Cytotoxic Alkaloids from Microcos paniculata with Activity at Neuronal Nicotinic Receptors

Natural products have a long history of use as leads for drug discovery. The World Health Organization (WHO) estimates that in Africa for example, up to 80% of the population relies on plant-derived traditional medicines to help meet primary healthcare needs (WHO, 2002). In the area of cancer, of the 175 new chemical entities in the western drug market from the 1940s to 2010, 48.6% were a combination of unaltered natural products, semisynthetic natural product derivatives, or obtained by total synthesis inspired by a natural product skeleton (Newman and Cragg, 2012). The question of why secondary metabolites are biosynthesized by organisms is still a topic of debate. However, ecological research has suggested that these compounds play a role as defense agents and are used also in communication (Williams et al., 1989; Yi et al., 2003; Chen et al., 2009). This dissertation work emcompasses bioactive alkaloids from the plant Microcos paniculata collected in Vietnam. Microcos paniculata is a large shrub or small tree that grows in several countries in South and Southeast Asia. Three new piperidine alkaloids, microgrewiapines A-C, and a number of known compounds were isolated from cytotoxic fractions of the separate chloroform-soluble extracts of the stem bark, branches, and leaves of M. paniculata. Comprehensive structure elucidation techniques were used to elucidate and identify the structure of the new compounds. In addition, H -C H2BC iii NMR spectroscopy and induced circular dichroism (ICD) were employed to confirm the structures of the known compounds maslinic acid and (-)-loliolide. A total synthesis of 2β-methyl-3,6β-(di-tert-butyldimethylsilyl) piperidine was accomplished to investigate the structural requirements for biological activity. The isolated compounds exhibited a range of cytotoxic potencies against the HT-29 human colon cancer cell line that was used to screen extracts, chromatographically purified fractions, and pure compounds. Structural similarities between the isolated piperidine alkaloids and certain previously identified nicotinic receptor (nAChR) antagonists, along with growing evidence to support the involvement of nAChRs in various aspects of cancer, provided a rationale to examine the effects of the isolates on nAChRs (Egleton et al., 2008; Schuller, 2009; González-Cestari et al., 2009; Henderson et al., 2010). When evaluated for their effects on human α3β4 or α4β2 nAChRs, several of these compounds were shown to have antagonistic activity for both receptor subtypes. As a result of this dissertation work, Microcos paniculata was collected as a tropical plant in Vietnam in 2008, leading to pure compounds being characterized showing activity at the cellular and receptor levels. The cytotoxicity and nAChR modulatory activity of compounds isolated in this investigation represent the first time such bioactivies have been shown for the constituents of a plant in the genus Microcos. Contemporary structure elucidation techniques were used and a preliminary analysis of the structural features contributing to the bioactivity of the isolated compounds has been