Molecular and Crystal Structures of Two N-alpha-Protected alpha-Aminoacyl Fluorides and Their Free Carboxylic Acid Synthetic Precursors

The molecular and crystal structures of N-para-toluenesulphonyl-alpha-aminoisobutyric acid fluoride (1), N-para-toluenesulphonyl-N-methyl-alpha-aminoisobutyric acid fluoride (2), N-para-toluenesulphonyl-alpha-aminoisobutyric acid (3), and N-para-toluenesulphonyl-N-methyl-alpha-amino- isobutyric acid (4) have been determined by x-ray diffraction. Crystal parameters: (1) monoclinic, space group P2(1)/a, a = 24.264(3) Angstrom, b = 8.386(2) Angstrom, c = 6.245(1) Angstrom, beta = 96.5(2)degrees, and Z = 4; (2) monoclinic, space group P2(1)/n, a = 7.606(1) Angstrom, b = 14.954(2) Angstrom, c = 11.969(2) Angstrom, beta = 92.3(2)degrees, and Z = 4; (3) monoclinic, space group P2(1)/n, a = 11.151(2) Angstrom, b = 6.168(1) Angstrom, c = 18.669(2) Angstrom, beta = 91.3(2)degrees, and Z = 4; (4) monoclinic, space group P2(1)/c, a = 11.993(2) Angstrom, b = 10.322(2) Angstrom, c = 12.123(2) Angstrom, beta = 115.9(2)degrees, and Z = 4. The structures were solved by direct methods. The least-squares refinements led to conventional R factors of 0.044, 0.059, 0.045, and 0.056 for (1)-(4), respectively. This is the first geometrical and conformational characterization at atomic resolution of two N-alpha-protected alpha-aminoacids containing the elusive acyl fluoride C-activating group.