An unexpected [5 + 1 + 3] cascade cyclization to the preparation of benzo[4,5]thieno[3,2-d]pyrimidine derivatives has been disclosed. In the new protocol, o-nitrochalcones reacted with elemental sulfur and guanidine promoted by NaOH, which reacted in EtOH for 20 min, providing structurally diverse benzo[4,5]thieno[3,2-d]pyrimidines with good yields (77-89%) and wide substrate compatibility (33 examples).