Brønsted acid-catalyzed formal [5+2+1] cycloaddition of ynamides and isoxazoles with water: access to oxygen-bridged tetrahydro-1,4-oxazepines

Abstract

A Brønsted acid-catalyzed formal [5+2+1] cycloaddition of ynamides and isoxazoles with water is described. This process provides atom-economical access to oxygen-bridged tetrahydro-1,4-oxazepines, where the bridged oxygen atom originates from water. The unique property of the Brønsted acid shows distinct chemoselectivity from the corresponding gold-catalyzed cycloadditions.

Iron‐Promoted Reductive Ring‐Opening and Isomerization Reactions of C4‐Alkynylisoxazoles Leading to 2‐Alkynylenaminones and Polysubstituted Furans

“…Other iron salts were screened, and FeCl 3 was the best one (Table 1, entries 1-4). The reaction conditions were further optimized by changing temperature (Table 1, entries 6,[9][10]. When FeCl 3 loading increased to 2.0 equivalent, the yield of 2 a increased to 79% (Table 1, entry 7).…”

“…[1] One of the key structural elements of isoxazoles depends on the fact that they possess a relatively labile NÀ O bond, which is easy to be cleaved into heterocyclic compounds. [1a] Recently, many research groups used isoxazoles as substrates to react with alkynes in the presence of gold, [6] platinum, [7] Zn(OTf) 2 , [8] and Tf 2 NH as catalysts, [9] affording various nitrogen-containing heterocycles. [1a] Recently, many research groups used isoxazoles as substrates to react with alkynes in the presence of gold, [6] platinum, [7] Zn(OTf) 2 , [8] and Tf 2 NH as catalysts, [9] affording various nitrogen-containing heterocycles.…”

“…[1,2] Therefore, many examples employing isoxazoles as the starting materials have been developed, including (a) reducing isoxazoles into enaminones in the presence of H 2 and metal catalyst, [3] (b) converting isoxazoles into β-keto nitriles under basic conditions, [4] (c) transforming isoxazoles into oxazole [5] or imidazole [1h] under photochemical conditions, and (d) halogen-substituted isoxazole reacting with vinyl ketone to synthesize 2azafluorenone in the presence of palladium catalyst. [1a] Recently, many research groups used isoxazoles as substrates to react with alkynes in the presence of gold, [6] platinum, [7] Zn(OTf) 2 , [8] and Tf 2 NH as catalysts, [9] affording various nitrogen-containing heterocycles. The isoxazoles also could react with salicylaldehyde [10] or 1,2,3-triazole [11] to produce secondary enamide or polysubstituted 3-aminopyrrole in the presence of a rhodium catalyst.…”

Synthesis Strategies for α‐, β‐, γ‐ and δ‐Carbolines

“…The nitride containing an electron‐withdrawing group on the nitrogen atom has been well proved to be a multi‐purpose structure for its combinatorial stability and special reactivity [27] . The cycloaddition reaction of imines with different precursors provides an effective method to construct complex skeletons [28] . Particularly, the [2+2+2] cycloaddition reaction of amides and nitriles catalyzed by transition metals is one of the most convenient and direct strategies to generate α‐carboline.…”

“…[27] The cycloaddition reaction of imines with different precursors provides an effective method to construct complex skeletons. [28] Particularly, the [2 + 2 + 2] cycloaddition reaction of amides and nitriles catalyzed by transition metals is one of the most convenient and direct strategies to generate α-carboline. In the presence of Lewis acid ZnCl 2 generated through the reduction of a Ni(II)Cl 2 L 2 (L = phosphine ligands) complex by zinc metal, the electrophilicity of the nitrile group of 37 was significantly enhanced and coupled with alkyne 38 via [2 + 2 + 2] cycloaddition to give nitrogen heterocycle 39 (Scheme 5A).…”