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Related Concept Videos

Photochemical Electrocyclic Reactions: Stereochemistry01:26

Photochemical Electrocyclic Reactions: Stereochemistry

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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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Thermal Electrocyclic Reactions: Stereochemistry01:17

Thermal Electrocyclic Reactions: Stereochemistry

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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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Diels–Alder Reaction Forming Cyclic Products: Stereochemistry01:28

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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Woodward–Hoffmann Selection Rules and Microscopic Reversibility01:34

Woodward–Hoffmann Selection Rules and Microscopic Reversibility

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Electrocyclic reactions, cycloadditions, and sigmatropic rearrangements are concerted pericyclic reactions that proceed via a cyclic transition state. These reactions are stereospecific and regioselective. The stereochemistry of the products depends on the symmetry characteristics of the interacting orbitals and the reaction conditions. Accordingly, pericyclic reactions are classified as either symmetry-allowed or symmetry-forbidden. Woodward and Hoffmann presented the selection criteria for...
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Thermal and Photochemical Electrocyclic Reactions: Overview01:26

Thermal and Photochemical Electrocyclic Reactions: Overview

2.8K
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.
2.8K
Reduction of Alkenes: Asymmetric Catalytic Hydrogenation02:17

Reduction of Alkenes: Asymmetric Catalytic Hydrogenation

3.7K
Catalytic hydrogenation of alkenes is a transition-metal catalyzed reduction of the double bond using molecular hydrogen to give alkanes. The mode of hydrogen addition follows syn stereochemistry.
The metal catalyst used can be either heterogeneous or homogeneous. When hydrogenation of an alkene generates a chiral center, a pair of enantiomeric products is expected to form. However, an enantiomeric excess of one of the products can be facilitated using an enantioselective reaction or an...
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Old yellow enzymes: structures and structure-guided engineering for stereocomplementary bioreduction.

Qinghua Shi1, Huibin Wang2, Junling Liu3

  • 1School of Life Sciences and Biopharmaceutical Sciences, Shenyang Pharmaceutical University, 103 Wenhua Road, Shenhe, Shenyang, 110016, People's Republic of China.

Applied Microbiology and Biotechnology
|August 25, 2020
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Summary

Old yellow enzymes (OYEs) enable valuable asymmetric reductions of alkenes. Protein engineering and structure analysis are key to controlling OYE stereoselectivity for novel biocatalytic applications.

Keywords:
BiocatalysisBiocatalytic reductionOld yellow enzymesProtein engineeringStereopreference switch

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Area of Science:

  • Biocatalysis and Organic Synthesis
  • Enzyme Engineering
  • Stereoselective Reactions

Background:

  • Old yellow enzymes (OYEs) are crucial biocatalysts for asymmetric alkene reduction.
  • Mining novel OYEs and protein engineering enhance their application potential.
  • Understanding OYE structure, active sites, and substrate recognition is vital for specificity.

Purpose of the Study:

  • To review OYE structures, active sites, and substrate recognition.
  • To analyze the OYE superfamily using sequence similarity networks.
  • To discuss structure-guided engineering for switching OYE stereoselectivity.

Main Methods:

  • Comparative analysis of OYE structures and active sites.
  • Construction and annotation of an OYE superfamily sequence similarity network.
  • Review of structure-guided engineering strategies for stereoselectivity control.

Main Results:

  • OYE substrate specificity and stereospecificity are linked to structural features.
  • Sequence similarity networks reveal the scope of characterized OYEs.
  • Structure-guided engineering successfully switched OYE stereoselectivity in several cases.

Conclusions:

  • OYE engineering offers a powerful approach to achieve stereocomplementary bioreductions.
  • Insights into OYE mechanisms guide the discovery and design of new biocatalysts.
  • This review highlights advancements in OYE engineering for synthetic chemistry applications.