Scheme 9. Later steps in the biosynthesis of blennolides A 59 (Panel A) and C 56 (Panel B), from their methyl ester precursors, 63 and 61 respectively. These pathways were proposed by Wei & Matsuda⁸⁰ based on in vitro assays with NsrK, NsrQ and NsrO. Replacing NsrO in the blennolide A in vitro assay (panel A) with the homologous ketoreductase from C. purpurea (Cp_05418) led to the additional generation of (−)-blennolide B 65 (red box). The authors recognised that the C-5 ketoreduction could occur either prior to, or after, cyclisation to give the xanthone core.⁸⁰.