Toblerols: Cyclopropanol‐Containing Polyketide Modulators of Antibiosis in Methylobacteria

Trans‐AT polyketide synthases (PKSs) are a family of biosynthetically versatile modular type I PKSs that generate bioactive polyketides of impressive structural diversity. In this study, we detected, in the genome of several bacteria a cryptic, architecturally unusual trans‐AT PKS gene cluster which eluded automated PKS prediction. Genomic mining of one of these strains, the model methylotroph Methylobacterium extorquens AM1, revealed unique epoxide‐ and cyclopropanol‐containing polyketides named toblerols. Relative and absolute stereochemistry were determined by NMR experiments, chemical derivatization, and the comparison of CD data between the derivatized natural product and a synthesized model compound. Biosynthetic data suggest that the cyclopropanol moiety is generated by carbon–carbon shortening of a more extended precursor. Surprisingly, a knock‐out strain impaired in polyketide production showed strong inhibitory activity against other methylobacteria in contrast to the wild‐type producer. The activity was inhibited by complementation with toblerols, thus suggesting that these compounds modulate an as‐yet unknown methylobacterial antibiotic.     

Enantioselective Catalytic Aldehyde α‐Alkylation/Semipinacol Rearrangement: Construction of α‐Quaternary‐δ‐Carbonyl Cycloketones and Total Synthesis of (+)‐Cerapicol

An enantioselective aldehyde α‐alkylation/semipinacol rearrangement was achieved through organo‐SOMO catalysis. The catalytically generated enamine radical cation serves as a carbon radical electrophile that can stereoselectively add to the alkene of an allylic alcohol and initiate ensuing ring‐expansion of cyclopropanol or cyclobutanol. This tandem reaction enables the production of a wide range of nonracemic functionalizable α‐quaternary‐δ‐carbonyl cycloketones in high yields and excellent enantioselectivity from simple aldehydes and allylic alcohols. As a key step, the intramolecular reaction was also successfully applied in the asymmetric total synthesis of (+)‐cerapicol.     

Cyclopropanol Warhead in Malleicyprol Confers Virulence of Human‐ and Animal‐Pathogenic Burkholderia Species

Burkholderia species such as B. mallei and B. pseudomallei are bacterial pathogens causing fatal infections in humans and animals (glanders and melioidosis), yet knowledge on their virulence factors is limited. While pathogenic effects have been linked to a highly conserved gene locus (bur/mal) in the B. mallei group, the metabolite associated to the encoded polyketide synthase, burkholderic acid (syn. malleilactone), could not explain the observed phenotypes. By metabolic profiling and molecular network analyses of the model organism B. thailandensis, the primary products of the cryptic pathway were identified as unusual cyclopropanol‐substituted polyketides. First, sulfomalleicyprols were identified as inactive precursors of burkholderic acid. Furthermore, a highly reactive upstream metabolite, malleicyprol, was discovered and obtained in two stabilized forms. Cell‐based assays and a nematode infection model showed that the rare natural product confers cytotoxicity and virulence.     

Copper‐Catalyzed Aerobic Oxidative Cyclization Cascade to Construct Bridged Skeletons: Total Synthesis of (−)‐Suaveoline

Based on the discovery of copper‐catalyzed cyclopropanol ring‐opening addition to iminium ions, an unprecedented catalytic aerobic C?H oxidation/cyclopropanol cyclization cascade using CuCl₂ as the multifunctional catalyst and air as the oxidant was developed to construct the azabicyclo[3.3.1]nonane skeleton, which is widespread in natural products and medicines. Using this method, concise asymmetric total synthesis of the indole alkaloid (?)‐suaveoline was achieved. This study not only provides an efficient, low‐cost, and environmentally benign method for constructing such bridged frameworks, but also enriches the realm of cyclopropanol chemistry and C?H functionalization.     

Oxidative Cyclopropanol Opening by Cob(III)alamin

Starting from the cyclopropanol 2 , the isomeric cyclopropanol 4 and the β, γ-unsasturated aldehydes 7 and 8 have been produced by a cobalamin-dependant transformation. In traces, the two acetoxycyclopropanes 3 and 6 , the saturated aldehydes 5 and 11 and the β,γ-unsaturated aldehyde 9 could be detected (cf. Structural Formulae and Table). Starting from 4 the same products in a rather similar distribution were obtained. The isomerization 2?4 as well as the transformations leading to 7,8 , and 9 are shown to be mediated by cob(III)alamin (1(III)) . The results are explained on the basis of rearranging Co-complexes. The migrations might be driven by the electrophilic nature of the central Co(d⁶)-atom.     

Temporal evolution of DMS and DMSP in Antarctic Coastal Sea water

The temporal evolution of concentrations of dimethylsulphide (DMS), its precursor dimethylsulphoniopropionate (DMSP) and chlorophyll a is surveyed weekly in the water column and in a landfast ice core at a coastal station of Gerlache Inlet (Terra Nova Bay, Antarctica) from 27 November 2000 to 14 February 2001. The DMS and DMSP profile concentrations in the water column are similar and show a clear temporal trend, with minimum values (<0.7?nM) at all depths occurring on 27 November 2000 and maximum values (4.8 × 10²?nM for DMS and 1.8 × 10²?nM for DMSP) in surface water on 27 December 2000 for DMS and on 19 December 2000 for DMSP. When the sea-ice cover is present, the temporal evolution of DMSP closely follows that of chlorophyll a in the water column, supporting the idea that DMSP, and therefore DMS, has a phytoplanktonic origin. However, when the ice cover breaks up during the late austral summer, a second phytoplankton bloom occurs, while the DMSP concentration in the sea-water column remains very low. In the ice core, the results show higher concentrations of DMSP than those of the underlying sea water, highlighting the important role of sea ice in the sulphur cycle of the Antarctic ecosystem.     

Dissecting Bottromycin Biosynthesis Using Comparative Untargeted Metabolomics

Bottromycin A₂ is a structurally unique ribosomally synthesized and post‐translationally modified peptide (RiPP) that possesses potent antibacterial activity towards multidrug‐resistant bacteria. The structural novelty of bottromycin stems from its unprecedented macrocyclic amidine and rare β‐methylated amino acid residues. The N‐terminus of a precursor peptide (BtmD) is converted into bottromycin A₂ by tailoring enzymes encoded in the btm gene cluster. However, little was known about key transformations in this pathway, including the unprecedented macrocyclization. To understand the pathway in detail, an untargeted metabolomic approach that harnesses mass spectral networking was used to assess the metabolomes of a series of pathway mutants. This analysis has yielded key information on the function of a variety of previously uncharacterized biosynthetic enzymes, including a YcaO domain protein and a partner protein that together catalyze the macrocyclization.     

Enantioselective Catalytic Aldehyde α-Alkylation/Semipinacol Rearrangement: Construction of α-Quaternary-δ-Carbonyl Cycloketones and Total Synthesis of (+)-Cerapicol

An enantioselective aldehyde α-alkylation/semipinacol rearrangement was achieved through organo-SOMO catalysis. The catalytically generated enamine radical cation serves as a carbon radical electrophile that can stereoselectively add to the alkene of an allylic alcohol and initiate ensuing ring-expansion of cyclopropanol or cyclobutanol. This tandem reaction enables the production of a wide range of nonracemic functionalizable α-quaternary-δ-carbonyl cycloketones in high yields and excellent enantioselectivity from simple aldehydes and allylic alcohols. As a key step, the intramolecular reaction was also successfully applied in the asymmetric total synthesis of (+)-cerapicol.     

A Homo-Mannich Reaction Strategy Enables Collective Access to Ibophyllidine,Aspidosperma, Kopsia,and Melodinus Alkaloids

We report here a homo-Mannich reaction of cyclopropanol with an iminium ion, generated by an asymmetric allylic dearomatization of indole, to construct a tricyclic hydrocarbazole core, which is shared by a variety of monoterpenoid indole alkaloids across families. Through this approach, an all-carbon quaternary stereogenic center as well as an allyl and a ketone group were installed. Using this functionalized hydrocarbazole as the structural platform, D ring and E rings of different sizes (i.e., five-, six-, and seven-membered) were successively or simultaneously assembled, leading to a collective asymmetric synthesis of seven alkaloids belonging to the ibophyllidine, Aspidosperma, Kopsia, and Melodinus alkaloid families.     

Reversible Product Release and Recapture by a Fungal Polyketide Synthase Using a Carnitine Acyltransferase Domain

Fungal polyketides have significant biological activities, yet the biosynthesis by highly reducing polyketide synthases (HRPKSs) remains enigmatic. An uncharacterized group of HRPKSs was found to contain a C‐terminal domain with significant homology to carnitine O ‐acyltransferase (cAT). Characterization of one such HRPKS (Tv6‐931) from Trichoderma virens showed that the cAT domain is capable of esterifying the polyketide product with polyalcohol nucleophiles. This process is readily reversible, as confirmed through the holo ACP‐dependent transesterification of the released product. The methyltransferase (MT) domain of Tv6‐931 can perform two consecutive α‐methylation steps on the last β‐keto intermediate to yield an α,α‐gem ‐dimethyl product, a new programing feature among HRPKSs. Recapturing of the released product by cAT domain is suggested to facilitate complete gem ‐dimethylation by the MT.     

Biosynthesis of the N–N‐Bond‐Containing Compound l‐Alanosine

The formation of a N?N bond is a unique biochemical transformation, and nature employs diverse biosynthetic strategies to activate nitrogen for bond formation. Among molecules that contain a N?N bond, biosynthetic routes to diazeniumdiolates remain enigmatic. We here report the biosynthetic pathway for the diazeniumdiolate‐containing amino acid l ‐alanosine. Our work reveals that the two nitrogen atoms in the diazeniumdiolate of l ‐alanosine arise from glutamic acid and aspartic acid, and we clarify the early steps of the biosynthetic pathway by using both in vitro and in vivo approaches. Our work demonstrates a peptidyl‐carrier‐protein‐based mechanism for activation of the precursor l ‐diaminopropionate, and we also show that nitric oxide can participate in non‐enzymatic diazeniumdiolate formation. Furthermore, we demonstrate that the gene alnA, which encodes a fusion protein with an N‐terminal cupin domain and a C‐terminal AraC‐like DNA‐binding domain, is required for alanosine biosynthesis.     

A convenient new method to construct 1-alkynyl cyclopropanol and its synthetic application to prepare trisubstituted dienones

A new synthetic route was developed via the nucleophilic addition of lithium alkynylide to 1-arylsulfonyl cyclopropanol 1 to afford 1-alkynyl cyclopropanol, which then reacted with aryl iodide to construct trisubstituted cross-conjugated dienones through a palladium-catalyzed process, where the key steps included the regioselective carbopalladation of arylpalladium(II) intermediate across the triple bond of 1-alkynyl cyclopropanol and the ring opening of the cyclopropyl group.     

Photochemical reactions. 140th Communication. Acid-catalyzed rearrangement and electron transfer photooxygenation of cyclopropanols and their silyl ethers

On acid-catalyzed hydrolysis, the tricyclic compounds 2 and 10 , incorporating cyclopropyl-silyl-ether moieties undergo rearrangement to the cis-decalones 3 and 7 , respectively. Hydrolysis of 2 and 10 in the presence of oxygen leads additionally to the formation of the 1,2-dioxolan-3-ols 9 and 13 , respectively, which involves an electron-transfer oxygenation process as could be demonstrated by photooxygenation of the silyl ether 10 and the cyclopropanol 14 in the presence of 9,10-dicyanoanthracene. The configurations of 3 and 9 were assigned by X-ray analysis of the latter compound as well as of the p-nitrobenzoate 8 of 3 .     

Morphology and Dynamics of Phase‐Separating Fluids with Viscosity Asymmetry

Summary: The effects of viscosity asymmetry of the components on morphology and dynamics of phase‐separating AB fluids are investigated numerically based on a modified Model H. For critical mixtures, in the early stage of phase separation the co‐continuous morphology with droplets of A in B and B in A is observed. In the late stage of phase separation, the viscosity asymmetry leads to morphological change from co‐continuous structure to completely dispersed structure where the less viscous component forms droplet. The pathway of this transformation is accompanied by the breakdown of balance of volume fraction between droplets with different viscosity. Domain growth is characterized by a crossover from a faster growth at intermediate time under the influence of hydrodynamics to Lifshitz–Slyozov behavior at late times. For off‐critical mixture, viscosity asymmetry only plays an important role for domain growth in the intermediate stage of phase separation and the domain growth depends on whether the more viscous phase is dispersed or continuous, and the late stage of domain growth follows Lifshitz–Slyozov power law independent of which phase is dispersed.

Result for the evolution of phase‐separating domains for critical fluid mixtures = 0.5 for t = 1 500 with viscosity asymmetry: η_A = 0.8, η_B = 0.2. A‐rich regions and B‐rich regions are represented by white and black, respectively.     

Competitive homolytic and heterolytic decomposition pathways of gas‐phase negative ions generated from aminobenzoate esters

An alkyl‐radical loss and an alkene loss are two competitive fragmentation pathways that deprotonated aminobenzoate esters undergo upon activation under mass spectrometric conditions. For the meta and para isomers, the alkyl‐radical loss by a homolytic cleavage of the alkyl‐oxygen bond of the ester moiety is the predominant fragmentation pathway, while the contribution from the alkene elimination by a heterolytic pathway is less significant. In contrast, owing to a pronounced charge‐mediated ortho effect, the alkene loss becomes the predominant pathway for the ortho isomers of ethyl and higher esters. Results from isotope‐labeled compounds confirmed that the alkene loss proceeds by a specific γ‐hydrogen transfer mechanism that resembles the McLafferty rearrangement for radical cations. Even for the para compounds, if the alkoxide moiety bears structural motifs required for the elimination of a more stable alkene molecule, the heterolytic pathway becomes the predominant pathway. For example, in the spectrum of deprotonated 2‐phenylethyl 4‐aminobenzoate, m/z 136 peak is the base peak because the alkene eliminated is styrene. Owing to the fact that all deprotonated aminobenzoate esters, irrespective of the size of the alkoxy group, upon activation fragment to form an m/z 135 ion, aminobenzoate esters in mixtures can be quantified by precursor ion discovery mass spectrometric experiments. Copyright © 2016 John Wiley & Sons, Ltd.     

The Energetics of a Three‐State Protein Folding System Probed by High‐Pressure Relaxation Dispersion NMR Spectroscopy

The energetic and volumetric properties of a three‐state protein folding system, comprising a metastable triple mutant of the Fyn SH3 domain, have been investigated using pressure‐dependent ¹⁵N‐relaxation dispersion NMR from 1 to 2500 bar. Changes in partial molar volumes (ΔV) and isothermal compressibilities (Δκ_T) between all the states along the folding pathway have been determined to reasonable accuracy. The partial volume and isothermal compressibility of the folded state are 100 mL mol^?1 and 40 μL mol^?1 bar^?1, respectively, higher than those of the unfolded ensemble. Of particular interest are the findings related to the energetic and volumetric properties of the on‐pathway folding intermediate. While the latter is energetically close to the unfolded state, its volumetric properties are similar to those of the folded protein. The compressibility of the intermediate is larger than that of the folded state reflecting the less rigid nature of the former relative to the latter.     

A Manifold Three‐Step Synthetic Route to Polycyclic Annulated Hydantoins via Cyclic Imines

A new three‐step synthetic pathway to generate polycyclic annulated hydantoins via rarely investigated heterocyclic imines is described. This procedure includes a one‐pot reaction forming imines as precursor structures (e.g., Asinger reaction), followed by an Ugi reaction to build up a bisamide structure that allows a ring‐closing reaction to the targeted hydantoins via substitution. This pathway leads to a multiplicity of substances with a potential pharmacological activity.     

Structure‐Based Design and Synthesis of the First Weak Non‐Phosphate Inhibitors for IspF,an Enzyme in the Non‐Mevalonate Pathway of Isoprenoid Biosynthesis

In this paper, we describe the structure‐based design, synthesis, and biological evaluation of cytosine derivatives and analogues that inhibit IspF, an enzyme in the non‐mevalonate pathway of isoprenoid biosynthesis. This pathway is responsible for the biosynthesis of the C₅ precursors to isoprenoids, isopentenyl diphosphate (IPP, 1 ) and dimethylallyl diphosphate (DMAPP, 2 ; Scheme 1). The non‐mevalonate pathway is the sole source for 1 and 2 in the protozoan Plasmodium parasites. Since mammals exclusively utilize the alternative mevalonate pathway, the enzymes of the non‐mevalonate pathway have been identified as attractive new drug targets in the fight against malaria. Based on computer modeling (cf. Figs. 2 and 3), new cytosine derivatives and analogues (Fig. 1) were selected as potential drug‐like inhibitors of IspF protein, and synthesized (Schemes 2–5). Determination of the enzyme activity by ¹³C‐NMR spectroscopy in the presence of the new ligands showed inhibitory activities for some of the prepared cytosine and pyridine‐2,5‐diamine derivatives in the upper micromolar range (IC₅₀ values; Table). The data suggest that it is possible to inhibit IspF protein without binding to the polar diphosphate binding site and the side chain of Asp56′, which interacts with the ribose moiety of the substrate and substrate analogues. Furthermore, a new spacious sub‐pocket was discovered which accommodates aromatic spacers between cytosine derivatives or analogues (binding to ‘Pocket III’) and rings that occupy the flexible hydrophobic region of ‘Pocket II’. The proposed binding mode remains to be further validated by X‐ray crystallography.