Conformational polymorphism of diethyl 1-(phthalylglycyloxy)2-phthalylaminoethenephosphonate

In course of the synthesis of phosphonic derivatives of bestatine, several products have been obtained. One of them turned out to occur in two polymor phic forms. Two crystal structures correspond to two molecular conformation s. The crystal structures are as follows: (1a) C24H21N2O9P, P2(1)/n, a = 8. 023(2) Angstrom, b = 12.092(2) Angstrom, c = 24.103(5) Angstrom, beta = 92. 39(3)degrees, Z = 4, (1b) C24H21N2O9P, P2(1)/c, a = 8.322(1) Angstrom, b = 16.378(2) Angstrom, c = 18.442(2) Angstrom, beta = 101.13(1)degrees, Z = 4. The main conformational difference consists in the mutual orientation of t wo phthalyl rings. The conformation and packing of sterically overcrowded a nd conformationally flexible molecules seem to be stabilized by very weak C -H...O hydrogen bonds. The extremely large thermal vibration amplitudes of the phosphonic group's atoms indicate the occurrence of disorder, probably of dynamical character. This disorder becomes frozen below room temperature . The differential scanning calorimetry (DSC) revealed the occurrence of ph ase transitions below room temperature in both crystals. (C) 2001 Elsevier Science B.V. All rights reserved.