Asymmetric [3 + 3] cycloaddition of cinnamaldehyde-derived N-aryl nitrones with 2-indolemethanols enabled by chiral phosphoric acid

We described a chiral phosphoric acid (CPA) catalyzed asymmetric [3 + 3] cycloaddition of cinnamaldehyde-derived N-aryl nitrones with 2-indolylmethanols to prepare various indole-fused 1,2-oxazines in high yields (up to 96%) with excellent enantioselectivity (>99% ee). Control experiments indicate that hydrogen bonding plays important roles in controlling the enantioselectivity of products. This strategy provides an efficient pathway to construct enantioenriched indole-fused 1,2-oxazines from N-aryl nitrones with 2-indolylmethanols.

Synergistic Rh(II)- and Zn(II)-Catalyzed [3 + 3] Annulation of Diazoenals and α-Hydroxy Ketones for the Direct Synthesis of 2H-Pyrans, A Gateway Toward γ-Pyrones

A new synergistic Rh(II)/Zn(II)-catalyzed [3 + 3] annulation has been developed between diazoenals and α-hydroxy ketones enabling the direct synthesis of 4-formyl-2H-pyrans. The reaction involves the formation of protic oxonium ylides from highly electrophilic Rh-enalcarbenoids followed by Zn-templated regioselective intramolecular aldol condensation. Subsequent investigations demonstrated that 4-formyl-2H-pyrans are unique precursors of γ-pyrones. The γ-pyrones were obtained via Et3N-mediated oxidative deformylation. Further, the triflic-acid-promoted double Schmidt reaction of 4-formyl-2H-pyrans provides synthetically important carboxamide-substituted 4-cyano 2H-pyrans.

[3+3]-Annulation of Cyclic Nitronates with Vinyl Diazoacetates: Diastereoselective Synthesis of Partially Saturated [1,2]Oxazino[2,3-b][1,2]oxazines and Their Base-Promoted Ring Contraction to Pyrrolo[1,2-b][1,2]oxazine Derivatives

A rhodium(II)-catalyzed reaction of cyclic nitronates (5,6-dihydro-4H-1,2-oxazine N-oxides) with vinyl diazoacetates proceeds as a [3+3]-annulation producing bicyclic unsaturated nitroso acetals (4a,5,6,7-tetrahydro-2H-[1,2]oxazino[2,3-b][1,2]oxazines). Optimization of reaction conditions revealed the use of Rh(II) octanoate as the preferred catalyst in THF at room temperature, which allows the preparation of target products in good yields and excellent diastereoselectivity. Under basic conditions, namely, the combined action of DBU and alcohol, these nitroso acetals undergo ring contraction of an unsaturated oxazine ring into the corresponding pyrrole. Both transformations can be performed in a one-pot fashion, thus constituting a quick approach to oxazine-annulated pyrroles from available starting materials, such as nitroalkenes, olefins, and diazo compounds.

B(C6F5)3-Catalyzed Diastereoselective and Divergent Reactions of Vinyldiazo Esters with Nitrones: Synthesis of Highly Functionalized Diazo Compounds

Herein we report a mild, transition-metal-free, highly diastereoselective Lewis acid catalyzed methodology toward the synthesis of isoxazolidine-based diazo compounds from the reaction between vinyldiazo esters and nitrones. Interestingly, the isoxazolidine products were identified to have contrasting diastereoselectivity to previously reported metal-catalyzed reactions. Furthermore, the same catalyst can be used with enol diazo esters, prompting the formation of Mukaiyama-Mannich products. These diazo products can then be further functionalized to afford benzo[b]azepine and pyrrolidinone derivatives.

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.

AgI -Catalyzed Reaction of Enol Diazoacetates and Imino Ethers: Synthesis of Highly Functionalized Pyrroles

An unprecedented Ag^I -catalyzed efficient method for the coupling of imino ethers and enol diazoacetates through a [3+2]-cycloaddition/C-O bond cleavage/[1,5]-proton transfer cascade process is reported. The general class of imino ethers that includes oxazolines, benzoxazoles and benzimidates are applicable substrates for these reactions that provide direct access to fully substituted pyrroles with uniformly high chemo- and regioselectivity. High variability in substitution at the pyrrole 2-, 5- and N-positions characterizes this methodology that also presents an entry point for further pyrrole diversification via facile modification of resulting N-functional pyrroles.