Configurational assignment of cyclic peroxy metabolites provides an insight into their biosynthesis: isolation of plakortolides, seco-plakortolides, and plakortones from the Australian marine sponge Plakinastrella clathrata.
Sixteen new compounds, comprising nine new plakortolides K-S (1-9), four seco-plakortolides (10-13), and three plakortones (14-16), were isolated from the Australian sponge Plakinastrella clathrata. Structural elucidation, including relative configurational assignment, was based on extensive spectroscopic analysis, while the absolute configurations of 1-4 were deduced from (1)H NMR analyses on MPA esters derived from Zn/AcOH reduction products. Diastereomeric sets of plakortolides, e.g., K and L, or M and N, differed in configuration at C-3/C-4 rather than at C-6, a stereochemical result that compromises a biosynthetic pathway involving Diels-Alder cycloaddition of molecular oxygen to a Δ(3,5)-diunsaturated fatty acid precursor. The biosynthesis may plausibly involve cyclization of a 6-hydroperoxydienoic acid intermediate following stereospecific introduction of the hydroperoxy group into a polyketide-derived precursor. Isolated seco-plakortolides converted under mild conditions into plakortones with full retention of configuration, suggesting C-6 hydroxy attack on an α,β-unsaturated lactone intermediate. The NMR data reported for the compound named plakortolide E are inconsistent with the current literature structure and are those of the corresponding seco-plakortolide (19). The reported conversion of the metabolite into a plakortone ether on storage is consistent with this structural revision.
School of Chemistry and Molecular Biosciences, The University of Queensland, Brisbane, QLD 4072, Australia.
Ken W L
KW
Yong
James J
JJ
De Voss
John N A
JN
Hooper
Mary J
MJ
Garson
D015107:4-Butyrolactone / Q000031:analogs & derivatives* / Q000737:chemistry / Q000302:isolation & purification; D000818:Animals; D001315:Australia; D004146:Dioxanes / Q000737:chemistry / Q000302:isolation & purification*; D008386:Marine Biology; D015394:Molecular Structure; D019906:Nuclear Magnetic Resonance, Biomolecular; D011161:Porifera / Q000737:chemistry*; D013237:Stereoisomerism
D004146:Dioxanes; C093753:plakortolide E; D015107:4-Butyrolactone