Wenger, Roland M., et al. “Structure of Cyclosporine and Its Metabolites: Total Synthesis of Cyclosporine Metabolites Formed by Oxidation at Positions 4 and 9 of Cyclosporine. Preparation of Leucine-4-Cyclosporine, (γ-Hydroxy)-N-Methyl-Leucine-9-Cyclosporine and Leucine-4-(γ-Hydroxy)-N-Methyl-Leucine-9-Cyclosporine”. CHIMIA, vol. 46, no. 7-8, Aug. 1992, p. 314, doi:10.2533/chimia.1992.314.