Unusual indole alkaloids from Ophiorrhiza blumeana Korth

Abstract

The alkaloid content of two different collections of Ophiorrhiza blumeana Korth. (Rubiaceae) was investigated. The first collection yielded the known alkaloid bracteatine 5 and two new alkaloids ophiorrhizine-12-carboxylate 6 and blumeanine 7. The second collection yielded the known alkaloid ophiorrhizine 1 as well as the two new alkaloids. The structures of 6 and blumeanine diacetate 8 followed from their CD and NMR spectral data. The former proved to be an unusual betaine with a carboxylate substituent on the benzenoid part of the indole and the structure was confirmed by single crystal X-ray structural determination. Bracteatine proved to be a dimeric Corynanthé alkaloid linked at the 9,12′ positions and is chiral about the biaryl linkage.