26488-24-4 Cyclo(D-phenylalanyl-L-prolyl) Cyclo(L-prolyl-D-phenylalanyl) NSC 255998 Phe-pro-diketopiperazine Phenylalanyl-prolyl diketopiperazine Pyrrolo(1,2-a)pyrazine-1,4-dione, hexahydro-3-(phenylmethyl)-, (3R-cis)-