Tetrodecamycin: An unusual and interesting tetronate antibiotic

AbstractThe tetrodecamycins are a group of secondary metabolites that are characterized by the presence of a tetronate ring in their structure. Originally discovered for their antibiotic activity against Photobacterium damselae ssp. piscicida, the causative agent of pseudotuberculosis in fish, this family of molecules has also been shown to have potent antibiotic activity against methicillin-resistant Staphylococcus aureus. Due to their small size and highly cyclized nature, they represent an unusual member of the much larger group of bioactive molecules called the tetronates. Herein, we review what is known about the mechanism of action of these molecules and also present a hypothesis for their biosynthesis. A deeper understanding of the tetrodecamycins will provide a more holistic view of the tetronate-family, provide new chemical probes of bacterial biology, and may provide therapeutic lead molecules

Approaches to the core structure of the squalestatins

The squalestatins are a new family of natural products which display potent cholesterol lowering effects. Common to all these natural products is the highly oxidised bicyclic core and the aim of this project was to achieve a concise synthetic route to this core unit. Initial studies were carried out using 2-benzyloxycyclohexanone as a model template. Following conversion to the 2-oxa-3-oxo-spirodecan-6-one via addition of the dianion of 3-(para- tolylsulphonyl)propionic acid, coupling of a C(2) fragment was explored. Addition of carboethoxymethylenetriphenylphosphorane, followed by oxidation to the diol and protection as the acetonide led to the formation of 4- Ethoxycarbonyl-(2,2-dimethyl-5"-oxodispiro[perhydro[l,3]dioxolane-4,r- cyclohexane-2',2"-(5"-H-furan)]-5-yl. The alternative order of addition of the C(4) and C(2) units has also been undertaken. Manipulation of the ester group to a silyl ether afforded a less reactive functionality and C(4) was manipulated to allow for the coupling of the next fragment to form the spiro lactone. The addition of the dianion of 3-(parc-tolylsulphonyl)propionic acid to4-((^t)butyldimethylsilyloxymethyl)-2,2-dimethyl-l,3-dioxa-spirodecan-6-one failed and another route to the spiro lactone was explored. Formation of 4 - ((^t)butyldimethy Isilyloxymethy l)-2,2-dimethyl-1,3,7-trioxa- dispirotetradecan-8-one (I) was achieved by allylation at C(4) followed by hydroboration of the double bond and subsequent oxidation. The C(l) side chain could be added to the spiro lactone using allyl magnesium bromide without compromising the other functionality present. Acid treatment of 4-((^t)butyldimethylsilyloxymethyl)-8-methoxy-2,2-dimethyl-8- propyl-l,3,7-trioxa-dispirotetradecane (II) promoted deprotection of the acetonide followed by concomitant cyclisation to the desired 6-hydroxy-9-propyl- 8,12-dioxatricyclododec-7-yl-l-methanol (III). This showed the viability of the retrosynthetic analysis as a route to core analogues of the squalestatins. Studies to the fully substituted core were commenced using cis-cyclohexadiene diol. The diol was protected as its p-anisaldehyde acetal before the formation of the Diels Alder adduct (IV) using 4-phenyl-l,3,5-triazolinone. However a lack of time prevented its manipulation to the a-alkoxy ketone species through Lewis Acid mediated cleavage of the acetal. In a second retrosynthetic plan 2-benzyloxycyclohexanone was coupled with methyl tetronate prepared following the procedure of Pelter. Preliminary studies towards the addition of the C(l) side chain have been undertaken and initial results seem promisin

Investigations of the biosynthesis and biomimetic synthesis of bioactive natural products

This thesis describes work towards the biomimetic synthesis and understanding the biosynthesis of two families of natural products: prodiginines and quartromicins. Prodiginines are a large family of red pigmented tripyrrole antibiotics. Although they have not been used clinically, the promising anti-cancer, immunosuppressive and antimalarial activity they display at non-toxic doses has generated renewed interest in their utilisation. The synthesis of an analogue of the proposed pyrrole-2-carboxyl-RedO intermediate in prodiginine biosynthesis has been achieved. The resulting NAC thioester and analogues of it have been used to investigate the prodiginine biosynthetic pathway in Streptomyces coelicolor, and to examine the production of prodiginine analogues by mutasynthesis. Quartromicins, novel anti-viral antibiotics, are a structurally unique group of spirotetronate natural products produced by Amycolatopsis species. They are unusual symmetric macro cyclic compounds which possess a 32-membered carbocyclic structure with four spirotetronic acid units connected by enone or dienone linkers in a head-to-tail fashion. These macrocyclic compounds are intriguing because they have alternating endo- and exo- spirotetronic acid units, with the opposite "comers" being identical. Although the quartromicins have therapeutic potential, very little is known about their biosynthesis. In this research a biosynthetic pathway to the quartromicins has been proposed based on hypothetical pathways to related natural products. The synthesis of the two putative key intermediates in quartromicin biosynthesis has been achieved. An improved method for the synthesis of exomethylene tetronates has been developed, and novel rearrangements have been discovered. The two putative key intermediates have been used to investigate the biomimetic synthesis of the carbon skeleton of the quartromicin algycone, and mass spectrometric evidence for formation of homo- and heterodimers, and a heterotetramer of the key intermediates has been obtained

Unexpected enzyme-catalysed [4+2] cycloaddition and rearrangement in polyether antibiotic biosynthesis

Enzymes that catalyse remarkable Diels–Alder-like [4+2] cyclizations have been previously implicated in the biosynthesis of spirotetronate and spirotetramate antibiotics. Biosynthesis of the polyether antibiotic tetronasin is not expected to require such steps, yet the tetronasin gene cluster encodes enzymes Tsn11 and Tsn15, which are homologous to authentic [4+2] cyclases. Here, we show that deletion of Tsn11 led to accumulation of a late-stage intermediate, in which the two central rings of tetronasin and four of its twelve asymmetric centres remain unformed. In vitro reconstitution showed that Tsn11 catalyses an apparent inverse-electron-demand hetero-Diels–Alder-like [4+2] cyclization of this species to form an unexpected oxadecalin compound that is then rearranged by Tsn15 to form tetronasin. To gain structural and mechanistic insight into the activity of Tsn15, the crystal structure of a Tsn15-substrate complex has been solved at 1.7 Å resolution

Review on abyssomicins:Inhibitors of the chorismate pathway and folate biosynthesis

Antifolates targeting folate biosynthesis within the shikimate-chorismate-folate metabolic pathway are ideal and selective antimicrobials, since higher eukaryotes lack this pathway and rely on an exogenous source of folate. Resistance to the available antifolates, inhibiting the folate pathway, underlines the need for novel antibiotic scaffolds and molecular targets. While para-aminobenzoic acid synthesis within the chorismate pathway constitutes a novel molecular target for antifolates, abyssomicins are its first known natural inhibitors. This review describes the abyssomicin family, a novel spirotetronate polyketide Class I antimicrobial. It summarizes synthetic and biological studies, structural, biosynthetic, and biological properties of the abyssomicin family members. This paper aims to explain their molecular target, mechanism of action, structure–activity relationship, and to explore their biological and pharmacological potential. Thirty-two natural abyssomicins and numerous synthetic analogues have been reported. The biological activity of abyssomicins includes their antimicrobial activity against Gram-positive bacteria and mycobacteria, antitumor properties, latent human immunodeficiency virus (HIV) reactivator, anti-HIV and HIV replication inducer properties. Their antimalarial properties have not been explored yet. Future analoging programs using the structure–activity relationship data and synthetic approaches may provide a novel abyssomicin structure that is active and devoid of cytotoxicity. Abyssomicin J and atrop-o-benzyl-desmethylabyssomicin C constitute promising candidates for such programs

Investigations of siderophore and tetronic acid biosynthesis in streptomyces scabies 87.22

Streptomyces are Gram-positive bacteria, usually found living within the soil and they are saphrophytes. Among this class of bacteria are some plant pathogenic species, which cause infection of the roots or the tubers of some plants. The model Streptomycete plant pathogen is Streptomyces scabies; this infects root crops, such as potato or radish and is a known cause of scab disease. Most Streptomyces species are producers of secondary metabolites, many of which possess important biological activities, such as antibacterial, iron-chelating, anticancer or immunosuppressant. One group of these secondary metabolites are called siderophores. These are small organic molecules, which can chelate ferric iron. The iron in the environment is mainly present as iron (III) hydroxide, which is not very water soluble and cannot, therefore, be taken up directly by microorganisms. Some bacteria solve this problem through production of siderophores. The siderophores are released into the environment by the microorganisms to chelate iron (III) from the environment and transport it into the cell across the cell membrane. Iron is required for many life processes. Analysis of the Streptomyces scabies genome sequence resulted in the identification of gene clusters predicted to direct the biosynthesis of known siderophores, e.g. desferrioxamines and pyochelin, as well as, potentially novel siderophores. A gene inactivation and comparative metabolic profiling approach has been employed to identify the metabolic products of these gene clusters. A PCR-targeting method was used to replace part of or whole genes in the S. scabies 87.22 putative secondary metabolite gene clusters. An internal fragment of the scabichelin biosynthetic gene scab85471 and the putative S. scabies desC gene were deleted using this method. The scabichelin and desC gene mutants were subsequently analysed by LC-MS allowing confirmation of the function of the genes investigated. Production of scabichelin by S. scabies 87.22 wild type was analysed by comparing it with the authentic standard. The chemical and genetic complementation of the Δ desC mutant was carried out to establish the involvement of the desC gene in the biosynthesis of desferrioxamines. The S. scabies 87.22 cryptic tetronate biosynthetic gene cluster predicted to encode a novel agglomerin-like product, which could potentially be involved in plant pathogenicity was also investigated. The expression of the gene cluster was first analysed using reverse transcriptase PCR (RT-PCR) which was carried out on the total RNA isolated from the wild type S. scabies. Following this, an attempt was made to disrupt the scab63021 gene, a putative transcriptional activator of the cryptic tetronate-like cluster in the S. scabies genome. Transcriptional analysis of the wild type S. scabies and the putative Δscab63021 mutant genomes did not show any difference in the expression of the tetronate genes between the wild type strain and the Δ scab63021 mutant

Phase Variable O Antigen Biosynthetic Genes Control Expression of the Major Protective Antigen and Bacteriophage Receptor in Vibrio cholerae O1

The Vibrio cholerae lipopolysaccharide O1 antigen is a major target of bacteriophages and the human immune system and is of critical importance for vaccine design. We used an O1-specific lytic bacteriophage as a tool to probe the capacity of V. cholerae to alter its O1 antigen and identified a novel mechanism by which this organism can modulate O antigen expression and exhibit intra-strain heterogeneity. We identified two phase variable genes required for O1 antigen biosynthesis, manA and wbeL. manA resides outside of the previously recognized O1 antigen biosynthetic locus, and encodes for a phosphomannose isomerase critical for the initial step in O1 antigen biosynthesis. We determined that manA and wbeL phase variants are attenuated for virulence, providing functional evidence to further support the critical role of the O1 antigen for infectivity. We provide the first report of phase variation modulating O1 antigen expression in V. cholerae, and show that the maintenance of these phase variable loci is an important means by which this facultative pathogen can generate the diverse subpopulations of cells needed for infecting the host intestinal tract and for escaping predation by an O1-specific phage