Fungal polyketide synthases (PKSs) can function collaboratively to synthesize natural products of significant structural diversity. Here we present a new mode of collaboration between a highly reducing PKS (HRPKS) and a PKS-nonribosomal peptide synthetase (PKS-NRPS) in the synthesis of oxaleimides from the Penicillium species. The HRPKS is recruited in the synthesis of an olefin-containing free amino acid, which is activated and incorporated by the adenylation domain of the PKS-NRPS. The precisely positioned olefin from the unnatural amino acid is proposed to facilitate a scaffold rearrangement of the PKS-NRPS product to forge the maleimide and succinimide cores of oxaleimides.