| Research Abstract |
Trichotomin (1), the dimeric indole-alkaloid pigment of Clerodendron trichotomum, is synthesized via 2, 3, 5, 6, 11, 11b-hexahydro-3-oxo-1H-indolizino [8, 7-b] indole-5, 11b-dicarboxylic acid (2) from L-tryptophan and 2-oxoglutaric acid. The precursor acid (2) exists in two diastereo-isomeric forms, namely the 11bα-form and the 11bβ-form, of which the 11bβ-form predominates in the plant. The stereochemistry of the 11b-position is determined by the steric course of the Pictet-Spengler reaction which leads to a tetrahydro-β-carboline intermediate. In homogeneous catalysis, the 11bβ-form is formed predominantly. In heterogeneous reactions in the use of solid acid catalysts such as zeolites, the steric course of the tetrahydro-β-carboline intermediate formation will be regulated by the reaction on the surface or within the pores of the catalysts. The steric course of photochemical reactions within the pores of catalysts will be affected greatly. In order to investigate the possibility of stereocontrol over the course of tetrahydro-β-carboline intermediate formation, the reactions of methyl tryptophanate with dimethyl 2-oxoglutarate in benzene were carried out using aerosil 200, alumina, zeolite (MCM-41), zeolites A-4 and F-9, molecular sieves 10Å, and mordenite (H-form) as solid acid catalysts, and the reaction products were analyzed by HPLC, TLC, and NMR spectroscopy. The heterogeneous reaction using the zeolites as catalysts gave the diester of the 11bα-form of the dicarboxylic acid (2) predominantly. Zeolite (MCM-41) gave a large amount of the tetrahydro-β-carboline intermediate of 11bβ-diester of (2), which showed the difficulty of lactam formation into the 11bβ-diester of (2) within the pores even though the pore size is 2.2nm.
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