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Regioselectivity of Electrophilic Additions to Alkenes: Markovnikov's Rule02:17

Regioselectivity of Electrophilic Additions to Alkenes: Markovnikov's Rule

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If a set of reactants can yield multiple constitutional isomers, but one of the isomers is obtained as the major product, the reaction is said to be regioselective. In such reactions, bond formation or breaking is favored at one reaction site over others.
The hydrohalogenation of an unsymmetrical alkene can yield two haloalkane products, depending on which vinylic carbon takes up the halogen. However, one product usually predominates, where hydrogen adds to the vinylic carbon bearing the...
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Regioselectivity of Electrophilic Additions-Peroxide Effect02:35

Regioselectivity of Electrophilic Additions-Peroxide Effect

10.3K
In the presence of organic peroxides, the addition of hydrogen bromide to an alkene yields the isomer that is not predicted by Markovnikov’s rule. For example, the addition of hydrogen bromide to 2-methylpropene in the presence of peroxides gives 1-bromo-2-methylpropane. This addition reaction proceeds via a free radical mechanism, which reverses the regioselectivity. The free radical reaction mechanism involves three stages: initiation, propagation, and termination.
10.3K
Regioselectivity and Stereochemistry of Hydroboration02:36

Regioselectivity and Stereochemistry of Hydroboration

9.4K
A significant aspect of hydroboration–oxidation is the regio- and stereochemical outcome of the reaction.
Hydroboration proceeds in a concerted fashion with the attack of borane on the π bond, giving a cyclic four-centered transition state. The –BH2 group is bonded to the less substituted carbon and –H to the more substituted carbon. The concerted nature requires the simultaneous addition of –H and –BH2 across the same face of the alkene giving syn stereochemistry.
9.4K
Sharpless Epoxidation02:57

Sharpless Epoxidation

5.0K
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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Acid-Catalyzed Ring-Opening of Epoxides02:24

Acid-Catalyzed Ring-Opening of Epoxides

8.7K
Epoxides that are three-membered ring systems are more reactive than other cyclic and acyclic ethers. The high reactivity of epoxides originates from the strain present in the ring. This ring strain acts as a driving force for epoxides to undergo ring-opening reactions either with halogen acids or weak nucleophiles in the presence of mild acid. The acid catalyst converts the epoxide oxygen, a poor leaving group, into an oxonium ion, a better leaving group, making the reaction feasible. The...
8.7K
Base-Catalyzed Ring-Opening of Epoxides02:26

Base-Catalyzed Ring-Opening of Epoxides

10.0K
Due to their highly strained structures, epoxides can readily undergo ring-opening reactions through nucleophilic substitution, either in the presence of an acid or a base. The nucleophilic substitution reactions in the presence of acid are called acid-catalyzed ring-opening reactions, and nucleophilic substitution reactions in the presence of a base are called base-catalyzed ring-opening reactions. Epoxides undergo base-catalyzed ring-opening reactions in the presence of a strong nucleophile...
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Functionalization and Dispersion of Carbon Nanomaterials Using an Environmentally Friendly Ultrasonicated Ozonolysis Process
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Unexpected Regiochemical Control in the Nugent-RajanBabu Reductive Epoxide Cyclization.

Nantamon Supantanapong1,2, Scott W Niman1, Christopher D Vanderwal1,3

  • 1Department of Chemistry, 1102 Natural Sciences II, University of California, Irvine, CA 92697, USA.

Synthesis
|September 25, 2025
PubMed
Summary

The Nugent-RajanBabu reductive epoxide cyclization offers new control over reaction pathways. Protecting group size dictates whether 6-endo or 5-exo cyclizations occur, enabling novel syntheses.

Keywords:
cyclizationsepoxideradicalregiocontrolterpenoidstotal synthesis

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Area of Science:

  • Organic Chemistry
  • Synthetic Methodology
  • Natural Product Synthesis

Background:

  • The Nugent-RajanBabu reductive epoxide cyclization is a key alternative to classical cationic polyene cyclizations.
  • This method typically exhibits exquisite regiochemical control, favoring 6-endo cyclization even against potential 5-exo pathways.
  • Applications include the synthesis of complex decalins and perhydrophenanthrenes.

Purpose of the Study:

  • To investigate the behavior of α-alkoxy epoxides in Nugent-RajanBabu reductive epoxide cyclizations.
  • To uncover a method for controlling the regioselectivity between 6-endo and 5-exo cyclization pathways.
  • To explore new synthetic routes toward complex, polyoxygenated terpenoid natural products and related structures.

Main Methods:

  • Evaluation of α-alkoxy epoxides in the Nugent-RajanBabu reductive epoxide cyclization.
  • Systematic variation of protecting group size on the α-oxygen substituent.
  • Analysis of reaction products to determine regiochemical outcomes (6-endo vs. 5-exo).

Main Results:

  • A novel method for controlling regioselectivity was discovered, dependent on the size of the α-oxygen protecting group.
  • Specific protecting group sizes were found to favor either the 6-endo or 5-exo cyclization pathway.
  • This control allows for predictable formation of different cyclic scaffolds.

Conclusions:

  • The size of the protecting group on α-alkoxy epoxides is a critical determinant of regioselectivity in Nugent-RajanBabu cyclizations.
  • This finding enables the targeted synthesis of highly oxygenated cyclopentane systems, expanding the utility of this reaction.
  • The methodology provides a new avenue for the rapid construction of complex molecular architectures relevant to natural product synthesis.