N^a-9-Fluorenylmethoxyloxcarbonyl amino acids – general procedure

(Protocols from Carpino and Han, 1972, Chang et al., 1980 and Fields et al., 1989)

²H-amino acid (50 mmol; Ala, Val, Leu, Ile, Gly; Phe. Trp) was dissolved with stirring in 10% Na₂CO₃ (100ml) and cooled in an ice-bath. Dioxane (50 ml) was added followed by the slow addition of a solution a solution of 9-fluorenylmethyl chloroformate (13g, 50.2 mmol) in dioxane (75 ml). The mixture was stirred for 1hr at 0°C and for 4 hrs or overnight at RT. The reaction time was monitored by TLC. The reaction mixture was poured into ice-water (1.5 liters) and extracted with ether (2 x 400 ml). The aqueous layer was chilled in an ice-bath and acidified with concentrated HCl to pH 2.0.

Crystalline precipitates dissolved in EtOAc and washed again with 0.1 N HCl and H₂O, followed by drying (MgSO₄), evaporation and crystallization from EtOAc-hexane (e.g. Gly, Ala, Phe). Most other Fmoc-amino acid derivatives precipitated as oils on acidification, and followed by the same procedures as described above. Instead CH₂Cl₂-hexane was used for crystallization. The products were then characterized by MS, solution NMR and TLC, if necessary, HPLC.

Yield %

Yield depends on the amino acids, for Gly, Leu, Ala, Gln, Trp, Phe, the %yield is > 90 %. More details on yield % of individual Fmoc-amino acid can be found in Chang et al., 1980 in Table 1.

²H-amino acids can then be used directly for solid phase peptide synthesis.

Reference:

Carpino, L. A., and Han, G. Y. (1972). J. Org. Chem. 37, 3404

Chang, C.-D., Waki, M., Ahmad, M., Meienhofer, J., Lundell, E. O. And Haug, J. D. (1980). Int. J. Peptide Protein Res. 15, 59

Fields, C. G., Fields, G. B., Fields, R. L., and Cross, T. A. (1989). Int. J. Peptide Protein Res. 33, 298