Search

The product of a revealed transformation-NHC-ethynyl coupling-was observed as a catalyst transformation pathway in the Sonogashira cross-coupling, catalyzed by Pd/NHC complexes. The 2-ethynylated azolium salt was isolated in individual form and fully characterized, including X-ray analysis. A number of possible intermediates of this transformation with common formulae (NHC)n Pd(C2 Ph) (n=1,2) were observed and subjected to collision-induced dissociation (CID) and infrared multiphoton dissociation (IRMPD) experiments to elucidate their structure. Measured bond dissociation energies (BDEs) and IRMPD spectra were in an excellent agreement with quantum calculations for coupling product π-complexes with Pd0 . Molecular dynamics simulations confirmed the observed multiple CID fragmentation pathways. An unconventional methodology to study catalyst evolution suggests the reported transformation to be considered in the development of new catalytic systems for alkyne functionalization reactions.

Uronic acids are important constituents of polysaccharides found on the cell membranes of different organisms. To prepare uronic-acid-containing oligosaccharides, uronic acid 6,3-lactones can be employed as they display a fixed conformation and a unique reactivity and stereoselectivity. Herein, we report a highly β-selective and efficient mannosyl donor based on C-4 acetyl mannuronic acid 6,3-lactone donors. The mechanism of glycosylation is established using a combination of techniques, including infrared ion spectroscopy combined with quantum-chemical calculations and variable-temperature nuclear magnetic resonance (VT NMR) spectroscopy. The role of these intermediates in glycosylation is assayed by varying the activation protocol and acceptor nucleophilicity. The observed trends are analogous to the well-studied 4,6-benzylidene glycosides and may be used to guide the development of next-generation stereoselective glycosyl donors.

RATIONALE: Deamidation of Asn and Gln residues is a primary route for spontaneous post-translational protein modification. Several structures have been proposed for the deamidation products of the protonated amino acids. Here we verify these structures by ion spectroscopy, as well as the structures of parallel and sequential fragmentation products. METHODS: Infrared ion spectroscopy using the free electron laser FELIX has been applied to the reaction products from deamidation of protonated glutamine and asparagine in a tandem mass spectrometer. IR spectra were recorded over the 800-1900 cm(-1) spectral range by infrared multiple-photon dissociation (IRMPD) spectroscopy. Molecular structures of the fragment ions are derived from comparison of the experimental spectra with spectra predicted for different candidate structures by density functional theory (DFT) calculations. RESULTS: [AsnH(+) - NH3](+) is found to possess a 3-aminosuccinic anhydride structure protonated on the amino group. The dissociation reaction involving loss of H2O and CO forms a linear immonium ion. For [GlnH(+)-NH3](+), the N-terminal nitrogen acts as the nucleophile leading to an oxo-proline product ion structure. For [GlnH(+)-NH3](+), a sequential loss of [CO + H2O] is found, leading to a pyrolidone-like structure. We also confirm by IR spectroscopy that dehydration of protonated aspartic acid (AspH(+)) and glutamic acid (GluH(+)) leads to identical structures as to those found for the loss of NH3 from AsnH(+) and GlnH(+). CONCLUSIONS: The structure determined for AsnH+ is in agreement with the suggested structure derived from measured and computed activation energies. IR ion spectra for the NH3-loss product from GlnH(+) establish that a different reaction mechanism occurs for this species as compared to AsnH(+). For both amino acids, loss of NH3 occurs from the side chain.