A compound having the general formula (I) wherein R1 represents C1-C6 alkyl, etc., R2 represents hydrogen, C2-C7 alkanoyl, C7-C11 arylcarbonyl, the formula R4—(CH2)l—, etc., R3 represents C6-C10 aryl, etc., X1, X2, X3, X4 and X5 each independently represents hydrogen, halogen, etc., and n represents an integer of 0 to 2, pharmacologically acceptable salts thereof or prodrugs thereof. They have excellent inhibition of platelet activation, etc. and are useful as a prophylactic or therapeutic agents for diseases related to thrombus or embolus formation.