Electron-impact fragmentation of N-substituted 2-aminobenzoxazoles and 2-aminobenzothiazoles

The mass spectra of twenty-eight N-substituted 2-aminobenzoxazoles and 2-aminobenzothiazoles have been recorded and the identity of various ions in the mass spectra have been established by high resolution measurement. The molecular compound of series, benzoxazole, undergoes loss of hydrogen cyanide and carbon monoxide from the molecular ion as the most important decomposition pathways. The mass spectra of N-substituted benzothiazoles showed a similar fragmentation as that of N-substituted benzoxazoles.     

Mass spectrometry of heterocyclic compounds—III. Electron-impact-induced fragmentation of 1,2-benzisothiazole and some 3-substituted derivatives

The mass spectra of fifteen 1,2-benzisothiazoles are reported; their fragmentation patterns have been investigated by labelling experiments, high resolution mass measurements and defocused metastable ion detection. The parent compound of the series, 1,2-benzisothiazole, eliminates HCN after partial inter-ring hydrogen scrambling. The extent of scrambling has been determined for normal daughter ions produced at different electron beam energies and also for metastable daughter ions, and is compared with the data reported for benzothiazole. Some 3-substituted derivatives show unusual fragmentation patterns. The mechanisms of these processes are under further investigation.     

Mass spectra of some 1-substituted-4-acetoacetyl-3-methylpyrazol-5-ols

The fragmentation of 1-phenyl-, l-(2′-pyridyl)- and 1-(4′-methyl-2′-quinolyl)-4-acetoacetyI-3-methyIpyrazol-5-ols (compounds 1, 2 and 3, respectively) on electron impact has been studied and the major processes interpreted. The common feature in the mass spectra of these compounds is the loss of ketene, acetonyl radical, acetone and two molecules of ketene from the molecular ion. Whereas the ion generated after the last process, which corresponds to 1-substituted-3-methyIpyrazol-5-ols, loses methyl cyanide in the case of 1, similar ions in the case of 2 and 3 lose ?₂HO moiety, necessitating an intramolecular hydrogen transfer followed by ring fission and subsequent loss of methyl cyanide. All these and other related processes have been substantiated with the help of accurate mass measurements of the fragment ions and B/E linked-scan spectra.     

High resolution mass spectrometry—I. Some substituted δ-lactones

The mass spectra of some δ-lactones substituted in various positions by methyl groups have been studied both by accurate mass measurements at high resulution and by deuterium labelling. The loss of carbon dioxide has been shown to be important only for a mono-substituted lactone. A fragmentation mode common to all the lactones studied is the elimination of the ring oxygen atom, plus the adjacent carbon atom with its substituents, as a neutral carbonyl molecule such as formaldehyde, acetaldehyde or acetone. Deuterium labelling has uncovered the existence of a rearrangement involving hydrogen transfer in an even-electron ion, and a mechanism involving a six-membered transition state is proposed for this. The bond fissions, common to all the lactones, by which the principal ions in the mass spectra arise, are summarised.     

Loss of hydrogen cyanide from monocyanopyridines upon electron impact: Dewar pyridine structures and ring opening—Ring closure reactions revealed by 13C and 15N Labelling

Extensive ¹³C and ¹⁵N labelling has shown that the molecular ions of 2-, 3- and 4-cyanopyridine with lifetimes up to 10^?6 s eliminate hydrogen cyanide originating predominantly from the ring (?65%). Moreover, this hydrogen cyanide loss occurs after an equilibrated positional interchange of the ring carbon atoms at positions interchange of the ring carbon atoms at positions 2, 4 and 6 via Dewar pyridine structures. In molecular ions with lifetimes of 10^?6–10^?5 s skeletal rearrangements have taken place in such a way that both nitrogen atoms have become equivalent prior to the loss of hydrogen cyanide. Arguments are put forward that this equivalence of nitrogen atoms is caused by the intermediacy of ions with a 1,4-dicyanobuta-1,3-diene structure. About 60% of these intermediate ions eliminate hydrogen cyanide in a fast process. The remaining 40% of these ions undergo ring closure again to a pyridine ring in which the carbon atoms of positions 2, 4 and 6 are positionally interchanged rapidly via Dewar pyridine structures followed by ring opening again and eventual loss of hydrogen cyanide. This interpretation of the ¹³C and ¹⁵N labelling results is further corroborated by a study of the loss of hydrogen cyanide from molecular ions of 1,4-dicyanobuta-1,3-diene labelled with ¹³C in both cyano groups.     

Electron-ionization-induced fragmentation of N-monosubstituted 2-phenylacetamides

Electron-ionization-induced mass spectra of N-monosubstituted 2-phenylacetamides were recorded and their fragmentation patterns were studied by metastable-ion analyses. Representative deuterated analogues of these compounds have also been synthesized and their mass spectra compared with those of the unlabelled parent compounds. The most typical fragmentation for N-alkyl-, N-isoalkyl- and N-cycloalkyl-2-phenylacetamides is cleavage of the bond alpha to the carbonyl function, resulting in an ion fragment of m/z 92, following the transfer of hydrogen and elimination of a corresponding ketene. The primary fragmentation process for N-aryl substituted 2-phenylacetamides is the loss of an aromatic hydrogen atom from the molecular ion. The other principal fragmentation processes observed with these compounds are discussed.     

Beitrag zum Mechanismus der stossinduzierten Fragmentierungen von quaternären Stickstoffverbindungen. 2. Mitteilung über stossinduzierte Fragmentierungen von felddesorbierten Kationen

Mechanistic studies on the collision-induced fragmentations of quaternary ammonium ions The collision-induced fragmentation behaviour of field desorbed quarternary ammonium ions has been investigated. A main reaction of these ions is the cleavage of the N? C bond accompanied by hydrogen rearrangement, i.e. alkane loss from the tetraalkyl substituted ammonium ions of the iodides 1, 2 and 3 , respectively, Deuterium labelling indicates that the hydrogen transfer to the leaving group occurs to the extent of about 80% from the α-position and about 20% from the other positions of an alkyl group. Pronounced heterolytic cleavage of the N? C bond is observed in the benzyl substituted ammonium ion of 4 . The β-phenylethyl substituted ammonium ion of 5 shows a homobenzylic heterolysis, possibly yielding the phenonium ion j.     

Fragmentation of dihydro- and tetrahydro-1,5-benzothiazepines under electron impact

The fragmentation mechanisms of dihydro- and tetrahydro-1,5-benzothiazepines under electron impact have been studied in detail using high resolution mass spectrometry, metastable decompositions and deuterium labelling techniques. Both kinds of the benzothiazepines possess high stability. The [M? SH]⁺ and the cyclic benzothiazole ions derived from the fragmentation and severe skeletal rearrangement of the molecular ion comprised the main features of the spectra. Some doubly charged ions were noticed in the low resolution electron impact mass spectra.     

Electron impact studies on oximes: Mass spectra of the condensation products of aniline,phenol and thiophenol with benzhydroximoyl chloride

The condensation products derived from aniline, phenol and thiophenol by treatment with benzhydroximoyl chloride, exhibited a stepwise (in the first two cases) or concerted (in the third case) loss of OH. and C6H5N from the corresponding skeletally rearranged molecular ions, affording [C6H5C?X]⁺ ions (where X = NH or o or S) when subjected to electron impact. The process has been substantiated by deuterium labelling and exact mass measurements. The base peaks in all the spectra were due to the aniline, phenol and thiphenol radical ions as the case may be, obtained by a shift prior to fragmentation of the oxime hydrogen to the heteroatom contained in these ions.     

The mass spectra of acetylenic ketones and their secondary amine adducts

The mass spectra of a number of alkyl and aryl acetylenic ketones of the type RCO·C?C·C?C·R′ have been examined. All the spectra exhibit molecular ions together with the expected intense fragments arising from cleavage α-to the carbonyl group. The most striking feature of the ketone spectra is the loss of CO from the molecular ion. This process dominates the spectra of the aryl ketones, the spectra below [M – 28]+˙. being virtually identical to those of the corresponding phenylacetylenes. The spectra of the piperidene and/or morpholine adducts of the alkyl ketones are very similar to those of the parent ketone whereas those of the aryl ketones are dominated by the loss HO˙ from the molecular ion. Deuterium labelling has established the site of the hydrogen involved as being the carbons α- to the nitrogen of the heterocyclic ring.     

Fragmentation of singly charged adenine induced by neutral fluorine beam impact at 3 keV

The fragmentation scheme of singly charged adenine molecule (H(5)C(5)N(5)(+)) has been studied via neutral fluorine impact at 3 keV. By analyzing in correlation the kinetic energy loss of the scattered projectile F(-) produced in single charge transfer process and the mass of the charged fragments, the excitation energy distribution of the parent adenine molecular ions has been determined for each of the main dissociation channels. Several fragmentation pathways unrevealed in standard mass spectra or in appearance energy measurements are investigated. Regarding the well-known hydrogen cyanide (HCN) loss sequence, we demonstrate that although the loss of a HCN is the dominant decay channel for the parent H(5)C(5)N(5)(+) (m = 135), the decay of the first daughter ion H(4)C(4)N(4)(+) (m = 108) involves not only the HNC (m = 27) loss but also the symmetric breakdown into two dimers of HCN.     

An unusual pathway for the elimination of hydrogen cyanide from benzonitrile upon electron impact as revealed by 13C and 15N labelling

It is shown by ¹⁵N and specific ¹³C labelling that ～50% of the molecules of hydrogen cyanide, eliminated within ～10^?6 s upon electron impact of benzonitrile, contains the original cyano carbon atom, whereas the remaining percentage contains one of the phenyl ring carbon atoms at random. This is even more dramatic for the molecular ions of benzonitrile which decompose in the first and second field-free regions of the VG Micromass ZAB-2F high-field mass spectrometer used. Then only 5–7% of the eliminated molecules of hydrogen cyanide contains the original cyano carbon atom. A cycloaddition-cycloreversion process in the molecular ions, leading to ionized 1-cyano-1,3-hexadien-5-yne as an intermediate in the hydrogen cyanide loss, is proposed to explain this.     

Etude de la Fragmentation du Benzisothiazole et de Quelques Benzoselenazoles et Benzisoselenazoles

The mass spectra of benzisothiazole, three benzoselenazoles and three benzisoselenazoles have been determined and compared with those of the corresponding benzothiazoles. The elimination of CS known to occur from these latter substances is confirmed in the case of benzisothiazole, but a different mechanism is proposed. The fragmentation of the selenium analogues led to ions of the same type as the benzothiazoles and benzisothiazole, through extrusion of molecules of the type CN—R, but loss of the heteroatom was not accompanied by loss of carbon. Furthermore, the isoselenazole structure seems to be more sensitive to electronolysis than its isomer. All six selenium heterocyclics investigated gave rise to two ions (130 and 132) resulting from a recombination of various fragments, suggesting that this is a characteristic feature of the electron-impact fragmentation of compounds of this type.     

SYNTHESE DU FOSTEDIL (4-(2-BENZOTHIAZOLYL) BENZYLPHOSPHONATE DE DIETHYLE) A PARTIR D'IMIDATES N-ACYLES

Fostedil (diethyl 4-(benzothiazol-2-yl) benzylphosphonate) 7 has been synthesized efficiently by treatment of 2-(4-bromomethylphenyl) benzothiazole 6 with triethylphosphite. The latter (6) has been prepared by bromation of benzothiazole 3c. N-acylimidates 1(a–d) react with 2-aminobenzenethiol 2 to lead to the corresponding benzothiazoles 3(a–d) after elimination of primary amide 5. The structure of these products have been unequivocally confirmed by means of IR, ¹H, ¹³C, and ³¹P NMR spectroscopy and mass spectra.     

Synthesis and Microbial Studies of New Pyridine Derivatives-Ill

2-Amino substituted benzothiazole 4a--4I and p-acetamidobenzenesulfonyl chloride 2 were used to prepare 2-（p-aminophenylsulfonamido） substituted benzothiazole 6a--6I using mixture of pyridine and acetic anhydride which formed an electrophilic complex （N-acetyl pyridinium） to facilitate condensation to give desired product by removal of HC1. 2-{p-[（3-Carboxypyrid-2-y1）amino]phenylsulfonamido}benzothiazoles 8a--81 were synthesized from 2-chloropyridine-3-carboxylic acid 7 and 6a--6I in 2-ethoxy ethanol using Cu-powder and K2CO3. Acid chlorides 9a--91 were condensed with 2-hydroxyethyl piperazine 10 and 2,3-dichloropiperazine 11 for amide deriva- tives 2-（p-（（3-（4-（2-hydroxyethy1）piperazin-1-ylcarbonyl）pyrid-2-y1）amino）phenylsulfonamido）benzothiazoes 12a -121 and 2-{p-[3-（2,3-dichloropiperazin-l-ylcarbonyl）pyrid-2-ylamino]phenylsulfonamido}benzothiazoles 13a- 131 respectively. The structures of the new compounds have been established on the basis of their chemical analysis and spectral data （IR, 1↑H NMR and mass）. All the compounds have been screened for their antibacterial and antifungal activities.     

Correlation of the mass spectral and photochemical behavior of some 1,3-dieneiron tricarbonyl complexes

The mass spectra of a number of 1,3-dieneiron tricarbonyl complexes have been investigated. It has been shown that complexes derived from substituted 1,3-cyclohexadienes lose hydrogen or methane in the mass spectrometer to lead predominantly to the formation of odd electron ions containing the iron atom bonded to an aromatic system. The fragmentation route followed by these complexes and those from acyclic dienes is compared.     

Chemical ionisation mass spectra of explosives

The chemical ionisation mass spectra of a number of representative explosives have been obtained using hydrogen as reagent gas. Fragmentation modes of the protonated molecular ions tend to be simple although some rearrangements have been noted. Loss of a radical from an even electron parent ion has been observed. The use of ‘ghost spectra’ for confirmation of the molecular weight of the sample is stressed: these consist of ions of low abundance resulting from reactions between sample ions and sample molecules. Labelling studies were carried out using deuterium as reagent gas.     

Mass spectrometric studies of some 4-acyl substituted 1-phenyl-3-methyl-5-pyrazolones

The electron-impact-induced fragmentation of 1-phenyl-3-methyl-5-pyrazolone and six 4-acyl substituted derivatives, have been studied with the aid of deuterium labelling, high resolution mass measurements and the metastable defocusing technique. The mass spectra of the pure keto and the pure enol forms do not show any significant differences. The predominant decomposition of the 4-acyl substituted pyrazolones is the α-cleavage with respect to the carbonyl group leading to intense peaks at m/e 201 by loss of the alkyl group. In the case of benzoyl and ethoxycarbonyl derivatives the α-cleavage is associated with a specific hydrogen rearrangement causing the loss of a benzene and an ethyl alcohol molecule, respectively.     

A comparative study between para‐aminophenyl and ortho‐aminophenyl benzothiazoles using NMR and DFT calculations

Ortho‐substituted and para‐substituted aminophenyl benzothiazoles were synthesised and characterised using NMR spectroscopy. A comparison of the proton chemical shift values reveals significant differences in the observed chemical shift values for the NH protons indicating the presence of a hydrogen bond in all ortho‐substituted compounds as compared to the para compounds. The presence of intramolecular hydrogen bond in the ortho amino substituted aminophenyl benzothiazole forces the molecule to be planar which may be an additional advantage in developing these compounds as Alzheimer's imaging agent because the binding to amyloid fibrils prefers planar compounds. The splitting pattern of the methylene proton next to the amino group also showed significant coupling to the amino proton consistent with the notion of the existence of slow exchange and hydrogen bond in the ortho‐substituted compounds. This is further verified by density functional theory calculations which yielded a near planar low energy conformer for all the o‐aminophenyl benzothiazoles and displayed a hydrogen bond from the amine proton to the nitrogen of the thiazole ring. A detailed analysis of the ¹H, ¹³C and ¹⁵N NMR chemical shifts and density functional theory calculated structures of the compounds are described. Copyright © 2014 John Wiley & Sons, Ltd.     

Hydrogen transfer in the retro Diels-Alder fragmentation of oxygen-containing heterocyclic Compounds. I—chromanones and 4-hydroxychromans

The mass spectra of 2,2-dimethylchromanones and 2,2-dimethyl-4-hydroxychromans show peaks corresponding to the ions formed by the retro Diels-Alder reaction with or without hydrogen transfer to the ion. The hydroxychromans also show peaks corresponding to an ion formed by the loss of hydrogen from the ion formed by the retro Diels-Alder reaction. Deuterium labelling showed that the hydrogen transferred in the chromanones arises from the gem-dimethyl groups on C-2, and the hydrogen atom lost in the case of the hydroxychromans is from C-4, the carbinyl carbon.