Ken W. L. Yong

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.

Oxidative processes in the Australian marine sponge Plakinastrella clathrata: Isolation of plakortolides with oxidatively modified side chains

Sixteen new cyclic peroxides (1-16) with a plakortolide skeleton and the methyl ester derivative of a didehydroplakinic acid (17) were isolated from the Australian sponge Plakinastrella clathrata Kirkpatrick, 1900. Structural elucidation and configurational assignments were based on spectroscopic analysis and comparison with data for previously isolated plakortolides and revealed both phenyl- and methyl-terminating side chains attached to the plakortolide core. Plakortoperoxides A-D (5-8) each contained a second 1,2-dioxine ring; a cis configuration for the side chain endoperoxide ring was determined by a low-temperature NMR study and by comparison of chemical shift values with those of reported compounds. An enantioselective HPLC study compared natural plakortoperoxide A with a synthetic sample prepared by cyclization of plakortolide P with singlet oxygen and revealed that the natural sample was a mixture of cis diastereomers at C-15/C18. Four other cyclic peroxides (9-12) possessed a C(9)-truncated side chain terminating in a formyl or carboxylic acid functionality, suggesting that these metabolites may have been formed by oxidative cleavage of the Δ(9,10) bond of diene-functionalized plakortolides. A final group of four metabolites (13-16) with hydroxy or the rare hydroperoxy functionality unexpectedly revealed a C(8) side chain, while the ester (17) represents further structural variation within the growing family of cyclic peroxy sponge metabolites.

Determination of hepatotoxic indospicine in Australian camel meat by ultra-performance liquid chromatography-tandem mass spectrometry.

Indospicine is a hepatotoxic amino acid found in Indigofera plant spp. and is unusual in that it is not incorporated into protein but accumulates as the free amino acid in the tissues (including muscle) of animals consuming these plants. Dogs are particularly sensitive to indospicine, and secondary poisoning of dogs has occurred from the ingestion of indospicine-contaminated horse meat and more recently camel meat. In central Australia, feral camels are known to consume native Indigofera species, but the prevalence of indospicine residues in their tissues has not previously been investigated. In this study, a method was developed and validated with the use of ultra-performance liquid chromatography-tandem mass spectrometry (UPLC-MS/MS) to determine the level of indospicine in camel meat samples using isotopically labeled indospicine as an internal standard. UPLC-MS/MS analysis showed that the method is reproducible, with high recovery efficiency and a quantitation limit of 0.1 mg/kg. Camel meat samples from the Simpson Desert were largely contaminated (≈50%) by indospicine with levels up to 3.73 mg/kg (fresh weight) determined. However, the majority of samples (95%) contained less than 1 mg/kg indospicine.

Isolation of norsesterterpenes and spongian diterpenes from Dorisprismatica (= Glossodoris) atromarginata

Ten new norscalarane metabolites (1-10) with the mooloolabene skeleton in which the C-8 methyl substituent of a scalarane is replaced by a C-7/C-8 double bond are described from the nudibranch Doriprismatica (= Glossodoris) atromarginata and characterized by extensive 1D and 2D NMR studies, together with MS data. Also isolated was the known scalarane 12-deacetoxy-12-oxo-deoxoscalarin together with 26 furanoterpenes, nine of which (11-19) are reported for the first time. The high diversity of chemical compounds and variation between individuals and locations could reflect a varied sponge diet or an enzymatic detoxification mechanism.

Absolute structures and conformations of the spongian diterpenes spongia-13(16), 14-dien-3-one, epispongiadiol and spongiadiol

The absolute configurations of spongia-13(16),14-dien-3-one [systematic name: (3bR,5aR,9aR,9bR)-3b,6,6,9a-tetramethyl-4,5,5a,6,8,9,9a,9b,10,11-decahydrophenanthro[1,2-c]furan-7(3bH)-one], C(20)H(28)O(2), (I), epispongiadiol [systematic name: (3bR,5aR,6S,7R,9aR,9bR)-7-hydroxy-6-hydroxymethyl-3b,6,9a-trimethyl-3b,5,5a,6,7,9,9a,9b,10,11-decahydrophenanthro[1,2-c]furan-8(4H)-one], C(20)H(28)O(4), (II), and spongiadiol [systematic name: (3bR,5aR,6S,7S,9aR,9bR)-7-hydroxy-6-hydroxymethyl-3b,6,9a-trimethyl-3b,5,5a,6,7,9,9a,9b,10,11-decahydrophenanthro[1,2-c]furan-8(4H)-one], C(20)H(28)O(4), (III), were assigned by analysis of anomalous dispersion data collected at 130 K with Cu Kalpha radiation. Compounds (II) and (III) are epimers. The equatorial 3-hydroxyl group on the cyclohexanone ring (A) of (II) is syn with respect to the 4-hydroxymethyl group, leading to a chair conformation. In contrast, isomer (III), where the 3-hydroxyl group is anti to the 4-hydroxymethyl group, is conformationally disordered between a major chair conformer where the OH group is axial and a minor boat conformer where it is equatorial. In compound (I), a carbonyl group is present at position 3 and ring A adopts a distorted-boat conformation.