Chuktabularins E−T, 16-Norphragmalin Limonoids from Chukrasia tabularis var. velutina
journal contributionposted on 28.05.2010, 00:00 authored by Jun Luo, Jun-Song Wang, Xiao-Bing Wang, Jian-Guang Luo, Ling-Yi Kong
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 tabularis var. 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 1 was 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 7 exists as an equilibrium mixture of conformational isomers in solution. The absolute configuration of 5 was determined by the CD exciton chirality method on its 11,12-di-p-chlorobenzoate (5a), and those of 1−4 and 6−16 were proposed by correlating with 5 spectroscopically and biogenetically.