First characterization at atomic resolution of the C-activating groups in a peptide synthesis acid chloride, acid azide and carboxylic-carboxylic mixed anhydride

Reactive intermediates in peptide synthesis. Molecular and crystal structures of reactive carboxylic amides

The molecular and crystal structures of N-para-toluenesulphonyl-N-methyl-α-aminoisobutyric acyl imidazole (1), and 3-(N-para-toluenesulphonyl-α-aminoisobutyric acyl)-1,3-thiazolidine-2-thione (2) have been determined by X-ray diffraction. Crystal parameters: (1) orthorhombic, space group P212121, a = 11.735(2) Å, b = 19.990(3) Å, c = 6.925(1) Å, and Z = 4; (2) orthorhombic, space group Pnc2, a = 9.507(2) Å, b = 16.902(2) Å, c = 10.347(2) Å, and Z = 4. The structures were solved by direct methods. The least-squares refinements led to conventional R factors of 0.036 and 0.031 for (1) and (2), respectively. This is the first geometrical and conformational characterization at atomic resolution of N α -protected α-aminoacids containing the elusive acyl imidazole and 3-acyl-1,3-thiazolidine-2-thione C-activating groups. Only the reactive carboxylic amide of (2) deviates significantly from planarity.

Molecular and crystal structures of two N α -protected α-aminoacyl fluorides and their free carboxylic acid synthetic precursors 1

The molecular and crystal structures of N-para-toluenesulphonyl-α-aminoisobutyric acid fluoride (1), N-para-toluenesulphonyl-N-methyl-α-aminoisobutyric acid fluoride (2), N-para-toluenesulphonyl-α-aminoisobutyric acid (3), and N-para-toluenesulphonyl-N-methyl-α-amino-isobutyric acid (4) have been determined by X-ray diffraction. Crystal parameters: (1) monoclinic, space group P21/a, a = 24.264(3) Å, b = 8.386(2) Å, c = 6.245(1) Å, β = 96.5(2)°, and Z = 4; (2) monoclinic, space group P21/n, a = 7.606(1) Å, b = 14.954(2) Å, c = 11.969(2) Å, β = 92.3(2)°, and Z = 4; (3) monoclinic, space group P21/n, a = 11.151(2) Å, b = 6.168(1) Å, c = 18.669(2) Å, β = 91.3(2)°, and Z = 4; (4) monoclinic, space group P21/c, a = 11.993(2) Å, b = 10.322(2) Å, c = 12.123(2) Å, β = 115.9(2)°, 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 α -protected α-aminoacids containing the elusive acyl fluoride C-activating group.[unk]