Mechanistic studies on PET-oxidative cyclization of unsaturated silyl enol ethers: dependence of the regioselectivity on alcohol addition and pressure effects

Ackermann L, Heidbreder A, Wurche F, Klarner FG, Mattay J. Mechanistic studies on PET-oxidative cyclization of unsaturated silyl enol ethers: dependence of the regioselectivity on alcohol addition and pressure effects. JOURNAL OF THE CHEMICAL SOCIETY-PERKIN TRANSACTIONS 2. 1999;(4):863-870.Unsaturated silyl enol ethers are irradiated in the presence of electron transfer sensitizers. The efficiency of the cyclization reaction using different sensitizers is investigated. The endolexo regiochemistry of the ring closure reaction can either be controlled by variation of the silyl group or by addition of alcohol. Furthermore, a dependence of the regiochemistry on pressure is revealed and it seems that it can be related to acetonitrile acting as a nucleophile at 1500 bar. As key intermediates radical cations and radicals are involved

Crystal doping aided by rapid expansion of supercritical solutions

The purpose of this study was to test the utility of rapid expansion of supercritical solution (RESS) based cocrystallizations in inducing polymorph conversion and crystal disruption of chlorpropamide (CPD). CPD crystals were recrystallized by the RESS process utilizing supercritical carbon dioxide as the solvent. The supercritical region investigated for solute extraction ranged from 45 to 100°C and 2000 to 8000 psi. While pure solute recrystallization formed stage I of these studies, stage II involved recrystallization of CPD in the presence of urea (model impurity). The composition, morphology, and crystallinity of the particles thus produced were characterized utilizing techniques such as microscopy, thermal analysis, x-ray powder diffractometry, and high-performance liquid chromatography. Also, comparative evaluation between RESS and evaporative crystallization from liquid solvents was performed. RESS recrystallizations of commercially available CPD (form A) resulted in polymorph conversion to metastable forms C and V, depending on the temperature and pressure of the recrystallizing solvent. Cocrystallization studies revealed the formation of eutectic mixtures and solid solutions of CPD+urea. Formation of the solid solutions resulted in the crystal disruption of CPD and subsequent amorphous conversion at urea levels higher than 40% wt/wt. Consistent with these results were the reductions in melting point (up to 9°C) and in the ΔHfvalues of CPD (up to 50%). Scanning electron microscopy revealed a particle size reduction of up to an order of magnitude upon RESS processing. Unlike RESS, recrystallizations from liquid organic solvents lacked the ability to affect polymorphic conversions. Also, the incorporation of urea into the lattice of CPD was found to be inadequate. In providing the ability to control both the particle and crystal morphologies of active pharmaceutical ingredients, RESS proved potentially advantageous to crystal engineering. Rapid crystallization kinetics were found vital in making RESS-based doping superior to conventional solvent-based cocrystallizations