Compounds of formula (I) and pharmaceutically acceptable salts thereof are selective agonists for 5-HT4 receptors. <IMAGE> (I) wherein Ak is a C3-C6 alkyl group, and R is a C2-C6 alkyl group, a C3-C6 alkenyl group, a C3-C6 alkynyl group, a C3-C7 cycloalkyl group or a C3-C6 cycloalkylmethyl group.
