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SN2 Reaction: Stereochemistry

S_N2 Reaction: Stereochemistry

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 observed.

Preparation and Reactions of Sulfides

Preparation and Reactions of Sulfides

Sulfides are the sulfur analog of ethers, just as thiols are the sulfur analog of alcohol. Like ethers, sulfides also consist of two hydrocarbon groups bonded to the central sulfur atom. Depending upon the type of groups present, sulfides can be symmetrical or asymmetrical. Symmetrical sulfides can be prepared via an SN2 reaction between 2 equivalents of an alkyl halide and one equivalent of sodium sulfide.

SN1 Reaction: Stereochemistry

S_N1 Reaction: Stereochemistry

This lesson provides an in-depth discussion of the stereochemical outcomes in an SN1 reaction.
In the first step of an SN1 reaction, the bond between the electrophilic carbon and the leaving group ionizes to generate the carbocation intermediate. The second step of the mechanism is the nucleophilic attack.
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...

Preparation of 1° Amines: Gabriel Synthesis

Preparation of 1° Amines: Gabriel Synthesis

Direct alkylation is not a suitable method for synthesizing amines because it produces polyalkylated products. Gabriel synthesis is the most preferred method to exclusively make primary amines. The method uses phthalimide, which contains a protected form of nitrogen that participates in alkylation only once to predominantly give primary amines.
Strong bases like NaOH or KOH deprotonate the phthalimide to form the corresponding anion, which acts as a nucleophile. Further, the anion attacks an...

Chirality at Nitrogen, Phosphorus, and Sulfur

Chirality at Nitrogen, Phosphorus, and Sulfur

Chirality is most prevalent in carbon-based tetrahedral compounds, but this important facet of molecular symmetry extends to sp3-hybridized nitrogen, phosphorus and sulfur centers, including trivalent molecules with lone pairs. Here, the lone pair behaves as a functional group in addition to the other three substituents to form an analogous tetrahedral center that can be chiral.
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Naming Enantiomers

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The naming of enantiomers employs the Cahn–Ingold–Prelog rules that involve assigning priorities to different substituent groups at a chiral center. Each enantiomer, being a distinct molecule, is assigned a unique name by the Cahn–Ingold–Prelog (CIP) rules, also called the R–S system. The prefix R- or S- attached to the chiral centers in an enantiomer is dependent on the spatial arrangement of the four substituents on the chiral center. The R–S system essentially comprises three steps:...

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Related Experiment Video

Updated: Jul 13, 2026

Efficient Construction of Drug-like Bispirocyclic Scaffolds Via Organocatalytic Cycloadditions of α-Imino γ-Lactones and Alkylidene Pyrazolones

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Published on: February 7, 2019

(+)-saxitoxin: a first and second generation stereoselective synthesis.

James J Fleming¹, Matthew D McReynolds, J Du Bois

¹Department of Chemistry, Stanford University, Stanford, CA 94305-0080, USA.

Journal of the American Chemical Society

|July 31, 2007

Summary

Researchers developed a stereoselective synthesis for (+)-saxitoxin (STX), a potent neurotoxin linked to paralytic shellfish poisoning. This novel approach utilizes a unique nine-membered ring guanidine intermediate, enabling efficient construction of the complex STX structure.

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Area of Science:

Organic Chemistry
Natural Product Synthesis
Toxicology

Background:

Saxitoxin (STX) is a potent neurotoxin responsible for paralytic shellfish poisoning.
The complex structure of STX presents significant challenges for chemical synthesis.

Purpose of the Study:

To develop a novel and efficient stereoselective synthesis of (+)-saxitoxin.
To explore new synthetic methodologies for constructing complex guanidine-containing natural products.

Main Methods:

The synthesis features an unusual nine-membered ring guanidine intermediate.
Key steps include Rh-catalyzed C-H amination and stereoselective acetylide dianion addition.
A four-electron alkene oxidation catalyzed by OsCl3 was employed.

Main Results:

The synthesis successfully produced (+)-saxitoxin.
The nine-membered ring intermediate was efficiently converted to the tricyclic STX skeleton in four steps.
Two distinct routes were established for constructing the monocyclic guanidine intermediate.
The overall synthesis achieved 14 linear steps from commercial materials.

Conclusions:

The developed synthetic strategy is effective for accessing (+)-saxitoxin.
The study introduces novel methods for synthesizing complex heterocyclic compounds.
This work provides a valuable route for obtaining STX for further research.