Process for producing 3-acylaminobenzofuran-2-carboxylic acid derivative

An excellent process for efficiently producing a benzofuran derivative or pyridofuran derivative or a pharmacologically acceptable salt of either, the derivatives being represented by the general formula [I]: [I] (wherein X represents -N= or -CH=; R1 represents hydrogen, halogeno, lower alkyl, lower alkoxy, cyano, or amino optionally substituted by lower alkyl; ring A represents a nitrogenous heterocyclic group; ring B represents an optionally substituted benzene ring or pyridine ring; and R3 represents hydrogen or lower alkyl). The derivatives are useful as an inhibitor against an activated blood coagulation factor X.