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Related Concept Videos

Thermal Electrocyclic Reactions: Stereochemistry01:17

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.
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.
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.
Aromatic Hydrocarbon Cations: Structural Overview01:18

Aromatic Hydrocarbon Cations: Structural Overview

Cycloheptatriene is a neutral monocyclic unsaturated hydrocarbon that consists of an odd number of carbon atoms and an intervening sp3 carbon in the ring. The three double bonds in the ring correspond to 6 π electrons, which is a Huckel number, and therefore satisfies the criteria of 4n + 2 π electrons. However, the intervening sp3 carbon disrupts the continuous overlap of p orbitals. As a result, cycloheptatriene is not aromatic.
Removing one hydrogen from the intervening CH2 group with both...
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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Related Experiment Video

Updated: Jun 14, 2026

A Two-Step Protocol for Umpolung Functionalization of Ketones Via Enolonium Species
08:12

A Two-Step Protocol for Umpolung Functionalization of Ketones Via Enolonium Species

Published on: August 16, 2018

Enantioselective thiourea-catalyzed cationic polycyclizations.

Robert R Knowles1, Song Lin, Eric N Jacobsen

  • 1Department of Chemistry & Chemical Biology, Harvard University, Cambridge, Massachusetts 02138, USA.

Journal of the American Chemical Society
|April 8, 2010
PubMed
Summary

A novel thiourea catalyst enables enantioselective cationic polycyclization of hydroxylactams. Catalyst performance, including yield and enantioselectivity, is optimized by specific aromatic groups that promote cation-pi interactions.

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Cercosporin-Photocatalyzed [4+1]- and [4+2]-Annulations of Azoalkenes Under Mild Conditions
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Chemoselective Preparation of 1-Iodoalkynes, 1,2-Diiodoalkenes, and 1,1,2-Triiodoalkenes Based on the Oxidative Iodination of Terminal Alkynes
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Published on: July 17, 2020

Area of Science:

  • Organic Chemistry
  • Catalysis
  • Polymer Science

Background:

  • Enantioselective synthesis is crucial for pharmaceuticals and fine chemicals.
  • Hydroxylactam cyclization offers a pathway to complex molecular architectures.
  • Developing efficient catalysts for these transformations remains an active research area.

Purpose of the Study:

  • To report a new thiourea catalyst for enantioselective cationic polycyclization of hydroxylactams.
  • To investigate the influence of catalyst structure on reaction outcomes.
  • To elucidate the mechanism underlying the observed enantioselectivity.

Main Methods:

  • Synthesis of a novel thiourea catalyst framework.
  • Enantioselective cationic polycyclization reactions using the developed catalyst.
  • Systematic variation of aromatic substituents on the catalyst.
  • Analysis of reaction products for yield and enantiomeric excess (ee).
  • Mechanistic studies to probe catalyst-substrate interactions.

Main Results:

  • The thiourea catalyst successfully mediated the enantioselective cationic polycyclization of hydroxylactams.
  • Both yield and enantioselectivity were highly sensitive to the aromatic residue on the catalyst.
  • Expansive and polarizable arenes on the catalyst framework yielded optimal results.
  • Evidence supports a mechanism involving cation-pi interactions as key to enantioselectivity.

Conclusions:

  • A new class of thiourea catalysts has been developed for enantioselective hydroxylactam polycyclization.
  • Catalyst design, specifically the nature of aromatic substituents, is critical for optimizing enantioselectivity.
  • Cation-pi interactions play a significant role in stabilizing intermediates and directing stereochemical outcomes.