Stereoselective crystallization of 3-(2,6-dimethylphenoxy)propane-1,2-diol: preparation of the single-enantiomer drug mexiletine

Crystal Structure of Chiral Drug Prenalterol and Its Precursor Prone to Spontaneous Resolution

Due to the chiral uniformity of proteins and carbohydrates, the basic building blocks of living matter, the mirror symmetry characteristics of drugs are of exceptional importance for medicinal chemistry. In this work, we present a new synthesis of the mono-enantiomeric chiral drug prenalterol 1 based on the symmetry-breaking phenomenon, namely, the spontaneous resolution of 4-hydroxyphenyl glycerol ether 2. The single crystal X-ray diffraction method was used to investigate both rac- and (S)-1 as well as (R)-2. A feature of the main crystal-forming supramolecular motif (SMM) for diol 2 is the participation of three different molecules representing different types of hydroxyl groups in the formation of its repeating unit. The type of prenalterol SMM, as in the case of the related drugs propranolol 3 and pindolol 4, appears to be a chirality driven property, and is dictated by the enantiomeric composition of the crystals. In single-enantiomeric forms, infinite one-dimensional chains are realized, organized around helical axes, while in racemates, zero-dimensional cycles are realized, organized around inversion symmetry elements. The results obtained again demonstrate the influence of the chiral polarization of a substance not only on the general (selection of a space group), but also on particular characteristics of matter crystal organization, namely selection of a specific SMM.

Stereoselective Crystallization of Chiral 3,4-Dimethylphenyl Glycerol Ether Complicated by Plurality of Crystalline Modifications

Spontaneous resolution of Pasteur’s salt was historically the first way to obtain pure enantiomers from the racemate. The current increase in interest in the direct racemates resolution during crystallization is largely due to the opened prospects for the industrial application of this approach. The chiral 3-(3,4-dimethylphenoxy) propane-1,2-diol 1 is a synthetic precursor of practically useful amino alcohols, the enantiomers of which exhibit different biological effects. In this work, it was first discovered that racemic diol 1 is prone to spontaneous resolution. However, the crystallization process is complicated by the existence, along with the conglomerate, of two other crystalline forms. Using the differential scanning calorimetry (DSC) approach, methods have been developed to obtain individual metastable phases, and all identified modifications ((R)-1, (R+S)-1, α-rac-1, β-rac-1) were ranked by energy. The IR spectroscopy and powder X-ray diffraction (PXRD) methods demonstrated the identity of the first two forms and their proximity to the third, while β-rac-1 is significantly different from the rest. The crystal structure of the forms (R)-1 and α-rac-1 was established by the single crystal X-ray diffraction (SC-XRD) method. Preliminary information on the structure of β-rac-1 phase was obtained by the PXRD approach. Based on the information received, the experimental conditions for a successful direct resolution of racemic 1 into individual enantiomers by a preferential crystallization procedure were selected.

Enantiodivergent syntheses of (+)- and (-)-1-(2,6-dimethylphenoxy)propan-2-ol: A way to access (+)- and (-)-mexiletine from D-(+)-mannitol

Chiron approach was used to acquire optically pure (R)- and (S)-1-(2,6-dimethylphenoxy)propan-2-ol, immediate precursors of (S)- and (R)-mexiletines, respectively. Two different routes were followed from a D-mannitol-derived optically pure common precursor to get the enantiomeric alcohols separately. Comparison of their specific rotation values with the corresponding literature values as well as exact mirror-image relationship between their CD curves proved their high enantiopurity. These alcohols were then transformed to the corresponding amine-drugs in an efficient one-step process instead of two steps described in the literature.

35Cl solid-state NMR spectroscopy of HCl pharmaceuticals and their polymorphs in bulk and dosage forms

Herein, we demonstrate the use of 35Cl SSNMR for the structural fingerprinting of HCl salts of pharmaceuticals in both bulk and dosage forms.