Graduate Thesis Or Dissertation
 

Phenolic oxidative coupling : an approach to the synthesis of pretazettine

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  • Various hypervalent iodine compounds were evaluated as reagents for intramolecular phenolic oxidative coupling. It was found that phenyliodine(III) bis(trifluoroacetate) was effective for the coupling of the monophenolic substrate 13a to 14a under neutral conditions. Vanadium oxytrifluoride and hypervalent iodine compounds were compared as oxidants for the conversion of 32 to 33. In an approach directed at a synthesis of pretazettine (17), phenacylamine 57 was elaborated in two steps to oxazole 50. However, this did not undergo catalytic hydrogenation to the desired oxazolidine 47. Oxazoline 60 proved to be too labile to be isolated upon dehydration of 65, which was prepared from di-octopamine (64) and the acid chloride of piperonylic acid (58). Finally, 64 and piperonal (69) were condensed and treated with ethyl chloroformate to provide trans-2,5-diaryloxazolidine 73. However, the latter was hydrolyzed under coupling conditions. Alternatively, when octopamine and piperonal were condensed and hydrogenated, amine 74 was obtained, which was in turn reacted with 1,1'-carbonyldiimidazole (75) to furnish the 2-oxazolidinone 76. Exposure of 76 to vanadium oxytrifluoride led to 78, which apparently arose from an undesired, spontaneous rear- rangement of the coupling product 77. Attempts to synthesize ether 79 starting with synephrine (81), 2-nitrostyrenes 86, and 4-hydroxymandelates 90 are described. The oxazolidine 97 was prepared from the condensation of synephrine 81 and treated with methyl chioroformate in the presence of methanol to afford acetal 100 in 71% yield. Phenyliodine(III) bis- (trifluoroacetate) was employed to couple 100 to 101 in 5 to 7% yield. Hydrolysis of the urethane moiety of 101 with potassium hydroxide in ethylene glycol gave the amine 105, which underwent a spontaneous cyclization to 107. Formation of the unnatural, cis B-D ring fusion stereochemistry in this ring closure was ascertained by nuclear magnetic resonance spectroscopy.
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