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Diels–Alder Reaction Forming Bridged Bicyclic Products: Stereochemistry

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

Diels–Alder Reaction Forming Cyclic Products: Stereochemistry

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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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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.

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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.

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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.

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Related Experiment Video

Updated: Apr 24, 2026

Preparation of Enantiopure Non-Activated Aziridines and Synthesis of Biemamide B, D, and epiallo-Isomuscarine

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Synthesis of (±)-terpendole E.

Takaaki Teranishi¹, Tetsuro Murokawa, Masaru Enomoto

¹a Laboratory of Applied Bioorganic Chemistry, Graduate School of Agricultural Science , Tohoku University , Sendai , Japan.

Bioscience, Biotechnology, and Biochemistry

|September 4, 2014

Summary

Researchers report the first synthesis of terpendole E, a mitotic kinesin Eg5 inhibitor. This 13-step process successfully created the racemate, a crucial step for further drug development.

Keywords:

antimitotic indole diterpene kinesin spindle protein terpendole total synthesis

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Published on: March 6, 2019

Area of Science:

Organic Synthesis
Medicinal Chemistry
Cell Biology

Background:

Mitotic kinesin Eg5 is a validated target for cancer therapy.
Terpendole E is a specific inhibitor of Eg5, but its synthesis is challenging.
Development of efficient synthetic routes is crucial for exploring Eg5 inhibitors.

Purpose of the Study:

To achieve the first synthesis of the racemate of terpendole E.
To establish a reliable synthetic pathway for potential pharmaceutical development.
To explore key chemical transformations for constructing complex molecular architectures.

Main Methods:

A 13-step synthetic sequence starting from a tricyclic dihydroxy ketone.
Diastereoselective installation of a C3 quaternary stereocenter using a cyclopropyl ketone intermediate.
Palladium-mediated two-step construction of the indole ring moiety.

Main Results:

Successful synthesis of racemic terpendole E.
Demonstration of diastereoselective control in stereocenter formation.
Efficient construction of the indole core structure.

Conclusions:

The developed 13-step synthesis provides a viable route to terpendole E racemate.
Key transformations enable the construction of complex molecules with quaternary stereocenters and indole moieties.
This synthesis is a foundational step for further investigation of terpendole E as an Eg5 inhibitor.