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  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
1

Biosynthesis of acivicin and 4-hydroxyacivicin

Ju, Shyh-chen 07 September 1988 (has links)
Graduation date: 1991
2

Oxidative coupling of dibenzylbutanolides catalyzed by plant cell culture extracts

Palaty, Jan January 1990 (has links)
This thesis aims to develop a new and inexpensive synthetic route to the anti-cancer drug etoposide (6) via 4'-demethylpodophyllotoxin (4) or 4'-demethylepipodophyllotoxin (5) involving the oxidative coupling of a dibenzylbutanolide catalyzed by a cell-free extract (CFE) from plant cell culture. This step was studied in depth using the Catharanthus roseus CFE-catalyzed biotransformation of frans-2-(3,5-dimethoxy-4-hydroxybenzyl)-3-(3-hydroxy-4-methoxybenzyl)butanolide (58) to 1-(3,5-dimethoxy-4-hydroxyphenyl)-6-hydroxy-3-hydroxymethyl-7-methoxy-1,2,3,4-tetrahydro-2-naphthoic acid γ lactone (59) as a model. The optimum values of reaction pH, enzyme:substrate ratio and co-factonsubstrate ratio were determined. The butanolide 58 was synthesized by a route involving the Stobbe condensation of 3-benzyloxy-4-methoxybenzaldehyde with dimethylsuccinate to yield 2-(3-benzyloxy-4-methoxybenzylidene)butanedioic acid 1-methyl ester (69). Hydrogenation of 69 to 2-(3-benzyloxy-4-methoxybenzyl)butanedioic acid 1-methyl ester (70) followed by reductive lactonization afforded 3-(3-benzytoxy-4-methoxybenzyl)butanolide (71). Alkylation of 71 with 4-benzyloxy-a-bromo-3,5-dimethoxytoluene (72) gave frans-2-(4-benzyloxy-3,5-dimethoxybenzyl)-3-(3-benzyloxy-4-methoxybenzyl)butanolide (73) which was then converted to the butanolide 58 by catalytic hydrogenolysis. In order to investigate the effect of different aromatic substituents on the oxidative coupling of butanolides, C. roseus CFE-catalyzed biotransformations of frans-2-(3,5-dimethoxy-4-hydroxybenzyl)-3-(3,4-methylenedioxybenzyl)butanolide (74) and frans-2-(3,5-dimethoxy-4-hydroxybenzyl)-3-(3,4-dihydroxy-a-hydroxybenzyl)butanolide (94) were also performed. The biotransformation of 74 gave 2-(3,5-dimethoxy-4-hydroxybenzylidene)-3-(3,4-methylenedioxybenzyl)butanoiide (76) as the sole isolated product. A pathway involving oxidative demethylatton is proposed to account for the balance of the unrecovered material. The butanolide 94, a potential precursor to etoposide, was prepared from piperonal. The lithium anion of 1-bis(phenylthio)methyl-3,4-methylenedioxybenzene (97) and the bromide 72 were added consecutively to but-2-en-4-olide to afford frans-2-(4-benzyloxy-3,5-dimethoxybenzyl)-3-(3,4-methylenedioxy-α,α-bis(phenylthio)benzyl)butanolide (96). A synthetic sequence involving the oxidation of 96 to frans-2-(4-benzyloxy-3,5-dimethoxybenzyl)-3-(3,4-methylenedioxybenzoyl)-butanolide (100), reduction to frans-2-(4-benzyloxy-3,5-dimethoxybenzyl)-3-(α-hydroxy-3,4-methylenedioxybenzyl)butanolide (109) and cleavage of the methylenedioxy and benzyl protecting groups gave the catechol 94. Unfortunately, the CFE-catalyzed oxidation of 94, following treatment with sodium borohydride, yielded 4-(3,4-dihydroxyphenyl)-5,7-dimethoxy-6-hydroxy-2-hydroxymethyl-1,2,3,4-tetrahydro-2-naphthoic acid γ lactone (103) as the sole isolated product. [Formulas omitted] / Science, Faculty of / Chemistry, Department of / Graduate
3

A new synthetic approach to the C-D ring portion of streptonigrin and its analogs.

Kilama, John Jolly. January 1988 (has links)
Two new synthetic methods for the construction of the C-D moiety of streptonigrin have been developed. The first is the cyclization of beta, gamma unsaturated ketals to cyanopyridines. These ketals were prepared from akylidenemalononitriles. The second method utilized is the ortho-directed metalation of benzamide or oxazoline derivative to give keto compounds. However, attempts to transform these keto compounds to akylidenemalononitriles by Knoevenagel condensations were unsuccessful. With the ease of the reaction and ready availability of starting materials, the beta, gamma unsaturated ketals offer versatile synthons for pyridine C ring synthesis.
4

Studies on the biosynthesis of podophyllotoxin:synthesis of labelled yatein and matairesinol, two potential precursors of podophyllotoxin

Neidigh, Kurt Alan 19 September 2009 (has links)
Podophyllotoxin, a naturally occurring lignan isolated from several species of Podophyllum, is used as a precursor to the clinical chemotherapeutic agents teniposide and etoposide. The biosynthesis of podophyllotoxin is not fully understood, but its optical activity, like that of most lignans, is suggestive of enzyme-mediated processes. It has been proposed that the formation of podophyllotoxin begins with stereo-controlled coupling of a hydroxy cinnamyl alcohol derivative and a substituted hydroxy Cinnamic acid, although no "coupling" enzyme has been isolated to date. Further biosynthetic modifications of the coupled compound could lead to matairesinol and/or yatein, which have been proposed as potential biological precursors of podophyllotoxin. Although no firm evidence has been obtained to date, conversion of matairesinol to yatein has been postulated. This conversion would, however, involve biosynthetic steps which, though common for hydroxycinnamates, are unprecedented at the dimeric level. Conversion of yatein to podophyllotoxin has been demonstrated, with the conversion involving a stereo-controlled cyclization and subsequent stereospecific hydroxylation. In order to investigate the biosynthesis of podophyllotoxin, leading from the postulated precursors matairesinol and yatein, a series of stereospecific deuterium-labelled matairesinol and yatein derivatives was proposed and the synthetic methodology for each compound developed. The methodology used to obtain deuterium-labelled compounds can be extended to generating tritium-labelled compounds as well. With sufficient quantities of a number of the deuterium-labelled compounds, feeding studies can now be carried out in Podophyllum plants. Isolation and analysis of podophyllotoxin, from plants fed with labelled yatein, will allow determination of the stereochemical nature of yatein cyclization. Isolation and analysis of yatein, from plants fed with labelled matairesinol, will indicate whether matairesinol is indeed a precursor to yatein (and, hence, podophyllotoxin). The information obtained from the synthesis and incorporation of such labelled compounds should then provide a clearer understanding of some interesting but, as yet, unestablished biotransformations. / Master of Science
5

Biosynthetic engineering of new pactamycins

Lu, Wanli 28 February 2013 (has links)
Among the myriad of naturally occurring bioactive compounds are the aminocyclopentitol-containing natural products that represent a family of sugar-derived microbial secondary metabolites, such as the antibiotics pactamycin, allosamidin, and trehazolin. Pactamycin, a structurally unique aminocyclitol antibiotic isolated from Streptomyces pactum, consists of a 5-membered ring aminocyclitol (cyclopentitol) unit, two aromatic rings (6-methylsalicylic acid (6-MSA) and 1-(3-Amino-phenyl)-ethanone or 3-aminoacetophenone) and a 1,1-dimethylurea. It has pronounced antibacterial, antitumor, antiviral, and antiplasmodial activities, but its development as a clinical drug was hampered by its broad cytotoxicity. Efforts to modulate its pharmacological and toxicity properties by structural modifications using synthetic organic chemistry have been difficult due to the complexity of its chemical structure. As part of our ongoing studies on the biosynthesis of aminocyclitol-derived bioactive natural products, we have identified the biosynthetic gene cluster of pactamycin in S. pactum ATCC 27456, which paves the way for a better understanding of pactamycin biosynthesis and generating novel pactamycin analogs through biosynthetic engineering. Through gene inactivations, feeding experiments, and in vitro enzymatic assay, we studied the biosynthesis of pactamycin, which include the modes of formation of the unique cyclopentitol unit, the 3-aminoacetophenone and the 6-methyl salicylic acid moieties. Armed with the tools needed to genetically engineer target strains of S. pactum, we were able to produce novel analogs of this untapped-class of natural products. TM-026 was generated from a ΔptmH (a radical SAM C-methyltransferase gene) mutant, whereas TM-025 was generated from a ΔptmH/ΔptmQ (a polyketide synthase gene) double knockout mutant. Both compounds show potent antimalarial activity, but lack significant antibacterial activity, and are about 10-30 times less toxic than pactamycin toward mammalian cells. The results suggest that distinct ribosomal binding selectivity or new mechanism(s) of action may be involved in their plasmodial growth inhibition, which may lead to the discovery of new antimalarial drugs and identification of new molecular targets within malarial parasites. TM-035 was also isolated from a ΔptmH mutant. However, we found that TM-035 showed no activity against bacteria, malarial parasites, and most tested mammalian cells, but it has potent growth inhibitory activity against two well-established human head and neck squamous cell carcinomas (SCC025 and SCC104) (IC₅₀ 725 nM) in an in vitro assay. More intriguingly, the compound is significantly less active against human primary epidermal keratinocytes (HPEK), demonstrating an interesting biological phenomenon and outstanding cell type selectivity, which may lead to the development of new anticancer chemotherapy. The production yield of pactamycin and its congeners under laboratory conditions is relatively low. This has hampered both mechanistic and preclinical studies of these promising compounds. To deepen our understanding of pactamycin biosynthesis and engineer mutant strains with improved production yields, we investigated pathway specific regulatory genes, ptmF and ptmE. Based on gene inactivation and RT-PCR studies, we found that the PtmF-PtmE system controls the transcription of the whole biosynthetic gene cluster. The results provide important insight into regulation of pactamycin biosynthesis and will contribute to future studies that aim at engineering high producing strains of S. pactum. / Graduation date: 2012 / Access restricted to the OSU Community at author's request from Feb. 28, 2012 - Feb. 28, 2013

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