A compound of the formula: ##STR1## or a pharmaceutically acceptable salt thereof, wherein X, O R, R.sup.1, R.sup.2, R.sup.3, and R.sup.4 are defined in the specification; compositions containing such compounds; intermediates used in the preparation of such compounds; and methods for treating a disease associated with the altered tone and/or motility of smooth muscle, for example chronic obstructive airways disease, asthma, urinary incontinence, irritable bowel syndrome, diverticular disease, oesophageal achalasia, and hypertension.