合成路线1

The tripeptide building block (XXVII) is prepared as follows. Cyclization of N-Boc-L-serine (XIX) to the beta-lactone (XX) is achieved by treatment with DEAD and PPh3. Subsequent ring opening with diphenyldiselenide in the presence of a reducing agent leads to selenide (XXI). Aminoacid (XXI) is then coupled to sarcosine ethyl ester (XXII) to afford dipeptide (XXIII). After acidic Boc group cleavage in (XXIII), the resultant carboxylic acid (XXIV) is coupled to N-Boc-D-aminobutyric acid (XXV) to give (XXVI). The N-Boc group of (XXVI) is then deprotected with trifluoroacetic acid, producing intermediate (XXVII).