Novel 29-nor-3,4-seco-cycloartane triterpene methyl esters from the aerial parts of Antirhea acutata

Four novel 29-nor-3,4-seco-cycloartanes, (6S)-hydroxy-25-methoxy-29-nor-3,4-seco-cycloart-4(30),23-dien-3-oic acid methyl ester (1), (6S,25)-dihydroxy-29-nor-3,4-seco-cycloart-4(30),23-dien-3-oic acid methyl ester (2), (6S)-hydroxy-24-oxo-29-nor-3,4-seco-cycloart-4(30),25-dien-3-oic acid methyl ester (3), and (6S)-hydroxy-(24ξ)-hydroperoxy-29-nor-3,4-seco-cycloart-4(30),25-dien-3-oic acid methyl ester (4), along with a semi-synthetic acetylated derivative, (6S,24ξ)-diacetyloxy-29-nor-3,4-seco-cycloart-4(30),25-dien-3-oic acid methyl ester (5), were isolated and characterized from the aerial parts of Antirhea acutata (DC.) Urb. (Rubiaceae). Their structures and absolute stereochemistry were elucidated by spectroscopic and chemical methods. Compounds 1–5 are based on the unprecedented 29-nor-3,4-seco-cycloartane skeleton. Compound 4 showed moderate inhibitory activities in cyclooxygenase-1 and -2 assays (IC50 45.7 and 18.4 μM, respectively), while the other four isolates were inactive.