Stereochemical Analysis of Trifluoroacetamide Derivatives Based on Through-Space 1H-19F Spin-Spin Couplings.

In this study, the conformational properties of tertiary trifluoroacetamides in dibenzoazepine (1a and 1b) and benzodiazepine (2a and 2b) derivatives, which exist as equilibrated E- and Z-amide conformers in solution, were investigated by 1H and 19F NMR spectroscopy. In all cases, one of the methylene protons neighboring the nitrogen atom of the minor conformer showed a finely split pattern due to coupling with the trifluoromethyl fluorine atoms, as confirmed by 19F-decoupling experiments. One-dimensional (1D) and two-dimensional (2D) 1H-19F heteronuclear Overhauser spectroscopy (HOESY) experiments were performed to confirm whether these couplings are attributable to through-bond spin-spin couplings (TBCs) or through-space spin-spin couplings (TSCs). HOESY cross-peaks between CF3 (19F) and one of the CH2-N protons of the minor conformers indicate that the two nuclei are spatially close to each other, thus establishing the stereochemistry of the major (E-) and minor (Z-) conformers. The E-amide preferences of the trifluoroacetamides are consistent with the results of density functional theory calculations and X-ray crystallographic analyses. Furthermore, the otherwise incomprehensible 1H NMR spectra were accurately assigned using the HOESY-determined TSCs. The 1H NMR assignments of the E- and Z-methyl signals of N,N-dimethyl trifluoroacetamide, the simplest tertiary trifluoroacetamide, were revised for the first time in half a century.

Conformational Preference of 2'-Fluoro-Substituted Acetophenone Derivatives Revealed by Through-Space 1H-19F and 13C-19F Spin-Spin Couplings.

The conformational properties of 2'-fluoro-substituted acetophenone derivatives were elucidated based on Hα-F and Cα-F through-space spin-spin couplings (TS-couplings), which occur between two atoms constrained at a distance smaller than the sum of their van der Waals radii. This study revealed that 2'-fluoro-substituted acetophenone derivatives in solutions form exclusively s-trans conformers by analyzing their NMR spectra focused on the TS-couplings. The magnitudes of the coupling constants 5J (Hα, F) and 4J (Cα, F) correlate linearly with the value of the dielectric constant of the solvents. Furthermore, s-trans conformations of the two derivatives were confirmed by X-ray crystallographic analysis. These conformational preferences were consistent with the DFT calculations. The s-cis conformer, in which fluorine and oxygen atoms lie in a syn-periplanar mode, may be subject to strong repulsion between the two polar atoms and become unstable. The s-trans preference of the 2'-fluoro-substituted acetophenone derivatives may be utilized in drug design.

Chemical synthesis of homogeneous glycopeptides and glycoproteins.

Oligosaccharides linked to proteins are known to play important roles in several biological events. However, oligosaccharides are heterogeneous, which has hindered detailed elucidation of oligosaccharide functions. In order to solve this problem, glycoproteins having homogeneous oligosaccharides have long been required. For this purpose, an efficient preparative method of complex-type oligosaccharides has been investigated from a natural source and this method was found to afford over 24 kinds of diverse complex-type oligosaccharides by use of chemical methods and branch-specific sequential glycosidase digestion. The sufficient amount of homogeneous complex type oligosaccharides obtained enabled us to examine the synthesis of homogeneous glycopeptides as well as glycoproteins by use of solid phase glycopeptide synthetic method and native chemical ligation. This review describes recent progress related to the efficient method of oligosaccharide preparation and synthesis of glycoproteins including bioactive erythropoietin.

Convenient synthesis of a sialylglycopeptide-thioester having an intact and homogeneous complex-type disialyl-oligosaccharide.

Access to glycopeptides with C-terminal thioester functionality is essential for the synthesis of large glycopeptides and glycoproteins through the use of native chemical ligation. Toward that end, we have developed a concise method for the synthesis of a glycopeptide thioester having an intact complex-type dibranched disialyl-oligosaccharide. The synthesis employed a coupling reaction between benzylthiol and a free carboxylic acid at the C-terminus of a glycopeptide in which the peptide side chains are protected. After construction of glycopeptide on the HMPB-PEGA resin through the Fmoc-strategy, the protected glycopeptide was released upon treatment with acetic acid/trifluoroethanol and then the C-terminal carboxylic acid was coupled with benzylthiol at -20 degrees C in DMF. For this coupling, PyBOP/DIPEA was found to be the best for the formation of the thioester, while avoiding racemization. Finally, the protecting groups were removed in good yield with 95% TFA, thus affording a glycopeptide-thioester having an intact and homogeneous complex-type disialyl-oligosaccharide.