Concise synthesis of the vasorelaxant alkaloids schwarzinicines A and B

A concise synthesis of the 1,4-diarylbutanoid-phenethylamine alkaloids, schwarzinicines A (1) and B (2), recently isolated from Ficus schwarzii, is reported. Key steps include a Claisen condensation to assemble the 1,4-diaryl-2-butanone intermediate, followed by a reductive amination to furnish the core skeleton of the target compounds. The overall synthetic yields of 1 and 2 were 9.1% and 3.5%, respectively. Synthetic (-)-1, (+)-1 and (+/-)-1 exhibited comparable vasorelaxation as natural schwarzinicine A on rat isolated aortic rings, suggesting that the observed vasorelaxant effects were not influenced by the chirality at C-2.