Studies on difficult intramolecular hydroaminations in the context of four syntheses of alkaloid natural products.

Examples of intramolecular alkene hydroaminations forming six-membered ring systems are rare, especially for systems in which the double bond is disubstituted. Such cyclizations have important synthetic relevance. Herein we report a systematic study of these cyclizations using recently developed Cope-type hydroamination methodologies. Difficult intramolecular alkene hydroaminations were used as key steps in syntheses of 2-epi-pumiliotoxin C, coniine, N-norreticuline and desbromoarborescidine A. This effort required the development of optimized hydroamination conditions to improve the efficiency of the cyclizations. Collectively, our results show that Cope-type cyclizations can be achieved on a variety of challenging substrates and proceed under similar conditions for both N-H and N-substituted hydroxylamines.

2-Aryl benzimidazoles: human SCD1-specific stearoyl coenzyme-A desaturase inhibitors.

A series of potent, benzimidazole-based SCD inhibitors which demonstrate selectivity for the hSCD1 enzyme over the hSCD5 isoform are described. The compounds possess suitable cellular activity and pharmacokinetic properties which render them capable of inhibiting liver SCD activity in a mouse pharmacodynamic assay. These 2-aryl benzimidazoles may serve as valuable tools for studying selective hSCD1-inhibition.

Intermolecular cope-type hydroamination of alkenes and alkynes using hydroxylamines.

The development of the Cope-type hydroamination as a method for the metal- and acid-free intermolecular hydroamination of hydroxylamines with alkenes and alkynes is described. Aqueous hydroxylamine reacts efficiently with alkynes in a Markovnikov fashion to give oximes and with strained alkenes to give N-alkylhydroxylamines, while unstrained alkenes are more challenging. N-Alkylhydroxylamines also display similar reactivity with strained alkenes and give modest to good yields with vinylarenes. Electron-rich vinylarenes lead to branched products while electron-deficient vinylarenes give linear products. A beneficial additive effect is observed with sodium cyanoborohydride, the extent of which is dependent on the structure of the hydroxylamine. The reaction conditions are found to be compatible with common protecting groups, free OH and NH bonds, as well as bromoarenes. Both experimental and theoretical results suggest the proton transfer step of the N-oxide intermediate is of vital importance in the intermolecular reactions of alkenes. Details are disclosed concerning optimization, reaction scope, limitations, and theoretical analysis by DFT, which includes a detailed molecular orbital description for the concerted hydroamination process and an exhaustive set of calculated potential energy surfaces for the reactions of various alkenes, alkynes, and hydroxylamines.

Diastereoselective construction of quaternary carbons directed via macrocyclic ring conformation: formal synthesis of (-)-mesembrine.

In this article, we report a highly diastereoselective new method for the generation of quaternary carbon centers through an anionic oxy-Cope/alkylation sequence where the diastereoselectivity is induced by the conformation of a macrocyclic tetrasubstituted enolate. The use of our methodology culminated in the formal total synthesis of (-)-mesembrine (34) in 11 steps from known starting materials.

Stereoselective synthesis of Z alkenyl halides via Julia olefination.

Julia olefination between alpha-halomethyl sulfones and a variety of aldehydes afforded alkenyl halides in good to excellent yields with high E/Z stereoselectivities. Sulfones were readily prepared in two or three steps from commercially available reagents in good yields. Optimization revealed that the nature of the solvent, the base, and the additive were crucial to obtain the desired alkenyl halides.