Chagit Denekamp

Preparative separation of stereoisomeric 1-methyl-4-methoxymethylcyclohexanecarboxylic acids by pH-zone-refining counter-current chromatography

The application of pH-zone-refining counter-current chromatography (CCC) to the preparative separation of stereoisomeric acids is described. The separation was accomplished on the basis of the difference in acidity of the two stereoisomers. pH-Zone-refining CCC of 400 mg of a crude synthetic mixture of stereoisomeric 1-methyl-4-methoxymethylcyclohexanecarboxylic acids yielded 49.5 and 40 mg of the pure Z- and E-stereoisomers respectively. The two-phase solvent system consisted of hexane-ethyl acetate-methanol-water (1:1:1:1). Trifluoro acetic and octanoic acids were used as retainer acids. The eluent base was aqueous ammonia. The eluted fraction were monitored by gas chromatography-mass spectrometry.

Homolytic cleavages in pyridinium ions, an excited state process

The favored fragmentation pathway for protonated and alkylated pyridinium cations of the general formula p-XC6H4CH2CH2CH=CH Py+R (R=H, Me; Py=pyridine) is a C-C homolytic cleavage. The tendency to form radicals is higher for alkylated pyridinium cations than for the protonated ones that can also afford closed-shell products. Theoretical calculations show that the singlet-triplet gap for transient structures with an elongated benzylic C-C bond is very low and the formation of radicals may result from mixing of these states. In addition to the notable substituent effect on the fragmentation efficiency of the cations under study, calculated results show a clear substituent effect on the singlet-triplet transitions. We also observe that triphenylphosphonium cations behave notably different. Thus, the pyridinium system that contains a p-chloro benzyl moiety loses a benzyl radical readily while the analogous triphenylphosphonium cation is very stable under the same conditions.

Towards a Fully Conjugated, Double-Stranded Cycle: A Mass Spectrometric and Theoretical Study

The two compounds, 1 and 5, are investigated by means of collision-induced dissociation experiments by using ion cyclotron resonance mass spectrometry and other mass spectrometric techniques as to their ability to act as precursors for the fully unsaturated double-stranded target compound 2. These experiments are complemented by flask-type pyrolyses of 5, the products of which are analyzed by mass spectrometry. For 1, no conditions were found under which the expected molecular ion of 2 at m/z 932 appeared, however, for its derivative 5 this was possible. This interesting finding is not in contradiction with the chemical structure of the long sought for compound 2 but calculations suggest that this compound may have isomerized into one where the conjugation is interrupted by hydrogen shift from the solubilizing alkyl chains into the cycles perimeter. The key driving force for such an isomerization would be the considerable relief of strain energy.

Mechanism of cross‐ring cleavage reactions in dirhamnosyl lipids: effect of the alkali ion

Liquid secondary ion mass spectrometry and high-energy collision-induced dissociation were used to analyze a dirhamnosyl lipid mixture. The negative fast-atom bombardment spectrum reveals a mixture of four homologous dirhamnosyl lipids with the following general structure: Rha-Rha-Cn-Cm (where Cn and Cm denote 3-hydroxy fatty acid moieties). The mass region 450-600 u in the collision-induced dissociation spectra of the negative [M - H]- ions shows product ions that can be rationalized by terminal loss of a 3-hydroxyalkanoic acid residue; these ions can be used for the characterization of the fatty acid substituents. A unique effect of alkali-metal ions on the course of fragmentation of dirhamnosyl lipid attachment ions was observed. The strong chelation of sodium is revealed from the stability of the [M - H + 2Na]+ ion that does not lose a sodium ion with the eliminated neutrals, contrary to what is observed for the dilithium adduct. Cross-ring cleavages occur during high-energy collision-induced dissociation of both positively and negatively charged precursor ions. The results suggest a concerted decomposition pathway involving the six-membered rings of the monosaccharide residues. The formation of cross-ring cleavage products, which retain the C10-C10 moiety during high-energy collision-induced dissociation of all the precursor ions that contain sodium or lithium, strongly supports a retro [2 + 2 + 2] mechanism.

Stereospecificity and Concertedness of Retro-Diels-Alder Fragmentation in Some Diester Systems Upon Chemical Ionization

Retro-Diels–Alder (RDA) fragmentation of cis- and trans-2,3-diethoxycarbonyl-5,6,7,8-dibenzo-bicyclo[2.2.2]octanes under isobutane and methane chemical ionization conditions is highly stereospecific, giving rise to protonated diethyl maleate and fumarate, respectively. This behaviour indicates a single-step concerted mechanism, analogous to the ground-state RDA process that occurs in neutral molecules in the condensed phase. The analogous dissociation is partially stereospecific in stereoisomeric endo-,exo- and trans-2,3-diethoxycarbonylbicyclo[2.2.1]heptanes and non-stereospecific in endo-,exo- and trans-2,3-diethoxycarbonyl-5,6-benzobicyclo[2.2.2]octanes, indicating the involvement of a stepwise mechanism in the latter two systems. The different behaviour of the above systems is explained in terms of the energy of the RDA fragmentation. The differentiation and quantitative estimates of protonated diethyl maleate and fumarate were obtained from collision-induced dissociation measurements.

Ion-neutral complexes formation and 1,3-proton transfer in the chemical ionization of alkylcyclohexyl benzoates

CI, CID, labelling experiments and DFT calculations are used for the elucidation of the mechanism for the decomposition of cyclohexyl benzoates, which proceeds through 1,3-H shift and two equilibrating ion-neutral complexes.