N-protected α-amino acids and O-protected α-hydroxy acids with 2,2'-carbonylbis (3,5-dioxo-4-methyl-1,2,4-oxadiazolidine) 6 provide a stable activated intermediate 7 suitable for the synthesis of β-ketoester 8. This activation was used in the preparation of the non-proteogenic units 1st 1 and Hip 4 present in the cyclodepsipeptides didemnins isolated from the tunicates Didemnidae.