Cytotoxic constituents of the bark of Plumeria rubra collected in Indonesia.

By bioactivity-directed fractionation, six cytotoxic constituents have been characterized from the bark of Plumeria rubra collected in Indonesia. Three iridoids, fulvoplumierin [1], allamcin [2], and allamandin [3], as well as 2,5-dimethoxy-p-benzoquinone [4], were found to be active constituents of the P. rubra petroleum-ether- and CHCl3-soluble extracts. Cytotoxic compounds isolated from the H2O-soluble extract of the bark were the iridoid plumericin [5], and the lignan liriodendrin [6]. Each of these substances was found to demonstrate general cytotoxic activity when evaluated with a panel of cell lines composed of murine lymphocytic leukemia (P-388) and a number of human cancer cell-types (breast, colon, fibrosarcoma, lung, melanoma, KB). Five additional iridoids, 15-demethylplumieride [7], plumieride [8], alpha-allamcidin [9], beta-allamcidin [10], and 13-O-trans-p-coumaroylplumieride [11], were obtained as inactive constituents. Compound 7 was found to be a novel natural product, and its structure was determined by spectroscopic methods and by conversion to plumieride [8]. The configuration of the C-4 stereocenter was unambiguously assigned for compounds 9 and 10, and certain nmr reassignments have been provided for compound 1.