Alkaloids from the Chinese vine Gnetum montanum.

During a high-throughput screening campaign of a prefractionated natural product library, fractions from the Chinese vine Gnetum montanum showed in vitro activity against Pseudomonas aeruginosa wild-type strain, PAO1. UV-directed isolation of the organic extract from the vine leaves resulted in the purification of the new natural products N-methyllaudanosolinium trifluoroacetate (1), 3'-hydroxy-N,N-dimethylcoclaurinium trifluoroacetate (2), 1,9,10-trihydroxy-2-methoxy-6-methylaporphinium trifluoroacetate (3), and 6a,7-didehydro-1,9,10-trihydroxy-2-methoxy-6-methylaporphinium trifluoroacetate (4). Compound 4 is described here for the first time, and this is the first report of compounds 1-3 as natural products. Compounds 1-3 were found to racemize over time. Starting from commercially available (+)-boldine, through a series of semisynthetic reactions, a mechanism for the racemization of the isolated compounds is proposed. The known natural products (-)-latifolian A (5) and magnocurarine (6) were also isolated during these studies. The antibacterial activity was explained by the presence of 5, which displayed an IC50 value of 9.8 μM (MIC = 35 μM).