Direct mass spectrometry of polymers. XII. Thermal fragmentation processes in poly-α-aminoacids

The thermal fragmentation processes in poly-α-aminoacids have been investigated by direct pyrolysis–Mass Spectrometry. The mass spectral data show that the pyrolytic breakdown of polyglycine, polysarcosine, and polyproline leads to the formation of cyclic oligomers. Polyalanine, polyphenylalanine, and polytyrosine decompose yielding compounds with olefin and nitrile end-groups. Finally, in the case of poly-α-methylglutamate, the primary thermal process is the loss of methanol with consequent formation, along the polymer chain, of pyroglutamic units, which yield cyclic dimer as main pyrolysis product.     

Direct mass spectrometry of polymers. XIV. Thermal fragmentation processes in poly-schiff bases

The thermal fragmentation processes in poly-Schiff bases have been investigated by direct pyrolysis–mass spectrometry. The mass spectral data show that the thermal fragmentation occurring in the polymers under investigation is characterized by hydrogen transfer reactions. In the case of a totally aromatic poly-Schiff base (polymer I ), the thermal fragmentation process involves hydrogen transfer irom the methyne group with formation of fragments bearing nitrile and/or phenyl end groups. In the case of aromatic-aliphatic poly-Schiff bases (polymers II–IV ), the hydrogen transfer process occurs from the aliphatic methylene groups. The latter process involves a lower energy and therefore occurs at lower temperatures with respect to the totally aromatic polymer I , with formation of thermal fragments bearing olefin and/or imine end groups. Beside these fragments, several thermal fragmentation compounds are also evolved by multiple hydrogen transfer reactions.     

Direct mass spectrometry of polymers. VII. Primary thermal fragmentation processes in polycarbonates

The primary fragmentation mechanisms in the thermal decomposition of several polycarbonates were studied by direct pyrolysis into the mass spectrometer. Our results indicate that ester exchange reactions predominate in the primary thermal fragmentation process of polycarbonates, causing the formation of cyclic oligomers.     

Direct mass spectrometry of polymers. VIII. Primary thermal fragmentation processes in polyurethanes

The primary fragmentation mechanisms in the thermal decomposition of several polyurethanes were studied by direct pyrolysis into the mass spectrometer. Ester exchange reactions predominate in the primary thermal fragmentation process, causing the formation of cyclic oligomers, which are subsequently cleaved to open-chain oligomers containing hydroxyl end groups.     

Direct mass spectrometry of polymers. XI. Primary thermal fragmentation processes in aromatic–aliphatic polyesters

The thermal decomposition of two series of isomeric aromatic–aliphatic polyesters was studied by direct pyrolysis-mass spectrometry. The results indicate that intramolecular exchange reactions predominate in the primary thermal fragmentation processes to cause the formation of cyclic oligomers. Several secondary thermal processes may occur after the primary step: hydrolytic cleavage of the ester bond, decarboxylation, and β-hydrogen transfer.     

Direct mass spectrometry of polymers. X. Primary thermal fragmentation processes in totally aromatic polyesters

The thermal decomposition of a series of isomeric poly-(oxphthaloyloxyphenylenes) (I–IV) and poly(m-hydroxybenzoic acid) (V) was studied by Direct Pyrolysis–Mass Spectrometry. The results indicate that intramolecular exchange reactions predominate in the primary thermal fragmentation processes, causing the formation of cyclic oligomers which are subsequently cleaved to open-chain fragments. The size and relative abundance of the cycles produced appear to be strongly influenced by steric factors, i.e., by the structure (para or meta) of the repeating unit in each polymer. Remarkably, in the case of poly(m-hydroxybenzoic acid) the formation of cyclic oligomers containing up to seven repeating units is observed.     

Variation of fragmentation with ring size in cyclic α-amino acids

High resolution mass measurements, defocused metastable ion detection and deuterium labeling experiments have been employed in an investigation of the electron-impact induced decomposition of a series of carbocyclic amino acids, which varied in ring size from three to eight carbon atoms. About 50 per cent of the total ion current is carried by [M ? COOH]⁺ ions in the spectra obtained from compounds with five to eight carbon atoms in the ring. This is analogous to the fragmentation of the open chain naturally occurring amino acids. The behavior of the compounds with the two smallest rings is anomalous, reflecting to a larger extent the influence of the ring itself. Special attention has been given the loss of H₂O and NH₄ from ionized species of 1-aminocyclopropanecarboxylic acid.     

Synthesis of chiral α-amino acids

A novel method for the synthesis of chiral α-amino acids has been developed where the acid functionality was constructed by oxidizing a hydroxymethyl group introduced by Evans’ method in the α-position of an appropriate acid substrate and the amino part came from the amide of the original carboxyl group following a modified Hofmann rearrangement reaction.     

Transformations of N-heteroarylformamidines into derivatives of β-heteroarylamino-α,β-dehydro-α-amino acids, β-heteroarylamino-α-amino acids,and dipeptides

Transformations of N'-heteroaryl-N,N-dimethylformamidines 1 as a general method for the preparation of β-heteroarylamino-α,β-dehydro-α-amino acids, β-heteroarylamino-α-amino acid derivatives 5–9 , and dipeptides 10 , are described.     

Nitrenium ions in N-chloro-α-amino acids decomposition?

This article deals with the kinetics of decomposition of four isomers: N? Cl-(L)-2-Aminohexanoic acid, N? Cl-(L)-Leucine, N? Cl-(L)-Isoleucine, and N? Cl-(L)-tert-Leucine, in mild acid and basic aqueous solution. An alternative mechanism for this process which takes place through a nitrenium-like ion transition state is proposed. This mechanism is in consonance with the experimental data and with those available from the literature. © 1993 John Wiley & Sons, Inc.     

An operational approach to N-Cl-α-amino acids decomposition

It has been usually thought that the rate of N-Cl-α-amino acids decomposition changes and that their coefficient of molar absorptivity falls when pH < 5, which has been explained on the basis of a hypothetical process of protonation at the N-Cl-amino group. We have proved: (i) the absence of protonation at the N-Cl-amino group, at least when pH > 2?; (ii) the effect of the method of mixing on the reaction rate; (iii) the absolute necessity of a very careful choice of buffers and solvents; and (iv) the need to check the accuracy of the mathematical model to describe the chemical processes. © 1993 John Wiley & Sons, Inc.     

The mass spectrometric fragmentation of α-diazosulphones

The fragmentation of four α-diazosulphones under conditions of electron-impact was studied with the aid of high resolution mass measurements and the metastable defocusing technique. An important difference from the spectra of the related α-diazoketones is the absence of peaks for [M ? N₂]^+˙. Metastables show, however, that these fragments probably exist as intermediates.     

Fluoro-α-amino acids. 1—use of 19F NMR spectroscopy for the configurational determination of β-fluoro-α-amino acids through complexation by 18-crown-6 ether

Erythro and threo configurational assignments have been made for 11 β-fluoro-α-amino acids or esters using the effect of complexation of the ammonium group by 18-crown-6 ether on the ¹⁹F NMR parameters. For the erythro configurations, ³J(HF) increases and a high-field ¹⁹F chemical shift is generally observed; these phenomena are accompanied by a decrease in ³J(HH) and ³J(CF). The opposite effects are observed for the threo configurations. These observations can be explained by a change in the relative population of the conformers around the Cα-Cβ bond on complexation of the ammonium group. This complexation impairs the interactions between the ammonium and fluorine groups and, concomitantly, the steric hindrance between the ammonium and R (methyl, phenyl or carboxylate) groups is increased.     

Studies in mass spectrometry. II—A common intermediate in the primary fragmentation process of α-phenylmethylenecylopropane

The variation of Z = [(F)⁺]/[(M)^+·] vs electron energy for the loss of H^· and CH₃^· from the molecular ion of 2-phenylmethylenecyclopropane (I) and α-phenylmethylenecyclopropane (II) suggests that they both rearrange to a common intermediate before fragmentation occurs. The structure of the intermediate is discussed with regard to the xperimental values found for the Z ratios and to the mass spectra of deuterium labelled compounds.     

Highly stereocontrolled boron-mediated synthesis of β-hydroxy-α-amino acids and dipeptides. Part 2

The chiral synthons 1(a–d) were submitted to boron-mediated asymmetric aldol condensation with acetaldehyde and benzaldehyde providing, in high diastereomeric excess (>95%), R,R-configured aldols 2(a–f) which are useful intermediates to enantiomerically pure β-hydroxy-α-amino acids.     

η-Pentamethylcyclopentadienylruthenium(II) complexes containing η-coordinated α-amino acids

Reaction of [Cp* RuCl₂]₂ with -alanine ( -alaH) in methanol at room temperature in the presence of NaOMe yields the complex Na[Cp* RuCl( -ala)] (1), which contains a five-membered N,O-coordinated chelate ring. The analogous complex Na[Cp* RuCl( -phe)] (2) is obtained under similar conditions but at 0°C in 90% yield. At temperatures above 20°C both 2 and the η⁶-coordinated complex [Cp* Ru( -pheH)]Cl (4) are obtained, with the proportion of the latter increasing with temperature. Compound 4 is obtained in 88% yield by refluxing [Cp* RuCl₂]₂ and -phenylalanine ( -pheH) in CH₃OH/CH₃ONa followed by separation from 2. The analogous ruthenium(II) sandwich complexes 5–10 were obtained from -tyrosine and -tryptophane and various derivatives. [Cp* Ru( -met)] (3), prepared by the reaction of [Cp* RuCl₂]₂ with -methionine ( -metH) in CH₃OH/CH₃ONa, displays N,O,S-coordination.     

The synthesis of β-heteroarylamino-α,β-dehydro-α-amino acid and β-heteroarylamino-α-amino acid derivatives

From heteroarylaminomethyleneoxazolones 4 , obtained from N-heteroarylformamidines 2 and 2-phenyl-5-oxo-4,5-dihydro-1,3-oxazole ( 3 ), the following β-heteroarylamino-α,β-dehydro-α-amino acid derivatives were prepared: methyl 8 and ethyl esters 9 , amides 10 and 11 , hydrazides 12 , and azides 15 . By catalytic hydrogenation the compounds 4 were converted into β-heteroarylamino substituted amides 18 and β-heteroarylamino-α-amino acids 20 .