O-Methyloscillatoxin D and its analogues were concisely synthesized by a bioinspired intramolecular Mukaiyama aldol reaction as a key step, which involves the construction of a novel spiro-ether moiety.

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Interest in the marine cyanobacteria natural products aplysiatoxin (ATX) and oscillatoxin (OTX) has been renewed recently due to the discovery of many new analogues, some exhibiting intriguing biological activities. We sought to develop a collective synthesis of these natural products, hypothesizing that ATX could serve as a common biosynthetic precursor. Herein, we reveal that the core structure of ATX has unique multiple reactivities giving access to the distinct ring structures of five of the analogues, depending upon the specific conditions used. Based on these findings, syntheses of the O-Me derivative of five analogues neo-deBr-ATX-B, OTX-H, OTX-D, neo-deBr-ATX-H, and OTX-I were achieved from the main fragment of ATX as a common intermediate in a few steps. These synthetic studies also led us to revise the relative configuration in the elucidated structures of neo-deBr-ATX-B and OTX-H, and obtain unnatural 8- and 12-membered lactones from the same intermediate.

Oscillatoxins (OTXs) and aplysiatoxins (ATXs) are biosynthetically related polyketides produced by marine cyanobacteria. We previously developed a synthetic route to phenolic O-methyl analogs of OTX-D and 30-methyl-OTX-D during collective synthesis of these natural products. According to our synthetic strategy, we achieved total synthesis of OTX-D, 30-methyl-OTX-D, OTX-E, and OTX-F by deprotecting the O-methyl group in an earlier intermediate, and determined their biological activities. Although OTX-D and 30-methyl-OTX-D have been reported to show anti-leukemic activity against L1210 cell line, we found that their cytotoxicity in vitro against this cell line is relatively weak (IC50: 29–52 μM). In contrast, OTX-F demonstrated cell line-selective anti-proliferative activity against DMS-114 lung cancer cells, which implies that OTXs target as yet unknown target molecules as part of this unique activity.

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