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Related Concept Videos

Cycloaddition Reactions: Overview01:16

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 Activation01:16

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.
Cycloaddition Reactions: MO Requirements for Photochemical Activation01:12

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

[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.
Thermal and Photochemical Electrocyclic Reactions: Overview01:26

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: Stereochemistry01:26

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

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Efficient Construction of Drug-like Bispirocyclic Scaffolds Via Organocatalytic Cycloadditions of &#945;-Imino &#947;-Lactones and Alkylidene Pyrazolones
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Efficient Construction of Drug-like Bispirocyclic Scaffolds Via Organocatalytic Cycloadditions of α-Imino γ-Lactones and Alkylidene Pyrazolones

Published on: February 7, 2019

The double [3 + 2] photocycloaddition reaction.

Clive S Penkett1, Jason A Woolford, Iain J Day

  • 1Department of Chemistry and Biochemistry, University of Sussex, Brighton BN1 9QJ, UK. c.s.penkett@sussex.ac.uk

Journal of the American Chemical Society
|December 17, 2009
PubMed
Summary

A novel double [3 + 2] photocycloaddition reaction creates complex fenestrane structures in one step. This efficient synthesis forms multiple rings and stereocenters, advancing organic synthesis methodologies.

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Cercosporin-Photocatalyzed [4+1]- and [4+2]-Annulations of Azoalkenes Under Mild Conditions
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Cercosporin-Photocatalyzed [4+1]- and [4+2]-Annulations of Azoalkenes Under Mild Conditions

Published on: July 17, 2020

Area of Science:

  • Organic Chemistry
  • Photochemistry
  • Synthetic Methodology

Background:

  • Photocycloaddition reactions are crucial for constructing complex molecular architectures.
  • Fenestranes represent a unique class of polycyclic compounds with strained ring systems.
  • Developing efficient synthetic routes to fenestranes remains a significant challenge in organic chemistry.

Purpose of the Study:

  • To report a novel double [3 + 2] photocycloaddition reaction for fenestrane synthesis.
  • To demonstrate the formation of multiple carbon-carbon bonds, rings, and stereocenters in a single synthetic operation.
  • To explore the mechanistic pathways of the photocycloaddition and subsequent photoreactions.

Main Methods:

  • Photochemical reactions utilizing aromatic acetals as starting materials.
  • Spectroscopic analysis (NMR, Mass Spectrometry) for structural elucidation.
  • Mechanistic studies involving isolation and characterization of reaction intermediates.

Main Results:

  • A remarkable double [3 + 2] photocycloaddition reaction successfully synthesized fenestrane 2 from aromatic acetal 1.
  • The one-pot process generated four new carbon-carbon bonds, five new rings, and seven new stereocenters.
  • Sequential reaction pathways from linear meta photocycloadduct 3 were observed, alongside an alternative fragmentation-translocation photoreaction of angular meta photocycloadduct 4 yielding tricycle 6.

Conclusions:

  • The reported photocycloaddition reaction provides an efficient and stereoselective route to complex fenestrane derivatives.
  • This methodology offers a powerful tool for constructing intricate polycyclic frameworks with high atom and step economy.
  • Understanding the competing reaction pathways enhances the control and predictability of photochemical transformations.