- Environmentally Benign Synthesis of Enamides via Waste-Free Catalytic Addition of Amides to Terminal Alkynes (2010)
- A number of natural products are known that contain an enamide as a key structural feature. This functionality is a very important subunit in various biologically active products and pharmaceutical drug lead compounds. In addition, enamides serve as highly versatile synthetic intermediates, particularly in the pericyclic reaction, formation of heterocycles, cross-coupling and in asymmetric synthesis. As a result, several protocols have been devised for the preparation of enamides. Traditional syntheses include condensation of aldehydes and ketones with amides or from hydroxylamines and acetic anhydride, require harsh conditions and yield mixtures of E/Z products. Several metal catalyzed approaches have been also investigated, such as isomerization of N-allylamides and catalytic cross-coupling of amides with vinyl halides or pseudohalides. These protocols proceed under milder conditions but suffer from the limited availability of these starting materials. The research described in this dissertation focuses on efficient and atom-economic preparation of enamides and thioenamides, using readily available starting materials. We developed catalyst systems generated in situ from bis(2-methallyl)-cycloocta-1,5-diene-ruthenium(II), phosphines and Lewis acid or base, efficiently catalyze the addition of primary amides and thioamides to terminal alkynes with exclusive formation of the anti-Markovnikov products in high yield and stereoselectivity under mild reaction conditions. The generality of the newly developed methodologies is demonstrated by common functional group tolerance. Furthermore, Markovnikov products were formed via phosphine-catalyzed addition of cyclic amides to phenylacetylene derivatives. The hydroamidation protocol of primary amides was successfully used in the synthesis of naturally occurring compounds, such as alatamide, lansiumamide A, botryllamides C and E, and the key intermediate in the synthesis of aristolactam. In order to investigate the reaction mechanism, the addition of various amides and carboxylic acids to terminal alkynes was performed using deuterium labeled starting materials and followed by in situ NMR and GC-MS studies.