Ph.D. Theses

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Browsing Ph.D. Theses by Subject "Chirality."

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    Asymmetric syntheses, ring opening and stereoselective reactions, absolute conformation and activation barrier determinations on axially chiral oxazolidine derivatives
    (Thesis (Ph.D.)-Bogazici University. Institute for Graduate Studies in Science and Engineering, 2008., 2008.) Ordu, Öznur Demir.; Doğan, İlknur.
    In this study, sterically hindered 5-methyl-3-(o-aryl)-2,4-oxazolidinediones, 5-methyl- 3-(o-iodophenyl)-2-thioxo-4-oxazolidinedione and 5,5-dimethyl-3-(o-aryl)-2,4- oxazolidinediones have been synthesized. Diastereomeric isomers of the 5-methyl-3-(oaryl)- 2,4-oxazolidinediones and 5-methyl-3-(o-iodophenyl)-2-thioxo-4-oxazolidinedione and enantiomeric isomers of the 5,5-dimethyl-3-(o-aryl)-2,4-oxazolidinediones have been identified by 1H NMR and 13C NMR. Activation barriers to hindered rotation around C-N single bond have been determined by using temperature dependent NMR or by thermal racemization on chiral sorbents by HPLC. The conformational preferences of the diastereomers have been investigated by one and two dimensional NMR spectroscopy and by HPLC on an optically active sorbent. It was found that when the bulky group on C-5 and the ortho substituent on the aromatic ring are on opposite sites the conformation becomes more stable. Absolute confomations of 5,5-dimethyl-3-(o-aryl)-2,4-oxazolidinediones have been determined by one and two dimensional 1H NMR in the presence of a chiral auxiliary (S)- (+)-1-(9-anthryl)-2,2,2-trifluoro ethanol ((S)-TFAE). A solvation model has been proposed for the determination absolute stereochemistry on the basis of interactions between the enantiomers and (S)-TFAE. vi Asymmetric alkylation and aldol reactions have been carried out on 5-methyl-3-(otolyl)- 2,4-oxazolidinedione, 5-methyl-3-(o-iodophenyl)-2,4-oxazolidinedione and 5- methyl-3-(o-iodophenyl)-2-thioxo-4-oxazolidinedione over the lithium enolate forms. The enolate formation could be achieved by lithium diisopropylamide at -78 0C under nitrogen. It was found that the o-aryl substituents had a stereodirecting effect on the reactions taking place at C-5 of the ring. The stereoselectivity of the reactions was determined by using chiral HPLC. Basic and reductive ring opening reactions of diastereomeric 5S-methyl-N-(o-aryl)- 2,4-oxazolidinediones and enantiomeric 5,5-dimethyl-N-(o-aryl)-2,4-oxazolidinediones have been done to examine the stability of the ring and to synthesize various organic compounds such as chiral –hydroxyamides, carbamoyloxyacids and carbamate derivatives via ring opening. It was observed that 5-methyl-2,4-oxazolidinedione ring cleaved more easily than 5,5-dimethyl-2,4-oxazolidinedione ring. 5-Methyl-2,4- oxazolidinedione ring was reduced to a primary alcohol after ring opening, whereas the reduction of 5,5-dimethyl-2,4-oxazolidinedione ring yielded a secondary alcohol asymmetrically without ring cleavage.
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    Axialy chiral n-(o-aryl)-2-thioxo-oxazolidine-4-one and rhodanine derivatives: enantiomeric separation and determination of racemization barriers
    (Thesis (Ph.D.)-Bogazici University. Institute for Graduate Studies in Science and Engineering, 2008., 2008.) Yılmaz, Esra Müjde.; Doğan, İlknur.
    In this study, sterically hindered 5,5-dimethyl-3-(o-aryl)-2-thioxo-4-oxazolidinones, 5,5-dimethyl-3-(o-aryl)-2-thioxo-4-thiazolidinones and 3-(o-aryl)-2-thioxo-4- thiazolidinones(rhodanines) have been synthesized as racemates by either the reaction of oaryl isothiocyanates with α-hydroxyisobutyrate or by treatment of o-arylisothiocyanates with the correponding thioglycolic acid ethyl ester. The 5,5-dimethylrhodanines have been synthesized according to Modified Kaluza synthesis. Chirality of the compounds has been proven by the presence of diastereotopic methylene protons or methyl groups on C-5 of the heterocyclic ring detected by 1H NMR or 13C NMR spectroscopy. The enantiomers of the synthesized compounds have been resolved or enriched micropreparetively on chiralpak AD-H column, packed with amylose tris-(3,5-dimethyl phenyl)carbamate. The activation barriers for interconversion between the enantiomers have been determined by either temperature dependent NMR or by thermal racemization followed by enantioresolution on a chiral sorbent via HPLC. The activation barriers to rotation around C-N bond showed a linear increase with the van der Waals radii of the ortho-halogen substitutents. Absolute conformations have been determined by analysis of NMR spectra of Nnaphthyl substituted derivatives of the series in the presence of the chiral auxiliary (S)-(+)- 1-(9-antryl)-2,2,2-trifluoro ethanol ((S)-TFAE). Plausible diastereomeric association models between the enantiomers of the compounds 3-(α-naphthyl)-2-thioxo-4- ii thiazolidinone and 3-(α-naphthyl)-2,4-thiazolidinedione and the chiral auxiliary (S)-TFAE have been proposed. Enantiodifferentiation of 3-aryl-2-thioxo-4-oxazolidinones by NMR was also studied by using a dirhodium tetracarboxylate complex as the chiral auxiliary. Determination of relative conformations of the series have also been attempted by applying the dirhodium method to the enantiomerically enriched samples, however this could not be achieved.