Synthesis of pyrazoles from sulfonyl hydrazone and benzyl acrylate under transition-metal-free conditions

Pyrazoles as an important class of heterocyclic compounds, are widely found in pharmaceuticals and bioactive natural products. Herein we report a [3 + 2] cycloaddition reaction for the synthesis of a series of pyrazoles, with the yield up to 77%. This approach exhibits many notable features, such as convenient operating conditions, excellent functional group compatibility and readily accessible raw materials, providing an alternative route for the construction of pyrazole derivatives.

Diazophosphonates: Effective Surrogates for Diazoalkanes in Pyrazole Synthesis

Diazophosphonates, readily prepared from α-ketophosphonates by oxidation of the corresponding hydrazones in batch or in flow, are useful partners in 1,3-dipolar cycloaddition reactions to alkynes to give N-H pyrazoles, including the first intramolecular examples of such a process. The phosphoryl group imbues a number of desirable properties into the diazo 1,3-dipole. The electron-withdrawing nature of the phosphoryl stabilizes the diazo compound making it easier to handle, whilst the ability of the phosphoryl group to migrate readily in a [1,5]-sigmatropic rearrangement enables its transfer from C to N to aromatize the initial cycloadduct, and hence its facile removal from the final pyrazole product. Overall, the diazophosphonate acts as a surrogate for the much less stable diazoalkane in cycloadditions, with the phosphoryl group playing a vital, but traceless, role. The cycloaddition proceeds more readily with alkynes bearing electron-withdrawing groups, and is regiospecific with asymmetrical alkynes. The potential of diazophosphonates for use in bioorthogonal cycloadditions is demonstrated by their facile addition to strained alkynes.

Synthesis of 1H-Pyrazol-5-yl-pyridin-2-yl-[1,2,4]triazinyl Soft-Lewis Basic Complexants via Metal and Oxidant Free [3 + 2] Dipolar Cycloaddition of Terminal Ethynyl Pyridines with Tosylhydrazides

Soft-Lewis basic complexants that facilitate chemoselective separation of the minor actinides from the lanthanides are critical to the closure of the nuclear fuel cycle. Complexants that modulate covalent orbital interactions with relevant metals of interest can facilitate desired outcomes in liquid-liquid separation, allowing for further transmutative processes that decrease issues related with storage of spent nuclear fuel from energy and weapons production. Synthesis of previously unexplored scaffolds seeks to improve performance over benchmark complexants. In the current work, an intermolecular, thermally initiated, and DBU-assisted [3 + 2] cycloaddition of 3-(6-ethynyl-pyridin-2-yl)-5,6-diphenyl-[1,2,4]triazine dipolarophiles with structurally diverse 4-methylbenzenesulfono-hydrazides afforded 21 yet-to-be reported examples in 42-68% yield and modest regioselectivity for the desired regioisomer. Preparation of requisite starting materials, method definition, dipole and dipolarophile scope, ten-fold scale-up reaction, and downstream functional group interconversion are reported herein.

Ring-Opening of Indoles: An Unconventional Route for the Transformation of Indoles to 1 H-Pyrazoles Using Lewis Acid

An unusual transformation of indoles to pyrazoles via an aromatic ring-opening strategy has been developed. The salient feature of this strategy involves the C2-N1 bond opening and concomitant cyclization reaction of the C2═C3 bond of the indole moiety with the tosylhydrazone, which proceeds under transition-metal and ligand free conditions. This ring-opening functionalization of indoles provides a wide scope of differently substituted pyrazoles.