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SN2 Reaction: Stereochemistry02:23

SN2 Reaction: Stereochemistry

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In an SN2 reaction, the nucleophilic attack on the substrate and departure of the leaving group occurs simultaneously through a transition state. As the nucleophile approaches the substrate from the back-side, the configuration of the substrate carbon changes from tetrahedral to trigonal bipyramidal and then back to tetrahedral, leading to an inversion in the configuration of the product.
If the substrate is an achiral molecule at the α-carbon, the inversion of configuration is not...
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This lesson provides an in-depth discussion of the stereochemical outcomes in an SN1 reaction.
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In the formed carbocation, the positively charged carbon is sp2 hybridized with a trigonal planar geometry. As all the three substituents lie on the same plane, a plane of symmetry for the...
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Ketones with α protons are deprotonated by strong bases like lithium diisopropylamide (LDA) to form enolate ions. The anion is stabilized by resonance, and its hybrid structure exhibits negative charges on the carbonyl oxygen and the α carbon. This ambident nucleophile can attack an electrophile via two possible sites: the carbonyl oxygen, known as O-attack, or the α carbon, known as C-attack. The nucleophilic attack via the carbanionic site is preferred. This is due to the...
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The conversion of allylic alcohols into epoxides using the chiral catalyst was discovered by K. Barry Sharpless and is known as Sharpless epoxidation. The use of a chiral catalyst enables the formation of one enantiomer of the product in excess. This chiral catalyst is mainly a chiral complex of titanium tetraisopropoxide and tartrate ester (specific stereoisomer). The stereoisomer used in the chiral catalyst dictates the formation of the enantiomer of the product. In other words, the use of...
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In an SN2 reaction, the reaction rate depends on both the type of nucleophile and the substrate. A hindered tertiary alkyl halide is practically inert to the SN2 mechanism despite using a strong nucleophile.
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The reaction between two different carbonyl compounds comprising α hydrogen in the presence of a strong base like lithium diisopropylamide (LDA) to form a crossed aldol product is known as a directed aldol reaction. The directed aldol reaction is depicted in Figure 1.
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Enzymatic Cascade Reactions for the Synthesis of Chiral Amino Alcohols from L-lysine
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Substrate control in stereoselective lanthionine biosynthesis.

Weixin Tang1, Gonzalo Jiménez-Osés2, K N Houk2

  • 1Department of Chemistry and Howard Hughes Medical Institute, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801, USA.

Nature Chemistry
|December 18, 2014
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Summary

In enzyme-catalyzed reactions, the substrate, not the enzyme, dictated stereoselectivity in lanthipeptide biosynthesis. A single mutation in the peptide substrate reversed the reaction

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Area of Science:

  • Biochemistry
  • Organic Chemistry
  • Enzymology

Background:

  • Enzymes are highly stereoselective catalysts.
  • Lanthipeptides are natural products containing thioether crosslinks.
  • These crosslinks form via cysteine attack on dehydrated amino acid residues.

Purpose of the Study:

  • To investigate the role of substrate in controlling enzyme stereoselectivity during lanthipeptide biosynthesis.
  • To identify the factors governing the face selectivity of Michael-type additions catalyzed by lanthionine synthetases.

Main Methods:

  • Enzyme kinetics and product analysis.
  • Site-directed mutagenesis of peptide substrates.
  • Quantum mechanical calculations.

Main Results:

  • Demonstrated substrate-controlled stereoselectivity in enzyme-catalyzed Michael-type additions.
  • Showed that a single point mutation in the peptide substrate inverted the stereochemical outcome.
  • Quantum mechanical calculations supported the experimental findings.

Conclusions:

  • Substrate conformation, not enzyme active site, dictates stereoselectivity in this system.
  • Amino acid sequence-imposed conformational restraints on transition states control face selectivity.
  • This provides a rare example of substrate-driven stereocontrol in enzymatic catalysis.