Acylglycerols (=Glycerides) from the Glandular Trichome Exudate on the Leaves of Paulownia tomentosa

Chemical investigations of the glandular trichome exudates on the leaves of Paulownia tomentosa (Scrophulariaceae) led to the identification of the thirty acylglycerols (=glycerides) 1–30, including five known ones (2, 3, 6, 9, and 15) (Fig. 1). Spectroscopic analysis combined with GC/MS studies of the glycerides and the liberated fatty acids, in the form of trimethylsilyl ether derivatives and trimethylsilylated methyl esters, respectively, established that the constituents belonged to 1,3-di-O-acetyl-2-O-(fatty acyl)glycerols, 1-O-acetyl-2-O-(fatty acyl)-sn-glycerols, and 2-O-(fatty acyl)glycerols, wherein the fatty acyl moiety was either an eicosanoyl or an octadecanoyl group bearing OH and/or AcO groups at the 3-, 3,6-, 3,7-, 3,8-, or 3,9-positions. The 1-O-acetyl-2-O-[(3R,6S)-3-(acetyloxy)-6-hydroxyeicosanoyl]-sn-glycerol (12; 20% of the total glycerides), 2-O-[(3R,8R)-3,8-bis(acetyloxy)eicosanoyl]glycerol (17; 14%), 2-O-[(3R,9R)-3,9-bis(acetyloxy)eicosanoyl]glycerol (18; 12%), and 2-O-[(3R)-3-(acetyloxy)eicosanoyl]glycerol (10; 12%) were relatively abundant constituents. The configurations of the stereogenic centers of the fatty acyl moieties were determined by 1H-NMR analysis of the monoesters obtained from (R)- and (S)-2-(naphthalen-2-yl)-2-methoxyacetic acid ((R)- and (S)-2NMAOH and the hydroxy-substituted fatty acid methyl esters (Fig. 2). The configuration at C(2) of the glycerol moiety of the 1-O-acetyl-2-O-(fatty acyl)glycerols was determined to be (2S) by chemical conversion of, e.g., G-2 (=2/3 1 : 10) to (+)-3-O-[tert-butyl)diphenylsilyl]-sn glycerol of known absolute configuration.