Lack of selectivity in the electrophilic addition of p-toluenesulfonylnitrene to tertiary amines. Conformational equilibrium in N-methylpiperidines

Ultrafast conformational dynamics of Rydberg-excited N-methyl piperidine

We have observed the ultrafast conformational dynamics of electronically excited N-methyl piperidine (NMP) using time-resolved Rydberg fingerprint spectroscopy. Optical excitation at various wavelengths ranging from 212 nm to 229 nm leads to the 3s or 3p Rydberg states and induces coherent oscillatory motions with periods of about 700 fs. These coherent motions survive the internal conversion from 3p to 3s but then dephase on a time scale of a few oscillations. Intramolecular vibrational energy redistribution on a picosecond time scale leads to an equilibrium between two conformeric structures that are separated in binding energy by 0.09 eV. Model calculations using the DFT-SIC method are in excellent agreement with the experiments and identify the conformers as the chair and twist structures of NMP. The analysis of the equilibrium parameters at long time delays as a function of excitation wavelength allows for the extraction of thermodynamic parameters for the conformeric transformation. We derive an enthalpy of the chair to twist reaction in the 3s excited state of 62 meV with an entropy of 19.70 J mol^-1 K^-1. An activation energy of 276 meV is also obtained with a kinetic model.

Mechanism of binding of fluoroquinolones to the quinolone resistance-determining region of DNA gyrase: towards an understanding of the molecular basis of quinolone resistance

We have studied the bacterial resistance to fluoroquinolones that arises as a result of mutations in the DNA gyrase target protein. Although it is known that DNA gyrase is a target of quinolone antibacterial agents, the molecular details of the quinolone-gyrase interaction remain unclear. The mode of binding of ciprofloxacin, levofloxacin, and moxifloxacin to DNA gyrase was analyzed by means of docking calculations over the surface of the QRDR of GyrA. The analysis of these binding models allows study of the resistance mechanism associated with gyrA mutations more commonly found in E. coli fluoroquinolone-resistant strains at the atomic level. Asp87 was found to be critical in the binding of these fluoroquinolones because it interacts with the positively charged nitrogens in these bactericidal drugs. The role of the other most common mutations at amino acid codon Ser83 can be explained through the contacts that the side chain of this residue establishes with fluoroquinolone molecules. Finally, our results strongly suggest that, although Arg121 has never been found to be associated with fluoroquinolone resistance, this residue makes a pivotal contribution to the binding of the antibiotic to GyrA and to defining its position in the QRDR of the enzyme.

Cyclic beta-amino acid derivatives: synthesis via lithium amide promoted tandem asymmetric conjugate addition-cyclisation reactions

The product distribution upon conjugate addition of homochiral lithium N-benzyl-N-alpha-methylbenzylamide to dimethyl-(E,E)-nona-2,7-dienedioate can be controlled to give either the cyclic 1,2-anti-1,6-anti-beta-amino ester (derived from conjugate addition and intramolecular enolate cyclisation) or the acyclic bis-beta-amino ester derivative (derived from double conjugate addition) in high de. The introduction of a protected nitrogen functionality into the diester skeleton facilitates, after conjugate addition and intramolecular enolate cyclisation, the asymmetric construction of piperidines in high de; variation in the N-protecting group indicates that the highest stereoselectivity is observed with alpha-branched N-substituents. Tandem conjugate addition-aldol reactions can also be achieved stereoselectively, with lithium amide conjugate addition to epsilon- and zeta-oxo-alpha,beta-unsaturated esters giving the corresponding five and six membered cyclic beta-amino esters in high de. N-deprotection by hydrogenolysis of the products arising from these reactions furnishes a range of polyfunctionalised transpentacin and transhexacin derivatives in high de and ee.