In Situ Generated Magnesium Cyanide as an Efficient Reagent for Nucleophilic Cyanation of Nitrosoalkenes and Parent Nitronates

[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.

Michael addition of P-nucleophiles to azoalkenes provides simple access to phosphine oxides bearing an alkylhydrazone moiety

β-Hydrazonophosphine oxides are precursors of useful organophosphorus compounds, including phosphorylated N-heterocycles, α-aminophosphonates, and vinylphosphonates. In this work, a general transition metal-free synthesis of β-hydrazonophosphine oxides was developed. The method relies on the Michael addition of phosphine oxides R₂P(O)H to reactive azoalkenes (1,2-diaza-1,3-butadienes), which are generated in situ from α-halohydrazones and Hunig's base. The reaction stereoselectively leads to Z-isomers of β-hydrazonophosphine oxides that are stabilized by intramolecular hydrogen bonding. The conversion of the products thus obtained into potential chelating ligands was showcased.

Deoxygenative Arylation of 5,6-Dihydro-4H-1,2-oxazine-N-oxides with Arynes

Six-membered cyclic nitronates (5,6-dihydro-4H-1,2-oxazine-N-oxides) react with Kobayashi's aryne precursors producing 3-(2-hydroxyaryl)-substituted 1,2-oxazines via deoxygenative C-H arylation. The process involves a hitherto unknown 1,3-dipolar cycloaddition of nitronate to the aryne to give an unusual tricyclic nitroso acetal, in which the N-O bond of the isoxazoline ring is selectively cleaved upon the action of a base (CsF) or an acid (TFA). The transient cycloadducts were isolated and characterized in some cases. The synthetic potential of the obtained 3-(2-hydroxyaryl)-substituted 1,2-oxazines was demonstrated by their stereoselective reduction to 1,4-amino alcohols and reductive 1,2-oxazine ring contraction to tetrahydrofuran derivatives.

Construction of Saturated Oxazolo[3,2-b][1,2]oxazines via Tandem [3+2]-Cycloaddition/[1,3]-Rearrangement of Cyclic Nitronates and Ketenes

Reaction of six-membered cyclic nitronates with disubstituted ketenes affords hitherto unknown saturated oxazolo[3,2-b][1,2]oxazines possessing up to four contiguous stereogenic centers. The process involves a tandem of [3+2]-cycloaddition across the C═O bond of ketene, followed by a spontaneous [1,3]-rearrangement of transient vinylidene-substituted bicyclic nitrosoacetals. DFT calculations of the mechanism suggest that the [1,3]-O,C-shift proceeds through a recyclization of a biradical intermediate formed by an unusually mild homolytic cleavage of the N-O bond. The resulting products can be utilized as precursors of other fused 1,2-oxazines derivatives, in particular 1,2-oxazino-1,2,4-triazin-3-ones.

Copper(II)-Dioxygen Facilitated Activation of Nitromethane: Nitrogen Donors for the Synthesis of Substituted 2-Hydroxyimino-2-phenylacetonitriles and Phthalimides

A simple and efficient method is explored for the synthesis of 2-hydroxyimino-2-phenylacetonitriles (2) and phthalimides (4), by using nitromethane as nitrogen donors. Both reactions are promoted by Cu(II) system with the participation of dioxygen as an oxidant. The scope of the method has been successfully demonstrated with a total of 51 examples. The flexible and diversified characteristics of reactions are introduced in terms of electronic effect, steric effect, position of substituted groups, and intramolecular charge transfer. Experimental studies suggest that the methyl nitrite could be a precursor in the path to the final products. A possible reaction mechanism is proposed, including the Cu(II)/O2-facilitated transformation of nitromethane to methyl nitrite, the base-induced formation of 2-hydroxyimino-2-phenylacetonitriles, and the base-dioxygen-promoted formation of phthalimides.

Electrophilically Activated Nitroalkanes in Reactions With Carbon Based Nucleophiles

Unusual reactivity of nitroalkanes, involving formation of aci-forms (nitronic acids or nitronates) and their subsequent interaction with carbon-based nucleophiles, is surveyed in this review.

Nucleophilic Halogenation of Cyclic Nitronates: A General Access to 3-Halo-1,2-Oxazines

In this article, comprehensive studies on the nucleophilic chlorination and bromination of readily available six-membered cyclic nitronates (1,2-oxazine-N-oxides) are reported. Under optimized conditions (POCl₃ or (COBr)₂ with Hünig's base), 3-halo-substituted 1,2-oxazines, which are difficult to access by other routes, were obtained in good to high yields. The latter were shown to be convenient precursors to other 3-substituted 1,2-oxazine derivatives using Lewis/Brønsted acid-assisted substitution of the halide atom for C-, S-, and N-nucleophiles.