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Synchronized Aromaticity Enables Syntheses of 2,4-Cyclohexadienones via [3 + 2]/[3,3]-Cascade Reaction

William A Carrick¹, Michael C Eng¹, Shubin Liu¹

¹Department of Chemistry, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599-3290, United States.

Organic Letters

|October 31, 2025

Abstract

None:

A [3+2]-dipolar cycloaddition of 2,6-disubstituted aryloxyallenes and 4-nitrobenzenesulfonyl azide (NsN₃) followed by a Claisen rearrangement driven by synchronized aromaticity affords novel 2,4-cyclohexadienones. The starting materials are easily synthesized, and the reaction proceeds under mild conditions in one pot and avoids the use of a significantly more dangerous azide that was used in prior work. The dearomatized products can then be functionalized via Diels-Alder cycloadditions, enabling the rapid generation of molecular complexity from planar arenes.

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Cycloadditions are one of the most valuable and effective synthesis routes to form cyclic compounds. These are concerted pericyclic reactions between two unsaturated compounds resulting in a cyclic product with two new σ bonds formed at the expense of π bonds. The [4 + 2] cycloaddition, known as the Diels–Alder reaction, is the most common. The other example is a [2 + 2] cycloaddition.

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The Diels–Alder reaction is an example of a thermal pericyclic reaction between a conjugated diene and an alkene or alkyne, commonly referred to as a dienophile. The reaction involves a concerted movement of six π electrons, four from the diene and two from the dienophile, forming an unsaturated six-membered ring. As a result, these reactions are classified as [4+2] cycloadditions.

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Thermal cycloadditions are reactions where the source of activation energy needed to initiate the reaction is provided in the form of heat. A typical example of a thermally-allowed cycloaddition is the Diels–Alder reaction, which is a [4 + 2] cycloaddition. In contrast, a [2 + 2] cycloaddition is thermally forbidden.

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Cycloheptatriene is a neutral monocyclic unsaturated hydrocarbon that consists of an odd number of carbon atoms and an intervening sp3 carbon in the ring. The three double bonds in the ring correspond to 6 π electrons, which is a Huckel number, and therefore satisfies the criteria of 4n + 2 π electrons. However, the intervening sp3 carbon disrupts the continuous overlap of p orbitals. As a result, cycloheptatriene is not aromatic.
Removing one hydrogen from the intervening CH2 group...

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