Compounds of the formula ##STR00001## wherein R.sub.1, R.sub.2, R.sub.3, R.sub.4, X and Y are as defined, which have a superior cGMP specific phosphodiesterase (PDE) inhibitory activity, and can be used as an agent for the treatment of cardiovascular diseases such as angina pectoris, heart failure, cardiac infarction, hypertension, arteriosclerosis, and the like; allergic diseases such as asthma, or disorders of male or female genital function and the like.