Diaryl ether hetrocycles

A diaryl ether heterocycle of the formula I

• wherein Ar.sup.1 is phenyl or naphthyl which may optionally bear one or more substituents selected from amino, halogeno, hydroxy, cyano, (1-6C)alkyl, (2-6C)alkenyl, (2-6C)alkynyl, (1-4C)alkoxy, (1-4C)alkylthio, (1-4C)alkysulphinyl, (1-4C)alkylsulphonyl, (1-4C)alkylamino, di-[(1-4C)alkyl]amino, (1-4C)alkoxycarbonyl, (2-4C)alkanoyl, hydroxy-(1-6C)alkyl, fluoro-(1-4C)alkyl, cyano-(1-6C)alkyl, fluoro-(1-4C)alkoxy, cyano-(1-4C)alkoxy, (1-4C)alkoxy-(1-4C)alkyl, (1-4C)alkylthio-(1-4C)alkyl, (1-4C)alkylsulphinyl-(1-4C)alkyl, (1-4C)alkylsulphonyl-(1-4C)alkyl, cyano-(4-6C)cycloalkyl, (2-4C)alkanoylamino, N-[(1-4C)alkyl]-(2-4C)alkanoylamino, N-(2,2,2-trifluoroethyl)-(2-4C)alkanoylamino, N-[(1-4C)alkoxycarbonyl-(1-2C)alkyl]-(2-4C)alkanoylamino, trifluoroacetyl, trifluoroacetamido, N-[(1-4C)alkyl]trifluoroacetamido, 2-oxopyrrolidinyl, 1,1,3,3-tetramethyl-2,2-dioxo-2-thiatrimethylene, tri-(1-4C)alkylsilyl, phenyl, benzoyl, benzamido and N-[(1-4C)alkyl]benzamido, and wherein said phenyl, benzoyl, benzamido or N-[(1-4C)alkyl]benzamido substituent may optionally bear a substituent selected from halogeno, (1-4C)alkyl and (1-4C)alkoxy;
• wherein X.sup.1 is oxy, thio, sulphinyl or sulphonyl;
• wherein Ar.sup.2 is phenylene which may optionally bear one or two substituents selected from halogeno, hydroxy, amino, nitro, cyano, carbamoyl, (1-4C)alkyl, (3-4C)alkenyloxy, (1-4C)alkoxy, (1-4C)alkylthio, (1-4C)alkylsulphinyl, (1-4C)alkylsulphonyl, (1-4C)alkylamino, di-[(1-4C)alkyl]amino, fluoro-(1-4C)alkyl, cyano-(1-4C)alkyl, (1-4C)alkoxycarbonyl, N-[(1-4C)alkyl]carbamoyl, N,N-di-[(1-4C)alkyl]carbamoyl, (2-4C)alkanoylamino, fluoro-(1-4C)alkoxy, cyano-(1-4C)alkoxy, carbamoyl-(1-4C)alkoxy, amino-(2-4C)alkoxy, (1-4C)alkylamino-(2-4C)alkoxy, di-[(1-4C)alkyl]amino-(2-4C)alkoxy and (1-4C)alkoxycarbonyl-(1-4C)alkoxy;
• wherein R.sup.1 is (1-6C)alkyl, (3-6C)alkenyl, (3-6C)alkynyl, cyano-(1-4C)alkyl or (2-4C)alkanoyl, or R.sup.1 is benzoyl which may optionally bear a substituent selected from halogeno, (1-4C)alkyl and (1-4C)alkoxy;
• and
• wherein R.sup.2 and R.sup.3 together form a group of the formula --A.sup.2 --X.sup.2 --A.sup.3 -- which, together with the carbon atom to which A.sup.2 and A.sup.3 are attached, defines a ring having 4 to 7 ring atoms, wherein A.sup.2 and A.sup.3, which may be the same or different, each is (1-4C)alkylene and X.sup.2 is oxy, and which ring may bear one, two or three substituents, which may be the same or different, selected from halogeno, hydroxy, cyano, (1-4C)alkyl, (1-4C)alkoxy, (1-4C)alkylthio, (1-4C)alkylsulphinyl, (1-4C)alkylsulphonyl and fluoro-(1-4C)alkyl;
• or a pharmaceutically-acceptable salt thereof.