Chuktabularins E−T, 16-Norphragmalin Limonoids from <i>Chukrasia tabularis</i> var. <i>velutina</i>

Abstract

Chuktabularins E−T (<b>1</b>−<b>16</b>), 16 new 16-norphragmalin limonoids, together with four known compounds, chuktabularins A−D, were isolated from the stem bark of <i>Chukrasia tabularis</i> var. <i>velutina</i>. 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 <b>1</b> was confirmed by a single-crystal X-ray diffraction experiment, representing the first verification of the skeleton of 16-norphragmalin limonoids. Chuktabularins K−O (<b>7</b>−<b>11</b>) were found to be the first 19-acetoxylated 16-norphragmalin limonoids. Variable-temperature ¹H NMR experiments suggested that <b>7</b> exists as an equilibrium mixture of conformational isomers in solution. The absolute configuration of <b>5</b> was determined by the CD exciton chirality method on its 11,12-di-<i>p</i>-chlorobenzoate (<b>5a</b>), and those of <b>1</b>−<b>4</b> and <b>6</b>−<b>16</b> were proposed by correlating with <b>5</b> spectroscopically and biogenetically.