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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.

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.

[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.

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.

Thermal Electrocyclic Reactions: Stereochemistry

Thermal Electrocyclic Reactions: Stereochemistry

The stereochemistry of electrocyclic reactions is strongly influenced by the orbital symmetry of the polyene HOMO. Under thermal conditions, the reaction proceeds via the ground-state HOMO.
Selection Rules: Thermal Activation
Conjugated systems containing an even number of π-electron pairs undergo a conrotatory ring closure. For example, thermal electrocyclization of (2E,4E)-2,4-hexadiene, a conjugated diene containing two π-electron pairs, gives trans-3,4-dimethylcyclobutene.

Diels–Alder Reaction Forming Cyclic Products: Stereochemistry

Diels–Alder Reaction Forming Cyclic Products: Stereochemistry

The Diels–Alder reaction is one of the robust methods for synthesizing unsaturated six-membered rings. The reaction involves a concerted cyclic movement of six π electrons: four π electrons from the diene and two π electrons from the dienophile.

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Related Experiment Video

Updated: Feb 18, 2026

Efficient Construction of Drug-like Bispirocyclic Scaffolds Via Organocatalytic Cycloadditions of α-Imino γ-Lactones and Alkylidene Pyrazolones

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Published on: February 7, 2019

Computational Insights into an Enzyme-Catalyzed [4+2] Cycloaddition.

Yiying Zheng¹, Walter Thiel¹

¹Max-Planck-Institut für Kohlenforschung , 45470 Mülheim an der Ruhr, Germany.

The Journal of Organic Chemistry

|November 14, 2017

Summary

The enzyme SpnF facilitates spinosyn A biosynthesis by favoring a concerted Diels-Alder reaction over other pathways. This enzyme significantly lowers the reaction barrier, acting as a Diels-Alderase.

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Area of Science:

Biochemistry
Enzymology
Computational Chemistry

Background:

SpnF is crucial for spinosyn A biosynthesis.
The reaction mechanism involves a formal [4+2] cycloaddition of a macrolactone.
Potential mechanisms include concerted [4+2] Diels-Alder or stepwise [6+4] cycloaddition with Cope rearrangement.

Purpose of the Study:

To elucidate the reaction mechanism catalyzed by SpnF.
To determine the favored pathway (Diels-Alder vs. stepwise) in the enzyme environment.
To understand how the enzyme lowers the activation barrier.

Main Methods:

Quantum mechanics/molecular mechanics (QM/MM) calculations.
Free energy simulations (OM2/CHARMM).
Analysis of optimized geometries and partial charges.

Main Results:

The concerted Diels-Alder pathway is favored in the enzyme.
A free energy barrier of 22 kcal/mol was predicted for the Diels-Alder process.
The enzyme significantly lowers the Diels-Alder barrier compared to the gas phase.
SpnF prearranges the substrate and utilizes a hydrogen bond (Thr196-substrate) to enhance the reaction rate.
Substrate conformational flexibility in the active site facilitates the reaction.

Conclusions:

SpnF functions as a Diels-Alderase.
The enzyme actively promotes the Diels-Alder pathway through substrate arrangement and specific interactions.
Computational methods provide insights into enzyme-catalyzed reaction mechanisms.