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Aromatic Hydrocarbon Cations: Structural Overview

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...

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

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.

Cycloaddition Reactions: MO Requirements for Photochemical Activation

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.

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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[DPEPhosbcpCu]PF6: A General and Broadly Applicable Copper-Based Photoredox Catalyst

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Oxidative cyclization reagents reveal tryptophan cation-π interactions.

Xiao Xie^1,2,3, Patrick J Moon¹, Steven W M Crossley¹

¹Department of Chemistry, University of California, Berkeley, Berkeley, CA, USA.

|March 6, 2024

Summary

Researchers developed a new method for selectively modifying tryptophan, the rarest amino acid, on proteins. This chemical ligation by cyclization (Trp-CLiC) technique offers efficient protein labeling and reveals new insights into protein interactions.

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Published on: August 12, 2019

Area of Science:

Biochemistry
Chemical Biology
Proteomics

Background:

Selective covalent modification of amino acids is crucial for protein studies.
Cysteine and lysine are typically targeted due to their reactivity.
Tryptophan, a rare amino acid, remains challenging to modify selectively.

Purpose of the Study:

To develop a novel redox-based strategy for selective tryptophan bioconjugation.
To enable efficient and specific labeling of tryptophan residues on peptides and proteins.
To profile hyper-reactive tryptophan sites and their functional roles in proteomes.

Main Methods:

Utilized oxaziridine reagents mimicking natural oxidative cyclization reactions.
Developed a method termed tryptophan chemical ligation by cyclization (Trp-CLiC).
Applied Trp-CLiC for selective payload attachment to tryptophan residues.

Main Results:

Achieved highly efficient and specific tryptophan labeling comparable to click reactions.
Enabled global profiling of hyper-reactive tryptophan sites across entire proteomes.
Identified tryptophan residues involved in cation-π interactions and protein phase separation.

Conclusions:

Trp-CLiC provides a powerful new tool for chemical biology and proteomics.
The method allows for precise manipulation and study of tryptophan residues.
This approach offers insights into protein function, particularly cation-π interactions and phase separation.