Structure and stereochemistry of constanolactones A-G, lactonized cyclopropyl oxylipins from the red marine alga Constantinea simplex

Article abstract of DOI:10.1021/jo00103a012

Extracts of the Oregon marine alga Constantinea simplex were found to contain a mixture of ω6 and ω3 unsaturated constanolactones, lactonized cyclopropyl-containing metabolites that logically derive from arachidonic and eicosapentaenoic acids, respectively. Detailed spectroscopic analysis of the isolated compounds, as natural products and various ester derivatives, afforded the planar structures of seven structurally related constanolactones. Constanolactones A-D possess 1,4-diol functionalities while constanolactones E-G contain a vicinal diol functionality. The absolute stereochemistry at all stereocenters in constanolactones A-D and at two stereocenters in constanolactones E and F were determined by chiral chromatography of fragments and chiroptical measurements of various mono- and dibenzoate derivatives and by comparable rotations within the two series (A-D and E-G). Isolation of these various diastereomeric diols, as well as of two presumed methanol adducts from CHCl₃/MeOH extraction of C. simplex, leads us to speculate on the occurrence of highly unstable allylic epoxides in this red alga.