Development of a new microwave-assisted cleavable backbone amide linker (BAL): a comparative study.

A thorough comparative study to demonstrate the properties of a new microwave labile backbone amide linker is presented. A cyclic pentapeptide, cyclo(Trp-Gln-Gly-beta-Ala-Phe), was used as a model and synthesized following 16 different conditions. The new backbone amide linker is stable towards acid and base at r.t., and can be cleaved at elevated temperature in trifluoroacetic acid under microwave irradiation, avoiding the use of aggressive reagents like HF. The new linker is compatible with Boc- as well as Fmoc-strategy and allows the cleavage of acid labile side chain protective groups at r.t., prior to cleavage of the cyclic pentapeptide from the resin.