Phosphonylpyrazoles from Bestmann−Ohira Reagent and Nitroalkenes: Synthesis and Dynamic NMR Studies

Abstract

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.

An Efficient and Novel Regioselective Synthesis of Phosphonylpyrazoles from Vinylazides and Bestmann-Ohira Reagent

“…13 C NMR spectra were recorded in CDCl 3 or DMSO-d 6 on a Varian Mercury-Plus (100/150 MHz) spectrometer and chemical shifts (d) are given in ppm. Unless otherwise specified, all reactions were carried out under an air atmosphere in oven dried RB flasks.…”

Regioselective synthesis of pyrazole and pyridazine esters from chalcones and α-diazo-β-ketoesters

“…17 Later on, this strategy was studied in depth by us which unveiled the scope and tautomeric equilibrium in phosphonylpyrazoles. 18 The utility of BOR as a diazoalkane equivalent in a one-pot multi-component reaction involving a domino Knoevenagel condensation/1,3-dipolar cyclization strategy for the synthesis of phosphonylpyrazoles was reported by Smietana et al 19 We and Smietana et al also employed enones and Rastogi et al employed ynones as dipolarophiles which led to the synthesis of carbonylated phosphonylpyrazoles. 20 More recently, Mohanan et al developed a domino reaction for the synthesis of phosphonylpyrazoles in which BOR reacted as a cycloaddition partner as well as a homologation agent.…”

NMR Spectroscopic Study of Tautomerism in Solution and in the Solid State

“…This review gives the perfect overview. This research was (and is) continued, for example, the pyrazole tautomerism of phosphonylpyrazoles was studied by low-temperature 1 H and 31 P NMR analysis [89]. Tautomerism of the 3-hydroxyindazoles in solution and in the solid state was studied by the Elguero group [90] as well.…”