Construction of pyrazole fused spiroketals by a (3 + 2) annulation reaction

Abstract

A stereoselective (3 + 2) annulation process between 4-bromo pyrazolones and benzofuran-derived azadienes was developed. A series of novel pyrazole fused spiroketals were synthesized in the presence of a base....

Organocatalytic Enantioselective Synthesis of Chiral Spiro‐indoline‐pyrazolones through a formal [4+1] Annulation Reaction of 4‐Bromopyrazolones and aza‐ortho‐Quinone Methides

“…Based on the previous reports on the use of 1a [17,18] and 2a , [27–29] a plausible reaction mechanism for the formal [4+1] cycloaddition is presented in Scheme 4. Chloroaniline 1a undergoes the elimination of HCl generating the aza‐ ortho ‐quinone methide intermediate in the presence of the inorganic base (Na 2 CO 3 ).…”

“…From the different strategies that have been reported for the synthesis of chiral spiropyrazolones, the use of 4‐bromopyrazolones represents a convenient approach. However, a very few examples have been described in the literature [27–29] . For example, Xu and coworkers described the synthesis of spiro‐benzofuran pyrazolones through an organocatalytic formal [4+1] cycloaddition of ortho ‐quinone methides with 4‐bromopyrazolones ( Scheme 1 b ) [27] .…”

Synthesis of Spiro[benzofuran-2,3′-pyrazol]-3-imines from Aurone-Derived Azadienes and Hydrazonoyl Chlorides via Regio- and Diastereospecific [2+3] Cycloaddition

“…5c Recently, a series of novel spiroannulation reactions have been developed to access chiral or non-chiral spirocycles with high regioselectivity. 5` e f g h i j k…”

Lewis Acid-Catalyzed [3 + 2]-Cyclization of Iodonium Ylides with Azadienes: Access to Spiro[benzofuran-2,2′-furan]-3-ones

“…First, HFIP as the solvent could significantly enhance the acidity of Lewis acid, thereby promoting the enol isomerization of iodonium ylides, and obtaining the Zn complex A , which was detected by high-resolution mass spectra (HRMS). Complex A and benzofuran-derived azadiene 1a underwent Michael addition to generate intermediate B . Subsequently, intramolecular nucleophilic substitution gave three-membered spiro cyclopropane D .…”