Six-Membered Cyclic Nitroso Acetals: Synthesis and Studies of the Nitrogen Inversion Process ofN-Silyloxy-3,6-dihydro-2H-1,2-oxazines

[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.

Challenges in the Highly Selective [3 + 1]-Cycloaddition of an Enoldiazoacetamide to Form a Donor-Acceptor Cis-Cyclobutenecarboxamide

A substituted donor-acceptor cyclobutenecarboxamide is synthesized with modest enantiocontrol through a chiral copper(I) complex catalyzed [3 + 1]-cycloaddition reaction of α-acyl diphenylsulfur ylides with 3-siloxy-2-diazo-3-butenamides. With a methyl substituent on the 4-position of the 3-butenamide, the cis-vicinal-3,4-disubstituted cyclobutenecarboxamide is formed with >20:1 diastereocontrol. Donor-acceptor 3-methyl-2-siloxycyclopropenecarboxamide is rapidly formed from the reactant enoldiazoamide and undergoes catalytic ring opening to give only the Z-γ-substituted metallo-enolcarbene. Elimination from 3-siloxy-2-diazo-3-pentenamide to form the conjugated 3-siloxy-2,4-pentadienamide is competitive but minimized at low temperature.

Rhodium-Catalyzed Enantioselective [4+2] Cycloadditions of Vinylcarbenes with Dienes

The reaction of 2-siloxycyclo-1,3-dienes with E-vinyldiazoacetates in the presence of the bulky chiral dirhodium tetracarboxylate catalyst, Rh₂ (R-p-PhTPCP)₄ results in an enantioselective [4+2] cycloaddition, in which three new stereogenic centers are formed. The [4+2] cycloadducts are generated as single diastereomers with high enantiocontrol (95-98 % ee). When the diene contains an additional stereogenic center, effective kinetic resolution can be achieved.

α-Electrophilic Reactivity of Nitronates

Chemistry of covalent nitronates regarding nucleophilic addition to C=N bond is described. Various types of electrophilic activation of nitronates and stability of formed products are discussed with main attention paid to authors' work in the area.

Cycloaddition reactions of enoldiazo compounds

Enoldiazo esters and amides have proven to be versatile reagents for cycloaddition reactions that allow highly efficient construction of various carbocycles and heterocycles. Their versatility is exemplified by (1) [2+n]-cycloadditions (n = 3, 4) by the enol silyl ether units of enoldiazo compounds with retention of the diazo functionality to furnish α-cyclic-α-diazo compounds that are themselves subject to further transformations of the diazo functional group; (2) [3+n]-cycloadditions (n = 1-5) by metallo-enolcarbenes formed by catalytic dinitrogen extrusion from enoldiazo compounds; (3) [2+n]-cycloadditions (n = 3, 4) by donor-acceptor cyclopropenes generated in situ from enoldiazo compounds that produce cyclopropane-fused ring systems. The role of dirhodium(ii) and the emergence of copper(i) catalysts are described, as are the different outcomes of reactions initiated with these catalysts. This comprehensive review on cycloaddition reactions of enoldiazo compounds, with emphasis on methodology development, mechanistic insight, and catalyst-controlled chemodivergence, aims to provide inspiration for future discoveries in the field and to catalyze the application of enoldiazo reagents by the wider synthetic community.

Catalytic Asymmetric [3+1]-Cycloaddition Reaction of Ylides with Electrophilic Metallo-enolcarbene Intermediates

The first asymmetric [3+1]-cycloaddition was successfully achieved by copper(I) triflate/double-sidearmed bisoxazoline complex catalyzed reactions of β-triisopropylsilyl-substituted enoldiazo compounds with sulfur ylides. This methodology delivered a series of chiral cyclobutenes in good yields with high enantio- and diastereoselectivities (up to 99 % ee, and >20:1 d.r.). Additionally, the [3+1]-cycloaddition of catalytically generated metallo-enolcarbenes was successfully extended to reaction with a stable benzylidene dichlororuthenium complex.