Selective Modification of the Terminal Amide Group of Linear Polyamides via 2-Oxazolin-5-ones as Intermediates.
Treatment of aqueous or alcoholic solutions of diamides of type 2 with HC1 leads to the formation of amide-acids and amide-esters of type 3 (Scheme 1 and Table). It has been shown, that 2-oxazolin-5-ones of type 4 are intermediates of this selective transformation of the disubstituted terminal amide group. The selectivity of the transformation is demonstrated by the reaction sequence shown in Scheme 3. No selectivity has been observed in the acid-catalyzed hydrolysis of triamide 9 with a monosubstituted terminal amide group (Scheme 4). Hydrolysis of the optically active dipeptide derivatives (+)-(L)-13and (+)-(L)-15 with HBr in nitromethane at 60-80° yields the pure enantiomer (+)-(L)-14 and (+)-(L)-16, respectively (Scheme 5), i.e,, no racemization takes place under the reaction conditions.
These results show the usefulness of the dimethylamide group as a protecting group for carboxylic acids for example in the peptide synthesis.