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Heterocyclic synthesis using Nitrile imines

Nitrile - Wikipedia

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Nomenclature and classification

The syntheses of an important class of hitherto unreported 1,3,5-pyrazoles, inspired by an unanticipated eliminatory ring opening are described. The reported pyrazole compounds were constructed through the Huisgen cyclization of 2-methylene-1,3,3-trimethylindoline and an in situ generated nitrile imine. The newly formed spiro-pyrazoline intermediate presumably then undergoes a ring opening/elimination process to afford a pyrazole, as evidenced by single X-ray crystal data. The current report constitutes the first formal observation of this kind of ring opening involving a spiro-pyrazoline intermediate.

Usually imines refer to compounds with the connectivity R 2 C=NR, as discussed below
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Heterocycles are popularly known for displaying a wide range of biological properties. The recent success of pyrazole based COX-II inhibitors and their application in medicinal chemistry have amplified the importance of pyrazoles to even a greater extent. Several pharmaceutical drugs including celecoxib and rimonabant utilize the pyrazole as their core molecular entity,, and a regioselective synthetic method for the synthesis of trisubstituted pyrazoles is still in demand.,- Pyrazoles and pyrazolines are often synthesized by the 1,3-dipolar cycloaddition reaction of nitrilimines with alkynes, alkyne surrogates,, or alkenes,,, and other methods. Special attention is warranted towards the synthetic design and development of pyrazoles because of their high demand in academic and pharmaceutical sectors. Herein we report the synthesis of 1,3,5-trisubstituted pyrazoles through a 1,3-dipolar cycloaddition/ring opening process.

Strecker Synthesis - Organic Chemistry Portal

Regiospecific synthesis of N2-aryl 1,2,3-triazoles from 2,5-disubstituted tetrazoles via photochemically generated nitrile imine intermediates
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In continuance of our interest in the 1,3-dipolar cycloaddition chemistry, we describe our endeavor towards the synthesis of 1,3,5-trisubstituted pyrazoles on the basis of the 1,3-dipolar cycloaddition protocol,, which was then followed by an unforeseen ring opening. During our investigation on the synthesis of spiro-pyrazolines, we discovered that when 2-methylene-1,3,3-trimethylindoline (1) was reacted with the in situ generated nitrile imine from hydrazonyl chloride (2), pyrazole (4) was the only product that was isolated rather than the expected spiro-pyrazoline (3) (). The formation of a trisubstituted pyrazole is in quite contrast to the expected spiro-pyrazoline product. On the basis of this result, we decided to investigate the generality of this 1,3-dipolar cycloaddition /ring opening reaction towards the synthesis of 1,3,5-trisubstituted pyrazoles.

Mechanism of the Strecker Synthesis

synthesis of 2-polyfluoroalkyl-2,3-dihydro-1,3,4-thiadiazoles via regioselective [3+2] cycloaddition of nitrile imines to polyfluoroalkanethioamides
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Amine Structure & Synthesis
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