Tricyclic heterocyclic compounds as 5-HT.sub.4 receptor antagonists

Compounds of formula (I), or pharmaceutically acceptable salts thereof:

• ##STR6## wherein X.sub.1 --(CH.sub.2).sub.x --X.sub.2 and the aromatic carbon atoms to which they are attached form a 5-7 membered ring
• X.sub.1 is O or S;
• X.sub.2 is O or S;
• or one of
• X.sub.1 and X.sub.2 is O, S or CH.sub.2 and the other is CH.sub.2 ;
• x is 1 or 2;
• R.sub.1 and R.sub.2 together are Q--CH.sub.2 --CH.sub.2, Q--CH.dbd.CH, or Q--CH.dbd.N where Q is linked either to the R.sub.1 or the R.sub.2 substitution position and Q is O, S or NR.sub.t wherein R.sub.t is hydrogen or C.sub.1-6 alkyl;
• R.sub.3 is hydrogen, halo, C.sub.1-6 alkyl, C.sub.1-6 alkoxy or amino;
• R.sub.4.sup.1 and R.sub.4.sup.2 are independently hydrogen or C.sub.1-6 alkyl;
• Y is O or NH;
• Z is of sub-formula (a):
• ##STR7## wherein n.sup.1 is 1, 2, 3 or 4;
• q is 0, 1, 2 or 3;
• R.sub.5 is hydrogen, C.sub.1-12 alkyl, aralkyl or R.sub.5 is (CH.sub.2).sub.z --R.sub.10 wherein z is 2 or 3 and R.sub.10 is selected from cyano, hydroxyl, C.sub.1-6 alkoxy, phenoxy, C(O)C.sub.1-6 alkyl, COC.sub.6 H.sub.5, --CONR.sub.11 R.sub.12, NR.sub.11 COR.sub.12, SO.sub.2 NR.sub.11 R.sub.12 or NR.sub.11 SO.sub.2 R.sub.12 wherein R.sub.11 and R.sub.12 are hydrogen or C.sub.1-6 alkyl;
• or R.sub.5 is straight or branched chain alkylene of chain length 1-6 carbon atoms terminally substituted by aryl, 3 to 8 membered cycloalkyl, 3 to 8 membered heterocyclyl, 5 or 6 membered monocyclic heteroaryl or 9 or 10 membered fused bicyclic heteroaryl linked through carbon, C.sub.2-7 alkoxycarbonyl, or secondary or tertiary hydroxy substituted C.sub.1-6 alkyl;
• and
• R.sub.6 is hydrogen or C.sub.1-6 alkyl;
• or a compound of formula (I) wherein the CO--Y linkage is replaced by a heterocyclic bioisostere.