Configurational assignment of fragment ions by CID. Stereochemistry and mechanism of retro-Diels–Alder fragmentation accompanied by hydrogen transfer in gas-phase cations

Low energy CID mass spectra of m/z 173, C₈H₁₃O₄⁺, obtained from the diethyl ester of cis,syn,cis-l,2,3,3a,4,5,5a,6,7,8-decahydroindacene-4,5-dicarboxylic acid and cis,syn,cis-l,2,3,4,5,6,7,8,8a,9,10,10a-dodecahydrophenanthrene-9,10-dicarboxylic acid indicate they have the structure of protonated diethyl maleate. This finding together with previous deuterium labelling results suggest that the formation of this ion from both precursors takes place by migration of a hydrogen atom from an allylic δ-position followed by the concerted cleavage of two allylic C? C bonds in analogy to the ground state retro-Diels–Alder fragmentation.     

Constituents of Catha edulis: Isolation and structure of cathidine D

Work which has recently appeared on the structures of Celastraceae alkaloids in addition to physical and chemical evidence adduced with respect to cathidine D permits formulation of structure 7a or 7b for this component of Catha edulis.     

Stereospecific elimination of acetic acid in 3- and 4-arylcyclohexyl acetates under electron impact

The elimination of CH₃COOH from the molecular ions of trans-3- and 4-arylcyclohexyl acetates takes place to a greater extent than in the cis isomers. Deuterium labelling shows that the elimination involves mainly the benzylic hydrogen in the trans-acetates, but not in the cis isomers. This behaviour is similar qualitatively to that of the corresponding alcohols and methyl ethers, but entirely different from that of t-butylcyclohexyl acetates, which do not exhibit any stereospecificity. Substituent effects on the elimination for both cis and trans isomers are discussed.     

Stereoselective H2O eliminations in 3- and 4-arcylcyclohexanols under electron impact

The elimination of H20 from the molecular ions of trans-4- and trans-3-arylcyclohexanois takes place to a greater extent than in the corresponding cis isomers. The remarkable differences in abundance taken together with substituent effects and the results of deuterium labelling, show that configuration is retained in the molecular ions which undergo the elimination, and that this process is a cis-1,4 and cis-1,3 elimination in the trans-4- and trans-3- arylcyclohexanois, respectively. Possible mechanisms for the elimination in the cis isomers are discussed.     

Structural assignment of isomeric 2-(2-quinolinyl)-1H-indene-1,3(2H)-dione mono- and disulfonic acids by liquid chromatography electrospray and atmospheric pressure chemical ionization tandem mass spectrometry

Positionally isomeric 2-(2-quinolinyl)-1H-indene-1,3(2H)-dione mono- and disulfonic acids give rise to similar electrospray ionization (ESI) and atmosphere pressure chemical ionization (APCI) mass spectra, which show very abundant MH(+) ions and negligible fragmentation. The MH(+) ions of these isomeric acids exhibit notably different behavior under collision-induced dissociation (CID) conditions. The acids with a sulfonic group at position 8' in the quinoline moiety, adjacent to the N-atom, exhibit highly abundant [MH - H(2)SO(3)](+) ions (m/z 272 for the mono- and m/z 352 for the disulfonic acids), which are of lower abundance in the CID spectra of isomers with the SO(3)H group at other positions, remote from the nitrogen atom. The latter isomers undergo efficient eliminations of SO(3) and HSO(3). The isomeric diacids with one SO(3)H group at position 4 of the indene-1,3(2H)-dione moiety, adjacent to one of the carbonyl groups, undergo highly efficient elimination of H(2)O. Mechanistic pathways, involving interactions between adjacent groups, are proposed for the above regiospecific fragmentations. Pronounced different behavior has been also observed in negative ion tandem mass spectrometric measurements of the sulfonic acids. The distinctive behavior of the isomeric acids was strongly pronounced when the measurements were performed with an ion trap mass spectrometer (LCQ), and much less so with a triple-stage quadrupole instrument (TSQ).     

Studies in mass spectrometry—XV: Retro-diels-alder fragmentation and double hydrogen migration in some polycyclic diketones

In contrast to adducts I of bi-1-cycloalken-1-yls and p-benzoquinone, their reduction products II do not exhibit a double hydrogen migration from δ positions accompanying a ‘retro-Diels-Alder’ type fragmentation. An ordinary retro-Diels-Alder fragmentation was found to take place, with charge retention in the diene portion of the molecule. A double hydrogen migration has been detected in II leading to m/e 112 ion c₂, which differed, however, from that in I in charge retention and in the origin of the migrating hydrogen atoms. Adducts III of di-1-cycloalken-1-yls and naphthoquinone behave similarly to II. They exhibit relatively low abundance ions a, however, due to a double hydrogen migration from δ positions, similarly to I. The origin of the migrating hydrogen atoms have been determined by deuterium labelling. Mechanistic suggestions are presented to explain the observed facts.     

Effect of intramolecular crosslinker properties on the mechanochemical fragmentation of covalently folded polymers

The mechanochemical stability of polymers in solution is enhanced if the chains are covalently folded. Under shear forces, the additional bonds absorb mechanical energy and inhibit unfolding, and as a result, slow down fragmentation. However, not all crosslinkers are equal in terms of their properties (length, strength, etc.). In order to understand the role of these added bonds in the polymers' stability under mechanical stress, a thorough study compares the rate of mechanochemistry on single-chain polymer nanoparticles which have been folded with crosslinkers with different lengths, strengths, positioning, and valencies. The usage of bonds with different mechanical strengths in the crosslinkers was found to be the most powerful way to change the mechanochemical fragmentation rate. In addition, positioning and valency also play significant role in the mechanical stabilization mechanism. © 2020 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2020 © 2020 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2020 , 58, 692–703