Synthesis of tribulusterine, a potent toxic alkaloid from Tribulus terrestris

Synthetic approaches to tribulusterine (15), a suspected toxic alkaloid from Tribulus terrestris, have been investigated via nucleophilic and electrophilic substitution reactions, and the Pictet-Spengler cyclisation reaction. Nucleophilic substitution reactions of the 9-(N,A^-dimethylsulfamoyl)-p-carboline-A^-oxide (50) with the furyllithium (46a) yielded the new 1-substituted P-carboline, l-(3- furyl)methoxy-9-(A^,A/'-dimethylsulfamoyl)-(3-carboline (52). Bromination of the Pcarboline derivative (51) afforded 1,3,6,8-tetrabromo-P-carboline and 3,6,8-tribromop- carboline as major products when the lithiated P-carboline (51) was treated with Nbromosuccinimide (NBS) and bromine, respectively. The Pictet-Spengler reaction approach may have yielded the required alkaloid, tribulusterine (15), based on some spectroscopic evidence. The new furan derivative, 3-(hydroxymethyl)-2-furaldehyde (66), required for the Pictet-Spengler approach, was synthesised via a lithiationmediated procedure. A precursor (33) for a palladium-catalysed Negishi-type crosscoupling approach was also prepared.