Structure of a Derivative of Streptovaricin C Triacetate. Crystal and Molecular Structure of the Atropisomer of the Cyclic p-Bromobenzeneboronate Ester of Streptovaricin C Triacetate: Methylene Dichloride 1 : 1 Solvate

The crystal and molecular structure of a p-bromobenzeneboronate ester of streptovaricin C triacetate has been determined by single-crystal x-ray methods. The derivative studied was of the atropisomer of the naturally occurring form of streptovaricin C triacetate and it crystallized as a 1:1 methylene dichloride solvate. The crystals are orthorhombic, a = 22.487 (8), b = 12.678 (5), and c = 19.723 (6) Å, with four molecules of C₅₂H₅₉BBrNO₁₇-CH₂Cl₂ in the space group P2₁2₁2₁. The structure has been refined to an R factor of 0.106 on 2148 nonzero reflections. A strong indication of the absolute configuration was obtained from the x-ray study. The relative configuration, absolute configuration, and conformation of the ansa ring in this derivative of streptovaricin are compared with those found in other derivatives of ansamycins that have been studied by x-rays. Some structural features that may have a bearing on the biological activity of these macrocyclic antibiotics are also described and discussed.