Enantioselective synthesis of 8-azabicyclo[3.2.1]octanes via asymmetric 1,3-dipolar cycloadditions of cyclic azomethine ylides using a dual catalytic system.

The first example of asymmetric 1,3-dipolar cycloadditions between diazo imine-derived cyclic azomethine ylides and acryloylpyrazolidinone using a rhodium(ii) complex/chiral Lewis acid binary system is reported. The asymmetric cycloadditions afforded optically active 8-oxabicyclo[3.2.1]octanes with high diastereo- and enantioselectivities (up to >99 : 1 dr, 99% ee). A switch of exo/endo-selectivity was observed depending on the diazo substrates.

Time-programmed release of fluoroscein isocyanate dextran from micro-pattern-designed polymer scrolls.

In this article we present a relevant strategy for a non-trivial time-programmed release of water-soluble macromolecules from biocompatible µ-containers. The system is based on self-scrolled chitosan acetate (CA) fibers, encapsulated in a poly(dimethylsiloxane) matrix. Mass transfer between a fiber and the external environment takes place via the only opened extremity of the fiber. Fluoroscein isocyanate dextran (FID) is initially deposited at the inner surface of the CA fiber according to a programmed pattern. The FID molecules became mobile after the arriving of the swelling front, which propagates along the fiber's axis upon the immersion of the system in aqueous solution. Diffusion of the macromolecules into the environment is enabled by the open-tube geometry of the swollen part of the fiber, while a programmed kinetics of the drug release is due to patterning of the polymer film prior to rolling. The release of the macromolecules can be retarded by a few hours according to the placement of the FID spot with respect to the fibers orifice. A pulsatile release kinetics is demonstrated for a discrete pattern. A few millimeter spacing of the FID spots results in a few hours time interval between the release impulses. Random walk model is plugged in the effective diffusion coefficient for Fick's law and the release kinetics are simulated.

Self-scrolling ability of differentially acetylated chitosan film.

Chitosan film cast on a glass slide was exposed to acetic anhydride vapor, resulting in an acetylation gradient in the film, with preferential acetylation of the exposed surface. The difference in degree of acetylation between the two surfaces of the peeled film was confirmed by attenuated total reflection infrared spectroscopy. Upon immersion of the film in water, differential swelling occurred because the more highly acetylated surface absorbed less water, and the resulting bending moment caused self-scrolling with the more highly acetylated surface inside. Simultaneous peeling and scrolling of films with a thickness of micrometer order, using dilute aqueous hydrofluoric acid, afforded tightly rolled chitosan microtubes. This simple self-scrolling mechanism is potentially applicable for micro-scale design with various naturally occurring polymers.