N-PHOSPHORYL AMINO ACIDS AND PEPTIDES: PART III. THE CRYSTAL AND MOLECULAR STRUCTURE OF N-(O,O-DIISOPROPYL PHOSPHORYL)-L-ALANINE

Abstract

The crystal and molecular structure of N-(O,O-diisopropyl phosphoryl)-L-alanine (N-DIPP-Ala) has been determined by X-ray diffraction analysis and is compared with N-(O,O-diisopropyl phosphoryl)-trans-4-hydroxy-D,L-proline (N-Dipp-Hyp) and N-acyl amino acids reported in the literature. The compound crystallized in the tetragonal system with space group P4₁2₁2₁, a = 15.571(2) Å, b = 15.571(2) Å, c = 12.392(3) Å, V = 3.004.56 ų, Z = 8, Dc = 1.12 g/cm³. The structure was solved by direct and Fourier method and refined to a discrepancy index R = 0.072. The P—N bond length of N-DIPP-Ala (1.60 Å) was found to be shorter than that of N-DIPP-HYp (1.62 Å), but these two compounds share the same value of the P = O bond distance (1.47 Å). It is thus postulated that in addition to the resonance interaction between the nonbonding electrons of the nitrogen and the P˭O π system in these two compounds, there might be another type of resonance interaction along the P—N bond which is proposed as N→P back donation effect. The conformation of the phosphoramide function in N-DIPP-Ala is favoured by the cis orientation of the P˭O bond with respect to the N—H bond. This situation is in contrast with N-acyl amino acids in which the conformation of the carboxamide function is favoured by the trans orientation of the C˭O bond with respect to the N—H bond.

Key Words:

• X-ray diffraction analysis
• N-(O,O-diisopropylphosphoryl)-L-alanine
• N-(O,O-diisopropylphosphoryl)-trans-4-hydroxy-D,L-proline
• comparison with N-acylaminoacids