Radical and palladium-catalyzed cyclizations to cyclobutenes: an entry to the BCD ring system of penitrem D

A novel approach toward the synthesis of the BCD ring system of penitrem D is described. The strategy capitalizes on the fast cyclization rates of aryl radicals into cyclobutenes and allows access to a variety of fused tricyclic structures. Radical/polar crossover reactions of precursors 24-29 promoted by samarium diiodide in the presence of HMPA and acetone allow access to the fully functionalized BCD ring system of penitrem D. The stereochemical implications of these processes are evaluated, and a Pd-mediated cyclization approach toward the penitrems is also introduced.     

A highly stereoselective synthesis of the C10–C31 (BCDEF ring) portion of pinnatoxin A

An efficient, highly stereoselective synthesis of the C10–C31 (BCDEF ring) portion of pinnatoxin A has been achieved utilizing tandem double hemiketal formation/intramolecular hetero-Michael addition to construct the 6,5,6-dispiroketal (BCD ring) system and subsequent intramolecular ketalization to form the 5,6-bicycloketal (EF ring) system as key steps.     

Total Synthesis and Biological Evaluation of (+)‐Gambieric Acid A and Its Analogues

In this study, we report the first total synthesis and complete stereostructure of gambieric acid A, a potent antifungal polycyclic ether metabolite, in detail. The A/B‐ring exocyclic enol ether 32 was prepared through a Suzuki–Miyaura coupling of the B‐ring vinyl iodide 18 and the alkylborate 33 and subsequent closure of the A‐ring by using diastereoselective bromoetherification as the key transformation. Suzuki–Miyaura coupling of 32 with acetate‐derived enol phosphate 49 , followed by ring‐closing metathesis of the derived diene, produced the D‐ring. Subsequent closure of the C‐ring through a mixed thioacetalization completed the synthesis of the A/BCD‐ring fragment 8 . The A/BCD‐ and F′GHIJ‐ring fragments (i.e., 8 and 9 ) were assembled through Suzuki–Miyaura coupling. The C25 stereogenic center was elaborated by exploiting the intrinsic conformational property of the seven‐membered F′‐ring. After the oxidative cleavage of the F′‐ring, the E‐ring was formed as a cyclic mixed thioacetal (i.e., 70 ) and then stereoselectively allylated by using glycosylation chemistry. Ring‐closing metathesis of the diene 3 thus obtained closed the F‐ring and completed the polycyclic ether skeleton. Finally, the J‐ring side chain was introduced by using a Julia–Kocienski olefination in the presence of CeCl₃ to complete the total synthesis of gambieric acid A ( 1 ), thereby unambiguously establishing its complete stereostructure. The present total synthesis enabled us to evaluate the antifungal and antiproliferative activities of 1 and several synthetic analogues.     

A new class of simplified phorbol ester analogues: synthesis and binding to PKC and eta PKC-C1B (eta PKC-CRD2)

[formula: see text] A unique class of simplified phorbol ester analogues is described for the first time. A highly efficient retro-annelation sequence was developed in order to remove the five-membered ring from the phorbol diterpene core, allowing access to BCD ring analogues of the phorbol esters. The binding of these analogues to protein kinase C (PKC) and the truncated peptide eta PKC-C1B (eta PKC-CRD2) is also reported.     

Studies on the total synthesis of lactonamycin: construction of model ABCD ring systems

Model studies on the synthesis of the tetracyclic ABCD ring system of lactonamycin (1) are described. The key step involved the double Michael addition reaction of alcohol 8 to propynoate esters to produce the BCD units 13 and 14 of the target 1. Alternatively, double Michael addition of alcohol 8 to di-tert-butyl acetylenedcarboxylate gave the corresponding BCD ring systems 36 and 37. Acid-mediated hydrolysis of the dihydroquinone monoketal units of 13 and 14 and 36 and 37 in the presence of air gave the corresponding quinones 7 and 39. These were converted into the tetracyclic ABCD units 6, 26a, 40, and 42 of lactonamycin (1) by either dihydroxylation or epoxidation and acid-catalyzed lactonization.     

Rapid construction of the unique BCD ring system of tricyclo[6.2.1.0] undecane in the C19-diterpenoid alkaloid aconitine

A model study leading to the preparation of the unique tricyclo [6.2.1.0] undecane BCD ring systems of aconitine is described. The synthesis features an unprecedented diastereoselective oxidative dearomatization/dimerization/retro-DA/IMDA cascade reaction and a highly efficient Wagner-Meerwein rearrangement.     

Synthetic studies towards the novel diterpenoid rameswaralide: RCM mediated acquisition of the tricyclic core

A synthetic approach towards the novel anti-inflammatory diterpenoid rameswaralide from the cis-Corey lactone involving a tandem RCM and Diels-Alder reaction has been conceived and endeavors so far have led to the acquisition of the BCD ring fragment of the natural product.     

Studies directed toward the total synthesis of pinnatoxin A: synthesis of the 6,5,6-dispiroketal (BCD ring) system by double hemiketal formation/hetero-Michael addition strategy

An efficient, highly stereoselective synthesis of the C10–C26 portion of pinnatoxin A has been achieved, wherein the key step is a highly stereoselective construction of the 6,5,6-dispiroketal (BCD ring) system by an intramolecular hetero-Michael addition of a hemiketal alkoxide reversibly formed under the influence of lithium methoxide.     

Synthesis of the BCD ring system of azaspiracid: construction of the trispiro ring structure by the thioether approach

The synthesis of the BCD ring system of azaspiracid 1 has been attained. Construction of the stereochemistry of the C₁₃ position was successfully controlled by connection between the B ring and the C ring with a sulfur atom.     

Ti(III)-induced radical cyclization. A stereoselective entry to the perhydrobenzo[e]indene unit of new protein farnesyltransferase inhibitors, andrastins A-D

An efficient approach for the synthesis of a fully functionalized perhydrobenzo[e]indene, the BCD ring system of andrastins A-D, is described. The synthesis commences from (±)-Wieland-Miescher ketone and features a Ti(III)-induced radical cyclization as the central step.     

The influence of bond rigidity and cluster diffusion on the self-diffusion of hard spheres with square well interaction

Hard spheres interacting through a square well potential were simulated by using two different methods: Brownian cluster dynamics (BCD) and event driven Brownian dynamics (EDBD). The structure of the equilibrium states obtained by both methods was compared and found to be almost identical. Self-diffusion coefficients (D) were determined as a function of the interaction strength. The same values were found by using BCD or EDBD. Contrary to EDBD, BCD allows one to study the effect of bond rigidity and hydrodynamic interaction within the clusters. When the bonds are flexible, the effect of attraction on D is relatively weak compared to systems with rigid bonds. D increases first with increasing attraction strength, and then decreases for stronger interaction. Introducing intracluster hydrodynamic interaction weakly increases D for a given interaction strength. Introducing bond rigidity causes a strong decrease in D which no longer shows a maximum as function of the attraction strength.     

Total Synthesis of Limonin

Limonoids are highly oxygenated C13α‐triterpenes and common secondary metabolites. Several hundred congeners have been isolated to date. The first total synthesis of (±)‐limonin, the flagship congener of the limonoids, is now reported and features 1) a tandem radical cyclization generating the BCD ring system with the C13α configuration that is essential to the limonoids and a Robinson annulation to construct the limonoid androstane framework, 2) a singlet‐oxygen cycloaddition and a Baeyer–Villiger oxidation to synthesize the highly oxidized D ring, and 3) a Suárez reaction to construct the unique AA′ ring system.     

Total Synthesis of Limonin

Limonoids are highly oxygenated C13α‐triterpenes and common secondary metabolites. Several hundred congeners have been isolated to date. The first total synthesis of (±)‐limonin, the flagship congener of the limonoids, is now reported and features 1) a tandem radical cyclization generating the BCD ring system with the C13α configuration that is essential to the limonoids and a Robinson annulation to construct the limonoid androstane framework, 2) a singlet‐oxygen cycloaddition and a Baeyer–Villiger oxidation to synthesize the highly oxidized D ring, and 3) a Suárez reaction to construct the unique AA′ ring system.     

Development of On-line Liquid Chromatography-Biochemical Detection for Soluble Epoxide Hydrolase Inhibitors in Mixtures

In this study, an end-point-based fluorescence assay for soluble epoxide hydrolase (sEH) was transformed into an on-line continuous-flow format. The on-line biochemical detection system (BCD) was coupled on-line to liquid chromatography (LC) to allow mixture analysis. The on-line BCD was based on a flow system wherein sEH activity was detected by competition of analytes with the substrate hydrolysis. The reaction product was measured by fluorescence detection. In parallel to the BCD data, UV and MS data were obtained through post-column splitting of the LC effluent. The buffer system and reagent concentrations were optimized resulting in a stable on-line BCD with a good assay window and good sensitivity (S/N > 60). The potency of known sEH inhibitors (sEHis) obtained by LC–BCD correlates well with published values. The LC–BCD system was applied to test how oxidative microsomal metabolism affects the potency of three sEHis. After incubation with pig liver microsomes, several metabolites of sEHis were characterized by MS, while their individual potencies were measured by BCD. For all compounds tested, active metabolites were observed. The developed method allows for the first time the detection of sEHis in mixtures providing new opportunities in the development of drug candidates.     

An IMDAF cycloaddition approach toward the synthesis of the lycopodium alkaloid (±)-fawcettidine

Using an intramolecular [4 + 2] cycloaddition/rearrangement cascade of 3-(1,4-dioxaspiro[4.4]non-7-en-7-yl)-N-furan-2-ylpropionamide (23) as the key step, the BCD core of the lycopodium alkaloid fawcettidine was constructed. Heating the initially formed Diels-Alder cycloadduct at 180 °C results in a nitrogen-assisted ring opening followed by a deprotonation/reprotonation of the ensuing zwitterion to give a rearranged hexahydroindolinone. Our attempts to induce a related intramolecular furan Diels-Alder reaction (IMDAF) from the corresponding ketone of 23 failed to give any cycloaddition product. Instead, the only product obtained corresponded to a cyclopentenone derivative derived by isomerization of the double bond into the thermodynamically more stable α,β-position. Efforts toward construction of the final skeleton of fawcettidine by ring A closure of the rearranged cycloadduct derived from furanyl amide 23 are discussed.     

Samarium(II) iodide mediated radical/polar crossover reactions of cyclobutenes. An efficient approach to the BCD ring system of the penitrems

[reaction: see text] Radical/polar crossover reactions of derivatives of 1-(2-cyclobutenyl)-2-(2-iodoaryl)ethanones with acetone promoted by samarium diiodide and HMPA provide 1-(1-hydroxy-1-methylethyl)-2,2a,4,8b-tetrahydro-1H-cyclobuta[a]naphthalen-3-one derivatives in about 50% isolated yield. This reaction shows promise for construction of the BCD ring fragment of the penitrems.     

Synthesis of the seco-Limonoid BCD Ring System Identifies a Hsp90 Chaperon Machinery (p23) Inhibitor

D-Ring-seco-limonoids (tetranortriterpenoids), such as gedunin and xylogranin B display anti-cancer activity, acting via inhibition of Hsp90 and/or associated chaperon machinery (e.g., p23). Despite this, these natural products have received relatively little attention, both in terms of an enabling synthetic approach (which would allow access to derivatives), and as a consequence their structure–activity relationship (SAR). Disclosed herein is a generally applicable synthetic route to the BCD ring system of the seco-D-ring double bond containing limonoids. Furthermore, cell based assays revealed the first skeletal fragment that exhibited inhibition of the p23 enzyme at a level which was equipotent to that of gedunin, despite being much less structurally complex.     

Development of a liquid chromatography-based screening methodology for proteolytic enzyme activity

A new methodology for the detection and isolation of serine proteases in complex mixtures has been developed. It combines the characterization of crude samples by electrospray tandem mass spectrometry (ESI-MS/MS) in a multi-substrate assay and the differentiated sensitive detection of the responsible enzymes by means of liquid chromatography hyphenated online to biochemical detection (BCD). First, active samples are identified in the multi-substrate assay monitoring the conversion of eight substrates in multiple reaction monitoring in parallel within 60 s. Hereby, the product patterns are investigated and the suitable peptide as substrate for BCD analysis is selected. Subsequently, the active proteases are identified online in the continuous-flow reactor serving as BCD after non-denaturing separation by size-exclusion chromatography and ion-exchange chromatography. For BCD, the selected para-nitroaniline (pNA) labeled peptide is added post-column and is cleaved by eluting proteases under release of the coloured pNA in a reaction coil (reaction time 5 min). The method was optimized and the figures of merit were characterized with trypsin and chymotrypsin serving as the model proteases. For trypsin, a limit of detection in LC–BCD of 0.1 U/mL corresponding to an injected amount of 0.4 ng protein (∼18 fmol) was observed. The BCD signal remained linear for an injected enzyme concentration of 0.3–10 U/mL (1.3–42 ng enzyme). The method was applied to the characterization of the crude venom of the pit viper Bothrops moojeni and the extracellular protease of the pathogenic amoeba Acanthamoeba castellanii. In the two samples, fractions with proteolytic activity potentially interfering with the blood coagulation cascade were identified. The described methodology represents a tool for serine protease screening in complex mixtures by a fast ESI-MS/MS identification of active samples followed by the separation and isolation of active sample constituents in LC–BCD.     

Stepwise acid-promoted double-Michael process: an alternative to Diels-Alder cycloadditions for hindered silyloxydiene-dienophile pairs

The hindered diene 1 reacts with 3-methylcyclohexenone 6 catalyzed by triflimide to produce the Mukaiyama Michael product 7 (low-temperature quenching) or the [4+2] cycloadduct 8 (quenching at 0 degrees C). Reaction of the hindered diene 23 with 2-methylcyclohexenone 12 with 5:1 AlBr3:AlMe3 afforded a 71% yield of a 1.9:1 mixture of two cycloadducts. Hydrolysis of the major isomer gave the dione 27', a model for the BCD ring system of pentacyclic triterpenes. [reaction: see text].     

Selective hydroxymethylation of guaiacol in the presence ofβ-cyclodextrin

Studies on the effect of-cyclodextrin (BCD) and its derivatives on the selectivity in hydroxymethylation of guaiacol by formaldehyde was carried out. Fairly high selectivity with respect to isovanillyl alcohol formation was achieved. Significantly, the selectivity-enhancing effects of 2,6-di-O-methyl-BCD was much larger, giving rise to 22% more of isovanillyl alcohol formation than BCD and its polymer. UV, fluorescence,¹H-NMR spectroscopic and potentiometric studies were also carried out to determine the orientation of guaiacol inside the BCD cavity.