Ratio Of Regioisomers Research Articles

Catalytic Regioselective Olefin Hydroarylation(alkenylation) by Sequential Carbonickelation-Hydride Transfer.

Alkene hydrocarbofunctionalization represents one of the most important classes of chemical transformations, but related branched-selective examples with unactivated olefins are scarce. Here, we report that catalytic amounts of a dimeric Ni(I) complex and an exogenous alkoxide base promote Markovnikov-selective hydroarylation(alkenylation) of unactivated and activated olefins using organo bromides or triflates derived from widely available phenols and ketones. Products bearing aryl- and alkenyl-substituted tertiary and quaternary centers could be isolated in up to 95% yield and >99:1 regioisomeric ratios. Contrary to previous dual-catalytic methods that rely on metal-hydride atom transfer (MHAT) to the olefin prior to carbofunctionalization with a cocatalyst, our mechanistic evidence points toward a nonradical reaction pathway that begins with site-selective carbonickelation across the C═C bond followed by hydride transfer using alkoxide as the hydride source. Utility of the single-catalyst protocol is highlighted through the synthesis of medicinally relevant scaffolds.

Synthesis, spectroscopy, and crystallography of mononuclear, five-coordinate aluminum complexes that act as cyclic ester polymerization initiators

Three ketoimines with pendant quinolyl moieties and a series of five-coordinate ketoiminate aluminum complexes were prepared and characterized spectroscopically and crystallographically. The synthesis of the ketoimines completed a series of ten regioisomers for which the product ratio varied between the product resulting from quinoline addition adjacent to the alkyl/aryl group or adjacent to the trifluoromethyl substituent. Quantitation of the ketoimine regioisomer ratios with 19F NMR demonstrated the role of steric encumbrance in the nucleophilic attack mechanistic step of the Schiff base condensation reaction and was further supported by linear free energy relationships with the Charton steric parameter and DFT calculations. Eight five-coordinate aluminum complexes were prepared in 57–92% yield from ketoimines and tris(2,6-dimethylphenoxide) aluminum dimer and characterized with 1H, 13C, 19F, and 27Al nuclear magnetic resonance spectroscopy, UV–visible absorbance spectroscopy, elemental analysis, and single-crystal X-ray crystallography. These mononuclear complexes differ from previously reported aluminum ion-pair complexes due to the symmetric cleavage of tris-dimethylphenoxide aluminum dimer by THF and were shown to have distorted trigonal bipyramidal coordination geometries (τ5 = 0.73–0.89). In a preliminary study, the aluminum complexes were demonstrated to be effective initiators for ring-opening polymerization of ε-caprolactone (εCL) to poly-caprolactone (PCL) and l-lactide (LA) to poly-lactic acid (PLA), reaching 77–99% conversion in 2 h for εCL and 10 h for LA at 100 °C.

Chemoselective Union of Olefins, Organohalides, and Redox-Active Esters Enables Regioselective Alkene Dialkylation.

Multicomponent catalytic processes that can generate multiple C(sp3)-C(sp3) bonds in a single step under mild conditions, particularly those that employ inexpensive catalysts and substrates, are highly sought-after in chemistry research for complex molecule synthesis. Here, we disclose an efficient Ni-catalyzed reductive protocol that chemoselectively merges alkenyl amides with two different aliphatic electrophiles. Starting materials are readily accessible from stable and abundant feedstock, and products are furnished in up to >98:2 regioisomeric ratios. The present strategy eliminates the use of sensitive organometallic reagents, tolerates a wide array of complex functionalities, and enables regiodivergent addition of two primary alkyl groups bearing similar electronic and steric attributes across aliphatic C═C bonds with exquisite control of site selectivity. Utility is underscored by the concise synthesis of bioactive compounds and postreaction functionalizations leading to structurally diverse scaffolds. DFT studies revealed that the regiochemical outcome originates from the orthogonal reactivity and chemoselectivity profiles of in situ generated organonickel species.

Separation and identification of diacylglycerols containing branched chain fatty acids by liquid chromatography-mass spectrometry

A combination of two chromatographic and two enzymatic methods was used for the analysis of molecular species of lipids from Gram-positive bacteria of the genus Kocuria. Gram-positive bacteria contain a majority of branched fatty acids (FAs), especially iso- and/or anteiso-FAs. Two strains K. rhizophila were cultivated at three different temperatures (20, 28, and 37°C) and the majority phospholipid, i.e., the mixture of molecular species of phosphatidylglycerols (PGs) was separated by means of hydrophilic interaction liquid chromatography (HILIC). After enzymatic hydrolysis of PGs by phospholipase C and derivatization of the free OH group, the sn-1,2-diacyl-3-acetyl triacylglycerols (AcTAGs) were separated by reversed phase HPLC. Molecular species such as i-15:0/i-15:0/2:0, ai-15:0/ai-15:0/2:0, and 15:0/15:0/2:0 (straight chains) were identified by liquid chromatography-positive electrospray ionization mass spectrometry. The tandem mass spectra of both standards and natural compounds containing iso, anteiso and straight chain FAs with the same carbons were identical. Therefore, for identification of the ratio of two regioisomers, i.e. i-15:0/ai-15:0/2:0 vs. ai-15:0/i-15:0/2:0, they were cleavage by pancreatic lipase. The mixture of free fatty acids (FFAs) and 2-monoacylglycerols (2-MAGs) was obtained. After their separation by TLC and esterification and/or transesterification, the fatty acid methyl esters were quantified by GC-MS and thus the ratio of regioisomers was determined.It has been shown that the ratio of PG (containing as majority i-15: 0 / i-15: 0, i-15: 0 / ai-15: 0 and / or ai-15: 0 / i-15: 0 and ai-15: 0 / ai-15: 0 molecular species) significantly affected the membrane flow of bacterial cells cultured at different temperatures.

Lipidomic analysis of diatoms cultivated with silica nanoparticles

Polar lipids from the diatoms Diadesmis gallica and Navicula atomus were separated and their structures were determined using high resolution tandem MS HILIC-LC/ESI. This method allowed us to identify 34 classes of lipids, each containing dozens of molecular species, including regioisomers. The largest differences were found in two sulfur-containing lipids, sulfoquinovosyldiacylglycerol and phosphatidylsulfocholine caused probably by the remodeling of lipid species. These diatoms have been found to use several mechanisms to resolve growth in extreme environments, i.e. silica starvation. The presence of insoluble nano-SiO2 leads to the replacement of cellular phospholipids with sulfolipids. Regioisomer ratios also vary depending on the concentration of nano-SiO2 in the culture medium, i.e. the biosynthesis of polar lipids via the prokaryotic (plastidial) and/or eukaryotic (explastidial) pathways. Complex analyses of polar lipids using high resolution HILIC-LC/ESI-tandem, as used for diatoms, can also be used for other photosynthetic microorganisms.

Selective Synthesis of Some Aminosugars via Catalytic Aminohydroxylation of Protected 2,3-Unsaturated d-Gluco- and d-Galacto-2-hexenopyranosides.

The aminohydroxylation of methyl 4,6-di-O-(tert-butyldimethylsilyl)-2,3-unsaturated α-d-glucopyranoside proceeds in the presence of chloramine-T, OsO4 (4 mol %), (DHQ)2PHAL (5 mol %), and triethylbenzylammonium chloride (TEBAC) in both a stereoselective and a regioselective manner to produce protected methyl α-d-mannosamide as the sole product. In contrast, the reaction of methyl 2,3-unsaturated β-d-galactopyranoside under the same conditions produced a mixture of regioisomers, although the stereochemistry was perfectly controlled. The regioisomeric ratio was dependent on the nature of the protecting group and the ligand used.

Modular Synthesis of Diarylalkanes by Nickel-Catalyzed 1,1-Diarylation of Unactivated Terminal Alkenes

A nickel-catalyzed 1,1-diarylation of electronically unbiased alkenes has been developed, providing straightforward access to diarylalkanes from readily available materials. Importantly, both the e...

Synthesis of poly(bicyclohexyldimethylene terephthalate): effect of regioisomer ratios on physical properties

A series of novel polyesters was synthesized from dimethyl terephthalate (DMT) and various ratios of two regioisomers of bicyclohexyldimethanol (BCD), 4,4′-BCD and 3,4′-BCD.

Amberlyst‐15‐catalyzed Procedure for the Synthesis of Novel 2,4‐Dihydroxybenzoyl‐1,2,3‐triazoles and Molecular Modelling Studies for Hsp‐90 Inhibition

AbstractNovel 4‐and 5‐(2,4‐dihydroxybenzoyl) triazoles were prepared from o‐alkynoylresorcinols with by the cycloaddition reaction performed with metal free one step protocol using Bronsted acid Amberlyst‐15 as catalyst for the first time. This procedure enables the synthesis of triazoles bearing o,p‐dihydroxybenzoyl group without the need to protection of hydroxyl groups. Products were obtained in good to excellent yields and with the moderate regioisomeric ratios. In addition, Hsp90 inhibition activities of synthesized compounds were examined by molecular modelling studies and promising results were obtained.

Transition-Metal (Pd, Ni, Mn)-Catalyzed C-C Bond Constructions Involving Unactivated Alkyl Halides and Fundamental Synthetic Building Blocks.

The catalytic construction of C-C bonds between organohalide or pseudohalide electrophiles and fundamental building blocks such as alkenes, arenes, or CO are widely utilized metal-catalyzed processes. The use of simple, widely available unactivated alkyl halides in these catalytic transformations has significantly lagged behind the use of aryl or vinyl electrophiles. This difference is primarily due to the relative difficulty of activating alkyl halides with transition metals under mild conditions. This Account details our group's work toward developing a general catalytic manifold for the construction of C-C bonds using unactivated alkyl halides and a range of simple chemical feedstocks. Critical to the strategy was the implementation of new modes of hybrid organometallic-radical reactivity in catalysis. Generation of carbon-centered radicals from alkyl halides using transition metals offers a solution to challenges associated with the application of alkyl electrophiles in classical two-electron reaction modes. A major focus of this work was the development of general palladium-catalyzed carbocyclizations and intermolecular cross-couplings of unactivated alkyl halides (alkyl-Mizoroki-Heck-type reactions). Initial studies centered on the use of alkyl iodides in these processes, but subsequent studies determined that the use of an electron-rich ferrocenyl bisphosphine (dtbpf) enables the palladium-catalyzed carbocyclizations of unactivated alkyl bromides. Mechanistic studies of these reactions revealed interesting details regarding a difference in mechanism between reactions of alkyl iodides and alkyl bromides in carbocyclizations. These studies were consistent with alkyl bromides reacting via an autotandem catalytic process involving atom-transfer radical cyclization (ATRC) followed by catalytic dehydrohalogenation. Reactions of alkyl iodides, on the other hand, involved metal-initiated radical chain pathways. Recent studies have expanded the scope of alkyl-Mizoroki-Heck-type reactions to the use of a first-row transition metal. Inexpensive nickel precatalysts, in combination with the bisphosphine ligand Xantphos, efficiently activate alkyl bromides for both intra- and intermolecular C-C bond-forming reactions. The reaction scope is similar to the palladium-catalyzed system, but in addition, alkene regioisomeric ratios are dramatically improved over those in reactions with palladium, solving one of the drawbacks of our previous work. Initial mechanistic studies were consistent with a hybrid organometallic-radical mechanism for the nickel-catalyzed reactions. The novel reactivity of the palladium catalysts in the alkyl-Mizoroki-Heck-type reactions have also paved the way for the development of other C-C bond-forming processes of unactivated alkyl halides, including aromatic C-H alkylations as well as low-pressure alkoxycarbonylations. Related hybrid organometallic-radical reactivity of manganese has led to an alkene dicarbofunctionalization using alkyl iodides.

A method for the quantitative determination of glycerophospholipid regioisomers by UPLC-ESI-MS/MS

Diacyl glycerophospholipids (GPs) belong to the most abundant lipid species in living organisms and consist of a glycerol backbone with fatty acyl groups in sn-1 and sn-2 and a polar head group in the sn-3 position. Regioisomeric mixed diacyl GPs have the same fatty acyl composition but differ in their allocation to sn-1 or sn-2 of the glycerol unit. In-depth analysis of regioisomeric mixed diacyl GP species composed of fatty acyl moieties that are similar in length and degree of saturation typically requires either chemical derivatization or sophisticated analytical instrumentation, since these types of regioisomers are not well resolved under standard ultra-performance liquid chromatography (UPLC) conditions. Here, we introduce a simple and fast method for diacyl GP regioisomer analysis employing UPLC tandem mass spectrometry (MS/MS). This GP regioisomer analysis is based both on minor chromatographic retention time shifts and on major differences in relative abundances of the two fatty acyl anion fragments observed in MS/MS. To monitor these differences with optimal precision, MS/MS spectra are recorded continuously over the UPLC elution profile of the lipid species of interest. Quantification of relative abundances of the regioisomers was performed by algorithms that we have developed for this purpose. The method was applied to commercially available mixed diacyl GP standards and to total lipid extracts of Escherichia coli (E. coli) and bovine liver. To validate our results, we determined regioisomeric ratios of phosphatidylcholine (PC) standards using phospholipase A2-specific release of fatty acids from the sn-2 position of the glycerol backbone. Our results show that most analyzed mixed diacyl GPs of biological origin exhibit significantly higher regioisomeric purity than synthetic lipid standards. In summary, this method can be implemented in routine LC-MS/MS-based lipidomics workflows without the necessity for additional chemical additives, derivatizations, or instrumentation.

Evaluation of Estrogenic Activity of Novel Bisphenol A Alternatives, Four Bioinspired Bisguaiacol F Specimens, by in Vitro Assays.

Alternatives to bisphenol A (BPA), such as lignin-inspired bisguaiacol F (BGF), are of interest for food contact materials due to increasing evidence of estrogenic activity (EA) and exposure-correlated harmful effects of BPA and its analogues. BGF has similar thermal and mechanical properties to BPA, but contains additional methoxy substituents that may significantly reduce its endocrine disruption potential. In this study, the EA of four BGF samples with different regioisomer ratios was quantified relative to 17β-estradiol at ten concentrations by using two in vitro assays: MCF-7 cell proliferation and VM7Luc4E2 transactivation (TA). The results suggest BGF mixtures with higher molar ratios of p, p'-BGF and o, p'-BGF regioisomers exhibited lower EA than BPA, while BGF samples containing higher molar ratios of m, p'-BGF had no detectable EA over a wide range of test concentrations. These findings suggest the potential of BGF as a viable alternative to BPA for use in more environmentally friendly materials.

Enantiomeric separation of triacylglycerols containing very long chain fatty acids

Enantiomers of triacylglycerols (TAGs) containing any combination of very long chain fatty acids (VLCFAs) and/or very long chain polyunsaturated fatty acids (VLCPUFAs) with diolein, dilinolein and didocosahexaenoin were synthesized. Gradient non-aqueous reversed-phase high-performance liquid chromatography/high resolution atmospheric pressure chemical ionization-tandem mass spectrometry (NARP-HPLC/HRMS2-APCI) and chiral liquid chromatography were used for the separation and identification of molecular species of these TAGs. Further, NARP-LC and chiral LC were used to separate natural mixtures of TAGs obtained from four natural sources, i.e. ximenia oil (Ximenia americana), green alga (Botryococcus braunii), breweŕs yeast (Saccharomyces pastorianus) and a dinoflagellate (Amphidinium carterae). The ratio of regioisomers and enantiomers in individual samples was determined and a hypothesis has been confirmed on the biosynthetic pathway of natural TAGs, which is based on the preferential representation of VLCFAs and VLCPUFAs in the sn-1 position of the glycerol backbone.

Simple approach for the regioselective synthesis of a bis (β-aminoalcohol) derived from polyoxyethylene: first report of fast ring-opening of polyoxyethylene diglycidyl ethers with sodium amide

An efficient oxirane ring opening of polyoxyethylene diglycidyl ethers with sodium amide has been achieved for the first time under catalyst-free conditions. This reaction is highly regioselective and affords the corresponding bis (β-aminoalcohols) polyoxyethylene in good yields as a mixture of regioisomers. Various other advantages are associated with this novel protocol, including short reaction time, no need for catalysts, and a simple workup procedure. Here, the wide scope and utility of this method is demonstrated, and the stereochemistry is discussed. The regioisomeric ratio is determined from binomial calculation based on 1H NMR data and is confirmed by GC–MS.

Arachidonic Acid Metabolism by Human Cardiovascular CYP2J2 Is Modulated by Doxorubicin.

Doxorubicin (DOX) is a chemotherapeutic that is used in the treatment of a wide variety of cancers. However, it causes cardiotoxicity partly because of the formation of reactive oxygen species. CYP2J2 is a human cytochrome P450 that is strongly expressed in cardiomyocytes. It converts arachidonic acid (AA) into four different regioisomers of epoxyeicosatrienoic acids (EETs). Using kinetic analyses, we show that AA metabolism by CYP2J2 is modulated by DOX. We show that cytochrome P450 reductase, the redox partner of CYP2J2, metabolizes DOX to 7-deoxydoxorubicin aglycone (7-de-aDOX). This metabolite then binds to CYP2J2 and inhibits and alters the preferred site of metabolism of AA, leading to a change in the ratio of the EET regioisomers. Furthermore, molecular dynamics simulations indicate that 7-de-aDOX and AA can concurrently bind to the CYP2J2 active site to produce these changes in the site of AA metabolism. To determine if these observations are unique to DOX/7-de-aDOX, we use noncardiotoxic DOX analogues, zorubicin (ZRN) and 5-iminodaunorubicin (5-IDN). ZRN and 5-IDN inhibit CYP2J2-mediated AA metabolism but do not change the ratio of EET regioisomers. Altogether, we demonstrate that DOX and 7-de-aDOX inhibit CYP2J2-mediated AA metabolism and 7-de-aDOX binds close to the active site to alter the ratio of cardioprotective EETs. These mechanistic studies of CYP2J2 can aid in the design of new alternative DOX derivatives.

Copper-catalyzed [1,2]-rearrangements of allylic iodides and aryl α-diazoacetates

The [1,2]- and [2,3]-rearrangements of iodonium ylides are synthetically useful reactions for the generation of functionalized α-iodoesters. Allylic iodides are coupled with α-diazoesters in the presence of a copper catalyst and a ligand to generate iodonium ylides, which undergo metal-mediated rearrangements. By fine-tuning the structure of the ligand, we have reversed the regioselectivity of copper-catalyzed reactions of iodonium ylides from [2,3]- to [1,2]-rearrangements with the use of alternate bipyridine ligands. The preference for [1,2]-rearrangements was further improved by using bulky aryl α-diazoester substrates. Several α-iodoesters with a diverse range of functional groups were generated in good yields (up to 88% yield) and high regioselectivities (up to >95:5 regioisomeric ratio). A deuterium-labeled substrate was utilized to gain insight into the mechanism of the reaction.

Variation in the flavonolignan composition of fruits from different Silybum marianum chemotypes and suspension cultures derived therefrom

Mature fruits collected from different milk thistle (Silybum marianum (L.) Gaertn.) plants, grown in various habitats in Europe, were analysed for silymarin content and variation in component composition. Two different German and Polish cultivars each, as well as fruits from Hungary and Bulgaria have been compared with respect to their ratio of flavonolignan regioisomers. Besides differences in total silymarin content (0.8%–4.9%), three distinct chemotypical variations in fruit flavonolignan regioisomer composition in the cultivars have been observed. Although the differences in the diastereomer ratios of silybin A/B and isosilybin A/B were not significant, they never appeared in a 1:1 ratio. In vitro cultures have been established from seedlings of three typical chemotypes for further insights into flavonolignan content and composition in suspension cultures and the release of these specialized compounds to the extracellular space. The differences in the three Silybum marianum chemotypes were also observed in the composition of the intracellular silymarin of suspension-cultured cells. Silymarin components released to the cell culture medium, however, showed a highly differing composition with only low amounts of silychristin and silydianin. Assays with crude protein extracts prepared from suspension cells or habituated medium of these three chemotypes did not result in differences in silymarin content or composition. In in vitro assays the formation of the regioisomers silydianin and silychristin were strongly influenced by the taxifolin:coniferyl alcohol concentration ratio.

Enantiomeric separation of triacylglycerols containing polyunsaturated fatty acids with 18 carbon atoms

Regioisomers and enantiomers of triacylglycerols (TAGs) containing any combination of stearidonic (18:4n-3) and octadecapentaenoic (18:5n-3) acids were prepared by organic synthesis. Gradient polar organic liquid chromatography/high resolution atmospheric pressure chemical ionization-tandem mass spectrometry (NARP-LC/HRMS2-APCI) and chiral liquid chromatography were used for the separation and identification of molecular species of these TAGs. Further, NARP-LC and chiral LC were used to separate natural mixtures of TAGs obtained from the haptophyte alga Coccolithophora sp. cultivated in a salinity range from 7.5 to 60‰. The ratio of regioisomers and enantiomers was found to change with increasing salinity of the culture medium. This can be explained by variable activity of acyltransferases in cells exposed to salt stress.

Ratios of Regioisomers of the Molecular Species of Triacylglycerols in Lesquerella (Physaria fendleri) Oil Estimated by Mass Spectrometry

AbstractThe ratios of regioisomers of 72 molecular species of triacylglycerols (TAG) in lesquerella oil were estimated using the electrospray ionization mass spectrometry of the lithium adducts of TAG in the HPLC fractions of lesquerella oil. The ratios of ion signal intensities (or relative abundances) of the fragment ions from the neutral losses of fatty acids (FA) as α‐lactones at the sn‐2 position (MS3) of the molecular species of TAG were used as the ratios of the regioisomers. The order of the preference of FA incorporation at the sn‐2 position of the molecular species of TAG in lesquerella was as: normal FA > OH18 (monohydroxy FA with 18 carbon atoms) > diOH18 > OH20 > diOH20, while in castor was as: normal FA > OH18 > OH20 > diOH18 > triOH18. Elongation (from C18 to C20) was more effective than hydroxylation in lesquerella to incorporate hydroxy FA at the sn‐1/3 positions. The block of elongation in lesquerella may be used to increase the content of hydroxy FA, e.g., ricinoleate, at the sn‐2 position of TAG and to produce triricinolein (or castor oil) for industrial uses. The content of normal FA at the sn‐2 position was about 95 %, mainly oleate (38 %), linolenate (31 %) and linoleate (23 %). This high normal FA content (95 %) at the sn‐2 position was a big space for the replacement of ricinoleate to increase the hydroxy FA content in lesquerella oil. The content of hydroxy FA at the sn‐1/3 positions was 91 % mainly lesquerolic acid (85 %) and the content of normal FA was 6.7 % at the sn‐1/3 position in lesquerella oil.

Determination of triacylglycerol regioisomers using differential mobility spectrometry.

Triacylglycerols (TG) contain three fatty acyls attached to the glycerol backbone in stereochemically numbered positions sn-1, 2 and 3. Isobaric TG with exchanged fatty acyl chains in positions sn-1/3 vs. sn-2 are referred to as regioisomers and the determination of their regioisomeric ratios is important for nutrition purposes. Differential mobility spectrometry (DMS) coupled to electrospray ionization mass spectrometry (ESI-MS) is applied for the separation of simple unsaturated TG regioisomers extracted from porcine adipose tissue using their silver-ion molecular adducts. Four pairs of TG regioisomers containing combinations of unsaturated and saturated fatty acyl chains are successfully separated using DMS with 1-butanol or 1-propanol as the chemical modifier. Various experimental parameters are carefully optimized, such as the separation and compensation voltages applied to DMS electrodes, the type and flow rate of chemical modifier and the dwell time of analyte ions in the DMS cell. The optimized DMS approach is used for the characterization of TG regioisomers in less than one minute, compared to tens of minutes typical for silver-ion or reversed-phase high-performance liquid chromatography/mass spectrometry approaches. The application of this method for the characterization of TG regioisomers in porcine adipose tissue shows the method suitability for analyses of other animal fats.