X-ray structure analysis of a solid solution of milbemycins A3 and A4.

Milbemycins A3 and A4 are pharmaceutically and agriculturally useful macrolides isolated from Streptomyces species. The molecular structures of the title compounds were unambiguously established by a single crystal X-ray analysis of the solid solution of both compounds. The crystals present trigonal system, space group P32 with Z = 3, unit cell dimensions: a = 12.2211(4), c = 17.5372(7) Å; V = 2268.4(1) Å(3), μ = 0.082 mm(- 1); d = 1.183 g cm(- 3). An interesting system of intramolecular hydrogen bonds and weak intermolecular CH…O type hydrogen bond was observed in the solid state.