The evolution of particle shape is an important consideration in many industrial crystallizations. This article describes the design of temperature-cycling experiments (between alternating positive and negative supersaturations) to substantially change crystal shape with only a small number of cycles. The growth and dissolution of monosodium glutamate crystals of varying shapes were monitored using in-process attenuated total reflection−Fourier transform infrared spectroscopy (ATR-FTIR), focused beam reflectance measurement (FBRM), particle vision and measurement (PVM), and off-line optical microscopy. The growth and dissolution kinetics were estimated in a multidimensional population balance model based on solute concentration and crystal dimension measurements. This model fitted the experimental data with a limited number of parameters of small uncertainty. In addition, with the estimated kinetic parameters, the model predicted the crystal size and shape distribution in a different temperature-cycling experiment reasonably well. In contrast to previous studies that have estimated kinetics along multiple crystal axes in mixed-tank crystallizers, this study implements dissolution terms in the multidimensional population balance model along multiple axes.
Both enantiomers of methyl dihydrojasmonate (-)-1 and (+)-1 were obtained by a short route using asymmetric Michael addition of dimethyl malonate onto pentyl enone 3, followed by nonracemizing demethoxycarbonylation. The key enantioselective step involves a new system of asymmetric solid-liquid phase-transfer catalysis using solvent-free conditions. Enantiomeric excess as high as 90% (91% yield) was achieved.
A concise scalable synthesis of a chiral dipeptide acid, key substructure of the HCV protease inhibitor faldaprevir, has been developed. A green process with an E-factor of 9.2 was achieved utilizing process analytical technology (PAT) to allow effective processing of multiple-steps in a one-stage operation. Mixed anhydride/oxazolone formation, peptide coupling, saponification, and then crystallization of the desired dipeptide acid were completed within 10 h. MultiMaxIR was used to detect the formation and consumption rates of key intermediates and to provide initial safety data which was subsequently confirmed by more comprehensive process safety testing. Further kinetic analysis was performed to determine the range of operability space to ensure conditions for a robust process.
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