Stereoselective and Stereospecific Triflate-Mediated Intramolecular Schmidt Reaction: Ready Access to Alkaloid Skeletons*

Formal Nitrene Insertion into the β-Vinyl C-H Bond of Acroleins: Synthesis of Enaminals

An efficient formal nitrene insertion reaction into the β-vinyl C-H bond of acroleins with an electron-rich organic azide was developed. The reaction protocol can produce secondary enaminals in high yield with a broad substrate scope. In the reaction, acid mediated [3 + 2] cycloaddition of organic azides with an acrolein generated intermediate protonated triazolines, which were selectively decomposed into enaminals with addition of a weakly Brønsted basic reagent such as methanol. The resulting secondary enaminal could be easily reduced into a γ-amino alcohol under mild hydrogenation conditions.

Copper-Nitrene-Catalyzed Desymmetric Oxaziridination/1,2-Alkyl Rearrangement of 1,3-Diketones toward Bicyclic Lactams

Although copper-nitrene has been extensively studied as a versatile active species in various transformations, asymmetric reactions involving copper-nitrene have been limited to the aziridination of olefins. Herein, we report the novel copper-nitrene-catalyzed desymmetric oxaziridination reaction of cyclic diketones with alkyl azides and the subsequent rearrangement of the resulting highly active intermediate, which produces a synthetically challenging chiral bicyclic lactam containing a quaternary carbon center. This procedure not only enriches the copper-nitrene-catalyzed asymmetric reactions, but also provides an alternative strategy to address the inherent challenges of catalytic asymmetric Schmidt reactions. This unique reaction could inspire the investigation of novel copper-nitrene-catalyzed asymmetric transformations and their reaction mechanisms.

Sulfonium ion-promoted traceless Schmidt reaction of alkyl azides

Schmidt reaction by sulfonium ions is described. General primary, secondary, and tertiary alkyl azides were converted to the corresponding carbonyl or imine compounds without any trace of the activators. This bond scission reaction through 1,2-migration of C-H and C-C bonds was accessible to the one-pot substitution reaction.

Stereoselective and Stereospecific Triflate-Mediated Intramolecular Schmidt Reaction: Ready Access to Alkaloid Skeletons*

The stereoselectivity and stereospecificity of the triflate-mediated intramolecular Schmidt reaction of substituted 3-(1-azidocyclohexyl)propanol derivatives leading to octahydro-1H-pyrrolo[1,2-a]azepine, the structural skeleton of several important families of alkaloids such as the Stemona alkaloids, has been examined. The reaction involves an initial intramolecular S_N 2 reaction between the azide moiety and the triflate affording an intermediate spirocyclic aminodiazonoium salt that undergoes the expected 1,2-shift/N₂ -elimination followed by hydride-mediated iminium salt reduction. Remarkably, chiral alcohols are converted to the azabicyclic derivative with no or limited racemization. The initial asymmetric alcohol center controls the diastereoselectivity of the whole process, leading to the formation of one out of the four possible diastereoisomers of disubstituted octahydro-1H-pyrrolo[1,2-a]azepine. The origin of the stereoselectivity is rationalized based on theoretical calculations. The concise synthesis of (-)-(cis)-3-propylindolizidine and (-)-(cis)-3-butyllehmizidine, two alkaloids found in the venom of workers of the ant Myrmicaria melanogaster, is reported.