Reaction of 3-(3-cyanopropylthio)[1]benzothiophene-2-carbonitrile with tert-BuONa gave 5-amino-1,2-dihydro[1]benzothieno[3,2-d]thieno[2,3-b]pyridine and 5-amino-2,3-dihydro[1]benzothieno[3,2-b]thiepin-4-carbonitrile. The latter compound served as a convenient scaffold for the synthesis of the new heterocycles, [1]benzothieno[3,2:2,3]thiepino[4,5-d]pyrimidines. All of our new tetracyclic products were evaluated for in vitro inhibitory activity on the formation of pentosidine, which is one of representative advanced glycation end products.