Shuxia Cao

Unexpected insertion of CO2 into the pentacoordinate P–N Bond: Atherton–Todd-type reaction of hydrospirophosphorane with amines

The Atherton-Todd-type reaction of pentacoordinate hydrospirophosphoranes with amines was investigated, and a novel CO2 insertion reaction into the pentacoordinate P-N bond under mild conditions was developed. The mechanism and stereochemistry of the CO2 insertion reaction between hydrospirophosphoranes and secondary amines were proposed via a carbon-13 labeling experiment, a (31)P NMR tracing experiment, and X-ray diffraction analysis. The chlorinated spirophosphorane intermediate was first generated with stereoretention of the configuration at phosphorus and subsequently was attacked by a carbamate anion formed from CO2 and a secondary amine. It was found that rear attack of nucleophilic substitution with stereoinversion at pentacoordinate phosphorus was the preferred route, although front attack happened for sterically hindered reactants. The configuration of the CO2 insertion product depended mainly upon the original phosphorus configuration of the hydrospirophosphoranes.

Fragmentation of deprotonated cyclic dipeptides by electrospray ionization mass spectrometry

The fragmentation pathways of deprotonated cyclic dipeptides have been studied by electrospray ionization multi-stage mass spectrometry (ESI-MSn) in negative mode. The results showed that the fragmentation pathways of deprotonated cyclic dipeptides depended significantly on the different substituents, the side chains of amino acid residues at the diketopiperazine ring. In the spectra of deprotonated cyclic dipeptides, the ion [M-H-substituent radical]- was firstly observed in the ESI mode. The characteristic fragment ions [M-H-substituent radical]- and [M-H-(substituent-H)]- could be used as the symbols of particular cyclic dipeptides. The hydrogen/deuterium (H/D) exchange experiment, the high-resolution mass spectrometry (Q-TOF) and theoretical calculations were used to rationalize the proposed fragmentation pathways and to verify the differences between the fragmentation pathways. The relative Gibbs free energies (DeltaG) of the product ions and possible fragmentation pathways were estimated using the B3LYP/6-31++G(d, p) model. The results have some potential applications in the structural elucidation and interpretation of the mass spectra of homologous compounds and will enrich the gas-phase ESI-MS ion chemistry of cyclic dipeptides.

ESI-MSn study on the fragmentation of protonated cyclic-dipeptides

Cyclic dipeptides are relatively simple compounds that can exhibit a great variety of important biological activities. The fragmentation pathways of protonated cyclic dipeptides have been studied by electrospray ionization multistage mass spectrometry (ESI-MS n ). The mass spectra studies of the cyclic dipeptides showed that the cyclic dipeptides with the similar substituents, the side chains of amino acid residues at the diketopiperazine ring, followed the same fragmentation pathway. In the fragmentation spectra of protonated cyclic dipeptides, some characteristic fragment ions were observed and could be used to distinguish the cyclic dipeptides. The hydrogen/deuterium (H/D) exchange experiment and the high-resolution mass spectrometry (Q-TOF) were used to verify and rationalize the proposed fragmentation pathways. These observations may have some potential applications in the structural elucidation and interpretation of the mass spectra of homologous compounds.

Fragmentation studies of pentacoordinated bisaminoacylspirophosphoranes by negative electrospray ionization mass spectrometry

Fragmentation pathways of a series of pentacoordinated bisaminoacylspirophosphoranes were elucidated by electrospray ionization multistage mass spectrometry (ESI-MS(n)) in negative mode. The deprotonated ions of pentacoordinated bisaminoacylspirophosphoranes tend to eliminate a corresponding amino acid to form base peak. The hydrogen/deuterium exchange experiment, the high-resolution mass spectrometry, (13)C stable isotope labeling experiment and theoretical calculations were used to rationalize the proposed fragmentation pathways and to verify the differences between the fragmentation pathways. The results indicate that the negative molecular ions of pentacoordinated bisaminoacylspirophosphoranes dissociate through its open-chain tricoordinated tautomers. The relative Gibbs free energies (ΔG) of the product ions and proposed fragmentation pathways were estimated using the B3LYP/6-31 + + G(d, p) model. The results have some potential applications in the identification structures of similar spirophosphorane compounds by ESI-MS(n).

Synthesis of a series of novel chiral smectic C (Sc*) phase shish-kebab type liquid crystalline polymers†

A series of chiral smectic C phase shish-kebab type liquid crystal polymers was synthesized by low-temperature solution condensation polymerization from 2,5-bis[4-((S)-alkoxyl)benzoyloxy]hydroquinone and aliphatic diacylchloride. The monomers and their precursors were identified by using elemental analysis, infrared spectrum, nuclear magnetic resonance and mass spectrometry. The polymers were characterized by gel permeation chromatography, differential scanning calorimetry, thermogravimetric analysis, temperature-variable X-ray diffraction, polarimeter and polarizing microscope (POM) with a heating stage. All the polymers entered into liquid crystal phase when heated to above their melting temperature. The Schlieren texture and sanded texture were observed on POM. All the chiral compounds and polymers showed high optical activity. Temperature-variable, X-ray diffraction study together with the POM and polarimetric analysis revealed that the polymers synthesized are chiral smectic C phase. Thus, the present report provides examples of shish-kebab type polymers that form a chiral smectic C phase. The change of the melting temperature and isotropization temperature with the variation in molecular structure was also discussed. Copyright

The 3JCCNP Coupling Constants of Pentacoordinate Spirophosphorane Derivatives: As a Method to Assign Relative Configuration

GRAPHICAL ABSTRACT Abstract The phosphorus-carbon coupling constant 3JCCNP was found to be sensitive to the relative configuration of pentacoordinate spirophosphorane diastereoisomers, while the 2JCNP value remained almost unchanged. The 3JCCNP value of the spirophosphorane with ΛP configuration at phosphorus was obviously larger than that with ΔP configuration. The rule, 3JCCNP (ΛP) > 3JCCNP (ΔP), for pentacoordinate spirophosphorane diastereoisomers was used to assign the configurations of new alkoxy spirophosphorane derivatives, and satisfactory results were achieved. The results showed that the coupling constant 3JCCNP might be used as a reliable probe for the configuration of pentacoordinate spirophosphorane derivatives.

The investigation of β-cyclodextrin noncovalent complex with protein or dipeptide by electrospray ionization mass spectrometry

Abstract Electrospray ionization ion trap mass spectrometry was used for the investigation of noncovalent complexes formed between β‐cyclodextrin (CD) and protein or dipeptide in aqueous solution. After one of those noncovalent complexes has formed, it was found that addition of the other ligand compound would produce the competitive noncovalent complexes. Measurements were obtained with four proteins, such as hen egg lysozyme, cytochrome c, insulin and RNase A, and three dipeptides, respectively. The data obtained from the investigated systems show that ESI‐MS is not only a reliable, sensitive, and fast technique for the analysis of the β‐CD/protein and β‐CD/dipeptide complexes, but also an interesting tool to observe the transformation from one complex to the other in aqueous solution.

Comparison of Non-covalent Interactions Between a Series of N-Phosphoryl Dipeptide or Methyl Esters and Protein by Electrospray Ionization Mass Spectrometry

The non-covalent interaction between a series of N-phosphoryl dipeptides (or methyl esters) (DPP) and protein was studied by ESI-MS and UV–vis spectrometer. The function of different groups in DPP and binding sites of protein were investigated. The results revealed that hydroxyl and aromatic ring in DPP were both important group for the interaction, and aromatic ring had double functions on the interaction. In addition, the molecular size, flexibility and steric hindrance showed obvious effects on the interaction, while, the chirality, sequence and length of carbon chains (changing 1–2C) of amino acid residue in DPP showed little effects on the interaction under the experimental conditions. Phosphoryl oligopeptides having extended structure, good molecular flexibility and smaller spatial hindrance could contract the protein conformation in solution. The aromatic, basic, acid and amide amino acid residues of protein may be the main binding sites and contributed to the survival of the complexes.

The investigation of substituent effects on the fragmentation pathways of pentacoordinated phenoxyspirophosphoranes by ESI-MSn

The fragmentation pathways of pentacoordinated phenoxyspirophosphoranes were investigated in the positive mode by electrospray ionization multistage mass spectrometry. The results demonstrate that the sodium adducts of the title compounds undergo two competitive fragmentation pathways, and the fragmentation patterns are heavily dependent on the various substituent patterns at the phenolic group. An electron-withdrawing substituent at the ortho-position always results in the removal of a corresponding phenol analogue, while cleavage by spiroring opening becomes the predominant fragmentation pathway if an electron-donating substituent is at the phenolic group. The substituent effects on the competitive fragmentation pathways were further elucidated by theoretical calculations, single crystal structure analysis, and high-resolution mass spectrometry. The results contribute to the understanding of the gas-phase fragmentation reactions and the structure identification of spirophosphorane analogues by electrospray ionization multistage mass spectrometry.

Fragmentation studies of sartans by electrospray ionization mass spectrometry

Sartans and related analogues with 5-oxo-l, 2, 4-oxadiazole ring and tetrazole ring are investigated in detail using collision-induced dissociation (CID) method in positive ion mode by electrospray ionization tandem mass spectrometry (ESI-MSn ). It is found that the protonated sartans and related analogues tend to form the N-substituted-3-substituted phenanthridin-6-amine ion which has a large conjugative structure. The possible fragmentation pathways were proposed for the first time, and the key structure of product ions was confirmed by high resolution tandem mass spectrometry and theoretical calculation. It is very helpful for understanding the intriguing roles of sartans analogues in fragmentation reactions and enriching the knowledge of the gas-phase chemistry of the oxadiazole and tetrazole ring. Copyright