Reaction of 1,2-Diaza-1,3-butadienes with Aminophosphorus Nucleophiles: A Practical Access to New Phosphorylated Pyrazolones

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.

Phosphonylpyrazoles from Bestmann-Ohira reagent and nitroalkenes: synthesis and dynamic NMR studies

Application of diethyl 1-diazo-2-oxopropylphosphonate (Bestmann-Ohira reagent) as a cycloaddition partner with nitroalkenes has been extensively investigated. Base-mediated reaction of the Bestmann-Ohira reagent with various nitroalkenes such as beta-substituted, alpha,beta-disubstituted, and nitroethylene that are part of a carbocyclic or heterocyclic ring provided functionalized phosphonylpyrazoles through a one-pot regioselective reaction at room temperature in high yield. The substituted nitroalkenes employed in these reactions also included Morita-Baylis-Hillman adducts of conjugated nitroalkenes with various electrophiles. Detailed dynamic NMR studies were performed on the prototropic tautomerism exhibited by the phosphonylpyrazoles using CDCl(3) and DMSO-d(6) as solvents and (1)H and (31)P as the probe nuclei. These studies unraveled the existence of two tautomers in solution with a small energy difference but considerable barrier to interconversion.