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Related Concept Videos

Thermal and Photochemical Electrocyclic Reactions: Overview01:26

Thermal and Photochemical Electrocyclic Reactions: Overview

2.4K
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.
2.4K
Thermal Electrocyclic Reactions: Stereochemistry01:17

Thermal Electrocyclic Reactions: Stereochemistry

2.0K
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.
2.0K
Photochemical Electrocyclic Reactions: Stereochemistry01:26

Photochemical Electrocyclic Reactions: Stereochemistry

1.9K
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
1.9K
[4+2] Cycloaddition of Conjugated Dienes: Diels–Alder Reaction01:16

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

10.3K
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.
10.3K
Diels–Alder Reaction Forming Cyclic Products: Stereochemistry01:28

Diels–Alder Reaction Forming Cyclic Products: Stereochemistry

3.9K
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.
3.9K
Diels–Alder Reaction Forming Bridged Bicyclic Products: Stereochemistry01:29

Diels–Alder Reaction Forming Bridged Bicyclic Products: Stereochemistry

4.6K
Diels–Alder reactions between cyclic dienes locked in an s-cis configuration and dienophiles yield bridged bicyclic products.
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Related Experiment Video

Updated: Jul 16, 2025

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

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Cytochrome P450 Mediated Cyclization in Eunicellane Derived Diterpenoid Biosynthesis.

Zengyuan Wang1, Qian Yang1, Jingyi He1

  • 1State Key Laboratory of Natural Medicines, School of Traditional Chinese Pharmacy, China Pharmaceutical University, Nanjing, 211198, China.

Angewandte Chemie (International Ed. in English)
|September 22, 2023
PubMed
Summary

Researchers discovered a novel two-step pathway for creating complex diterpenoids using a terpene cyclase and a rare cytochrome P450 enzyme. This finding expands our understanding of natural product biosynthesis and enzyme catalysis.

Keywords:
BiosynthesisCytochrome P450 EnzymesEunicellane DiterpenoidsTerpene Cyclization

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Cercosporin-Photocatalyzed [4+1]- and [4+2]-Annulations of Azoalkenes Under Mild Conditions
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Transient Expression in Nicotiana Benthamiana Leaves for Triterpene Production at a Preparative Scale
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Transient Expression in Nicotiana Benthamiana Leaves for Triterpene Production at a Preparative Scale
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Area of Science:

  • Biochemistry
  • Natural Product Biosynthesis
  • Enzymology

Background:

  • Terpene cyclization is a complex natural reaction typically catalyzed by terpene cyclases (TCs).
  • Cytochrome P450 enzymes acting as TCs are rare but known.
  • Bacterial terpenoid gene clusters offer a source for discovering novel biosynthetic pathways.

Purpose of the Study:

  • To investigate a cryptic bacterial gene cluster (ari) from *Amycolatopsis arida*.
  • To characterize novel diterpenoids and elucidate their unique biosynthetic pathway.
  • To expand the known catalytic capabilities of cytochrome P450s in terpene biosynthesis.

Main Methods:

  • Genome mining of *Amycolatopsis arida* for the ari gene cluster.
  • Heterologous production and isolation of aridacins A-C.
  • In vivo and in vitro biochemical assays to determine the biosynthetic pathway.
  • Quantum chemical computations to elucidate the reaction mechanism.

Main Results:

  • Three novel eunicellane-derived diterpenoids, aridacins A-C, with a 6/7/5-fused tricyclic scaffold were identified.
  • A noncanonical two-step pathway involving a class I TC (AriE) and a cytochrome P450 (AriF) was established.
  • AriE cyclizes geranylgeranyl diphosphate (GGPP) to a bicyclic intermediate, and AriF catalyzes the final cyclization via C2-C6 bond formation.
  • A mechanism involving hydrogen abstraction, electron transfer, and carbocation ring closure was proposed for AriF.

Conclusions:

  • The study reveals an unprecedented biosynthetic logic for diterpene skeleton construction.
  • This work expands the catalytic diversity of cytochrome P450s in natural product biosynthesis.
  • The findings provide a foundation for studying P450-mediated carbocation generation in terpenoid pathways.