Biosynthetic incorporation of advanced precursors into dehydrocurvularin, a polyketide phytotoxin from Alternaria cinerariae

Biosynthesis of the polyketide, dehydrocurvularin 1, by Alternaria cinerariae was examined by incorporation experiments with N-acetylcysteamine (NAC) thiolesters of potential labeled di-, tri-, tetra-, and pentaketide assembly intermediates, 5–10. The results show that diketide and ,β-unsaturated tetraketide precursors can be utilized intact, whereas a saturated tetraketide can not, thereby suggesting that 1 is the initial PKS product. Curvularin 2 and 1 could not be interconverted by A. cinerariae, but 8-hydroxycurvularin 3 and 1 are transformed into each other by component(s) in the fermentation media.     

Synthesis and complete stereochemical assignment of psymberin/irciniastatin A

We describe a concise and flexible synthetic avenue for the preparation of compounds with structures relevant to those proposed for the novel marine-derived differential cytotoxins psymberin and irciniastatin A. Our efforts led to their complete stereochemical assignment and the notion that psymberin and irciniastatin A are identical compounds. Our total synthesis features an interesting termini-differentiating lactolization of a dialdehyde obtained from a C2-symmetrical bis-olefin precursor, a mild platinum-catalyzed hydrolysis of an epimerizable nitrile, a novel protocol to prepare sensitive methyl imidates, and a one-pot conversion of these imidates to N-acyl aminals. Starting from fragments 5-7 (prepared in 7-8 steps each, 30-49% overall yield) the synthesis of psymberin/irciniastatin A was completed in an additional 9 steps and 30% yield (17 steps longest linear sequence, 8.9% overall).     

Synthesis and stereochemical assignment of (+)-chamuvarinin

A stereocontrolled total synthesis of (+)-chamuvarinin, isolated from the root extract of Uvaria Chamae, utilizes a convergent modular strategy to construct the adjacently linked C15-C28 ether array, followed by a late-stage Julia-Kocienski olefination to append the butenolide motif. This constitutes the first total synthesis of (+)-chamuvarinin, defining the relative and absolute configuration of this unique annonaceous acetogenin.     

Total synthesis and stereochemical assignment of burkholdac B, a depsipeptide HDAC inhibitor

Three diastereomers of burkholdac B were prepared by total synthesis, enabling the full stereochemical assignment of the natural product. It is proposed that burkholdac B is identical to thailandepsin A independently isolated by Cheng from the same strain of Burkholderia thailandensis . Burkholdac B is the most potent among depsipeptide histone deacetylase inhibitors in growth inhibition of the MCF7 breast cancer cell line with an IC(50) of 60 pM.     

Synthesis and stereochemical assignment of brasilibactin A

[structure: see text] Brasilibactin A, a naturally occurring siderophore related to the mycobactins, has been synthesized in six steps. Use of asymmetric titanium-mediated aldol reactions allowed the preparation of both diastereomers from a common synthetic intermediate, thus allowing the relative stereochemistry of the natural product to be assigned. Brasilibactin A exhibits no inhibition of histone deacetylases (HDACs) in spite of the N-formyl-N-hydroxy lysine moiety that is expected to affect the activity of these metal-dependent lysine-modifying enzymes.     

Structure of phaseolinone,a novel phytotoxin from macrophomina phaseolina

Structure of phaseolinone, a phytotoxin isolated from the fungus $\text{Macrophima phaseolina}$ (Tassi) Gold, has been established as $\text{1}$.     

Perylene bisimide atropisomers: synthesis, resolution, and stereochemical assignment

The macrocyclization of the tetra-hydroxyphenoxy-substituted perylene bisimide 4 bearing two (R)-configured 2-octyl substituents in the imide positions by etherification with diethylene glycol ditosylate afforded both the diagonally bridged (1,7- and 6,12-linkage) and laterally bridged (1,12- and 6,7-linkage) regioisomers 6 and 7. The atropo-diastereomers of the diagonally bridged macrocycle 6 were separated by semipreparative HPLC on a chiral column, and their absolute configurations were determined by circular dichroism (CD) spectroscopy in combination with quantum chemical CD calculations. The isolated epimers (P,R,R)-6 and (M,R,R)-6 represent the first examples of diasteriomerically pure perylene bisimide atropisomers. The optical and chiroptical properties of these epimers were investigated by UV/vis, fluorescence, and CD spectroscopy, and their conformational properties have been explored by temperature-dependent 1H NMR studies.     

Azedaralide: total synthesis, relative and absolute stereochemical assignment

Azedaralide, a potentially advanced intermediate for the total synthesis of various tetranortriterpenes, was constructed utilising the Fernández-Mateos protocol and assigned both relative and absolute stereochemistries. Both asymmetric aldol and classical chiral resolution attempts failed to deliver pure enantiomers whereas preparative chiral chromatography resolved racemic azedaralide with ease.     

Total synthesis, stereochemical assignment, and antimalarial activity of gallinamide A

The total synthesis and stereochemical assignment of gallinamide A, an antimalarial depsipeptide of cyanobacterial origin, is described. Synthesis of the four possible N-terminal diastereoisomers of gallinamide A (including the natural product symplostatin 4) was achieved using a divergent strategy from a common imide fragment. The natural product and corresponding diastereoisomers were synthesized in 30-33% overall yield in a longest linear sequence of 8 steps. Comparative NMR spectroscopic studies of the four synthetic diastereoisomers with the isolated natural product demonstrated that gallinamide A possesses a dimethylated L-isoleucyl residue at the N-terminus. As such, we have shown that gallinamide A is structurally and stereochemically identical to symplostatin 4. Gallinamide A and its N-terminal diastereoisomers were also shown to possess significant antimalarial activity with IC(50) values in the nanomolar range against the 3D7 strain of Plasmodium falciparum.     

Total synthesis,stereochemical assignment,and biological evaluation of L-755,807

The relative and absolute configurations of L-755,807 were established through total synthesis. All four possible stereoisomers were prepared via a convergent synthetic strategy, including a novel diastereoselective Darzens reaction of an α-alkoxy aldehyde with di-tert-butyl bromomalonate, an E-selective Horner–Wadsworth–Emmons reaction, and late-stage coupling of the ring and side-chain segments. Additionally, biological evaluation of the synthesized compounds revealed their potent inhibitory activities (IC₅₀?=?5–21?μM) against amyloid-β aggregation for the treatment of Alzheimer's disease.     

Enantiomeric resolution,stereochemical assignment and toxicity evaluation of TPA enantiomers

Tetrahydro‐α ‐(1‐methylethyl)‐2‐oxo‐1(2H)‐pyrimidineacetic acid (TPA) is a critical intermediate in the synthesis of HIV protease inhibitors. A simple and efficient method for the separation and determination of TPA enantiomers was developed. The TPA was separated into its enantiomers with an enantiomeric purity of 99% using an HPLC system equipped with a Chiralpak OD‐H column. Semi‐preparative HPLC enantioseparations were carried out for further enrichment of the enantiomers. The validity of this method was evaluated on the basis of its precision, accuracy, linearity and recovery. The method was observed to be suitable for the rapid separation and semi‐preparation of TPA isomers. The separated enantiomers were identified by optical rotation and high‐resolution electrospray ionization mass spectrometry. Furthermore, the stereochemical structures of the TPA enantiomers were definitively confirmed using a combination of experimental and calculated electronic circular dichroism spectra. The toxicity of the separated pure enantiomers against Oryzias melastigma was evaluated using the median lethal concentration (LC₅₀) values. The results indicated that (S )‐(−)‐TPA is ~2.5 times more toxic than its enantiomorphism.     

Synthesis and stereochemical assignment of (+)-Cladospolide D

Cladospolides A-D are 12-membered, α,β-unsaturated lactones isolated from various species of Cladosporium. Cladospolide D is unique in its γ-keto functionality and possesses antifungal activity; however, the stereochemistry of Cladoapolide D was unknown. We report the asymmetric syntheses to generate both possible diastereomers of Cladospolide D. Two regioselective cross-metatheses were applied to form the carbon skeleton, and the two olefins were differentiated by Michael addition, hydrogenation, and elimination. Later, the macrocycle was achieved through the Yamaguchi protocol. After comparing the spectroscopic data of the synthetic Cladospolide D with the reported values, the stereochemistry of Cladospolide D is confirmed as (2E,5R,11S).     

Design,synthesis, resolution,and stereochemical assignment of a conformationally rigid chiral ligand derived from anthracene and a dienophile containing a pyridine moiety

The conformationally rigid chiral ligand, trans-12-(pyridin-2-yl)-9,10-dihydro-9,10-ethanoanthracene-11-carboxylic acid ethyl ester, 1, was designed and synthesized in racemic form. Both isomers were successfully obtained in enantiomerically pure form through classical resolution using l-(+)-tartaric acid in acetonitrile. The nature of the diastereomeric complex formed in this resolution was elucidated using single crystal X-ray crystallographic studies. The absolute configuration of (+)-1 was unambiguously assigned as (11S,12S) by single crystal structural analysis of salt 5 formed from (+)-1 and l-(+)-tartaric acid.     

Structure of turbicoryn, a new glucoside from Turbina corymbosa

From the seeds of the plant known as Ololiuqui (Turbina corymbosa),¹ a new glucoside, turbicoryn, has been isolated and the structure of its aglucone, turbicorytin, established.¹     

Synthesis and stereochemical assignment of DNA spore photoproduct analogues

Investigation of the DNA repair process performed by the spore photoproduct (SP) lyase repair enzyme is strongly hampered by the lack of defined substrates needed for detailed enzymatic studies. The problem is particularly severe because the repair enzyme belongs to the class of strongly oxygen-sensitive radical (S)-adenosylmethionine (SAM) enzymes, which are notoriously difficult to handle. We report the synthesis of the spore photoproduct analogues 1 a and 1 b, which have open backbones and are diastereoisomers. In order to solve the problem of stereochemical assignment, two further derivatives 2 a and 2 b with closed backbones were prepared. The key step of the synthesis of 2 a/b is a metathesis-based macrocyclization that strongly increases the conformational rigidity of the synthetic spore photoproduct derivatives. NOESY experiments of the cyclic isomers furnished a clear cross-peak pattern that allowed the unequivocal assignment of the stereochemistry. The results were transferred to the data for isomers 1 a and 1 b, which were subsequently used for enzymatic-repair studies. These studies were performed with the novel spore photoproduct lyase repair enzyme from Geobacillus stearothermophilus. The studies showed an accordance with a recent investigation performed by us with the spore photoproduct lyase from Bacillus subtilis, in that only the S isomer 1 a is recognized and repaired. The ability to prepare a defined functioning substrate now paves the way for detailed enzymatic studies of the SP-lyase lesion recognition and repair process.     

Total synthesis and absolute stereochemical assignment of kibdelone C

Kibdelones are hexacyclic tetrahydroxanthones and potent anticancer agents isolated from an Australian microbe. Herein, we describe the synthesis of a chiral, nonracemic iodocyclohexene carboxylate EF ring fragment of the kibdelones employing an intramolecular iodo halo-Michael aldol reaction and its merger with an ABCD ring fragment to afford the congener kibdelone C.     

Structure of eucommiol, a new cyclopentenoid-tetrol from Eucommia Ulmoides

Isolation of Eucommiol a new cylopentenoid-tetrol from Eucommia Ulmoides (Eucommiaceae) is described. Its structure and absolute stereochemistry have been demonstrated.