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Using electron-rich or electron-poor N-substituted oxazolidines as substrates, selective formation of either ammonium or oxonium ylides is possible in the presence of α-imino carbenes. As such, treatment of 5-membered oxazolidine precursors with N-sulfonyl-1,2,3-triazoles under dirhodium catalysis (2 mol%) affords the regiodivergent synthesis of either 8-membered 1,3,6- or 1,4,6-oxadiazocines upon the initial N or O reactivity with the carbene.

An original and straightforward entry to polysubstituted indenes from readily available ynamides is reported. Upon reaction with a NHC-gold complex under mild conditions, activated keteniminium ions are generated whose unique electrophilicity triggers a [1,5]-hydride shift and a subsequent cyclization. The presence of an endocyclic enamide in the densely functionalized resulting indenes was shown to be especially useful and versatile, offering a range of opportunities for their further post-functionalization., info:eu-repo/semantics/published

Hexahydropyrazinoindoles were prepared in a single step from N-sulfonyl triazoles and imidazolidines. Under dirhodium catalysis, α-imino carbenes were generated and formed nitrogen ylide intermediates that, after subsequent aminal opening, afforded the pyrazinoindoles predominantly via formal [1,2]-Stevens and tandem Friedel–Crafts cyclizations. Of mechanistic importance, a regiodivergent reactivity was engineered through the use of a specific unsymmetrically substituted imidazolidine that promoted the exclusive formation of 8-membered ring 1,3,6-triazocines. Based on DFT calculations, an original Curtin–Hammett-like situation was demonstrated for the mechanism. Further derivatizations led to functionalized tetrahydropyrazinoindoles in high yields., Hexahydropyrazinoindoles are prepared in a single step from N-sulfonyl triazoles and imidazolidines. Of mechanistic importance, a regiodivergent reactivity can be engineered towards the exclusive formation of 8-membered ring 1,3,6-triazocines.

Using N-sulfonyl triazoles as substrates, compounds as diverse as 2-imino tetrahydrofurans, 13- and 15-membered ring aza-macrocycles can be prepared selectively via formal [1 + 4], [5 + 4 + 4] and [3 + 4 + 4 + 4] condensations of α-imino carbenes and oxetanes under Rh(ii)-catalysis or thermal activation. Spirocyclic N-heterocycles are also accessible by means of Buchwald-Hartwig and Pictet-Spengler cyclizations. By reaction control, substrate selection or further derivatization, a large variety of chemical structures is thus achievable. Finally, using triazoles reacting under thermal activation, interesting mechanistic insight was obtained.

Polycyclic indoline‐benzodiazepines are afforded in one step by the reaction of Troger bases with N‐sulfonyl‐1,2,3‐triazoles under Rh(II) catalysis. After α‐imino carbene formation, the process involves a cascade of [1,2]‐Stevens rearrangement, Friedel‐Crafts, Grob fragmentation, and aminal formation reactions. It is highly diastereoselective (d.r. >49:1, four stereocenters incl. two bridgehead N‐atoms). However and in contrast with other reported carbene additions to these moieties, full racemization occurs when enantiopure Troger bases are used as substrates. To pinpoint the origin of this unexpected behavior, a key elemental step of the mechanism was evaluated and tested. Interestingly, it is not only the initial ring‐opening but also the latter reversible Mannich reaction of the imino‐substituted ethano Troger base intermediate that is responsible for the loss of enantiospecificity.

Polycyclic indoline‐benzodiazepines can be accessed through the intermolecular reaction of Tröger bases with N‐sulfonyl‐1,2,3‐triazoles. Under RhII catalysis, α‐imino carbenes are generated and a subsequent cascade of [1,2]‐Stevens, Friedel–Crafts, Grob, and aminal formation reactions yield the polycyclic heterocycles as single isomers (d.r.>49:1, four stereocenters including two bridgehead N atoms). Further ring expansion by insertion of a second α‐imino carbene leads to elaborated polycyclic 9‐membered‐ring triazonanes.