Pyranosides as templates for the synthesis of oxygenated tetrahydrofurans.
Item
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Title
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Pyranosides as templates for the synthesis of oxygenated tetrahydrofurans.
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Identifier
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AAI9917696
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identifier
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9917696
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Creator
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Seepersaud, Mohindra.
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Contributor
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Adviser: David R. Mootoo
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Date
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1999
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Language
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English
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Publisher
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City University of New York.
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Subject
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Chemistry, Organic
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Abstract
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The synthesis of 3-oxygenated-2,5-dialkyl-tetrahydrofurans (THF's) has received interest because of their use as precursors for the synthesis of complex THF containing natural products such as the eunecellin and kumausyne groups and certain polyethers. A common strategy for the synthesis of these THF's is electrophillic cyclization of penten-3,5-diols, wherein the allylic alcohol substituent provides a stereocontrolling element.;Our contribution to the area involves the use of C-6 allylated pyranosides as precursors for these 3-oxygenated THF ring systems. Specifically, R and S allylic alcohol pyranosides were prepared from tri-O-acetyl-D-glucal in 6 steps. These substrates were then subjected to halocyclization using iodonium dicollidine perchlorate (IDCP). For the R series high selectivity for the cis-2,5-substituted THF was observed while the S substrates showed poor selectivity. Trityl pyranosides led to higher selectivity than substrates with less bulky aglycones.;The study was then extended to a racemic series of syn and anti homoallylic pyranosides which were prepared in 4 steps from acrolein. Halocyclization of t-butyl glycosides of both the syn (corresponding to R) and anti (corresponding to S) gave moderate selectivity (65--70%) for the cis-2,5-THF. The corresponding trityl glycoside gave higher (85--90%) cis -2,5-THF selectivity. These results are unlike the allylic case where high selectivity was observed for only the R substrate. A stereochemical model which is based on previous results by Houk and Guindon, was postulated.;The allylic pyranoside methodology was applied to the synthesis of a THF precursor of the eunecellin group of natural products. The pyranoside alkene was prepared from nerol in 11 steps. Cyclization of this precursor gave the key functionalized THF precursor with high selectivity. However, attempts at converting this intermediate to the macrocyclic skeleton of the eunecellin was not successful. The iodocyclization of a model Z-alkene pyranoside for the kumausyne framework was carried out as part of this study. High selectivity for the cis-2,5-disubstituted THF was observed.;In summary a novel synthesis of complex, oxygenated THF's from carbohydrate alkene precursors was demonstrated. This methodology was applied to the construction of THF subunits of the eunecellin and kumausyne groups of natural products.
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Type
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dissertation
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Source
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PQT Legacy CUNY.xlsx
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degree
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Ph.D.
