Siliquapyranone: a tannic acid Tetrahydropyran-2-one isolated from the leaves of carob (Ceratonia siliqua) by pressurised hot water extraction

Studies of the phytochemistry of carob (<i>Ceratonia siliqua</i>) leaf material are extremely limited. This report features the second natural product isolation study of carob leaves and the first such investigation of <i>C. siliqua</i> grown in Australia. Investigation of leaf material from seven carob cultivars using pressurised hot water extraction (PHWE) revealed the presence of high levels of myricitrin, D-pinitol, and sucrose in addition to the previously unreported natural product siliquapyranone {(2R,3R,4S,5R,6R)-5-hydroxy-6-(hydroxymethyl)-2-(((2S,4S)-2-methyl-6-oxotetrahydro-2<i>H</i>-pyran-4-yl)oxy)tetrahydro-2<i>H</i>-pyran-3,4-diyl bis(3,4,5-trihydroxybenzoate)}. Siliquapyranone represents a biosynthetic fusion of conventional 2,3-gallate esters on a b-glucose tannin with a hydroxytetrahydro-2<i>H</i>-pyran-2-one-related non-tannin tetrahydropyran-2-one, parasorboside. Preliminary biological testing indicates that siliquapyranone exhibits weak activity against both tumour and normal cell lines. This suggests that, like most tannins and parasorboside, siliquapyranone may act as an antifeedant.