Y is O, S, SO2, or NR4; and wherein Het is of sub-formula (i), (ii), (iii), (iv) or (v): The compounds are phosphodiesterase (PDE) inhibitors, in particular PDE4 inhibitors. Also provided is the use of a compound of formula (I), or a pharmaceutically acceptable salt thereof, in the manufacture of a medicament for the treatment and/or prophylaxis of an inflammatory and/or allergic disease in a mammal such as a human, for example chronic obstructive pulmonary disease (COPD), asthma, or allergic rhinitis.