Herein, we describe the synthesis and structure–activity relationships of dehydroaltenusin derivatives as inhibitors of a mammalian DNApolymerase α. We have newly synthesized nine dehydroaltenusin derivatives modified at the side chains or benzoquinone moiety. We also achieved the first synthesis of desmethylaltenusin and desmethyldehydroaltenusin, metabolites of Alternaria sp. or Talaromyces flavus