Base-catalyzed reaction of 2,6-dibenzylidenecyclohexanone and alkylguanidines gave 2-alkylamino-8-benzylidene-4-phenyl-3,4,5,6,7,8-hexahydroquinazolines IVa-c, which were oxidized to 2-alkylamino-8-benzylidene-4-phenyl-5,6,7,8-tetrahydroquinazolines Va-c. The acylamino (VIa-c, VIIa-d, IXa,b) and diacylamino derivatives (VIIIa,b) of the 2-amino-8-benzylidene-4-pheny]-3,4,5,6,7,8-hexahydroquinazoline (II), 2-amino-8-benzylidene-4-phenyl-5,6,7,8-tetrahydroquinazoline (III) and compounds Va-c have also been prepared. These compounds having the E-configuration were converted into the Z isomers XIIa-e by photoisomerization. The structures were confirmed by spectroscopic methods (IR, 1H NMR, 13C NMR).

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The intrinsic mechanism of circumambulatory rearrangements of 5‐bromo‐5‐methyl‐1,2,3,4‐tetramethoxycarbonylcyclopentadiene, 5‐bromo‐1,2,3,4 5‐pentamethoxycarbonylcyclopentadien and 5‐bromo‐ 1,2,3,4,5‐pentaphenylcyclopentadiene due to sigmatropic shifts of bromine over the cyclopentadiene ring was proved, using the dynamic 13C and 1H NMR technique, to be governed by successive intramolecular 1,5‐sigmatropic shifts. Semi‐empirical AM1 and MINDO/3 calculations of reactions paths performed for fluoro‐, chloro‐ and bromocyclopentadienes are in accord with the conclusion of a preference for a 1,5‐ over a 1,3‐shift reaction path of halogen migration over a cyclopentadiene system. Intramolecular 1,5‐sigmatropic shifts of chlorine in 5‐chloro‐5‐methyl‐1,2,3,4‐tetramethoxycarbonylcyclopentadiene with the free energy barrier of ΔG 298+ = 26.1 kcal mol−1 were studied using 1H NMR spectroscopy.

PACS. 73.20.At -Surface states, band structure, electron density of states. PACS. 73.23.Ad -Ballistic transport. PACS. 73.50.-h -Electronic transport phenomena in thin films.Abstract. -We present fully quantum-mechanical magnetotransport calculations for shortperiod lateral superlattices with one-dimensional electrostatic modulation. A non-perturbative treatment of both magnetic field and modulation potential proves to be necessary to reproduce novel quantum oscillations in the magnetoresistance found in recent experiments in the resistance component parallel to the modulation potential. In addition, we predict oscillations of opposite phase in the resistance component perpendicular to the modulation not yet observed experimentally. We show that the new oscillations originate from the magnetic miniband structure in the regime of overlapping minibands.

The solution structure of eight cyclic pentapeptides has been determined by two-dimensional 1H-NMR spectroscopy combined with spectra simulations and restrained molecular dynamic simulations. Six of the cyclic pentapeptides were derived from the C-terminal cholecystokinin fragment CCK-4 enlarged with Asp1 resulting in the sequence (Asp-Trp-Met-Asp-Phe), one L-amino acid after the other was substituted by its D-analog. In addition, two peptides, including an all-L-amino-acid-containing cyclic pentapeptide, cyclo(Asp-Phe-Lys-Ala-Thr) and cyclo(Asp-Phe-Lys-Ala-D-Thr) were investigated. All D-amino-acid-containing peptides show beta II'-turn conformations with the D-amino acid in the i + 1 position, excepting the D-aspartic-acid-containing peptides. These two peptides are characterized by the lack of beta-turns at pH values less than 4, suggesting that D-aspartic acid in the full-protonized state avoids the formation of beta-turns in these compounds. At pH values greater than 5, a conformational change into the beta II'-turn conformation was also observed for these peptides. Conformations without beta-turns are expected for cyclic all-L pentapeptides, but both cyclo(Asp-Phe-Lys-Ala-Thr) and the D-Thr analog cyclo(Asp-Phe-Lys-Ala-D-Thr) exhibit beta II'-turn conformations around Thr-Asp and D-Thr-Asp. Thus cyclic all-L pentapeptides and those with one D-amino acid are able to form similar structures preferably with a beta II'-turn. The beta-turn formation in cyclic pentapeptides containing a D-aspartic acid is dependent on the ionization state. The relevance of the work to the design of beta'-turn mimetics is discussed.