Phoslactomycins (PLMs) are a group of natural products belonging to a polyketides class. These polyketides are synthesized by sequential reaction catalyzed by a collection of enzymes activities called polyketide synthases. A polyketide is a large class of diverse compounds that are characterized by more than two carbonyl groups connected by single intervening carbon atoms. In other words, a polyketide is a polymer whose monomer is a ketide. The PLMs are also known as phosphazomycins or phospholines. These compounds were isolated based on antifungal and antitumor activities. This array of promising biological activities has stimulated research into the field of PLMs for treatment of various diseases such as aspergillosis. A significant success has been reported in understanding and manipulating PLM biosynthesis. However, its post-polyketide biosynthetic mechanism remains to be elucidated. In this study, we established steps needed to pave the way for the elucidation of the post-polyketide synthase tailoring steps in the phoslactomycin biosynthetic pathway. Various, biological activities of polyketide natural products are often linked with specific structural motifs, biosynthetically introduced after construction of the polyketide core. Therefore, investigation of such "post-polyketide synthase (PKS)" modifications is important, and the accumulated knowledge on these processes can be applied for combinatorial biosynthesis to generate new polyketide derivatives with enhanced biological activity. In this study, the enzymes and genes responsible for the modification of the phoslactomycin moiety have been investigated to verify their functions and to study how they are coordinated to achieve the desired phoslactomycin. The proposed modification steps in the PLM biosynthesis pathway involves, PlmT4 a cytochrome P450 monooxygenase, PlmT5, a kinase, and PlmT8 an oxidoreductase. These enzymes were successfully cloned, overexpressed, and purified from an overexpression vector. Mutant strains for two genes plmT4 and plmT8 were either constructed or studied. The function of PlmT4 tailoring enzyme was characterized, by gene disruption and an in vitro enzyme activity assay. The isolation of PLM 1 an intermediate analog from plmT4 mutant strain and the observation of a malonylated PLMs, suggests that the malonyl side chain is introduced during polyketide chain formation These results, will pave the way to delineate the intermediary steps between the PLM PKS product(s) that is released from the PLM PKS and the formation of the final phoslactomycin.

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Chamuvarinin (22) is a unique annoanceaeous acetogenin isolated from the roots of Senegalese medicinal plant Uvaria chamae by Laurens and co-workers in 2004. It displays highly potent cytotoxicity towards the cervical cancer cell lines (KB 3-1, IC₅₀= 0.8 nM). Structurally, chamuvarinin is the first reported acetogenin to contain an adjacently linked bis-THF-THP ring system spanning the C15-C28 carbon backbone. However, initial efforts to assign the relative and absolute configuration within this stereochemical array, on the basis of ¹H and ¹³C NMR analysis, provided only partial information pertaining to the relative configuration of C15-C19 region. As a consequence, 32 diastereomeric structural possibilities exist for the highly unusual structure of chamuvarinin; an unrealistic target for total synthesis. The synthesis of the central core tricyclic (BCD) intermediate represents the most challenging aspect in the entire synthesis, which in turn will aid ultimate structural proof. At the outset of the project the stereochemical configuration of C15-C28 (BCD) of chamuvarinin was uncertain and a library approach was proposed to enable structure elucidation (Scheme A-1). Chapter 2 and Chapter 3 detail the synthesis of possible diastereomers of the C9-C21 (51) and C22-C34 fragments (52). Chapter 4 details the intial strategy to couple the key diastereomeric fragments in a series of model studies. Chapter 5 describes the successful coupling strategy via an revised synthetic approach to reach the advanced C9-C34 intermediate 251 (Scheme A-2).

In 2004, the polyketide natural product, chamuvarinin (72) was isolated by Laurens et al. from the roots of Uvaria chamae, a member of the Annonaceae plant family. This unique tetrahydropyran containing acetogenin displayed potent levels of cytotoxic activity against the KB 3-1 cell line with an ED50 value of 0.8 nM. Upon initial isolation the relative and absolute stereochemical assignment of chamuvarinin (72) was unable to be readily achieved through ¹H and ¹³C NMR analysis. The initial synthetic route described herein has enabled the relative and absolute stereochemical determination of chamuvarinin (72) through the first total synthesis completed in 20 longest linear steps in 1.5% overall yield. A revised synthetic strategy towards chamuvarinin (72) was completed in 17 longest linear steps in 2.2% overall yield. The revised route facilitated the assembly of non-natural chamuvarinin-like analogues and their trypanocidal and cytotoxic activities have been assessed. The synthesis of these analogues has formed the basis of a more focussed study through the design and synthesis of simplified triazole (295), isoxazole (325) and butenolide triazole (305) analogues as potential Trypanosoma brucei (causative agent in African Sleeping sickness) inhibitors.