Isolation and structural analysis of novel conformational isomers of the m-xylylene-bridged calix[6]arenes: the ‘partial cone’ and ‘inverted cone’ isomers

In the benzylation of a m-xylylene-bridged calix[6]arene tetrol, the first example of the ‘partial cone’ isomer of a calix[6]arene was obtained in addition to the corresponding cone and 1,2,3-alternate isomers, and its structure was established by X-ray crystallographic analysis. The synthesis and crystal structure of the ‘inverted cone’ isomer as well as its thermal conversion to the ‘normal cone’ isomer are also described.     

A solvent-controlled highly efficient Pd-C catalyzed hydrogenolysis of benzaldehydes to methylbenzenes via a novel ‘acetal pathway’

Pd-C catalyzed hydrogenolysis of benzaldehydes to methylbenzenes has been described to proceed via a ‘benzenemethanol pathway’. In this article, a novel ‘acetal pathway’ was first revealed by a systematic study when lower alcohols were used as solvents and a solvent-controlled highly efficient procedure was established.     

Synthesis of a ‘twisted’ transition-state analogue of biotin

A facile, racemic synthesis of a ‘twisted’ transition-state analogue of biotin is described. A key reaction is the electronically assisted ring-closure of the sulfur containing ring by displacement of an in situ generated mesylate by a suitably positioned 4-methoxybenzyl sulfide. The crystal structure of tricyclic compound 6 shows the AB ring system to indeed be twisted. The ‘twist’ was introduced to examine the possible involvement of sulfur participation in biotin biochemistry.     

Electrochemically induced dethreading of a 2-pseudorotaxane based on the 1,2-bis(4,4′-pyridinium)ethane/24-crown-8 ether motif

Reversible electrochemical regulation of dethreading and rethreading of a 2-pseudorotaxane complex composed of a dibenzo-24-crown-8 ‘wheel’ noncovalently bound to the ethyl bridge of a bis-viologen ‘axle’ is demonstrated by cyclic voltammetry.     

Reinforcing effect of bi- and tri-cyclopolyprenols on ‘primitive’ membranes made of polyprenyl phosphates

Cholesterol plays the role of membrane reinforcer in eukaryotes, whereas hopanoids play the same role in bacteria. Which components could have reinforced ‘primitive’ membranes? We describe here an efficient biomimetic synthesis of bi- and tri-cyclopolyprenols and demonstrate that these compounds reinforce the membranes of polyprenyl phosphate, which we speculated as ‘primitive’ membranes.     

N-Dansyl-carbazoloquinone; a chemical and electrochemical fluorescent switch

A new ‘push-pull’ molecule having an efficient fluorophore (dansyl) in electronic communication with an active redox quencher (phenyl-carbazoloquinone) through an NH-bridge was designed and synthesized. This all-organic molecule is suggested as a highly reversible ‘on/off’ molecular switching system. Chemical and electrochemical inter-conversion between the quinone acceptor and the dansyl donor were demonstrated via UV-vis, cyclic voltammetry and fluorescence measurements.     

The ‘fully protected backbone’ approach as a versatile tool for a new solid-phase PNA synthesis strategy

The ‘fully protected backbone’ (FPB) strategy has been efficiently adapted to the solid-phase synthesis of homothymine, homocytosine and ‘mixed’ pyrimidine PNAs. This versatile and simple method avoids the preparation of PNA monomers and relies on easy available starting materials, highly efficient backbone elongations and effective nucleobase units condensations.     

Pre-analytical method for NMR-based grape metabolic fingerprinting and chemometrics

Although metabolomics aims at profiling all the metabolites in organisms, data quality is quite dependent on the pre-analytical methods employed. In order to evaluate current methods, different pre-analytical methods were compared and used for the metabolic profiling of grapevine as a model plant. Five grape cultivars from Portugal in combination with chemometrics were analyzed in this study. A common extraction method with deuterated water and methanol was found effective in the case of amino acids, organic acids, and sugars. For secondary metabolites like phenolics, solid phase extraction with C-18 cartridges showed good results. Principal component analysis, in combination with NMR spectroscopy, was applied and showed clear distinction among the cultivars. Primary metabolites such as choline, sucrose, and leucine were found discriminating for ‘Alvarinho’, while elevated levels of alanine, valine, and acetate were found in ‘Arinto’ (white varieties). Among the red cultivars, higher signals for citrate and GABA in ‘Touriga Nacional’, succinate and fumarate in ‘Aragonês’, and malate, ascorbate, fructose and glucose in ‘Trincadeira’, were observed. Based on the phenolic profile, ‘Arinto’ was found with higher levels of phenolics as compared to ‘Alvarinho’. ‘Trincadeira’ showed lowest phenolics content while higher levels of flavonoids and phenylpropanoids were found in ‘Aragonês’ and ‘Touriga Nacional’, respectively. It is shown that the metabolite composition of the extract is highly affected by the extraction procedure and this consideration has to be taken in account for metabolomics studies.     

‘O-Acyl isopeptide method’: racemization-free segment condensation in solid phase peptide synthesis

We disclosed a novel ‘racemization-free segment condensation’ based on the ‘O-acyl isopeptide method’ in which an N-segment including C-terminal O-acyl isopeptide structure with urethane-protected Ser/Thr residue was employed for the segment condensation, suggesting that the use of this method contributes to the effective convergent synthesis of long peptides/proteins.     

Derivatization of amines with chloroformate esters for gas chromatographic analysis

Summary The gas chromatographic analysis of amines after conversion to electron capture sensitive carbamates in two-phase systems has been studied. Hydrophilic compounds, for instance methylamine, are reacted with 2,2,2-trichloro-tert. butyl chloroformate. Quantitation below 10^–7 M can be made by thermionic or electron capture detection. A hydrophobic amine, namely N,N-dimethyl-n-octylamine, was derivatized with 2,4,6-tribromophenyl chloroformate with addition of iodide ion to the aqueous phase. The favorable effect of iodide ion as well as the choice of pH and chloroformate ester is discussed.Presented at the 14th International Symposium on Chromatography London, September, 1982     

Hooker’s ‘lapachol peroxide’ revisited

The structure of ‘lapachol peroxide’ was published by Hooker in 1936; however, we recently discovered a mistake in the originally proposed structure; the correct structure, according to X-ray spectroscopy, is given here.     

‘O-Acyl isopeptide method’ for the efficient synthesis of difficult sequence-containing peptides: use of ‘O-acyl isodipeptide unit’

A novel ‘O-acyl isodipeptide unit’, Boc-Thr(Fmoc-Val)-OH 5 has been successfully used for the efficient synthesis of a difficult sequence-containing pentapeptide based on the ‘O-acyl isopeptide method’, in which racemization-inducible esterification could be omitted, suggesting that the use of O-acyl isodipeptide units allows the application of this method to fully automated protocols for the synthesis of long peptides or proteins.     

Heptakis-6-(5-methylene-thioureido-5′-methyl-2,2′-bipyridyl)-β-cyclodextrin: synthesis and metal complexation study

A new heptapode heptakis-6-(5-methylene-thioureido-5′-methyl-2,2′-bipyridyl)-β-cyclodextrin was prepared and its complexation properties towards metal cations were investigated. Substituting the urea functions by the corresponding thioureas promoted the inversion of the metal coordination selectivity. Preliminary results showed the heptapode unable to complex lanthanides but authorise selective complexation of ‘soft’ and ‘borderline’ metal cations.     

DNA and RNA binding properties of an arginine-based ‘Extended Chiral Box’ Peptide Nucleic Acid

Lys-based ‘chiral box’ Peptide Nucleic Acids (PNAs with three adjacent 2D-Lys-based chiral monomers) have shown unsurpassed specificity in DNA recognition. In this Letter, the binding performances of arginine-based chiral PNAs were evaluated for PNAs containing in the middle part of the strand either a 2D,5L-Arg monomer or three adjacent 2D-; 2D,5L-; 5L-Arg monomers (‘Extended Chiral Box’), a combination never studied before. The binding performances of the PNAs were studied by evaluating the melting temperatures of fullmatch and mismatch PNA-DNA and PNA-RNA hybrids and by studying their structure by circular dichroism (CD). The data indicated that the arginine side chains inserted in the PNA structure are perfectly equivalent to lysine side chains as far as oligonucleotide recognition is concerned. The insertion of an ‘Extended Chiral Box’ into PNA differently influences the binding properties to DNA and RNA: the additional side chains had no observable effect on binding affinity and selectivity toward DNA, whereas, seemed to slightly disturb the binding affinity to RNA but at the same time highly enhancing the recognition selectivity.     

Prenyl carbamates: preparation and deprotection

Prenyloxycarbonylimidazole (PreocIm) and prenyl p-nitrophenyl carbonate (PreocOC₆H₄p-NO₂), two substitutes for the unstable prenyl chloroformate, allowed an efficient introduction of the prenyloxycarbonyl group to a variety of primary and secondary amines. Deprotection of prenyl carbamates was readily achieved by, first their conversion to 2-iodo-3-methoxy-3-methylbutyl carbamates with iodine in methanol followed by reductive β-elimination with zinc powder. These reaction conditions are compatible with the presence of a number of functional groups such as Boc and Cbz carbamates, sulfides, double bonds, indoles and aromatic ethers.     

F-Induced ‘turn-on’ fluorescent chemosensor based on 1,3-alt thiacalix[4]arene

A new thiacalix[4]arene based fluorescent sensor 1 bearing two naphthyl groups has been synthesized in 1,3-alternate conformation. In the absence of fluoride ion, the receptor 1 is in ‘off-state’ showing no fluorescence emission. The presence of fluoride ion triggered the fluorescence emission to ‘on-state’. The receptor shows pronounced selectivity for fluoride ions. In THF, the presence of F⁻ ions induces the formation of a 1:1 (G:H) complex.     

Ruthenium trichloride catalyzed synthesis of 2,3-unsaturated-N-glycosides via Ferrier azaglycosylation

An efficient, economical and mild protocol for the synthesis of 2,3-unsaturated-N-glycosides has been developed using ruthenium(III) chloride. The Ferrier azaglycosylation of glycals with various N-nucleophiles such as sulfonamides, benzamides, carbamates and N-substituted sulfonamides proceeded smoothly to afford the corresponding 2,3-unsaturated-N-glycosides or ‘N-pseudoglycals’ in good yields (64–98%). High α-anomeric selectivity was observed with N-substituted sulfonamides such as N-benzyl or N-phenyl sulfonamides under similar conditions.     

Persistent carbocations from 4,4′-dimethylaminodiphenylethylenes within CaY zeolite: intrazeolite-water controls the structure of the carbocation

Inclusion of 4,4′-dimethylaminodiphenylethylene and related olefins within activated CaY results in the formation of persistent monomer carbocations. The structure of the cation, consequently the color of the zeolite, is controlled by water molecules present within zeolites. Under ‘dry’ conditions the zeolite is light yellow and under ‘wet’ conditions it is blue colored. Blue color results from the quininoidal structure adopted by 1,1-bis-(4-dimethylaminophenyl)ethyl cation through participation of the lone pair electrons present on the amino substituent at the para position.     

Synthesis of extended ethynylnaphthalene-based ruthenium(II) 2,2′:6′,2″-terpyridine complexes

A ‘synthesis-at-metal’ approach is described for the preparation of extended ethynylnaphthalene-based ruthenium(II) 2,2′:6′,2″-terpyridine complexes.