Author's School

Graduate School of Arts & Sciences

Author's Department/Program

Chemistry

Language

English (en)

Date of Award

January 2010

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Chair and Committee

Kevin Moeller

Abstract

Anodic oxidation has been shown to be a powerful method for initiating umpolung reactions that allow the coupling of two nucleophilic functional groups. Most of the work in this area concentrates on making carbon-carbon bond. Hence, the reactions are usually terminated with olefin nucleophiles. The use of heteroatom-based coupling partners to form carbon-heteroatom bond is less explored despite the wide occurrence of heterocyclic compounds as natural products, pharmaceuticals, and synthetic building blocks. Therefore, this thesis focuses on the application of heteroatom-based coupling partners in intramolecular anodic olefin coupling reactions for the synthesis of N- and O-heterocyclic compounds. First, a synthesis of: -)-crobarbatic acid was accomplished in 10 steps by employing as the key step anodic coupling of an alcohol and a ketene dithioacetal. The key to the success of the oxidative cyclization reaction is the use of lithium methoxide as a base instead of the commonly used weak base, 2,6-lutidine. The synthesis demonstrates that a vinyl substituted ketene dithioacetal is compatible with anodic cyclizations and that it is possible to reverse the stereochemistry of the newly formed stereogenenic center in the electrolysis product. Next, the anodic coupling of sulfonamides and electron-rich olefins was developed in order to build cyclic amino acid derivatives. Low yields were obtained under reaction conditions employing 2,6-lutidine as a base. However, with the use of a stronger base, LiOMe, a variety of electron-rich olefins such as enol ethers, vinyl sulfides, ketene dithioacetals, and allylsilanes, are compatible with the cyclizations affording proline and pipecolic acid derivatives in good to excellent yields and diastereoselectivity. The mechanism of the reactions under the basic reaction conditions was investigated using competition studies. The harsh reaction conditions associated with the removal of the tosyl group from the electrolysis products prompted us to study the use of primary amines as coupling partners. The oxidative coupling of unprotected amines with ketene dithioacetals afforded proline and pipecolic acid derivatives with alpha-tetrasubstituted carbon atoms despite the oxidation potential of the product being significantly lower than either functional group in the substrate. The competing oxidation of the product was avoided because the oxidation potential of the substrate is lowered by a fast cyclization. One of the limitations of the oxidative cyclization reactions is the failure to afford cyclic amino acids that do not have alpha-tetrasubstituted carbon atoms. Hence, a complementary method, base induced intramolecular hydroamination of ketene dithioacetals, was developed to address this issue. The catalytic cyclizations afforded both five and six-membered ring products with an aldehyde equivalent at C2 in excellent yield and diastereoselectivity. Amides such as O-benzyl hydroxamate and N-phenyl amides were employed successfully in intramolecular anodic olefin coupling reactions to provide lactams. This study demonstrates that amidyl radicals can be generated easily from N-H amides using anodic oxidation. Compared with established methods for the generation of amidyl radicals, which usually involve the fragmentation of amide derivatives under relative high temperatures, the electrochemical oxidation method generates amidyl radicals at room temperature and avoids the preparation of amide derivatives, which are usually unstable and hard to prepare. In addition to the synthesis of N-heterocycles, carboxylic acids were used as nucleophiles in anodic coupling reactions to provide lactones with a carbonyl equivalent at the C2 position. Such structures can be found in many natural products. The cyclizations afforded both gamma-butyro- and delta-valerolactones in good to excellent yields and diastereoselectivity.

Comments

Permanent URL: http://dx.doi.org/10.7936/K7S75DCK

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