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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.
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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.
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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.
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The cationic polymerization mechanism consists of three steps: initiation, propagation, and termination. In the initiation step of the polymerization process, the π bond of a monomer gets protonated by the Lewis acid catalyst, which is formed from boron trifluoride and water. The protonation of the π bond generates a carbocation stabilized by the electron‐donating group. In the propagation step, the π bond of the second monomer acts as a nucleophile and attacks the...
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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.
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The Cope rearrangement is classified as a [3,3] sigmatropic shift in 1,5-dienes, leading to a more stable, isomeric 1,5-diene. The reaction involves a concerted movement of six electrons, four from two π bonds and two from a σ bond, via an energetically favorable chair-like transition state.
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Terpene cyclization catalysed inside a self-assembled cavity.

Q Zhang1, K Tiefenbacher1

  • 1Department Chemie, Technische Universität München, Lichtenbergstraße 4, D-85747 Garching, Germany.

Nature Chemistry
|February 21, 2015
PubMed
Summary
This summary is machine-generated.

Researchers achieved the first tail-to-head terpene (THT) cyclization within a supramolecular structure, mimicking enzyme catalysis. This breakthrough offers new insights into terpene cyclase mechanisms and enables controlled synthesis of complex natural products.

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Area of Science:

  • Organic Chemistry
  • Supramolecular Chemistry
  • Biochemistry

Background:

  • Terpene natural products are vital in nature, synthesized via tail-to-head terpene (THT) cyclization.
  • Enzymes facilitate THT cyclization by stabilizing cationic intermediates within their active sites.
  • Achieving selective THT cyclizations using synthetic catalysts in solution remains a significant challenge.

Purpose of the Study:

  • To report the first successful THT cyclization within a synthetic supramolecular structure.
  • To mimic the catalytic environment of terpene cyclase enzymes.
  • To develop a novel catalytic system for selective THT cyclizations.

Main Methods:

  • Utilized a supramolecular structure to encapsulate and promote the THT cyclization reaction.
  • Employed geranyl acetate as the substrate for the catalytic cyclization.
  • Investigated the reaction mechanism and intermediate stabilization within the supramolecular assembly.

Main Results:

  • Demonstrated the first catalytic non-stop THT cyclization within a supramolecular environment.
  • Successfully mimicked the stabilizing effect of enzyme pockets on cationic intermediates.
  • Provided evidence for the feasibility of direct geranyl cation isomerization to the cisoid isomer.

Conclusions:

  • Supramolecular chemistry offers a viable platform for mimicking and controlling complex enzymatic reactions like THT cyclization.
  • The findings challenge previous assumptions about the terpene cyclization mechanism, particularly regarding cation isomerization.
  • This work opens new avenues for the synthetic and catalytic production of terpene natural products.