Structural investigation of cyclo-dioxo maleimide cross-linkers for acid and serum stability

The size of the acetal ring and the length of the carbon chain strongly influence the serum stability of cyclo-dioxo-based amino-to-thiol coupling reagents.

Two New Triterpenoids from Gelsemium Elegans and Aglaia odorata

Eleganoside A (1) and odoratanone A (15), a triterpenoid trisaccharide glycoside and a nortriterpenoid, together with twelve known compounds (2–13) and a mixture of cerebrosides (14) were isolated from Gelsemium elegans and Aglaia odorata. Their structures were elucidated by extensive spectroscopic and spectrometric analysis. Eleganoside A (1) features a 3- O-α-L-rhamnopyranosyl (1→4)-β-D-glucopyranosyl (1→4)-β-D-glucopyranoside of a peculiar 3,16-dihydroxyl-lanosta-8,24-dien-26-oic acid triterpenoid skeleton, and odoratanone A (15) is a 29-norcycloartane-type triterpenoid bearing an unusual five-membered methyl acetal ring. Anti-acetylcholinesterase/butyrylcholinesterase (AChE/BChE) assay indicated that at 50 μM, ethyl caffeate (5) was promising as a dual inhibitor of AChE and BChE, and paeonol (3) and 24-hydroperoxy-24-vinylcholesterol (9) exhibited BChE-selective inhibition.

Stereochemical rearrangements in tricarbonylrhenium(I) halide complexes of the nonracemic chiral ligand 2-[(4R,5R)-dimethyl-1,3-dioxan-2-yl]pyridine (L): a dynamic NMR study

Tricarbonylrhenium(I) halide complexes of the nonracemic chiral ligand 2-[(4R,5R)-dimethyl-1,3-dioxan-2-yl]pyridine (L), namely fac-[ReX(CO)3L] (X = Cl, Br, or I), have been prepared and their latent fluxionality studied by dynamic NMR techniques in the slow and intermediate exchange regimes. In solution, these complexes give rise to four diastereoisomers, depending on the configuration at the metal and at the acetal-carbon atom, respectively; the relative populations are in the order SR > RR >>; RS > SS. At moderate temperatures, a reversible "acetal ring flip" leads to formal inversion of configuration at the acetal-carbon atom; the free energies of activation are in the range 84-88 kJ mol-1 at 298 K. Above ca. 370 K, reversible ligand dissociation also occurs, leading to an exchange of all four diastereoisomers on the NMR chemical shift time-scale.Key words: dynamic NMR, fluxionality, tricarbonylrhenium(I) complexes, chiral acetal ligands.