Thesis (Ph.D.)--Chulalongkorn University, 2009

In phytochemical investigation for bioactive compounds from Thai medicinal plants, Gnetum macrostachyum and Alangium salviifolium were selected for isolation, purification and structure elucidation. The chromatographic separation of the acetone extract of G. macrostachyum led to the isolation of a new flavonoid, 5,7,2[superscript ’]-trihydroxy-5[superscript ’]-methoxyflavone (1.1) and a new tetramers stilbenoid, (1.10) along with a known flavonoid 5,7,4[superscript ’] -trihydroxy-3[superscript ’]-methoxyflavanone (1.2) and seven known stilbenoids including, resveratrol (1.3), 3-methoxyresveratrol (1.4), shegansu B (1.5), gnetulin (1.6), gnetuhainin C (1.7), parvifolol B (1.8) and pallidol (1.9). Nine isolated compounds (1.1-1.9) were tested for antioxidant activity (DPPH radical scavenging). Compounds 1.3, 1.4, 1.6 and 1.9 showed DPPH radical scavenging activity with IC[subscript 50] values of 0.40, 0.30, 0.21 and 0.53 mM, respectively. The chromatographic separation of the butanolic crude extract of A. salviifolium led to the isolation of five known alkaloids including, psychotrine (2.1), alangiside (2.2), 3-O-demethyl-2-O-methylalangiside (2.3), alangicine (2.4) and demethylalangiside (2.5). While 1[superscript ’] ,2[superscript ’] -dehydrotubulosine (2.6) and demethylpsychotrine (2.7) were also yielded from the water crude extract. All isolated alkaloids were evaluated for cytotoxicity against KB and HeLa cells. Compounds 2.1 and 2.4 showed significant cytotoxicity against KB cell with IC [subscript 50] values of 1.00 and 9.00 µg/mL, respectively. In addition, compound 2.1 also showed the highest cytotoxicity against Hela cell with IC50 value of 0.70 µg/mL. The structures of all isolated compounds were elucidated by spectroscopic methods as well as comparison with previous literature data. Curcumin (3.1), demethoxycurcumin (3.2) and bisdemethoxycurcumin (3.3) isolated from the CH[subscript 2] Cl[subscript 2 ] crude extract from rhizomes of Curcuma longa (Zingiberaceae), and their derivatives 3.4-3.14 were evaluated for alpha-glucosidase inhibitory activity. The results indicated that curcuminoids 3.1-3.3 showed inhibitory activities against alpha-glucosidase with the IC[subscript 50] values of 0.81, 0.31 and 0.47 mM, respectively, which were comparable to that of Acarbose[superscript (R)] and exhibited more potent inhibitory activity than their derivatives 3.4-3.14. In addition, the derivatives 3.10-3.12, whose C-4 was fully methylated, showed weak α-glucosidase inhibition with the IC[subscript 50] values of 1.39, 0.89 and 5.27 mM, respectively. On the other hand, the asymmetrical curcuminoids 3.5, 3.8 and 3.11 exhibited more potent inhibitory activity than the symmetrical curcuminoid congeners 3.6, 3.7, 3.9, 3.10 and 3.12-3.14. In conclusion, simple curcumins having at least one hydroxyl group attached to a aromatic moiety possessed enhanced inhibitory effect whereas the introduction of alkyl groups at hydroxyl groups of curcuminoids reduced activities. In addition, the presence of C-4 methylene proton is not associated with blocking enzyme function.