Synthesis and crystal structure of erythromycin A 9-(1-iso-propoxycyclohexyl) oxime

E- and Z-isomers of erythromycin A 9-(1-iso-propoxycyclohexyl) oxime have been synthesized and characterized. The crystal structure of E-isomer was determined by X-ray single-crystal structure analysis. The crystal belongs to orthorhombic system, P2₁2₁2₁ space group with a = 2.3101 (5) nm, b = 2.3761 (5) nm, c = 0.97066 (19) nm, V = 5.3279 (18) nm³, Z = 4, μ = 0.088 mm^-1, F (000) = 2064, D _c = 1.176 g/cm³, R₁ = 0.0505, wR₂ = 0.0775 for 6747 unique reflections. The torsion angles of its fourteen-member erythrolide clearly elucidated the origin of regioselectivity occurring at 6-hydroxyl in the O-methylation of erythromycin A.