Synthesis of (2S)-isopropyl-5-alkynylpyrimidin-2-ones: precursors of β-aminoacids

The efficient palladium-catalyzed Suzuki-Miyaura cross-coupling reaction of (2S)-isopropyl-5-iodo-2,3-dihydro-4(H)-pyrimidin-4-one with, arylethynyl-, heteroarylethynyl-, and alkylethynyltrifluoroborate salts is reported. The standard protocol was evaluated and optimized in order to gain access to suitable precursors of enantiopure 2-substituted β-amino acids. The scope and limitations of this methodology are discussed.     

Crystal effects in the neutralization of He+ ions in the low energy ion scattering regime

Investigating possible crystal effects in ion scattering from elemental surfaces, measurements of the positive ion fraction P+ are reported for He+ ions scattered from single and polycrystalline Cu surfaces. In the Auger neutralization regime, the ion yield is determined by scattering from the outermost atomic layer. For Cu(110) P+ exceeds that for polycrystalline Cu by up to a factor of 2.5, thus exhibiting a strong crystal effect. It is much less pronounced at higher energies, i.e., in the reionization regime. However, there a completely different angular dependence of the ion yield is observed for poly- and single crystals, due to massive subsurface contributions in nonchanneling directions.     

Second-row anomeric interactions: The involvement of phosphorus

Proton NMR spectroscopy and x-ray crystallographic studies demonstrate the predominance of the axial conformer of 2-(diphenylphosphinoyl)-1,3-dithiane ( 1 ). Chemical equilibration of anancomeric models ( 3 ⇌ 4 ) allows quantitative determination of the conformational free energy in 1 , 1.0 kcal/mol, which corresponds to an anomeric effect of 2.64–3.74 kcal/mol, depending on the method used for estimating it. This value indicates that the anomeric effect operative in 1 may be among the largest yet measured. Five distinct rationalizations are considered to account for such a strong S-C-P(O) anomeric effect: (a) dipole-dipole interactions, (b) electrostatic, attractive interaction between the phosphoryl oxygen and the syn-axial hydrogens in 1 -axial, (c) delocalization of the lone pair on the endocyclic heteroatom into the antiperiplanar (axial) adjacent polar C-P bond, (d) through-space 3p-3d electron donation from sulfur to phosphorus, and (e) repulsive interaction between the lone pairs on sulfur and on the phosphoryl oxygen. All these hypotheses have some experimental and/or the-oretical support, but evidence contrary to expectation can be found also to argue against each one of them. The conformational preference of the diphenylthiophosphinoyl group in the 1,3-dithian-2-yl ring was determined by nuclear magnetic resonance (NMR) analysis and by chemical equilibration of diastereomeric models. The slight predominance of axial 2-(diphenylthiophosphinoyl)-1,3-dithiane ( 13 -ax) over 13 -eq reflects nonetheless the influence of a strong S-C-P anomeric interaction, worth ca. 2.3 kcal/mol. Spectroscopic evidence for the predominance of the equatorial conformers in 2-(diphenylphosphinoyl)-1,3-oxathiane ( 19 ) and 2-(diphenylphosphinoyl)-1,3-dioxane ( 22 ) was confirmed by the study of derivatives containing counterpoise substitutents, or by chemical equilibration on anancomeric models. ΔD°_27°C [P(O)Ph₂] = -3.23 kcal/mol was determined for the dioxane (chloroform solution), and ΔG°C_55°C [P(O)Ph₂] = -1.42 kcal/mol for the oxathiane (ethanolic solution). Nevertheless, evaluation of the different steric requirements in 1, 13 and 22 reveals that the magnitude of the O-C-P(O) and S-C-P(O) anomeric effects in the heterocycles is, in fact, quite similar, close to kcal/mol.