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Thermal and Photochemical Electrocyclic Reactions: Overview

Thermal and Photochemical Electrocyclic Reactions: Overview

Electrocyclic reactions are reversible reactions. They involve an intramolecular cyclization or ring-opening of a conjugated polyene. Shown below are two examples of electrocyclic reactions. In the first reaction, the formation of the cyclic product is favored. In contrast, in the second reaction, ring-opening is favored due to the high ring strain associated with cyclobutene formation.

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.

Photochemical Electrocyclic Reactions: Stereochemistry

Photochemical Electrocyclic Reactions: Stereochemistry

The absorption of UV–visible light by conjugated systems causes the promotion of an electron from the ground state to the excited state. Consequently, photochemical electrocyclic reactions proceed via the excited-state HOMO rather than the ground-state HOMO. Since the ground- and excited-state HOMOs have different symmetries, the stereochemical outcome of electrocyclic reactions depends on the mode of activation; i.e., thermal or photochemical.
Selection Rules: Photochemical Activation

Catalysis

Catalysis

The presence of a catalyst affects the rate of a chemical reaction. A catalyst is a substance that can increase the reaction rate without being consumed during the process. A basic comprehension of a catalysts’ role during chemical reactions can be understood from the concept of reaction mechanisms and energy diagrams.

Catalytically Perfect Enzymes

Catalytically Perfect Enzymes

The theory of catalytically perfect enzymes was first proposed by W.J. Albery and J. R. Knowles in 1976. These enzymes catalyze biochemical reactions at high-speed. Their catalytic efficiency values range from 108-109 M-1s-1. These enzymes are also called 'diffusion-controlled' as the only rate-limiting step in the catalysis is that of the substrate diffusion into the active site. Examples include triose phosphate isomerase, fumarase, and superoxide dismutase.

Most enzymes...

Keto–Enol Tautomerism: Mechanism

Keto–Enol Tautomerism: Mechanism

The keto and enol forms are known as tautomers and they constantly interconvert (or tautomerize) between the two forms under acid or base catalyzed conditions. Both the reactions involve the same steps—protonation and deprotonation— although in the reverse order.

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Updated: Jul 13, 2025

A Customizable Approach for the Enzymatic Production and Purification of Diterpenoid Natural Products

A Customizable Approach for the Enzymatic Production and Purification of Diterpenoid Natural Products

Published on: October 4, 2019

Decoding Catalysis by Terpene Synthases.

Joshua N Whitehead¹, Nicole G H Leferink², Linus O Johannissen¹

¹Manchester Institute of Biotechnology, Department of Chemistry, The University of Manchester, Manchester, M1 7DN, United Kingdom.

ACS Catalysis

|October 12, 2023

Summary

This review updates the field of terpene synthase/cyclase (TS) biocatalysis, detailing recent advances in understanding TS reaction chemistry and engineering. Future goals include the rational design of "designer terpene synthases" for novel applications.

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A Customizable Approach for the Enzymatic Production and Purification of Diterpenoid Natural Products

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Area of Science:

Biochemistry and Synthetic Biology
Enzyme Catalysis and Engineering

Background:

Terpene synthases (TSs) are crucial enzymes for producing diverse terpenoids.
A 2017 review summarized foundational TS discoveries; this work updates the field since then.

Purpose of the Study:

To review recent TS literature, focusing on reaction chemistry and engineering strategies.
To explore the future of rational design for terpene synthases.

Main Methods:

Literature review of TS research published after 2017.
Analysis of combined experimental and computational approaches to understand TS catalysis.
Examination of recent TS engineering strategies, including machine learning.

Main Results:

Identification of catalytic motifs governing TS product outcomes.
Detailed exploration of how TS enzymes convert simple substrates into complex terpenoids.
Emerging data-driven approaches for TS engineering.

Conclusions:

Rational and predictive engineering of TSs is becoming a realistic goal.
Understanding catalytic motifs is key to deciphering TS complexity.
"Designer terpene synthases" are on the horizon.