Influence of intramolecular hydrogen bonds in the enzyme-catalyzed regioselective acylation of quinic and shikimic acid derivatives

Selective mono-functionalization of 3-epi, 4-epi-, and 5-epi quinic and shikimic acid derivatives has been accomplished by enzymatic acylation with Candida antarctica lipase A (CAL-A). We propose that the selectivity of this lipase is related to both the inherent receptor selectivity and the degree of intramolecular hydrogen bonding in the ligand. Conformational analysis of the hydroxyl protons has been carried out by ¹H NMR spectroscopy. We have shown that exchange of the hydroxyl protons by acid catalysis provides a useful method for the detection of intramolecular hydrogen bonds. The interpretation of exchange rates and coupling constants determines the direction of the H-bonds as conditioned by the relative acceptor and donor properties of the hydroxyl groups. The selectivity of the acylation agrees fully with the effectiveness of H-bonding networks in polyol compounds and with the higher reactivity of the equatorial hydroxyl groups.     

Unexpected B-ring regioselective di-nitration of diosmetin, a Citrus flavonoid

Nitration in position C-8 of diosmetin, an easily available citroflavonoid, was studied in order to gain access to original analogs. The one-step nitration proved impossible, as mono or di-nitration on C-2′ and C-6′ positions on the lateral B ring of the molecule was exclusively observed. This surprisingly straightforward di-nitration of ring B, showing a lack of reactivity of ring A despite its high activation, has never been mentioned to date. Nitration in position C-8 was therefore performed in five steps, requiring selective deactivation of ring B.     

Borinic acid-catalyzed regioselective acylation of carbohydrate derivatives

Reversible covalent interactions of organoboron compounds are exploited as the basis for regioselective borinic acid-catalyzed acylations of polyols. This catalytic protocol enables differentiation of the secondary OH groups of a wide range of carbohydrate derivatives with diverse acid chloride and chloroformate reagents, using a structurally simple diarylborinic acid-derived catalyst.     

GC-MS identification of the flavonoid aglycones isolated from propolis

Summary Between six and nine flavonoids have been identified by GC-MS in five samples of propolis and in one sample of bud exudate from poplar (Populus nigra), all collected in Poland. On the basis of experimental data and data obtained by use of Drylab software, the optimum temperature programme was found for separation of reference samples of pinostrobin chalcone, pinocembrin, tectochrysin, genkwanin, chrysin, galangin, 5-hydroxy-4′,7-dimethoxyflavone, and pilloin.     

Mass spectrometric profiling of flavonoid glycoconjugates possessing isomeric aglycones

In fields such as food and nutrition science or plant physiology, interest in untargeted profiling of flavonoids continues to expand. The group of flavonoids encompasses several thousands of chemically distinguishable compounds, among which are a number of isobaric compounds with the same elemental composition. Thus, the mass spectrometric identification of these compounds is challenging, especially when reference standards are not available to support their identification. Many different types of isomers of flavonoid glycoconjugates are known, i.e. compounds that differ in their glycosylation position, glycan sequence or type of interglycosidic linkage. This work focuses on the mass spectrometric identification of flavonoid glycoconjugate isomers possessing the same glycan mass and differing only in their aglycone core. A non‐targeted HPLC‐ESI‐MS/MS profiling method using a triple quadrupole MS is presented herein, which utilizes in‐source fragmentation and a pseudo‐MS³ approach for the selective analysis of flavonoid glycoconjugates with isomeric/isobaric aglycones. A selective MRM‐based identification of the in‐source formed isobaric aglycone fragments was established. Additionally, utilizing the precursor scanning capability of the employed triple quadrupole instrument, the developed method enabled the determination of the molecular weight of the studied intact flavonoid glycoconjugate. The versatility of the method was proven with various types of flavonoid aglycones, i.e. anthocyanins, flavonols, flavones, flavanones and isoflavones, along with their representative glycoconjugates. The developed method was also successfully applied to a commercially available sour cherry sample, in which 16 different glycoconjugates of pelargonidin, genistein, cyanidin, kaempferol and quercetin could be tentatively identified, including a number of compounds containing isomeric/isobaric aglycones. Copyright © 2015 John Wiley & Sons, Ltd.     

Enantiomeric resolution of cyclobutanones and related derivatives by enzyme-catalyzed acylation and hydrolysis

A method for the regio- and enantioselective protection and deprotection of a number of cyclobutanone derivatives employing the isolated enzyme porcine pancreatic lipase (PPL) has been developed. PPL catalyzes the regioselective acylation or deacylation of the C-3 substituent in (2S,3R)-(+)-bis[hydroxymethyl]-1,1-dimethoxycyclobutanone and its enantiomer. Photochemical ring expansion of the corresponding cyclobutanones in methanol gave anomeric mixtures of the methyl furanosides with stereochemical retention of the ring substituents. The PPL-catalyzed hydrolysis of the acetal derived from (2S,3R)-bis[acetoxymethyl]cyclobutanone resulted in a regioselective reaction of the C-3 acetoxymethyl group. PPL exhibits no hydrolysis activity toward the acetal derived from the enantiomeric cyclobutanone diacetate.

Racemic 2-acetoxymethyl-3,3-dimethylcyclobutanone was converted to its enantiomerically enriched (S)-alcohol by PPL-catalyzed hydrolysis. The corresponding methyl furanoside obtained from the photochemical ring expansion of racemic 2-acetoxymethyl-3,3-dimethylcyclobutanone in methanol is not an effective substrate for PPL mediated hydrolysis.     

Fabrication of a nano-biocatalyst for regioselective acylation of arbutin

A nano-biocatalyst has been successfully prepared via coordination between zinc ion and the nitrogen atoms of the amide groups in the Candida sp. 99-125 lipase. The as-prepared nanoparticle has an excellent hierarchical structure with a large number of separate petals. In regioselective acylation of arbutin, the nano-biocatalyst showed enhanced enzyme activities to about 4.3-folds than the free enzyme. Meanwhile, the prepared nano-biocatalyst exhibited good operational reusability and only about 6% of enzyme activity has lost during 10 continuous reaction-recycles.     

Chemoenzymatic synthesis of structured triacylglycerols by highly regioselective acylation

A highly efficient two-step chemoenzymatic synthesis of structured triacylglycerols comprising a pure n-3 polyunsaturated fatty acid at the mid-position and a pure saturated fatty acid located at the end-positions is described. In the first step an immobilized Candida antarctica lipase was observed to display an excellent regioselectivity toward the end-positions of glycerol at 0-4°C using vinyl esters as acylating agents. The n-3 fatty acids were introduced into the remaining mid-position highly efficient and in excellent yields using EDCI coupling agent.     

Mass spectra of trimethylsilyl derivatives of naturally occurring flavonoid aglycones and chalcones

Electron impact mass spectra of the trimethylsilyl derivatives of a series of flavonoid aglycones and chalcones are reported. The spectra show prominent ions arising from fragmentation of the trimethylsilyl (TMS) groups. Inter-actions between adjacent TMS groups, and between TMS groups in the 3- or 5-position (6′-position for the chalcones) and the C-ring carbonyl, yield structurally significant ions. Few fragments associated with the retro-Diels-Alder cleavage of the C-ring characteristic of the underivatized compounds, are observed. The TMS derivatives thus provide complementary information for the identification of flavonoid aglycones and chalcones in biological systems.     

Synthesis of photochromic 1,2-dihetarylethene using regioselective acylation of thienopyrroles

[formula: see text] The influence of catalysts, acid chlorides, and solvents in the acylation of methyl 2-methyl-4H-thieno[3,2-b]pyrrole-5-carboxylate was studied. Conditions for the regioselective acylation processes were found. Thienopyrrole-based photochrome was synthesized for the first time.     

Studies on regioselective acylation of cellulose with bulky acid chlorides

The regioselective esterification of cellulose by reaction with bulky acyl halides including pivaloyl chloride, adamantoyl chloride and 2,4,6-trimethylbenzoyl chloride was studied. Functionalization conditions to achieve a given degree of substitution (DS) and the resulting ester substitution pattern were described in detail. One- and two-dimensional NMR spectroscopy techniques were used to confirm the structure of the esters obtained. We compared the effects on substitution of using different esterifying reagents and solvent systems including DMAc/LiCl, DMSO/TBAF, and ionic liquids (ILs).     

Metal-free chemo- and regioselective acylation of pyridine derivatives with alcohols in water

A straightforward acylaltion of pyridine derivatives has been developed using K₂S₂O₈ as the oxidant and water as a green solvent. The corresponding 2-acylpyridines were synthesized with high chemo- and regioselectivity in good to high yields. This efficient and practical method could serve as a new tool for the convenient synthesis of 2-benzoylpyridines of interest for future pharmaceutical and chemical applications.     

A catalytic one-step process for the chemo- and regioselective acylation of monosaccharides

An organocatalytic method for the chemo- and regioselective acylation of monosaccharides has been developed. Treatment of octyl beta-D-glucopyranoside with isobutyric anhydride in the presence of 10 mol % of a C2-symmetric chiral 4-pyrrolidinopyridine catalyst (1) at -50 degrees C gave the 4-O-isobutyryl derivative as the sole product in 98% yield. Thus, chemoselective acylation, favoring a secondary hydroxyl group in the presence of a free primary hydroxyl group, and regioselective acylation, favoring one of three secondary hydroxyl groups, took place with perfect selectivity. A competitive acylation between octyl beta-D-glucopyranoside and a primary alcohol (2-phenylethanol) with 1.1 equiv of isobutyric anhydride in the presence of 1 gave the 4-O-isobutyrate of octyl beta-D-glucopyranoside with 99% regioselectivity in 98% yield, which indicates that acylation of the secondary hydroxyl group at C(4) of the carbohydrate proceeds in an accelerative manner. A possible mechanism, involving multiple hydrogen-bonding between 1 and the monosaccharide, is proposed for the chemo- and regioselective acylation.     

Preparative regioselective acylation of glycols by enzymatic transesterification in organic solvents

Porcine pancreatic lipase can catalyze transesterification reac tions in organic solvents in a highly regioselective manner. Lipase powder was suspended in solutions of various diols in ethyl carboxylates, and the mixtures were shaken at 30°C; as a result, pri mary monoesters of glycols were produced on a preparative scale.     

Insertion of arynes into carbon-halogen sigma-bonds: regioselective acylation of aromatic rings

Arynes were found to insert into carbon-halogen sigma-bonds of various acid halides, enabling acyl and halogen moieties to be introduced simultaneously into adjacent positions of aromatic rings.     

Unexpected regioselective diels-alder cycloaddition reactions between 3-fluorobenzyne and 2-alkylfurans

The Diels-Alder reaction between 3-fluorobenzyne (1) and 2-alkylfurans (2) gives a mixture of syn adduct 3 and anti adduct 4. The syn cycloadduct invariably predominates and the regioselectivity increases in the order: R = Me < Et < i-Pr < t-Bu, to a maximum of 90% syn adduct 3d for 2-tert-butylfuran. These results are rationalized in terms of a concerted nonsynchronous transition state that reflects an alkyl steric effect and a polarized aryne 1.     

Determination of flavonoid aglycones in several food samples by mixed micellar electrokinetic chromatography

The application of mixed micellar electrokinetic chromatography to the separation of ten flavonoid aglycones (catechin, epicatechin, naringenin, morin, fisetin, quercetin, kaempferol, galangin, apigenin, and chrysin) belonging to four different classes (flavanols, flavanones, flavonols, and flavones), and expected to be prominent in commonly consumed foods, has been developed. A micellar system composed of 25 mM SDS and 25 mM sodium cholate buffered at pH 7.0 provided a simultaneous separation of all compounds in less than 20 min. The procedure could be easily adapted to the determination of some flavonoids from each of these classes in real complex samples (propolis, Ginkgo biloba, etc.). The LODs of these compounds were in the range of 1.2-4 microg/mL, and the peak area and migration time repeatabilities were below 6.0 and 3.1%, respectively.     

Acylation of pyrrolyltricarbonylmanganese. Structure of the acylation product

The acylation of pyrrolyltricarbonylmanganese (PTM) is studied. The reaction proceeds in an unusual manner; introduction of an acetyl group into the α-position of the PTM pyrrolyl ligand is accompanied by extensive rearrangement of bonds resulting in the formation of a binuclear complex (π-C₄H₄N)Mn(CO)₃(CH₃COC₄H₃N)Mn(CO)₃ (I)Coordination of the acetylpyrrolyl ligand to the manganese atom is of the chelate type and involves atoms of heterocyclic nitrogen and of acetyl oxygen. The ligand, therefore, is not a five-electron but a three-electron donor. The manganese atom is additionally coordinated to three carbonyl groups and one molecule of the initial PTM (by a donor—acceptor bond through a nitrogen atom), thus acquiring a 18-electron shell. A scheme for complex I formation is proposed.