Koichiro Ota

Total Synthesis of Marine Eicosanoid (−)‐Hybridalactone

(-)-Hybridalactone (1) is a marine eicosanoid isolated from the red alga Laurencia hybrida. This natural product contains cyclopropane, cyclopentane, 13-membered macrolactone and epoxide ring systems incorporating seven stereogenic centers. Moreover, this compound has an acid-labile skipped Z,Z-diene motif. In this paper, we report on the total synthesis of (-)-hybridalactone (1). The unique eicosanoid (-)-hybridalactone (1) was synthesized starting from optically active γ-butyrolactone 2 in a linear sequence comprising 21 steps with an overall yield of 21.9%. A key step in the synthesis of (-)-hybridalactone (1) is the methyl phenylsulfonylacetate-mediated one-pot synthesis of the cis-cyclopropane-γ-lactone derivative. This reaction provided an efficient and stereoselective access to cis-cyclopropane-γ-lactone 12. Further elaboration of the latter compounds through desulfonylation, epoxidation, oxidation, Wittig olefination and Shiina macrolactonization afforded (-)-hybridalactone.

Total Synthesis and Absolute Configuration of the Marine Norditerpenoid Xestenone

Xestenone is a marine norditerpenoid found in the northeastern Pacific sponge Xestospongia vanilla. The relative configuration of C-3 and C-7 in xestenone was determined by NOESY spectral analysis. However the relative configuration of C-12 and the absolute configuration of this compound were not determined. The authors have now achieved the total synthesis of xestenone using their developed one-pot synthesis of cyclopentane derivatives employing allyl phenyl sulfone and an epoxy iodide as a key step. The relative and absolute configurations of xestenone were thus successfully determined by this synthesis.

Amitorines A and B, Nitrogenous Diterpene Metabolites of Theonella swinhoei: Isolation, Structure Elucidation, and Asymmetric Synthesis

Two new nitrogenous prenylbisabolanes never before found in Lithistid sponges have been isolated from Theonella swinhoei. These new diterpenes, named amitorine A (1) and amitorine B (2), containing a prenylbisabolane skeleton have been characterized by spectroscopic analyses, and the relative and absolute configurations of 1 and 2 were determined by asymmetric synthesis of both diastereomers via the common bicyclic lactone 6 intermediate.

BRIAROXALIDES : NOVEL DIEPOXYBRIARANE DITERPENES FROM AN OKINAWAN GORGONIAN BRIAREUM SP.

Seven new 8,17and 11,12-diepoxybriarane diterpenoids, briaroxalides A-G (1-7), were isolated from an Okinawan gorgonian Briareum sp. The structures of the diterpenoids were determined on the basis of spectroscopic analysis, chemical conversions and X-ray diffraction analysis.

Total Synthesis of ent-Ascospiroketal B

Ascospiroketal B was isolated from a marine-derived fungus as a structurally unique polyketide possessing a rare tricyclic core including 5,5-spiroketal-γ-lactone. An asymmetric total synthesis of ent-ascospiroketal B was achieved using an original synthetic route. The synthesis included the stereoselective construction of 5,5-spiroketal for ascospiroketal B and stereocontrolled construction of a quaternary asymmetric carbon by rearrangement of a trisubstituted epoxide.