Binary system of (R)- and (S)-nitrendipine - Polymorphism and structure

Three monotropically related modifications of (RS)-nitrendipine (RS- NTD) were investigated. With the aid of the optical antipodes of NTD, it was possible to construct a binary phase diagram, from which it is obvious that the thermodynamically stable mod. I of RS-NTD (mp 156-158.5 °C, heat of fusion 41.1 ± 0.2 kJ mol^-1) is a racemic compound, while mod. II (mp 130- 134 °C, heat of fusion 29.4 kJ mol^-1) and mod. III (mp 124-126 °C, heat of fusion 27.2 kJ mol^-1) occur as conglomerates of different modifications of R- and S-NTD. The eutectic points (calculated) are at 150 °C and at 19% and 81% molar fraction of an enantiomer, respectively. Two modifications of the enantiomers are characterized (En-mod. I, thermodynamically stable form, mp 156-158 °C, heat of fusion 34.5 ± 1.3 kJ mol^-1 and En-mod. III, mp 121 °C), whereas a third modification (En-mod. II) must be hypothesized pursuant to the phase diagram (mp ca. 152 °C). IR spectra, DSC curves, and X-ray powder diffractograms of the different phases are described. The crystal structure of the enantiomer En-mod. I is reported (monoclinic, P2₁, density 1.346 g cm^-3) and compared with the X-ray structure of the racemic compound (monoclinic, P2₁/c, density 1.356 g cm^-3). A comparison with similar studies on nimodipine has been made to emphasize the parallels to a substance of this chemical group.