Gas-phase fragmentation reactions of protonated benzofuran- and dihydrobenzofuran-type neolignans investigated by accurate-mass electrospray ionization tandem mass spectrometry

Electrospray ionization tandem mass spectrometry of 4-aryl-3,4-dihydrocoumarins

We have investigated the gas-phase fragmentation reactions of 11 synthetic 4-aryl-3,4-dihydrocoumarins by electrospray ionization tandem mass spectrometry (ESI-MS/MS) on a quadrupole-time-of flight (Q-TOF) hybrid mass spectrometer. We have also estimated thermochemical data for the protonated coumarins (precursor ion A) and product ion structures by computational chemistry at a B3LYP level of theory to establish the ion structures and to rationalize the fragmentation pathways. The most abundant ions in the product ion spectra of coumarins 1-11 resulted from C8H8O2, CO2, C4H4O3, C8H10O3, C8H8O2, and CH3OH eliminations through retro-Diels-Alder (RDA) reactions, remote hydrogen rearrangements (β-eliminations), and β-lactone ring contraction. Although the investigated coumarins shared most of the fragmentation pathways, formation of a benzylic product ion and its corresponding tropylium ion was diagnostic of the substituents at ring C. The thermochemical data revealed that the nature and position of the substituents at ring C played a key role in the formation of this product ion and determined its relative intensity in the product ion spectrum. The results of this study contribute to knowledge of the gas-phase ion chemistry of this important class of organic compounds.

Study on 2-arylbenzo[b]furans from Itea omeiensis and their fragmentation patterns with Q-Orbitrap mass spectrometry

One new 2-arylbenzo[b]furan named iteafuranal F (1) as well as two known analogues (2-3) were isolated from the 95% EtOH extract of aerial parts of Itea omeiensis. Their chemical structures were constructed based on extensive analyses of UV, IR, 1D/2D NMR and HRMS spectra. Antioxidant assays revealed significant superoxide anion radical scavenging capacity of 1 with IC₅₀ value of 0.66 mg/mL, which was comparable to the efficiency of positive control of luteolin. In addition, the preliminary MS fragmentation patterns in negative ion mode were established to distinguish 2-arylbenzo[b]furans with C-10 in different oxidation states: the characteristic loss of CO molecule [M-H-28]^- was observed for 3-formyl-2-arylbenzo[b]furans, and the loss of CH₂O fragment [M-H-30]^- for 3-hydroxymethyl-2-arylbenzo[b]furans, and the loss of CO₂ fragment [M-H-44]^- for 2-arylbenzo[b]furan-3-carboxylic acids.

Electrospray ionization tandem mass spectrometry of deprotonated dihydrobenzofuran neolignans

RATIONALE

Although dihydrobenzofuran neolignans (DBNs) display a wide diversity of biological activities, the identification of their in vivo metabolites using liquid chromatography/electrospray ionization tandem mass spectrometry (LC/ESI-MS/MS) remains a challenge to be overcome. Recently, ESI-MS/MS data of protonated DBNs have been reported, but they were shown to be limited due to the scarcity of diagnostic ions.

METHODS

The gas-phase fragmentation pathways of a series of biologically active synthetic benzofuran neolignans (BNs) and DBNs were elucidated by means of negative ESI accurate-mass tandem and sequential mass spectrometry, and thermochemical data estimated using computational chemistry and the B3LYP/6-31+G(d,p) model.

RESULTS

Deprotonated DBNs produced more diagnostic product ions than the corresponding protonated molecules. Moreover, a series of odd-electron product ions (radical anions) were detected, which has not been reported for protonated DBNs. Direct C₂ H₃ O₂ ^• elimination from the precursor ion (deprotonated molecule) only occurred for the BNs and can help to distinguish these compounds from the DBNs. The mechanism through which the [M - H - CH₃ OH]^- ion is formed is strongly dependent on specific structural features.

CONCLUSIONS

The negative ion mode provides much more information than the positive ion mode (at least one diagnostic product ion was detected for all the analyzed compounds) and does not require the use of additives to produce the precursor ions (deprotonated molecules).

Electrospray ionization tandem mass spectrometry of monoketone curcuminoids

RATIONALE

Although monoketone curcuminoids (MKCs) have been largely investigated due to their biological activities, data on the gas-phase fragmentation reactions of protonated MKCs under collision-induced dissociation (CID) conditions are still scarce. Here, we combined electrospray ionization tandem mass spectrometry (ESI-MS/MS) data, multiple-stage mass spectrometry (MSⁿ ), deuterium exchange experiments, accurate-mass data, and thermochemical data estimated by computational chemistry to elucidate and to rationalize the fragmentation pathways of eleven synthetic MKCs.

METHODS

The MKCs were synthesized by Claisen-Schmidt condensation under basic (1-9) or acidic (10-11) conditions. ESI-CID-MS/MS analyses and deuterium-exchange experiments were carried out on a triple quadrupole mass spectrometer. MSⁿ analyses on an ion trap mass spectrometer helped to elucidate the fragmentation pathways. Accurate-mass data and thermochemical data, obtained at the B3LYP/6-31+G(d,p) level of theory, were used to support the ion structures.

RESULTS

The most intense product ions were the benzyl ions ([C₇ H₂ R₁ R₂ R₃ R₄ R₅ ]⁺ ) and the acylium ions ([M + H - C₈ H₃ R₁ R₂ R₃ R₄ R₅ ]⁺ ), which originated directly from the precursor ion as a result of two competitive hydrogen rearrangements. Product ions [M + H - H₂ O]⁺ and [M + H - C₆ HR₁ R₂ R₃ R₄ R₅ ]⁺ , which are formed after Nazarov cyclization, were also common to all the analyzed compounds. In addition, •Br and •Cl eliminations were diagnostic for the presence of these halogen atoms at the aromatic ring, whereas •CH₃ eliminations were useful to identify the methyl and methoxy groups attached to this same ring. Nazarov cyclization in the gas phase occurred for all the investigated MKCs and did not depend on the presence of the hydroxyl group at the aromatic ring. However, the presence and the position of a hydroxyl group at the aromatic rings played a key role in the Nazarov cyclization mechanism.

CONCLUSIONS

Our results reinforce some aspects of the fragmentation pathways previously published for 1,5-bis-(2-methoxyphenyl)-1,4-pentadien-3-one and 1,5-bis-(2-hydroxyphenyl)-1,4-pentadien-3-one. The alternative fragmentation mechanism proposed herein can explain the fragmentation of a wider diversity of monoketone curcuminoids.