Synthesis of esters of diaminotruxillic bis-amino acids by Pd-mediated photocycloaddition of analogs of the Kaede protein chromophore

Orthopalladated imidazolones and thiazolones: synthesis, photophysical properties and photochemical reactivity

The reaction of Pd(OAc)2 with (Z)-5-arylidene-4-(4H)-imidazolones (2a-e) and (Z)-4-arylidene-5(4H)-thiazolones (3a-e) in trifluoroacetic acid results in the corresponding orthopalladated dinuclear complexes (4a-e, imidazolones; 11a-d, thiazolones) with trifluoroacetate bridges through regioselective C-H activation at the ortho position of the 4-arylidene group. Compound 4e, which contains an imidazolone substituted at 2- and 4-positions of the arylidene ring with methoxide groups and exhibits strong push-pull charge transfer, is an excellent precursor for the synthesis of fluorescent complexes with green yellowish emission and remarkable quantum yields. Breaking the bridging system with pyridine yields the mononuclear complex 5e (ΦF = 5%), while metathesis of trifluoroacetate ligands with chloride leads to the dinuclear complex 6e, also a precursor of fluorescent complexes by breaking the chloride bridging system with pyridine (7e, ΦF = 7%), or by substitution of chloride ligands with pyridine (8e, ΦF = 15%) or acetylacetonate (9e, ΦF = 2%). In addition to notable photophysical properties, dinuclear complexes 4 and 11 also exhibit significant photochemical reactivity. Thus, irradiation of orthopalladates 4a-c and 11a-c in CH2Cl2 with blue light (465 nm) proceeds via [2 + 2] photocycloaddition of the CC double bonds of imidazolone and thiazolone ligands, yielding the corresponding cyclobutane-bridging diaminotruxillic derivatives 10a-c and 12a-c, respectively.

Structural properties of Phoenix oolong tea polysaccharide conjugates and the interfacial stability in nanoemulsions

BACKGROUND

Tea polysaccharide conjugate (TPC) is a naturally occurring active substance that is extracted from tea. Owing to its benefits in enhancing human immunity and antioxidant effects, TPC is widely used in culinary products. The binding mode of polysaccharides and proteins in TPC, however, has not been well studied; it may be closely related to their functional properties, especially emulsification.

RESULTS

The molecular weights and monosaccharide compositions of TPC were determined by ion chromatography and high-performance gel permeation chromatography. Although the functional groups of polysaccharides and proteins were confirmed by infrared spectroscopy, the presence of proteins could not be detected by sodium dodecyl sulfate polyacrylamide gel electrophoresis and ultraviolet spectroscopy. It was hypothesized that the hydrophobic groups of the proteins in TPC were wrapped by polysaccharide chains, thus making the proteins undetectable. The rheology and interfacial protein adsorption results show that TPC forms a viscoelastic film at the oil-water interface to prevent the aggregation of oil droplets, thereby enhancing the stability of the emulsion. Based on these structural and emulsifying properties of TPC, the binding mode of polysaccharides and proteins along with their phase behavior at the oil-water interface of the emulsion was speculated.

CONCLUSION

In TPC, the hydrophilic groups of the proteins are linked to polysaccharides by covalent interactions, where the hydrophobic groups are wrapped with the polysaccharide chains with the help of hydrophobic forces to form a hydrophobic core. The unique binding of polysaccharides and proteins in TPC enhances its amphiphilic properties, which can be effectively distributed at the oil-water interface and form stable emulsions. © 2023 Society of Chemical Industry.

Fluorescence Amplification of Unsaturated Oxazolones Using Palladium: Photophysical and Computational Studies

Weakly fluorescent (Z)-4-arylidene-5-(4H)-oxazolones (1), Φ_PL < 0.1%, containing a variety of conjugated aromatic fragments and/or charged arylidene moieties, have been orthopalladated by reaction with Pd(OAc)₂. The resulting dinuclear complexes (2) have the oxazolone ligands bonded as a C^N-chelate, restricting intramolecular motions involving the oxazolone. From 2, a variety of mononuclear derivatives, such as [Pd(C^N-oxazolone)(O₂CCF₃)(py)] (3), [Pd(C^N-oxazolone)(py)₂](ClO₄) (4), [Pd(C^N-oxazolone)(Cl)(py)] (5), and [Pd(C^N-oxazolone)(X)(NHC)] (6, 7), have been prepared and fully characterized. Most of complexes 3-6 are strongly fluorescent in solution in the range of wavelengths from green to yellow, with values of Φ_PL up to 28% (4h), which are among the highest values of quantum yield ever reported for organometallic Pd complexes with bidentate ligands. This means that the introduction of the Pd in the oxazolone scaffold produces in some cases an amplification of the fluorescence of several orders of magnitude from the free ligand 1 to complexes 3-6. Systematic variations of the substituents of the oxazolones and the ancillary ligands show that the wavelength of emission is tuned by the nature of the oxazolone, while the quantum yield is deeply influenced by the change of ligands. TD-DFT studies of complexes 3-6 show a direct correlation between the participation of the Pd orbitals in the HOMO and the loss of emission through non-radiative pathways. This model allows the understanding of the amplification of the fluorescence and the future rational design of new organopalladium systems with improved properties.

Stereospecific Formation of the rctt Isomer of Bis-crown-Containing Cyclobutane upon [2 + 2] Photocycloaddition of an (18-Crown-6)stilbene Induced by Self-Assembly via Hydrogen Bonding

The formation and the spectroscopic and structural properties of 1:1 and 2:1 (ligand-to-dication) complexes of an (18-crown-6)stilbene with ethane-1,2-diammonium diperchlorate in MeCN were studied by UV-vis and NMR spectroscopy and by density functional theory calculations. Prolonged UV irradiation of 2:1 mixtures of the crown stilbene and the diammonium salt led to the formation of two main photoproducts, namely, the single syn-"head-to-head" photodimer of the crown stilbene (rctt cyclobutane) due to supramolecular-assisted [2 + 2] photocycloaddition and a crown ether derivative of phenanthrene due to a photoinduced electrocyclization reaction. The rctt cyclobutane was isolated by preparative photolysis, followed by chromatography. The selectivity of the [2 + 2] photocycloaddition is explained by supramolecular pre-organization of crown stilbene molecules into the 2:1 complexes that have a pseudo-sandwich structure with stacking interactions between the stilbene moieties.

Stereoselective, Ruthenium-Photocatalyzed Synthesis of 1,2-Diaminotruxinic Bis-amino Acids from 4-Arylidene-5(4H)-oxazolones

The irradiation of (Z)-2-phenyl-4-aryliden-5(4H)-oxazolones 1 in deoxygenated CH₂Cl₂ at 25 °C with blue light (465 nm) in the presence of [Ru(bpy)₃](BF₄)₂ (5% mole ratio) as a triplet photocatalyst promotes the [2+2] photocycloaddition of the C=C bonds of the 4-arylidene moiety, thus allowing the completely regio- and stereoselective formation of cyclobutane-bis(oxazolone)s 2 as single stereoisomers. Cyclobutanes 2 have been unambiguously characterized as the μ-isomers and contain two E-oxazolones coupled in an anti-head-to-head form. The use of continuous-flow techniques in microreactors allows the synthesis of cyclobutanes 2 in only 60 min, compared with the 24–48 h required in batch mode. Ring opening of the oxazolone heterocycle in 2 with a base affords the corresponding 1,2-diaminotruxinic bis-amino esters 3, which are also obtained selectively as μ-isomers. The ruthenium complex behaves as a triplet photocatalyst, generating the reactive excited state of the oxazolone via an energy-transfer process. This reactive excited state has been characterized as a triplet diradical ³(E/Z)-1* by laser flash photolysis (transient absorption spectroscopy). This technique also shows that this excited state is the same when starting from either (Z)- or (E)-oxazolones. Density functional theory calculations show that the first step of the [2+2] cycloaddition between ³(E/Z)-1* and (Z)-1 is formation of the C(H)–C(H) bond and that (Z) to (E) isomerization takes place at the 1,4-diradical thus formed.

Reactivity of (Z)-4-Aryliden-5(4H)-thiazolones: [2 + 2]-Photocycloaddition, Ring-Opening Reactions, and Influence of the Lewis Acid BF3

The irradiation of (Z)-2-phenyl-4-aryliden-5(4H)-thiazolones 2 with blue light (465 nm) in CH2Cl2 solution promotes [2 + 2]-photocycloaddition of the exocyclic C═C bonds and the formation of the dispirocyclobutanes 3. This reaction takes place with high stereoselectivity, given that the ε-isomer (1,3 head-to-tail syn coupling) is formed in more than 90% yield in most of the cases. However, irradiation of 5(4H)-thiazolones 2 with blue light (456 nm) in dry MeOH in the presence of BF3·OEt2 leads to the monospirocyclobutanes 4 with full stereoselectivity, also affording the ε-isomer. A ring-opening reaction of only one of the thiazolone rings appears to have taken place in 4 upon methanolysis, leading to the corresponding ester and thioamide groups. The treatment of free 4-aryliden-5(4H)-thiazolones 2 with a base in alcohol (NaOR/ROH) also produces a ring-opening reaction of the heterocycle by methanolysis, although, under these reaction conditions, further intramolecular S-attack at the exocyclic C(H)═C bond and cyclization is observed, forming the dihydrothiazoles 5 or 6 as mixtures of cis (RS/SR)- and trans (RR/SS)-isomers with high diastereomeric excess. trans-(RR/SS)-Dihydrothiazoles 6 can be isolated as pure diastereoisomers by column chromatography. Surprisingly, dihydrothiazoles 5 can also be obtained by the treatment of 4-aryliden-5(4H)-thiazolones 2 with BF3·OEt2 in methanol in the absence of a base.

Pd-Oxazolone complexes conjugated to an engineered enzyme: improving fluorescence and catalytic properties

Different Pd-complexes containing orthometallated push-pull oxazolones were inserted by supramolecular Pd-amino acid coordination on two genetically engineered modified variants of the thermoalkalophilic Geobacillus thermocatenolatus lipase (GTL). Pd-lipase conjugation was performed on the solid phase in the previously immobilized form of GTL under mild conditions, and soluble conjugated Pd-GTL complexes were obtained by simply desorbing by washing with an acetonitrile aqueous solution. Three different Pd complexes were incorporated into two different genetically modified enzyme variants, one containing all the natural cysteine residues changed to serine residues, and another variant including an additional Cys mutation directly in the catalytic serine (Ser114Cys). The new Pd-enzyme conjugates were fluorescent even at ppm concentrations, while under the same conditions free Pd complexes did not show fluorescence at all. The Pd conjugation with the enzyme extremely increases the catalytic profile of the corresponding Pd complex from 200 to almost 1000-fold in the hydrogenation of arenes in aqueous media, achieving in the case of GTL conjugated with orthopalladated 4a an outstanding TOF value of 27 428 min^-1. Also the applicability of GTL-C114 conjugated with orthopalladated 4b in a site-selective C-H activation reaction under mild conditions has been demonstrated. Therefore, the Pd incorporation into the enzyme produces a highly stable conjugate, and improves remarkably the catalytic activity and selectivity, as well as the fluorescence intensity, of the Pd complexes.

Fluorescent Orthopalladated Complexes of 4-Aryliden-5(4H)-oxazolones from the Kaede Protein: Synthesis and Characterization

The goal of the work reported here was to amplify the fluorescent properties of 4-aryliden-5(4H)-oxazolones by suppression of the hula-twist non-radiative deactivation pathway. This aim was achieved by simultaneous bonding of a Pd center to the N atom of the heterocycle and the ortho carbon of the arylidene ring. Two different 4-((Z)-arylidene)-2-((E)-styryl)-5(4H)-oxazolones, the structures of which are closely related to the chromophore of the Kaede protein and substituted at the 2- and 4-positions of the arylidene ring (1a OMe; 1b F), were used as starting materials. Oxazolones 1a and 1b were reacted with Pd(OAc)₂ to give the corresponding dinuclear orthometalated palladium derivates 2a and 2b by regioselective C–H activation of the ortho-position of the arylidene ring. Reaction of 2a (2b) with LiCl promoted the metathesis of the bridging carboxylate by chloride ligands to afford dinuclear 3a (3b). Mononuclear complexes containing the orthopalladated oxazolone and a variety of ancillary ligands (acetylacetonate (4a, 4b), hydroxyquinolinate (5a), aminoquinoline (6a), bipyridine (7a), phenanthroline (8a)) were prepared from 3a or 3b through metathesis of anionic ligands or substitution of neutral weakly bonded ligands. All species were fully characterized and the X-ray determination of the molecular structure of 7a was carried out. This structure has strongly distorted ligands due to intramolecular interactions. Fluorescence measurements showed an increase in the quantum yield (QY) by up to one order of magnitude on comparing the free oxazolone (QY < 1%) with the palladated oxazolone (QY = 12% for 6a). This fact shows that the coordination of the oxazolone to the palladium efficiently suppresses the hula-twist deactivation pathway.