Chuktabularins E−T, 16-Norphragmalin Limonoids from Chukrasia tabularisvar. velutina

Chuktabularins E−T (1−16), 16 new 16-norphragmalin limonoids, together with four known compounds, chuktabularins A−D, were isolated from the stem bark of Chukrasia tabularisvar. velutina. These compounds possess a biosynthetically extended propionyl or acetyl group at C-15 and a characteristic ketal moiety between the limonoid skeleton and the acyl substituent at C-15. The structures of these compounds were established on the basis of detailed spectroscopic analysis, and that of 1was confirmed by a single-crystal X-ray diffraction experiment, representing the first verification of the skeleton of 16-norphragmalin limonoids. Chuktabularins K−O (7−11) were found to be the first 19-acetoxylated 16-norphragmalin limonoids. Variable-temperature 1H NMR experiments suggested that 7exists as an equilibrium mixture of conformational isomers in solution. The absolute configuration of 5was determined by the CD exciton chirality method on its 11,12-di-p-chlorobenzoate (5a), and those of 1−4and 6−16were proposed by correlating with 5spectroscopically and biogenetically.