3-Alkoxycarbonyl-2-oxazolones and Their Homopolymers as Highly Preservable Amino-Protecting Reagents. tert-Butoxy-carbonylation and Benzyloxycarbonylation of Amino Groups

Highly preservable amino protecting reagents derived from the 2-oxazolone moiety as a common activating mediator have been developed. 3-Alkoxycarbonyl-2-oxazolones serve as easily handled reagents for amino protection, including tert-butoxycarbonylation, benzyloxycarbonylation, p-methoxybenzyloxycarbonylation, methoxycarbonylation and ethoxycarbonylation. For example, high yield N-protection of α-amino acids has been smoothly performed by the use of 3-tert-butoxycarbonyl [Boc-Ox] and 3-benzyloxycarbonyl-2-oxazolones Cbz-Ox] in aqueous solution at room temperature. A series of homopolymers, poly(3-alkoxycarbonyl-2-oxazolone), is readily obtainable by radical-initiated chain reaction of the corresponding 4,5-unsubstituted oxazolone monomers (except for the tert-butoxy derivative, which failed to give polymeric compounds), and these were successfully used for amino protection as well. Use of the polymer reagents greatly simplifies the purification procedure, though a longer reaction time is required.