Copper‐Catalyzed Chemo‐ and Diastereoselective 1,3‐Dipolar Cycloaddition of Carbonyl Ylide and Aldehyde‐Tethered‐Cyclohexadienone to Access Polycyclic Systems

Microwave-assisted [3 + 2] cycloaddition reactions of dicyanoepoxides with benzylidene Meldrum's acids

The [3 + 2] cycloaddition of dicyanoepoxides with benzylidene Meldrum's acid under microwave irradiation and solvent-free conditions was explored for the synthesis of trioxaspirodecanes. This method presents a highly diastereo- and regioselective route to spiro cycloadducts, delivering similar stereoselectivity to conventional reflux in toluene but with shorter reaction times and improved yields.

Electrochemical Dearomatizing Methoxylation of Phenols and Naphthols: Synthetic and Computational Studies

The electrochemical oxidative dearomatizing methoxylation of phenols and naphthols was developed. It provides an alternative route for the preparation of methoxycyclohexadienones, important and versatile synthetic intermediates, that eliminates the need for stoichiometric high-energy chemical oxidants and generates hydrogen as a sole by-product. The reaction proceeds in a simple constant current mode, in an undivided cell, and it employs standardized instrumentation. A collection of methoxycyclohexadienones derived from various 2,4,6-tri-substituted phenols and 1-substituted-2-naphthols was obtained in moderate to excellent yields. These include a complex derivative of estrone, as well as methoxylated dearomatized 1,1'-bi-2-naphthols (BINOLs). The mechanism of the reaction was subject to profound investigations using density functional theory calculations. In particular, the reactivity of two key intermediates, phenoxyl radical and phenoxenium ion, was carefully examined. The obtained results shed light on the pathway leading to the desired product and rationalize experimentally observed selectivities regarding a side benzylic methoxylation and the preference for the functionalization at the para over the ortho position. They also uncover the structure-selectivity relationship, inversely correlating the steric bulk of the substrate with its propensity to undergo the side-reaction. Moreover, the loss of stereochemical information from enantiopure BINOL substrates during the reaction is rationalized by the computations.

Unexpected Copper-Catalyzed Cascade Reaction of 1,6-Enynes with Sulfoxonium Ylides

An unprecedented copper-catalyzed cascade reaction of 1,6-enynes with sulfoxonium ylides is reported, providing a series of structurally intriguing 2,3-disubstituted indolines bearing a conjugated dienone functionality at the 3-position in moderate to excellent yields with good chemo-, regio-, and diastereoselectivities under mild reaction conditions. Importantly, sulfoxonium-ylide-derived copper-carbene herein exhibits quite different reactivity from that of diazo copper-carbene. A rational mechanism, an initial ammonium ylide rather than allene formation, is proposed.

Copper-Catalyzed Cycloadditions of Diazo Compounds with Imidazolidines/Hexahydropyrimidines for the Syntheses of N-Heterocycles

Reported herein are the unprecedented copper-catalyzed formal [n + 1]/[n + 3] (n = 5, 6) cycloadditions of diazo compounds with imidazolidines/hexahydropyrimidines, thus providing a general, economical, and efficient route to construct different sized (six- to nine-membered) diaza-heterocycles in moderate to excellent yields under mild reaction conditions. This strategy features the use of copper catalyst to accomplish such diverse annulations and the utilization of imidazolidines/hexahydropyrimidines as stable 1,5-/1,6-dipoles.

Enantioselective Synthesis of α-Aryl-β-Aminocyclopropane Carboxylic Acid Derivatives via Rh(II)-Catalyzed Cyclopropanation of Vinylsulfonamides with α-Aryldiazoesters

The reaction of vinylsulfonamides with donor-acceptor carbenes derived from α-aryldiazoesters, catalyzed by the tert-butyl glycine-derived dirhodium complex Rh₂(S-4-Br-NTTL)₄, has been reported. This method provides a variety of α-aryl-β-aminocyclopropane carboxylic acid derivatives bearing one quaternary carbon stereogenic center vicinal to the amino-substituted carbon in high yields with excellent diastereo- and enantioselectivities. Vinylsulfonamides showed complementary advantages over the well-developed vinylamides or vinylcarbamates for this Rh(II)-catalyzed cyclopropanation strategy. Moreover, these conformationally restricted α-aryl-β-aminocyclopropyl carboxylic acid derivatives can be readily incorporated into dipeptides.