Abstract
In this study, the phytochemical profile of Conchocarpus mastigophorus was investigated, leading to the isolation of four pyranoquinolinone alkaloids, including huajiaosimuline (1) and three new compounds: 3'-acetoxy-4'-hydroxyzanthosimuline (2), epoxyzanthosimuline (3), and mastigophorine (4). All new compounds possess two chiral centers and exist as a 1:1 mixture of epimers, differing in the configuration of a chiral center at the pyran ring. The absolute configurations of the epimers 2 and 3 were determined by data comparison with identical synthetic compounds, revealing mixtures of (2S,3'R)- and (2R,3'R)-configured epimers in both cases. By employing Sharpless asymmetric dihydroxylation and Shi epoxidation as key stereoselective steps, stereocontrol at C3 was achieved in the synthesis of compounds 2 and 3, respectively. Our synthesis approach provided 3'R- and 3'S-configured 2 in three steps (41-46% overall yield, 95% dr) and 3'R-configured 3 in 2 steps (61% overall yield, 90% dr) from 4-hydroxy-1-methyl-2(1H)-quinolinone and citral.