Crystal form and transformation of clarithromycin

Form II of clarithromycin can easily be obtained by the process where by a salt of clarithromycin with citric acid is neutralized in ethanol-water, acetone-water, and isopropanol-water (1:1), or a salt with tartaric acid is neutralized in ethanol-water, acetone-water (1:1), followed by drying at 60°C and on normal pressure. The process obviates the condition of vacuum and higher temperature drying, as reported in the literatures. DSC analysis proved the crystal to be of the Form II. Additionally, the single crystal structure of clarithromycin acetonitrile adduct was obtained in monoclinic system, P2₁ space group, a = 1.0644(2) nm, b = 1.8099(4) nm, c = 1.1628(2) nm, α = γ = 90°, β = 95.38(3)°, V = 2.2301(8) nm³, Z = 2, D_c = 1.175 g/cm³, μ = 0.087 mm^-1, F(000) = 860, R₁ = 0.034, wR₂ = 0.062.