Mass spectrometry in structural and stereochemical problems–CXCIV: The mass spectrometric fragmentations of steroidal sapogenins

The characteristic mass spectral fragmentation patterns of the basic structure of the steroidal sapogenin, (25R)-5α-spirostan, have been elucidated through the preparation of analogs with deuterium labels at positions 11, 12, 14, 15, 16, 17, 20, 21, 23, 24, 25, 26 and 27. In addition, the effects of a change of stereochemistry at positions 14 and 20, of the introduction of oxygencontaining functionalities mostly in ring F, and of the incorporation of olefinic unsaturation have been determined through synthesis of many examples.     

Mass spectrometry in structural and stereochemical problems—CLXXIV:1 Electron-impact induced fragmentation of some aryl- and alkylsulfonylthioureas

The mass spectra of five aryl (I to V) and four alkylsulfonylthioureas (VI toIX) have been recorded and mechanistic rationalizations are suggested for their principal fragmentation processes. The aryl analogs exhibited peaks in their mass spectra corresponding to skeletal rearrangements with elimination of SO₂ from their molecular ions but this fragmentation was absent in those alkylsulfonylthioureas (VI to IX) examined.     

Mass spectrometry in structural and stereochemical problems—CLVIII electron impact promoted fragmentation of triphenylmethyl ethers

The triphenylmethyl (trityl) moiety is frequently used for the protection of alcohols but the mass spectra of such trityl ethers have hitherto escaped scrutiny. It has now been found that triphenylmethyl derivatives of primary alcohols yield abundant molecular ions which permit the determination of the isotopic purity of the parent alcohol. Upon electron impact the triphenyl-methyl entity directs the fragmentation of trityl ethers as demonstrated by a detailed study of n-pentyl trityl ether and its deuterated analogs. Ions formed by migration of phenyl groups were observed in the mass spectra of the trityl ethers investigated as well as in the spectrum of triphenyl-methane itself.     

Mass spectrometry in structural and stereochemical problems—XXXIII Colchicine alkaloids

Mass spectra have been measured for a series of colchicine alkaloids and correlations between spectra and structure have been made. The results indicate that mass spectra will be of assistance in structure determination of such naturally occurring tropolones.     

Mass spectrometry in structural and stereochemical problems—CXCIX Contrasting fragmentations of singly- and doubly-charged o-, m- and p-hydroxyalkylphenones and their trimethylsilyl ethers

High resolution mass spectrometry, metastable defocusing and deuterium labeling of trimethylsilyl (TMS) ethers have been used to study the electron-impact induced fragmentations of o-, m- and p-hydroxyalkylphenones and their TMS ether derivatives. These derivatives have proven useful in contrasting the fragmentation patterns of singly- and doubly-charged ions because of the competing fragmentations: α-cleavage and a McLafferty rearrangement from the ketone moiety and methyl cleavage from the TMS group. A proximity effect was responsible for a markedly increased methyl radical loss from the o-TMS ether. This fragmentation was minor with the m- and p-isomers. Significantly intense doubly-charged ions were formed from ketonic cleavage and by the loss of a TMS methyl radical. The sequence of fragmentation depended on the size of the alkyl group attached to the ketone carbonyl. There was no evidence found for a McLafferty rearrangement occurring from the doubly-charged molecular ion of the TMS ethers of the hydroxyalkylphenones but the rearrangement occurred from the doubly-charge molecular ion of bis-3-(1-oxopentyl)-4-hydroxy-phenyl-methane and, of course, from the singly charged [M]^+. The bis-p-hydroxyphenylmethane derivatives were studied in an effort to increase the intensity of the doubly-charged ions as it was expected that the charges would be separated by a longer distance.     

Mass spectrometry in structural and stereochemical problems–CXCIII: On the combined hydroxyl and water loss in nitrophenylhydrazones

The mass spectrum of valeraldehyde o-nitrophenylhydrazone displays only a low intensity peak (3% relative abundance) corresponding to the loss of 35 mass units from its molecular ion contrary to data previously published. Mention is also made of the fragmentation of 5-nonanone 2,4-dinitrophenylhydrazone in view of a recent challenge to a published rationalization for the electron impact promoted fragmentation for this compound.     

Mass spectrometry in structural and stereochemical problems—CCXXIII:ortho interactions of nitro groups

The mass spectra of C¹³ labeled o- nitrobenzoic acid and o-nitrobenzaldehyde were studied. Earlier proposed fragmentations involving group migration to a charge-carrying vacant ortho position were verified. A completely unexpected interaction in o- nitroanisole was uncovered by O¹⁸labeling.     

Mass spectrometry in structural and stereochemical problems—XXXVII. Tropone and tropolone derivatives

The general observation is made that on electron impact the molecular ions of tropone derivatives eliminate carbon monoxide as a neutral molecule and the resultant ions behave as benzene derivatives. Exceptions from this rule can be explained in terms of the presence of specific structural features.     

Mass spectrometry in structural and Stereochemical problems—XXXIV Aromatic methyl and ethyl ethers

The behavior under electron impact of a series of aromatic methoxy and ethoxy compounds has been observed. The more common fragmentation processes are interpreted and in some cases the differences among spectra of positional isomers can be explained.     

Mass spectrometry in structural and stereochemical problems—CLXXII: The electron-impact promoted fragmentation of 1,2-cyclohexanediol

The mass spectra of 1,2-cyclohexanediol and important data from the spectra of four deuterium labeled analogs have been recorded and mechanistic rationalizations are given for the most important degradation processes observed. Multiple mechanisms are required to explain the indicated peak shifts in the labeled analogs. Particularly relevant is the initial loss of water from the molecular ion, which involves at least four distinct processes.     

Mass spectrometry in structural and stereochemical problems—CLXXXVII A study of skeletal rearrangements in chromans by combined 13C and deuterium labeling

Chroman (I), 2,2-dimethylchroman (II), flavan (III) and 2,2-diphenylchroman (IV) were labeled with ¹³C or deuterium at every position of importance. The mass spectra of these labeled compounds make possible a detailed interpretation of some very complex carbon skeleton rearrangements. The combination of deuterium and ¹³C-labeling revealed that loss of CH₃, C₂H₅ and C₇H₇ fragments from chroman (I), 2,2-dimethylchroman (II) and 2,2-diphenylchroman (IV) respectively, occurs by multiple pathways involving aryl or alkyl migrations with little hydrogen randomization prior to fragmentation. Although ejection of C₇H₇ from flavan (III) occurs to a small extent by a route similar to those mentioned above, its [M ? C₇H₇] fragment can also be formed by a pathway involving no phenyl migration but extensive hydrogen scrambling. The validity of the schemes proposed on the basis of the ¹³C labeling data was checked by using them to predict the spectra of the deuterium labeled compounds.     

Mass spectrometry in structural and stereochemical problems—CLXXVIII: The electron-impact promoted fragmentation of 1,2-cyclohexene oxide

The mass spectra of 1,2-cyclohexene oxide and three deuterium labeled analogs are discussed and mechanistic rationalizations are given for the major fragmentation processes observed. Multiple mechanisms are required to explain the indicated peak shifts in the labeled analogs for many of these ions, thus demonstrating again the care that needs to be exercised in the interpretation of the mass spectra of relatively simple molecules.     

Mass spectrometry in structural and stereochemical problems—CLXIV. Electron impact promoted fragmentation of some thiosemicarbazones

The mass spectral fragmentation of thiosemicarbazone derivatives of some typical aliphatic, alicyclic and aromatic aldehydes and ketones has been rationalized using high resolution mass spectrometry supplemented by deuterium labeling. Thiosemicarbazone derivatives of carbonyl compounds yield mass spectra which have little in common with those generated by their semicarbazone analogs.     

Mass spectrometry in structural and stereochemical problems—CLXXXIV: Charge localization in fragment ions of amino ketones and esters

The mass spectral fragmentation of a number of specially synthesized amino esters and ketones has been studied in which the distance between the two functionalities has been varied. The principal fragments formed following initial ionization are the even-electron immonium ions a and b. Further fragmentation of ion a in the amino esters occurs via non-specific processes. Therefore it is difficult to establish the degree of mobility of the positive charge in the fragment ions, notably whether the species with the charge present on the oxygen atom makes a significant contribution to the fragmentation. The β-(IIb) and γ-(IV) amino ketones show no McLafferty rearrangements after α-fission, thus demonstrating that uncoupling of the carbonyl π electrons is not a feasible trigger in lieu of charge localization.     

Mass spectrometry in structural and stereochemical problems—CCXVI: Anomalous cleavage ions in bifunctional compounds resulting from participative interaction

Two types of remote group interaction after electron-impact are possible: one involving the transfer of an atom or group of atoms and the other involving expulsion of a group of atoms concomitant with ring closure. These interactions were examined in a homologous series of polyfunctional compounds at high and low ionizing voltages. Fragmentation resulting from remote group interaction in hydroxy ethyleneketals occurs exclusively via 5-, 6- and 7-membered rings. This is contrasted with the remote group interaction of keto esters, diesters and other literature examples in which ring size (e.g. 5-15-membered) plays no significant role.     

Mass spectrometry in structural and stereochemical problems–CXCV: The structure of the [C6H6O]+˙ ion in the spectrum of phenyl alkyl ethers

The [C₆H₆O]⁺˙ ion from C₆H₅OC₄H₉ is structurally similar to the molecular ion from phenol as demonstrated by ¹³C labeling.     

Mass spectrometry in structural and stereochemical problems—CLXVI. The electron impact promoted fragmentation of some aliphatic 1,2-glycols

The mass spectra of nineteen aliphatic 1,2-glycols containing a variety of substituents have been recorded and mechanistic rationalizations are presented for the more important degradation processes observed. Fragmentation of the hydroxylated carbon-carbon bond in these compounds occurred to an appreciable degree often accompanied by the transfer of a hydroxylic hydrogen atom.     

Mass spectrometry in structural and stereo-chemical problems—CCXXIV: Fragmentation of cyclopropanes

Generalizations for interpreting the mass spectra of cyclopropanes are enumerated and are based on the examination of many deuterium labeled substrates. Most cyclopropyl fragmentations can be visualized as resulting from ionization of the most substituted cyclopropyl bond in which the positive charge is localized on the most substituted carbon atom. Subsequent hydrogen shifts and/or simple scissions produce an array of predictable fragment ions.