Compounds of formula (I) and pharmaceutically acceptable salts thereof are selective agonists for 5-HT.sub.4 receptors. ##STR1## wherein Ak is a C.sub.3 -C.sub.6 alkyl group, and R is a C.sub.2 -C.sub.6 alkyl group, a C.sub.3 -C.sub.6 alkenyl group, a C.sub.3 -C.sub.6 alkynyl group, a C.sub.3 -C.sub.7 cycloalkyl group or a C.sub.3 -C.sub.6 cycloalkylmethyl group.