THE SYNTHESIS OF SOME NATURAL PRODUCTS OF MEDICINAL VALUE USING ORGANOMETALLIC COMPLEXES

Abstract

The challenge of accomplishing C-C bond formation while protecting sensitive functional groups and overcoming problems of stereo-control has led to the use of transition-metal organometallic compounds in organic compounds synthesis. Nucleophilic additions to cationic dienyliron carbonyl complexes provide a convenient means of C-C bond formation. It is highly regioselective and stereospecific making it useful for the synthesis of natural products and compounds with biological activities. Previous research showed the use of synthetic ligands as nucleophiles but none has used naturally isolated compounds. Therefore, this study was designed to synthesise and characterise new natural products through demetallation of adducts with selected natural products isolated from medicinal plants as nucleophiles. Additionally, to investigate the antimicrobial and electronic properties of these compounds. The reaction of parent dienylium cations ([1-5-η-(dienyl)Fe(CO)3]BF4(Dienyl = C6H7,2- MeOC6H6)) with selected natural products were synthesised and their eight adducts were purified using standard procedure. The existing natural products: gedunin, khivorin, polyavolensinol and 7-ketokhivorin were used as nucleophiles. The resulting complexes were demetallated to obtain new products. The synthesised compounds were characterised using Infrared (IR), Nuclear Magnetic Resonance (NMR) and Mass Spectroscopic (MS) techniques. The energy band gap (ΔE) was calculated at Density Functional Theory (DFT) level using hybrid Beckie-3-Lee Yang Parr (B3LYP) functional density with 6-31G (d) and pseudo potential basis sets in gaseous state. The adducts, ligands and the demetallated products were also screened from 2.5 to 405 mg/mL for antimicrobial properties using the disc diffusion method against Bacillus cereus ATTC 14579, Bacillus subtilis ATTC 33923, Proteus mirabilis ATTC 21784, Salmonella typhi ATTC 14028 and Candida albican MTTC 227. The Minimum Inhibitory Concentrations (MICs) were determined by agar dilution method. The reaction of dienylium cations with the natural products yielded the corresponding 1,3- diene substituted derivatives. The adducts exhibited strong and intense IR √(CO) bands at 2050 and 1975 cm-1 which are characteristic absorptions of neutral tricarbonyl(1,3-dienesubstituted) iron derivatives. The complete disappearance of these bands in all the demetallated products confirmed cleavage of the iron tricarbonyl moiety. The 1HNMR spectra of all the products showed overlapping characteristic resonances of the outer and UNIVERSITY OF IBADAN LIBRARY iii inner 1,3-diene protons. All the natural products attacked the dienylium cations at C5' via the α-furan and β-indole carbon fragments in their structures. The loss of αβ-unsaturated ketone fragment at m/z 151(C10H15O) was responsible for the expected absence of the parent peaks in the MS data. There was also loss of M-173 fragment in all the compounds indicating the weakness of the C(diene)-C(natural products) bond and the calculated energy band gap (ΔE) indicated the reactivities of the compounds. The zones of inhibition of the demetallated compounds ranged from 7.5 to 22.5 mm while that of the ligands ranged from 8.0 to 12.5 mm. The adducts show no activity. The MIC values ranged from 0.01 to 29.17 mg/mL. The demetallation of adducts from nucleophilic addition to dienylium cations provided a convenient method of achieving C-C bond formation and this enabled the synthesis of structurally complex natural products of biological importance. The synthesised compounds could serve as leads in drug development. Keywords: Dienylium cations, Demetallated adducts, Molecular orbital Word counts: 496