The meso Helix: Symmetry and Symmetry-Breaking in Dynamic Oligourea Foldamers with Reversible Hydrogen-Bond Polarity.

Oligoureas (up to n=6) of meso cyclohexane-1,2-diamine were synthesized by chain extension with an enzymatically desymmetrized monomer 2. Despite being achiral, the meso oligomers adopt chiral canonical 2.5-helical conformations, the equally populated enantiomeric screw-sense conformers of which are in slow exchange on the NMR timescale, with a barrier to screw-sense inversion of about 70 kJ mol(-1) .

Huge electro-/photo-/acidoinduced second-order nonlinear contrasts from multiaddressable indolinooxazolodine.

In this work, linear and nonlinear optical properties of electro-/acido-/photoswitchable indolino[2,1-b]oxazolidine derivatives were investigated. The linear optical properties of the closed and open forms have been characterized by UV-visible and IR spectroscopies associated with DFT calculations. Nonlinear optical properties of the compounds have been obtained by ex situ and in situ hyper-Rayleigh experiments in solution.

Unambiguous Determination of the Absolute Configuration of Dimeric Stilbene Glucosides from the Rhizomes of Gnetum africanum.

Dimeric stilbene glucosides 1-3 [two diastereomers of (-)-gnemonoside A (1a and 1b), (-)-gnemonoside C (2), and (-)-gnemonoside D (3)] as well as a mixture of the two enantiomers of gnetin C (4) were isolated from the rhizomes of Gnetum africanum. The two enantiomers of gnetin C, (+)-4 and (-)-4, were obtained from the aglycones of 1a and 1b, respectively. The configurations of these stilbenoids were investigated by NMR and vibrational circular dichroism (VCD) experiments.

Chiroptical properties of nona- and dodecamethoxy cryptophanes.

Enantiopure cryptophane derivatives bearing nine (2, 3) and 12 (4) methoxy substituents attached on the six aromatic rings were separated by HPLC using chiral stationary phases. The chiroptical properties of compounds 2-4 were determined from polarimetry, electronic circular dichroism (ECD), and vibrational circular dichroism (VCD) experiments and were compared to those of cryptophane-A (1) derivative. ECD spectra of 1 and 4 were calculated by time-dependent density functional theory (TDDFT) to determine the absolute configuration (AC) of cryptophane derivatives.

Raman optical activity of enantiopure cryptophanes.

Raman optical activity (ROA) and density functional theory (DFT) calculations were used to determine the absolute configuration of enantiopure cryptophane molecules and to obtain conformational information about their three ethylenedioxy linkers. ROA spectra recorded in chloroform solution for the two resolved enantiomers of cryptophanes derivatives bearing five (2), six (1), nine (3 and 4), and 12 (5) methoxy substituents are presented for the first time. The number of methoxy substituents (cryptophanes 1, 3, and 5) and the arrangement of the three linkers (anti for 3 and syn for 4) are two important parameters that significantly affect the ROA spectra.