Determination of absolute configuration of clerodane-type diterpenes using experimental and theoretical electronic circular dichroism techniques / Ahmad Nazif Aziz … [et al.]

Clerodane-type diterpenes isolated from plants are chiral compounds with specific stereochemistry. The compounds have complex structure with many stereo centres which gives them unique 3-dimensional features, not uncommonly found for natural products. These 3-dimensional (3D) features are in fact what makes natural product compounds unique and, in many cases, contributes to their biological functions. In exhibiting their biological properties, the natural product compounds very often bind with enzymes and proteins, which take place in lock and key manner. Stereochemistry of the compounds becomes crucial and may help explain important questions in the mechanism of action for a particular biological activity. In this project, absolute configurations of these compounds were determined using Electronic Circular Dichroism spectroscopy (ECD) techniques. Circular Dichroism (CD) may be regarded as one of the most powerful techniques for stereochemical analysis. It is sensitive to the absolute configuration as well as to conformational features, which are often completely obscured in the ordinary absorption spectrum. CD is a measurement of how an optically active compound absorbs right- and left-handed circularly polarized light. Optically active compounds containing chromophores exhibit different extinction coefficients for R and L circularly polarized light waves produced characteristic absorption bands. Electronic Circular Dichroism (ECD) spectroscopy exploits this phenomenon to probe the handedness of the environment of the chromophores. ECD is a very sensitive diagnostic tool for determining not only absolute configuration and conformation, but also for monitoring intermolecular interactions where chiral systems are involved. The experimental ECD spectra were verified by predicting the theoretical CD spectra using a semi-empirical ab initio calculation employing time-dependent density functional theory (TD-DFT) using Gaussian09 software. The conformational analysis of the compounds was implemented prior to the TD-DFT calculations, using the Boltzmann method. This method will find all possible minimum structures which are populated at the room temperature. The structures within 3 kcal mol-1 energy cut-off were taken into consideration. Boltzmann method is also useful in uncovering any errors in the assignment of absolute configuration by experimental ECD. The assignments of absolute configuration of the natural diterpenoids were finalized by comparing the experimental and theoretical ECD data. The developments of the density functional theory (DFT) method have revolutionized the determination of the absolute Conï¿¿guration of chiral molecules using these chiroptical properties. This method is applicable for other chiral compounds with suitable chromophores since it is measured within the Ultraviolet (UV) range wavelength. A multi stereo centred clerodane- type diterpenoid namely crovatin were selected for the determination of its absolute configurations. This compound was isolated from the dichloromethane extracts of Croton laevifolius collected from Dungun, Terengganu, Malaysia. The experimental CD was done in HPLC grade acetonitrile solvent. Conformational analysis resulted with five conformers within 3 kcal mol-1 energy cut-off. TDDFT calculations were done at B3LYP/6-31G+ (d, p) level, by considering the normal and end configurations of this clerodane diterpene. The comparison of the experimental and calculated ECD spectra of crovatin has resulted in the conclusion that this compound follows the normal clerodane configuration and its absolute configurations were successfully determined.