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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.

Pericyclic Reactions: Introduction

Pericyclic Reactions: Introduction

Pericyclic reactions are organic reactions that occur via a concerted mechanism without generating any intermediates. The reactions proceed through the movement of electrons in a closed loop to form a cyclic transition state, where rearrangement of the σ and π bonds yields specific products.
Pericyclic reactions can be classified into three categories: electrocyclic reactions, cycloaddition reactions, and sigmatropic rearrangements. Electrocyclic reactions and sigmatropic...

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.

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

Woodward–Hoffmann Selection Rules and Microscopic Reversibility

Woodward–Hoffmann Selection Rules and Microscopic Reversibility

Electrocyclic reactions, cycloadditions, and sigmatropic rearrangements are concerted pericyclic reactions that proceed via a cyclic transition state. These reactions are stereospecific and regioselective. The stereochemistry of the products depends on the symmetry characteristics of the interacting orbitals and the reaction conditions. Accordingly, pericyclic reactions are classified as either symmetry-allowed or symmetry-forbidden. Woodward and Hoffmann presented the selection criteria for...

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.

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Updated: Jul 8, 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

Class II terpene cyclases: structures, mechanisms, and engineering.

Xingming Pan¹, Jeffrey D Rudolf², Liao-Bin Dong¹

¹State Key Laboratory of Natural Medicines, School of Traditional Chinese Pharmacy, China Pharmaceutical University, Nanjing 211198, China. ldong@cpu.edu.cn.

Natural Product Reports

|December 18, 2023

Summary

Class II terpene cyclases (TCs) initiate complex natural product biosynthesis. Understanding their mechanisms is key for enzyme engineering and creating novel terpenoids.

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Last Updated: Jul 8, 2025

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

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Transient Expression in Nicotiana Benthamiana Leaves for Triterpene Production at a Preparative Scale

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Unraveling Entropic Rate Acceleration Induced by Solvent Dynamics in Membrane Enzymes

Unraveling Entropic Rate Acceleration Induced by Solvent Dynamics in Membrane Enzymes

Published on: January 16, 2016

Area of Science:

Biochemistry
Natural Product Biosynthesis
Enzyme Engineering

Background:

Terpene cyclases (TCs) are crucial enzymes responsible for synthesizing over 95,000 known terpenoid natural products.
Canonical TCs are classified into two types based on structure, function, and reaction mechanisms.
Class II TCs uniquely mediate acid-base-initiated cyclization reactions of various isoprenoid substrates.

Purpose of the Study:

To review the diverse class II terpene cyclases found in nature.
To explore their roles in the biosynthesis of important natural products like phytohormones and fungal meroterpenoids.
To highlight their potential in enzyme engineering for novel molecule discovery.

Main Methods:

Comprehensive literature review of class II TCs up to July 2023.
Analysis of sequences, structures, and reaction mechanisms of various class II TCs.
Examination of structure-guided engineering studies.

Main Results:

Class II TCs are vital in early biosynthetic steps for biologically active compounds.
These enzymes are instrumental in producing gibberellins and fungal meroterpenoids.
Their catalytic mechanisms and biocatalytic potential are of significant interest.

Conclusions:

A thorough understanding of class II TCs is essential for expanding terpenoid structural diversity.
Engineering class II TCs offers exciting prospects for creating novel bioactive molecules.
Further research into their mechanisms will drive innovation in biosynthesis and organic chemistry.