Fmoc-OASUD: A New Reagent for the Preparation of Fmoc-Amino Acids Free from Impurities Resulting from Lossen Rearrangement.

Abstract A new reagent, 9-fluorenylmethoxy-carbonyl- N -hydroxy-3-azaspiro[5,5]undecane-2,4-dione ester (Fmoc-OASUD) for the preparation of Fmoc-amino acids is described. The Fmoc-OASUD reacts with amino acids at room temperature in the presence of a base and gives Fmoc-amino acids in high yields and purity. The Fmoc-amino acids prepared using Fmoc-OASUD are free from impurities due to Lossen rearrangement, which are generally present when Fmoc-OSu is used. This is mainly because of the higher stability of Fmoc-OASUD compared to Fmoc-OSu.