Reactions of vinyl sulfone with α-diazo-β-ketosulfone and Bestmann–Ohira reagent for the regioselective synthesis of highly functionalized pyrazoles

Regioselective synthesis of functionalized pyrazole-chalcones via a base mediated reaction of diazo compounds with pyrylium salts

A base-mediated reaction of triaryl/alkyl pyrylium tetrafluoroborate salts with α-diazo-phosphonates, sulfones and trifluoromethyl compounds affords the corresponding functionalized pyrazole-chalcones as 5-P-5 and 3-P-3 tautomeric mixture. The reaction proceeds through an initial nucleophilic addition of diazo substrates to pyrylium salts followed by a base-mediated pyrylium ring-opening and intramolecular 1,5-cyclization to afford formal 1,3-dipolar cycloaddition products. The products underwent a Nazarov-type cyclization upon hydride reduction followed by acidic-workup, furnishing the corresponding indenyl-pyrazoles in high yields.

The Bestmann-Ohira Reagent and Related Diazo Compounds for the Synthesis of Azaheterocycles

Azaheterocycles are one of the most prevalent classes of compounds present in numerous bioactive compounds, natural products, and agrochemicals, and undoubtedly, new methods to access them are always in high demand. Among the methods available, the 1,3-dipolar cycloaddition reactions involving diazo compounds are particularly attractive because of their ability to rapidly construct densely functionalized azaheterocycles in a regioselective manner. In this context, the Bestmann-Ohira reagent has become a well-known reagent for the 1,3-dipolar cycloaddition reactions to produce phosphonylated heterocycles, besides its widespread use as a homologating agent for the conversion of aldehydes to alkynes. This account details our efforts toward broadening the synthetic utility of the Bestmann-Ohira reagent and related compounds for the preparation of azaheterocycles such as pyrazoles, spirooxindoles, triazoles, triazolines, and spiropyrazolines, emphasizing on domino multicomponent reactions employing readily available feedstock reagents.

Synthesis of Functionalized Pyrazoles via 1,3-Dipolar Cycloaddition of α-Diazo-β-ketophosphonates, Sufones and Esters with Electron-Deficient Alkenes

1,3-Dipolar cycloaddition of diazo compounds with olefinic substrates is a promising atom-economic strategy for the construction of functionalized pyrazoles. Over the last few years, our group has been engaged in the synthesis of phosphonyl/sulfonylpyrazoles and pyrazole esters by employing Bestmann-Ohira Reagent (BOR) and its sulfur and ester analogs as 1,3-dipole precursors with various dipolarophiles. This account describes the novel synthetic methods developed in our laboratory, in the perspective of closely related work by others, for the synthesis of phosphonyl/sulfonylpyrazoles, pyrazole esters and the total synthesis of Withasomnine, a natural product, by using 1,3-dipolar cycloaddition as the key step.

Rapid and selective synthesis of spiropyrazolines and pyrazolylphthalides employing Seyferth–Gilbert reagent

An efficient protocol for the selective synthesis of spiropyrazoline phosphonates and pyrazolylphthalides employing Seyferth–Gilbert reagent is reported.

Base mediated 1,3-dipolar cycloaddition of α-substituted vinyl phosphonates with diazo compounds for synthesis of 3-pyrazolylphosphonates and 5-pyrazolcarboxylates

5-Aryl-substituted pyrazol-3-ylphosphonates have been conveniently synthesized by 1,3-dipolar cycloaddition of 1-formamidovinylphosphonates and aryldiazomethanes under K₂CO₃/MeOH conditions at room temperature. These pyrazoles are formed in one pot via spontaneous elimination of formamide. Basic conditions prevent competitive formation of cyclopropylphosphonates. 3-Aryl substituted pyrazol-5-carboxylates can be synthesized by the same methodology from 1-arylvinylphosphonates and ethyl diazoacetate, although a stronger base NaH is necessary to ensure the success of the aromatization stage with the elimination of the diethoxylphosphoryl moiety.

Metal-Free Three-Component Domino Approach to Phosphonylated Triazolines and Triazoles

An efficient, three-component domino reaction between aldehydes, amines, and the Bestmann-Ohira reagent is reported that enables a general, mild, and straightforward access to 1,4,5-trisubstituted 1,2,3-triazolines and triazoles. The reaction proceeds through a domino-condensation/1,3-dipolar cycloaddition sequence to afford the triazoline derivatives with excellent diastereoselectivity. Moreover, when both amine and aldehyde employed for this reaction are aromatic, a spontaneous oxidation afforded 1,4,5-trisubstituted triazoles in moderate yields.

[3 + 2] cycloaddition and subsequent oxidative dehydrogenation between alkenes and diazo compounds: a simple and direct approach to pyrazoles using TBAI/TBHP

Herein, an efficient [3 + 2] cycloaddition and oxidative dehydrogenation reaction of diazo compounds and alkenes was well developed using TBAI/TBHP, leading to pyrazoles with moderate to high yields.

An efficient one pot regioselective synthesis of a 3,3'-spiro-phosphonylpyrazole-oxindole framework via base mediated [1,3]-dipolar cycloaddition reaction of the Bestmann-Ohira reagent with methyleneindolinones

A one pot, highly regioselective synthesis of racemic 3,3'-spiro-phosphonylpyrazole-oxindole by 1,3-dipolar cycloaddition of an in situ generated anion of dialkyl 1-diazomethylphosphonate from the Bestmann-Ohira reagent (BOR) & methyleneindolinones has been developed. The synthesis affords the highly functionalized pyrazole scaffolds in good yields with excellent regioselectivity under mild reaction conditions within a short reaction time.

Substrate-controlled product-selectivity in the reaction of the Bestmann–Ohira reagent with N-unprotected isatin-derived olefins

A mild and efficient reaction of the Bestmann–Ohira reagent withN-unprotected isatin-derived olefins has been developed for the selective synthesis of spiro-phosphonylpyrazoline-oxindoles and phosphonylpyrazoloquinazolinones.

Domino reaction involving the Bestmann–Ohira reagent and α,β-unsaturated aldehydes: efficient synthesis of functionalized pyrazoles

A mild, efficient and rapid domino reaction involving the Bestmann–Ohira reagent (BOR) and α,β-unsaturated aldehydes has been developed for the synthesis of densely functionalized vinylpyrazoles.