Studies on antitumor cyclic hexapeptides RA obtained from rubiae radix, rubiaceae. VI. Minor antitumor constituents.

New antitumor cyclic hexapeptides RA-IV, RA-III, RA-II and RA-I were isolated as minor constituents of Rubia cordifolia. Their structures were elucidated by means of various instrumental and chemical analyses. From a comparison of the antitumor activities of 7 products derived by oxidizing RA-V with 2, 3-dichloro-5, 6-dicyano-p-benzoquinone and 6 native cyclic hexapeptides isolated from R. cordifolia, it is surmised that the active site in the RA-series is present at the α-side of RA molecules.