Compounds of formula (I) in which R1 is 1-6C-alkoxy, 3-7C-cycloalkoxy, 3-7C-cycloalkylmethoxy, benzyloxy or 1-4C-alkoxy which is completely or predominantly substituted by fluorine, R2 is 1-4C-alkyl and R3 is hydrogen or 1-4C-alkyl, or in which R2 and R3, together and including the two carbon atoms to which they are bonded, are a spiro-linked 5-, 6- or 7-membered hydrocarbon ring, if desired interrupted by an oxygen atom or an imino (-NH-) group, R4 is hydrogen, hydroxyl, nitro, cyano, carboxyl, 1-4C-alkoxy or 1-4C-alkoxycarbonyl, A is B, -CH(R5)- or > C = N-R6, where B is oxygen (-O-), imino (-NH-), sulfinyl (-S(O)-), sulfonyl, (-S(O)2-) or carbonylimino (-C(O)NH-) and R5 is carboxyl, 1-4C-alkoxycarbonyl, aminocarbonyl, mono- or di-1-4C-alkylaminocarbonyl, hydroxylaminocarbonyl (-C(O)NHOH), 1-4C-alkoxyaminocarbonyl or a radical of formula (a) where R51 is hydrogen, 1-4C-alkyl or aryl and R52 is hydrogen, 1-4C-alkyl or aryl, where aryl is a phenyl, pyridyl, benzyl or phenethyl radical which can be unsubstituted or mono- or disubstituted by hydroxyl, 1-4C-alkoxy, halogen, amino, cyano or nitro, R6 is hydroxy or 1-4C-alkylcarbonyloxy, and the salts of these compounds are efficacious PDE inhibitors.