Mass spectral fragmentation studies in usnic acid and related compounds

A study of the fragmentation patterns of the naturally occurring lichen substance, usnic acid (I) and various other chemically and biochemically derived compounds is reported and discussed. From these results it is demonstrated that the characteristic fragmentation patterns observed are of considerable utility in determining not only the substitution patterns on these highly oxygenated molecules, but also the structural elucidation of new compounds within the series.     

Mass spectral fragmentation of some phosphaaromatic compounds

The behaviour of 2-phosphanaphthalene, 3-methyl-phosphanaphthalene and 10-methyl-9-phosphaanthracene upon electron-impact has been compared with that of the carbon and nitrogen analogues. A close resemblance to the fragmentation pattern of the [M ? H]+ ions of 2-methyl-and 2-ethylbenzo[b]thiophene has been observed. It is concluded from the σ₄₀ intensities of the molecular ions, the relatively high intensities of the doubly charged molecular ions and the fragmentation patterns, that these phosphorin derivatives behave as aromatic compounds under electron-impact. From the molecular ion of 2-phosphanaphthalene a remarkable expulsion of a phosphours atom is observed.     

Mass spectral fragmentation pathways in cyclic difluoramino and nitro compounds

The recently synthesized compounds 4, 4-bis(difluoramino)-1-nitropiperidine (I), 1,4,4-trinitropiperidine (II), 1,1,4,4-tetranitrocyclohexane (III), 1,1,4, 4-tetrakis(difluoramino)cyclohexane (IV) and 3,3,7, 7-tetrakis(difluora-mino)octahydro-1,5-dinitro-1,5-diazocine (HNFX, V) are being considered as potential energetic materials. The mass spectra of these compounds were studied using electron ionization (EI) mass spectrometry. A collision-induced dissociation (CID) study of the major EI peaks was carried out using a Finnigan TSQ 700 tandem mass spectrometer. The mass fragmentation pathways are constructed and discussed. The decomposition of HNFX (V), under EI, appeared to parallel the thermal decomposition of nitramines where N-NO(2) cleavage is often the first step. However, the two nitramines with a six-membered ring structure (I and II) underwent initial loss of a geminal substituent; loss of a nitramine nitro group was the secondary step. The two cyclohexane structures (III and IV) showed similar initial fragmentation pathways, featuring successive losses of nitro or difluoramino groups. Copyright 2000 John Wiley & Sons, Ltd.     

Mass spectral fragmentation reactions of angiotensin-converting enzyme (ACE) inhibitors

A variety of mass spectrometric techniques have been employed in the study of a series of structurally similar compounds used in the treatment of hypertension. The compounds, known collectively as angiotensin-converting enzyme (ACE) inhibitors, all share the amino acid residue proline or some variant thereof, as a common structural element. The gas phase fragmentation behavior of these compounds has been explored systematically using various instruments and techniques. An interesting dissociation process (rearrangement) unique to one of the compounds, lisinopril, has been investigated using isotopic labeling experiments and exact mass measurements. The general nature of the process has been probed through both the positive and negative ion analyses of fourteen related compounds exhibiting structural homology.     

Mass spectral fragmentation of the intravenous anesthetic propofol and structurally related phenols

Propofol (2,6-diisopropyl phenol) is a widely used intravenous anesthetic. To define its pharmacokinetics and pharmacodynamics, methods for its quantitation in biological matrixes have been developed, but its pattern of mass spectral fragmentation is unknown. We found that fragmentation of the [M - H](-) ion (m/z 177) of propofol in both APCI MS/MS and ESI MS/MS involves the stepwise loss of a methyl radical and a hydrogen radical from one isopropyl side chain to give the most intense product ion, [M -H - CH(4)](-), at m/z 161. This two-step process is also the preferred mode of fragmentation for similar branched alkyl substituted phenols. This mode of fragmentation of the [M - H](-) ion is supported by three independent lines of evidence: (1) the presence of the intermediary [M - H - CH(3)](-) radical ion under conditions of reduced collision energy, (2) the determination of the mass of the predominant [M - H - CH(4)](-) product ion by high resolution mass spectrometry, and (3) the pattern of product ions resulting from further fragmentation of the [M - H - CH(4)](-) product ion. Phenols with a single straight chain alkyl substituent, in contrast, undergo beta elimination of the alkyl radical irrespective of the length of the alkyl chain, yielding the most intense product ion at m/z 106. This product ion represents a special case of a stable intermediary radical for the two-step process described for branched side chains, because further elimination of a hydrogen radical from the beta carbon is not possible.     

Mass spectral fragmentation of 4-oxo-4H-pyrazole 1-oxides and 1,2-dioxides

The characteristic fragmentation pattern in the mass spectra of 4-oxo-4H-pyrazole 1-oxides and 4-oxo-4H-pyrazole 1,2-dioxides involves scission of the ring into two fragments by cleavage of the nitrogen-nitrogen bond and one of the bonds to the carbonyl carbon. Loss of carbon monoxide or oxygen from the molecular ion is not observed.     

Mass spectral fragmentation of functionalized lanostanes-I

Mass spectra of various functionalized lanostanes and their deuterated analogs are compared. It is proposed that the transfer of a hydrogen from the 18-methyl group to the 11-oxo group via a McLafferty rearrangement is made geometrically possible by prior ionization of the 13,14 bond which allows an optimum interatomic distance between the oxygen and hydrogen to be acquired; the 8,9 double bond is a requisite for electronic induction of this specific process. Transfer of the hydrogen from the 7β-hydroxy group to position 14 is implicated in the mass spectrum of 3β-acetoxy-7β-hydroxy-5α-lanostan-11-one.     

Mass spectral fragmentation pattern of 3-methyl-4-arylaminomethyleneisoxazol-5-ones

The mass spectral fragmentation patterns of 3-methyl-4-arylaminomethyleneisoxazol-5-ones obtained by electron impact have been elucidated. The base peaks are due to the molecular ions. The main fragmentation routes involve loss of H, OH, H₂O, CO₂ and COOH from the molecular ions as well as rupture of the exocyclic CH-NH bond.     

Mass spectral fragmentation and rearrangement of isatin derivatives

Mass spectral fragmentation pathways were formulated for 1-alkylisatins, 1-alkyl-3-arylimino-2-indolinones and 3-arylimino-2-indolinons bearing no substituent at the 1-position. Deuterium-labelled and ¹³C-labelled compounds were utilized in this study. An interesting rearrangement of the 1-alkyl compounds, in which the 1-alkyl substituent is incorporated into a fulvene ion, was established.     

Mass spectral fragmentation of benzofurazan-1-oxide

Fragmentations in the mass spectrum of benzofurazan-1-oxide have been studied using linked scan, accelerating voltage scan and mass-analysed ion kinetic energy spectrometric techniques. Major pathways involve NO·+ NO· and NO·+CO loss, these double losses occurring in such rapid succession as to appear ‘concerted’ in some experiments. Minor pathways are loss of CO₂, C₂N₂O₂, or C₂HN₂O₂ from the molecular ion. The major fragment ion, m/z 76, in the conventional mass spectrum is not detected in a mass-analysed ion kinetic energy spectrometric experiment with the molecular ion until collision activation is provided. The conventional electron impact spectrum invariably includes ions from benzofurazan which is produced by thermal deoxygenation in the source.     

Mass spectral and pyrolytic fragmentation paths for o-nitroanisole

The mass spectral and pyrolytic dissociations of o-nitroanisole are reported. The parent ion dissociates by two pathways, principally through loss of CH₂O, but a significant fraction through loss of NO. The parent-less-30 peak is therefore a doublet, as shown by high resolution measurements. The thermal decomposition at 600°C and above proceeds also by two pathways, but these are the loss of HNO and HNO₂, to form 1,2-methylenedioxybenzene and benzaldehyde respectively.     

Mass spectral studies on aryl-substituted N-carbamoyl/N-thiocarbamoyl narcotine and related compounds

Positive ion mass spectral fragmentation of new N-carbamoyl/N-thiocarbamoyl derivatives of narcotine and compounds closely related to it are reported and discussed. The techniques used include electron impact (EI), fast-atom bombardment (FAB), matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) and electrospray ionization tandem mass spectrometry (ESI-MS/MS). Prominent peaks in the mass spectra of these compounds appear to involve C-C bond cleavage beta to the amine nitrogen with loss of the 4,5-dimethoxy(1H)isobenzofuranone moiety from their molecular ions, along with another prominent peak at m/z 382. No molecular ion peaks of these compounds were recorded in EI, whereas intense [M + H]+ ion peaks were observed in FAB and ESI spectra. MALDI also yielded [M + H]+ ion peaks in good agreement with FAB and ESI studies.     

Mass spectral fragmentation of metal dithiocarbamate complex salts

A summary of the mass spectra of metal dithiocarbamate complex salts (ML₂ and ML₃) is presented. Only divalent metal dithiocarbamate ions without an electronic configuration containing an inert s-orbital electron pair exhibited both expulsion of a ligand radical (L) and the neutral even electron species (L–H) generated from the ligand via hydrogen transfer to the metal-containing fragment ion. Divalent metal dithiocarbamate ions can be generated either by direct electron impact ionization of gas phase ML₂ molecules or ionization of ML₃ molecules followed by loss of a ligand radical. A highly stable sp² hybridized, gas phase ion of a monobidentate lead dithiocarbamate complex is proposed.     

Electron-impact-induced fragmentation of morellin and related compounds

The mass spectra of morellin and other related natural products, isolated from the various Garcinia species, have been recorded and the data has been rationalised. The characteristic cleavage is the opening of the bicyclo(2.2.2) octenone ring system by a retro-Diels-Alder reaction. The γ-pyrone ring also undergoes the retro-Diels-Alder fragmentation. These two modes of cleavage indicate the different substituents on the main skeleton of morellin. The hydrogenated derivatives show slight variations in their fragmentation modes. The mass spectral data assists considerably in the structural elucidation of similar complex molecules, if isolated in minute quantities.     

Mass spectral fragmentation pathways in some dinitroaromatic compounds studied by collision-induced dissociation and tandem mass spectrometry

A collision-induced dissociation study of a series of dinitroaromatic compounds was carried out using a tandem BB mass spectrometer. Fragmentation pathways were determined in the electron impact mode. Loss of NO₂˙ from the molecular ion was observed In most of the investigated compounds. In some compounds loss of NO₂˙ occurred only after loss of OH˙. In other compounds it was not observed at all because of competitive processes, such as loss of NO˙, CO₂, CH₂O, C₂H₄ or H₂O. Loss of NO˙ was a major decomposition pathway, forming ‘dished peaks’ in some of the compounds having a nitro group ortho to a phenyl group, indicating a release of kinetic energy associated with the decomposition. Loss of OH˙ due to an ‘ortho effect’ occurred in compounds where a nitro group was ortho to a group containing a labile hydrogen, but was not observed when competitive processes such as loss of NO˙, NO₂˙ or H₂O occurred. ‘Nitro to nitrite’ isomerization was suggested to explain the decarboxylation process in 2,4- and 2,5-dinitrobenzoic acid and the loss of COH₂ in 2,4-dinitroanisole.     

Mass spectral fragmentation studies in monomeric and dimeric coumarins

A study on the behavior of a large number of monomeric and dimeric coumarins in the mass spectrometer is reported and discussed. The results illustrate that the observed characteristic fragmentation patterns are of considerable utility in the application of mass spectrometry to structure elucidation in this series.     

Chemistry of thienopyridines. XXXIV. Mass spectral fragmentation patterns of some secondary amines and related imines

The mass spectra of some secondary amines and aldimines in the thieno[2,3-b]pyridine ( 3 ) system are presented. α, α-Bis(4-thieno[2,3-b]pyridylamino)toluene ( 1 ) undergoes (a) electron impact-induced dissociation at 80° to give the spectrum of 4-amino- 3 (3b) and (b) thermal dissociation at 200° to give a composite spectrum of 3b and its benzylidene derivative. Spectra of benzylamino and dimethylaminobenzylamino derivatives of 3 are dominated by benzyl-type fragments, while benzylideneimino derivatives show more varied fragmentation.