PHENOLIC AND TERPENOID CONSTITUENTS OF SOME SINGAPORE PLANTS

Abstract

Chapter One provides a brief introduction to the study of natural products. The various nuclear magnetic resonance (NMR) techniques are outlined and their applications in structure elucidation of natural products are discussed with emphasis on two-dimensional NMR spectroscopy. The remaining chapters are concerned with the isolation and characterization of natural products from plants which are found in Singapore. Structure elucidation of these compounds was achieved mainly by a combination of chemical and NMR (COSY, HMQC, HMBC, NOE, 1H and 13C) studies. Chapter Two is dedicated to the chemical investigation of four species of the Guttiferae. The bark of Cratoxvlum cochinchinense (Lour.) Blume contains six novel compounds, the bicarbocyclic triterpenoid polypoda8(26 ),13, 17,21-tetrnen-3?-ol and the xanthone derivatives 2-geranyl-1,3,7-trihydroxy-4-( 3-methylbut-2-enyl )-xanthone, cratoxylone, 11-hydroxy- 1 isomangostin, cratoxyxanthone and 5'--demethoxycadensin G. 2-Geranyl-1,3,7-trihydroxy-4-(3- methylbut-2-enyl)-xanthone showed moderate antiproliferative activity against human cervical cancer, human leukaemia and mouse lymphoma cell lines. 11-Hydroxy-1-isomangostin is a hydroxylated derivative of the known constituent of Garcinia, 1-isomangostin. Although cyclization of a 3-methylbut-2-enyl unit with an ortho hydroxyl group is common among xanthones, the presence of a hydroxylated chroman, as in 11-hydroxy- 1-isomangostin, is rare. Cratoxylone is an isomer of the known xanthone, garcinone D. The isomers differ in that the positions of the side chains are interchanged. Cratoxyxanthone was proven to be a bisxanthone, the first to be reported in this species. The xanthone units are linked through a cyclized C5 side chain. 5'-Demethoxycadensin G is a xanthonolignoid, a class of xanthone derivatives which is found only in species of Hypericoideae and Kielmeyeroideae. The isolation of 5' -demethoxycadensin G from Cratoxylum cochinchinense supports its inclusion in the sub-family Hypericoideae. The twigs and branches of Garcinia dulcis (Roxb.) Kurtz yielded the known bidavonoids podocarpusflavone-A and bisdehydro GBla as well as the flavanone 3' -prenylnaringenin, and a xanthone, 1,4,6-trihydroxy-5-methoxy-7-(3-methylbut-2- enyl)-xanthone. The twigs and branches of Garcinia forbesii King were found to contain a new 1,3,6, 7-tetraoxygenated chromenoxanthone, forbexanthone. Two new oleanane triterpenoids were isolated from the bark of Ploiorium aliernifolium (Vahl) Melchior, one of which possesses a ?-lactone ring. Chapter Three deals with the chemical investigation of Bhesa paniculata Arn. (Celastraceae). In contrast to previous results which indicate that members of this family produce biologically active alkaloids and sesquiterpene polyol esters, an investigation of Bhesa paniculata revealed a complete absence of these compounds. A novel sugar derivative was isolated in addition to the known compounds ?-sitosterol, glycerol 1-9',12' -octadecadienoate, pinoresinol, vanillic acid, methyl 3,4-dihydroxybenzoate, anofinic acid and 2-(1'-methyl-ethenyl)-benzofuran-5- carboxylic acid. Chapter Four is concerned with the study of Tetracera indica (Houtt. ex. Christm. and Panz.) Merr. (Dilleniaceae). The major constituent was found to be 5,7-dihydroxy-8-methoxyflavone.