Microwave-Assisted Cyclization of Unprotected Dipeptides in Water to 2,5-Piperazinediones and Self-Assembly Study of Products and Reagents

Dipeptides and their cyclized 2,5-piperazinedione (or diketopiperazine, DKP) derivatives are attractive building blocks for supra­molecular hydrogels. The Phe-Phe, (p-nitro)-Phe-Phe, and Phe-Val dipeptides and their corresponding DKPs are studied for self-assembly in water. The DKPs were obtained in high yields by microwave-assisted cyclization­ of the dipeptides in water, demonstrating that use of their methyl ester derivatives as reported in the literature is not necessary for successful cyclization. Single-crystal XRD structures are reported for two DKPs as well as stable hydrogels at neutral pH.