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Cycloaddition Reactions: Overview

Cycloaddition Reactions: Overview

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.

Diels–Alder Reaction Forming Bridged Bicyclic Products: Stereochemistry

Diels–Alder Reaction Forming Bridged Bicyclic Products: Stereochemistry

Diels–Alder reactions between cyclic dienes locked in an s-cis configuration and dienophiles yield bridged bicyclic products.

[4+2] Cycloaddition of Conjugated Dienes: Diels–Alder Reaction

[4+2] Cycloaddition of Conjugated Dienes: Diels–Alder Reaction

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.

Aromatic Hydrocarbon Cations: Structural Overview

Aromatic Hydrocarbon Cations: Structural Overview

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...

Cycloaddition Reactions: MO Requirements for Thermal Activation

Cycloaddition Reactions: MO Requirements for Thermal Activation

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.

Cycloaddition Reactions: MO Requirements for Photochemical Activation

Cycloaddition Reactions: MO Requirements for Photochemical Activation

Some cycloaddition reactions are activated by heat, while others are initiated by light. For example, a [2 + 2] cycloaddition between two ethylene molecules occurs only in the presence of light. It is photochemically allowed but thermally forbidden.

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Light-Mediated Tandem Giese/C-H Functionalizations Toward Cyclopenta[b]indoles.

David M Kitcatt¹, Eva Pogacar¹, Daniel U Kleinjan²

¹Institute of Chemical Sciences, Heriot-Watt University, Edinburgh EH14 4AS, United Kingdom.

Organic Letters

|September 15, 2025

Summary

Researchers developed a new method to create cyclopenta[b]indoles using indole-α-ketoacids. This visible light-driven process avoids metals, photocatalysts, and bases for efficient synthesis.

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Published on: August 12, 2019

Area of Science:

Organic Chemistry
Synthetic Chemistry
Photochemistry

Background:

Cyclopenta[b]indoles are important heterocyclic compounds with diverse biological activities.
Efficient synthetic routes to cyclopenta[b]indoles are crucial for medicinal chemistry and materials science.
Existing methods often require harsh conditions, expensive catalysts, or multiple steps.

Purpose of the Study:

To develop a novel, efficient, and mild method for the direct synthesis of cyclopenta[b]indoles.
To utilize readily available 3-indole-α-ketoacids as starting materials.
To explore a metal-free, photocatalyst-free, and base-free tandem reaction strategy.

Main Methods:

A tandem Giese/C-H functionalization reaction was employed.
Photoactive 3-indole-α-ketoacids were used as radical precursors.
Visible light irradiation was utilized to initiate the radical cascade reaction.

Main Results:

Direct formation of cyclopenta[b]indoles from 3-indole-α-ketoacids was achieved.
The reaction proceeded efficiently under mild conditions using visible light.
The method demonstrated broad substrate scope and good functional group tolerance.
The process was metal-free, photocatalyst-free, and base-free.

Conclusions:

A novel and efficient visible-light-mediated tandem Giese/C-H functionalization strategy has been established for cyclopenta[b]indole synthesis.
This method offers a sustainable and practical approach, avoiding the need for transition metals, photocatalysts, or bases.
The developed protocol provides a valuable tool for accessing diverse cyclopenta[b]indole derivatives.