Enantioselective Transfer Hydrogenation of α-Methoxyimino-ß-keto Esters.

α-Methoxyimino-ß-keto esters are reported to undergo highly enantioselective catalytic transfer hydrogenation using the Noyori-Ikariya complex RuCl(p-cymene)[(S,S)-Ts-DPEN] in a mixture of formic acid-triethylamine and dimethylformamide at 25 °C. The experimental study performed on over 25 substrates combined with computational analysis revealed that a Z-configured methoxyimino group positioned alpha to a ketone carbonyl leads to higher reactivity and mostly excellent enantioselectivity within this substrate class. Density functional theory calculations of competing transition states were used in rationalizing the origins of enantioselectivity and the possible role of the methoxyimino group in the reaction outcome.

Unexpectedly regioselective Diels-Alder reactions of new unsymmetrical benzoquinones: A convenient synthetic entry to uniquely substituted decalins.

We report flexible synthesis of new unsymmetrically 2,6-disubstituted benzoquinones (33 examples) and a systematic study of their reactivity in the Diels-Alder reaction. The Diels-Alder reactions of selected unsymmetrical benzoquinones with seemingly similar substituents were found to proceed with high regioselectivity and the formation of selected experimentally observed main products was rationalized by theoretical (DFT) calculations. The findings can be exploited in the convenient preparation of densely substituted and stereochemically defined decalins with unique angular substituents at ring fusion. We also demonstrate the usefulness of this methodology in complex molecule synthesis through the total synthesis of a novel forskolin analog possessing an ethyl group at the fusion of the rings B and C.

A Platform for the Synthesis of Oxidation Products of Bilirubin.

Bilirubin is the principal product of heme catabolism. High concentrations of the pigment are neurotoxic, yet slightly elevated levels are beneficial. Being a potent antioxidant, oxidative transformations of bilirubin occur in vivo and lead to various oxidized fragments. The mechanisms of their formation, intrinsic biological activities, and potential roles in human pathophysiology are poorly understood. Degradation methods have been used to obtain samples of bilirubin oxidation products for research. Here, we report a complementary, fully synthetic method of preparation. Our strategy leverages repeating substitution patterns in the parent tetracyclic pigment. Functionalized ready-to-couple γ-lactone, γ-lactam, and pyrrole monocyclic building blocks were designed and efficiently synthesized. Subsequent modular combinations, supported by metal-catalyzed borylation and cross-coupling chemistries, translated into the concise assembly of the structurally diverse bilirubin oxidation products (BOXes, propentdyopents, and biopyrrins). The discovery of a new photoisomer of biopyrrin A named lumipyrrin is reported. Synthetic bilirubin oxidation products made available in sufficient purity and quantity will support future in vitro and in vivo investigations.

Convergent Assembly of the Tricyclic Labdane Core Enables Synthesis of Diverse Forskolin-like Molecules.

We report a new synthetic strategy for the flexible preparation of forskolin-like molecules. The approach is different from the previously published works and employs a convergent assembly of the tricyclic labdane-type core from pre-functionalized cyclic building blocks. Stereoselective Michael addition enabled the fragment coupling with excellent control over three newly created contiguous stereocenters, all-carbon quaternary centers included. Silyl enol ether-promoted ring-opening metathesis paired with ring closure were the other key steps enabling concise assembly of the tricyclic core. Late-stage functionalization sequences transformed the tricyclic intermediates into a set of different forskolin-like molecules. The modular nature of the synthetic scheme described herein has the potential to become a general platform for the preparation of analogs of forskolin and other complex tricyclic labdanes.

Short Synthesis of (+)-Actinobolin: Simple Entry to Complex Small-Molecule Inhibitors of Protein Synthesis.

We report a concise synthesis of the naturally occurring protein synthesis inhibitor (+)-actinobolin (1). The densely functionalized and stereochemically complex molecular structure of 1 was assembled from (-)-quinic acid, L-threonine, and L-alanine as the principal components. Our route is based around a convergent strategy that features conjugate addition of an α-amino radical in the key fragment-coupling step. The dramatically simplified synthesis of (+)-actinobolin proceeding in 9 steps with 18 % overall yield has practical implications for analog preparation, as demonstrated herein.

Photochemistry of (Z)-Isovinylneoxanthobilirubic Acid Methyl Ester, a Bilirubin Dipyrrinone Subunit: Femtosecond Transient Absorption and Stimulated Raman Emission Spectroscopy.

Bilirubin (BR) is an essential metabolite formed by the catabolism of heme. Phototherapy with blue-green light can be applied to reduce high concentrations of BR in blood and is used especially in the neonatal period. In this work, we studied the photochemistry of (Z)-isovinylneoxanthobilirubic acid methyl ester, a dipyrrinone subunit of BR, by steady-state absorption, femtosecond transient absorption, and stimulated Raman spectroscopies. Both the (Z)- and (E)-configurational isomers of isovinylneoxanthobilirubic acid undergo wavelength-dependent and reversible photoisomerization. The isomerization from the excited singlet state is ultrafast (the lifetimes of (Z)- and (E)-isomers were found to be ∼0.9 and 0.1 ps, respectively), and its efficiencies increase with increased photon energy. In addition, we studied sensitized photooxidation of the dipyrrinone subunit by singlet oxygen that leads to the formation of propentdyopents. Biological activities of these compounds, namely, effects on the superoxide production, lipoperoxidation, and tricarboxylic acid cycle metabolism, were also studied. Finally, different photochemical and biological properties of this BR subunit and its structural analogue, (Z)-vinylneoxanthobilirubic acid methyl ester, studied before, are discussed.

Stereocontrolled Synthesis of Pseurotin A2.

We report synthesis of two diastereomeric structures previously proposed for the complex secondary metabolite pseurotin A2. Both structures were accessed from the same building blocks taking advantage of a stereodivergent nickel(II)-diamine-catalyzed 1,4-addition of a chiral 2-alkoxycarbonyl-3(2H)-furanone. Late-stage Csp-Csp3 cross-coupling of a highly functionalized bromoalkyne featured in the pseurotin A2 side-chain assembly. The work supports the 2016 stereochemical revision of pseurotin A2 and represents the first chemical synthesis of this natural product.

Conformational Control of the Photodynamics of a Bilirubin Dipyrrinone Subunit: Femtosecond Spectroscopy Combined with Nonadiabatic Simulations.

The photochemistry of bilirubin has been extensively studied due to its importance in the phototherapy of hyperbilirubinemia. In the present work, we investigated the ultrafast photodynamics of a bilirubin dipyrrinone subunit, vinylneoxanthobilirubic acid methyl ester. The photoisomerization and photocyclization reactions of its (E) and (Z) isomers were studied using femtosecond transient absorption spectroscopy and by multireference electronic structure theory, where the nonadiabatic dynamics was modeled with a Landau-Zener surface hopping technique. The following picture has emerged from the combined theoretical and experimental approach. Upon excitation, dipyrrinone undergoes a very fast vibrational relaxation, followed by an internal conversion on a picosecond time scale. The internal conversion leads either to photoisomerization or regeneration of the starting material. Further relaxation dynamics on the order of tens of picoseconds was observed in the ground state. The nonadiabatic simulations revealed a strong conformational control of the photodynamics. The ultrafast formation of a cyclic photochemical product from a less-populated conformer of the studied subunit was predicted by our calculations. We discuss the relevance of the present finding for the photochemistry of native bilirubin. The work has also pointed to the limits of semiclassical nonadiabatic simulations for simulating longer photochemical processes, probably due to the zero-point leakage issue.

Wavelength-Dependent Photochemistry and Biological Relevance of a Bilirubin Dipyrrinone Subunit.

Phototherapy is a standard treatment for severe neonatal jaundice to remove toxic bilirubin from the blood. Here, the wavelength-dependent photochemistry of vinylneoxanthobilirubic acid methyl ester, a simplified model of a bilirubin dipyrrinone subunit responsible for a lumirubin-like structural rearrangement, was thoroughly investigated by liquid chromatography and mass and absorption spectroscopies, with the application of a multivariate curve resolution analysis method supplemented with quantum chemical calculations. Irradiation of the model chromophore leads to reversible Z → E photoisomerization followed by reversible photocyclization to a seven-membered ring system (formed as a mixture of diastereomers). Both the isomerization processes are efficient (ΦZE ∼ ΦEZ ∼ 0.16) when irradiated in the wavelength range of 360-410 nm, whereas the E-isomer cyclization (Φc = 0.006-0.008) and cycloreversion (Φ-c = 0.002-0.004) reactions are significantly less efficient. The quantum yields of all processes were found to depend strongly on the wavelength of irradiation, especially when lower energy photons were used. Upon irradiation in the tail of the absorption bands (490 nm), both the isomers exhibit more efficient photoisomerization (ΦZE ∼ ΦEZ ∼ 0.30) and cyclization (Φc = ∼0.07). In addition, the isomeric bilirubin dipyrrinone subunits were found to possess important antioxidant activities while being substantially less toxic than bilirubin.

Stereocontrolled Synthesis of (-)-Bactobolin A.

A stereoselective synthesis of the ribosome-binding antitumor antibiotic (-)-bactobolin A is reported. The presented approach makes effective use of (-)-quinic acid as a chiral pool starting material and substrate stereocontrol to establish the five contiguous stereocenters of (-)-bactobolin A. The key steps of the synthesis include a stereoselective vinylogous aldol reaction to introduce the unusual dichloromethyl substituent, a completely diastereoselective rhodium(II)-catalyzed C-H amination reaction to set the configuration of the axial amine, and an intramolecular alkoxycarbonylation to build the bicyclic lactone framework. The developed synthetic route was used to prepare 90 mg of (-)-bactobolin A trifluoroacetate in 10% overall yield.

Natural pseurotins and analogs thereof inhibit activation of B-cells and differentiation into the plasma cells.

BACKGROUND: The frequency of allergic diseases is constantly rising. Dysregulated production of isotype E immunoglobulins is one of the key factors behind allergic reactions and its modulation is therefore an important target for pharmacological intervention. Natural products of the pseurotin family were reported to be inhibitors of IgE production in B-cells. Mechanistic details underlying these effects are however not well understood. PURPOSE: In the present study, we synthesized new analogs of natural pseurotins and extensively investigated their inhibitory effects on activation, proliferation and differentiation of B-cells, as well as on the production of IgE. STUDY DESIGN: Effects of two natural pseurotins (pseurotins A and D) and a collection of fully synthetic pseurotin analogs were studied on mouse B-cells stimulated by the combination of IL-4 and E. coli lipopolysaccharide. The IgE production was determined along with cell viability and cell proliferation. The phosphorylation of selected members of the STAT transcription factor family was subsequently investigated. Finally, the in vivo effect of pseurotin D on the ovalbumin-induced delayed type hypersensitivity response was tested in mice. RESULTS: We discovered that several fully synthetic pseurotin analogs were able to decrease the production of IgE in stimulated B-cells with potency comparable to that of pseurotins A and D. We found that the two natural pseurotins and the active synthetic analogs inhibited the phosphorylation of STAT3, STAT5 and STAT6 proteins in stimulated B-cells, resulting in the inhibition of B-cell proliferation and differentiation into the plasma cells. In vivo, pseurotin D decreased ovalbumin-induced foot pad edema. CONCLUSION: Our results advance the current mechanistic understanding of the pseurotin-induced inhibition of IgE production in B-cells by linking the effect to STAT signaling, and associated modulation of B-cell proliferation and differentiation. Together with our finding that structurally simpler pseurotin analogs were able to reproduce the effects of natural pseurotins, the presented work has implications for the future research on these secondary metabolites in the context of allergic diseases.

Enantioselective Conjugate Additions of 2-Alkoxycarbonyl-3(2 H)-furanones.

Enantioselective conjugate additions of in situ generated 2-alkoxycarbonyl-3(2 H)-furanones to three distinct types of π-electrophiles (terminal alkynones, α-bromo enones, and α-benzyl nitroalkenes) are reported. Catalysis by a nickel(II)-diamine complex provided alkynone-derived adducts with high enantioselectivity, preferentially as the Z-isomers, and completely suppressed the undesired O-alkylation pathway. A cupreidine-based catalyst enabled extension of the enantioselective conjugate additions to α-bromo enones and α-benzyl nitroalkenes. The densely functionalized adducts that result are useful precursors to synthetic analogs of spirocyclic natural products pseurotins.

A Concise Synthesis of Forskolin.

A 24-step synthesis of (±)-forskolin is presented, which delivered hundred milligram quantities of this complex diterpene in one pass. Transformations key to our approach include: a) a strategic allylic transposition, b) stepwise assembly of a sterically encumbered isoxazole ring, and c) citric acid-modified Upjohn dihydroxylation of a resilient tetrasubstituted olefin. The developed route has exciting potential for the preparation of new forskolin analogues inaccessible by semisynthesis.

Enantioselective Synthesis of Cephalimysins B and C.

The first synthesis of cephalimysins B and C is reported. The route features a Ni(II)-diamine-catalyzed enantioselective conjugate addition of a densely substituted 3(2H)-furanone and an efficient dihydroxylation-lactonization sequence as key steps in the assembly of the spirocyclic core. The fully synthetic strategy is amenable to analog preparation.

Biology-oriented synthesis of a withanolide-inspired compound collection reveals novel modulators of hedgehog signaling.

Biology-oriented synthesis employs the structural information encoded in complex natural products to guide the synthesis of compound collections enriched in bioactivity. The trans-hydrindane dehydro-δ-lactone motif defines the characteristic scaffold of the steroid-like withanolides, a plant-derived natural product class with a diverse pattern of bioactivity. A withanolide-inspired compound collection was synthesized by making use of three key intermediates that contain this characteristic framework derivatized with different reactive functional groups. Biological evaluation of the compound collection in cell-based assays that monitored biological signal-transduction processes revealed a novel class of Hedgehog signaling inhibitors that target the protein Smoothened.

Diastereoselective additions of allylmetal reagents to free and protected syn-α,ß-dihydroxyketones enable efficient synthetic routes to methyl trioxacarcinoside A.

Two routes to the 2,6-dideoxysugar methyl trioxacarcinoside A are described. Each was enabled by an apparent α-chelation-controlled addition of an allylmetal reagent to a ketone substrate containing a free α-hydroxyl group and a ß-hydroxyl substituent, either free or protected as the corresponding di-tert-butylmethyl silyl ether. Both routes provide practical access to gram quantities of trioxacarcinose A in a form suitable for glycosidic coupling reactions.

A multiply convergent platform for the synthesis of trioxacarcins.

Many first-line cancer drugs are natural products or are derived from them by chemical modification. The trioxacarcins are an emerging class of molecules of microbial origin with potent antiproliferative effects, which may derive from their ability to covalently modify duplex DNA. All trioxacarcins appear to be derivatives of a nonglycosylated natural product known as DC-45-A2. To explore the potential of the trioxacarcins for the development of small-molecule drugs and probes, we have designed a synthetic strategy toward the trioxacarcin scaffold that enables access to both the natural trioxacarcins and nonnatural structural variants. Here, we report a synthetic route to DC-45-A2 from a differentially protected precursor, which in turn is assembled in just six steps from three components of similar structural complexity. The brevity of the sequence arises from strict adherence to a plan in which strategic bond-pair constructions are staged at or near the end of the synthetic route.

Anti-selective epoxidation of methyl alpha-methylene-beta-tert-butyldimethylsilyloxycarboxylate esters. Evidence for stereospecific oxygen atom transfer in a nucleophilic epoxidation process.

Methyl alpha-methylene-beta-tert-butyldimethylsilyloxycarboxylate esters are found to undergo diastereoselective epoxidation in the presence of potassium tert-butoxide-tert-butyl hydroperoxide to form anti products. In an effort to better understand mechanistic details of the transformation and the basis of diastereoselectivities observed, we studied the epoxidation of substrates with alpha-methylene groups containing (trans) deuterium labels and discovered that oxygen-atom transfer proceeds with > or = 95% stereospecificity in all cases examined. These and other experiments suggest that the mechanism of epoxidation is not distinguishable from a concerted process.

Reactivity of cyclic sulfamidates towards sulfur-stabilised enolates. Stereocontrolled synthesis of functionalised lactams.

A structurally representative series of 1,2- and 1,3-cyclic sulfamidates react with enolates derived from methyl alpha-phenylthioacetate 9b to give 5- and 6-substituted alpha-phenylthio lactams 20-24. These products provide, via the corresponding sulfoxides, an entry to alpha,beta-unsaturated lactams e.g. 12, 27, 29 and their alpha-phenylthio analogues e.g. 26 and 30. With the enantiomerically pure 1,2-cyclic sulfamidates 10, 15 and 17, these reactions all proceed with no detectable loss of stereochemical integrity.

Cyclic sulfamidates as vehicles for the synthesis of substituted lactams.

[reaction: see text] A structurally diverse series of mono- and disubstituted 1,2- and 1,3-cyclic sulfamidates react with stabilized enolates, including malonate and phosphonoacetate variants, to provide, after lactamization, substituted and alpha-functionalized pyrrolidinone and piperidinone derivatives.