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Related Concept Videos

Five-Membered Heterocyclic Aromatic Compounds: Overview01:13

Five-Membered Heterocyclic Aromatic Compounds: Overview

Heterocyclic aromatic compounds are cyclic compounds that are aromatic and have one or more heteroatoms—atoms other than carbon, in the ring. Depending upon the number of atoms present in the ring, they can be either five or six-membered. Examples of five-membered heterocyclic aromatic compounds include pyrrole, furan, thiophene, and imidazole. Pyrrole consists of one nitrogen atom having one lone pair of electrons. Furan and thiophene have one oxygen and one sulfur heteroatom, respectively.
Criteria for Aromaticity and the Hückel 4n + 2 Rule01:20

Criteria for Aromaticity and the Hückel 4n + 2 Rule

Like benzene, cyclobutadiene and cyclooctatetraene are cyclic compounds with alternate single and double bonds. However, their chemical behavior differs from benzene, as they are unstable and not aromatic. So, what are the structural characteristics of unsaturated compounds categorized as aromatic?
For the first time, Eric Hückel, a German chemical physicist, derived a set of structural features for a compound to be classified as aromatic. This is now known as Hückel’s rule or the 4n + 2 rule.
Cycloaddition Reactions: Overview01:16

Cycloaddition Reactions: Overview

Cycloadditions are one of the most valuable and effective synthesis routes to form cyclic compounds. These are concerted pericyclic reactions between two unsaturated compounds resulting in a cyclic product with two new σ bonds formed at the expense of π bonds. The [4 + 2] cycloaddition, known as the Diels–Alder reaction, is the most common. The other example is a [2 + 2] cycloaddition.
Aromatic Hydrocarbon Cations: Structural Overview01:18

Aromatic Hydrocarbon Cations: Structural Overview

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.
Removing one hydrogen from the intervening CH2 group with both...
Pericyclic Reactions: Introduction01:17

Pericyclic Reactions: Introduction

Pericyclic reactions are organic reactions that occur via a concerted mechanism without generating any intermediates. The reactions proceed through the movement of electrons in a closed loop to form a cyclic transition state, where rearrangement of the σ and π bonds yields specific products.
Pericyclic reactions can be classified into three categories: electrocyclic reactions, cycloaddition reactions, and sigmatropic rearrangements. Electrocyclic reactions and sigmatropic rearrangements are...
Frost Circles for Different Conjugated Systems01:18

Frost Circles for Different Conjugated Systems

The inscribed polygon method is consistent with Hückel’s 4n + 2 rule and helps to learn whether the given cyclic compound is aromatic or not. The compound is stable and aromatic if every bonding molecular orbital (MO) is completely filled with a pair of electrons. However, if the non-bonding or antibonding orbitals are filled with electrons, the compound is unstable and not aromatic. Consider the Frost circle diagrams for cycloalkenes containing 4 to 8 carbons.

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Related Experiment Video

Updated: Jun 5, 2026

Solid-phase Synthesis of [4.4] Spirocyclic Oximes
05:15

Solid-phase Synthesis of [4.4] Spirocyclic Oximes

Published on: February 6, 2019

Polycyclic compounds by Ugi-Pictet-Spengler sequence.

Wei Wang1, Sarah Ollio, Eberhardt Herdtweck

  • 1Drug Discovery Institute, University of Pittsburgh, Biomedical Science Tower 3, 3501 Fifth Avenue, Pittsburgh, Pennsylvania 15261, United States.

The Journal of Organic Chemistry
|December 31, 2010
PubMed
Summary

This study introduces a novel two-step synthesis for complex polycyclic structures using Ugi multicomponent reactions and Pictet-Spengler cyclizations. The method efficiently produces compounds with potential applications as biological probes.

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

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Cercosporin-Photocatalyzed [4+1]- and [4+2]-Annulations of Azoalkenes Under Mild Conditions
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Cercosporin-Photocatalyzed [4+1]- and [4+2]-Annulations of Azoalkenes Under Mild Conditions

Published on: July 17, 2020

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Last Updated: Jun 5, 2026

Solid-phase Synthesis of [4.4] Spirocyclic Oximes
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Solid-phase Synthesis of [4.4] Spirocyclic Oximes

Published on: February 6, 2019

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

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Cercosporin-Photocatalyzed [4+1]- and [4+2]-Annulations of Azoalkenes Under Mild Conditions
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Cercosporin-Photocatalyzed [4+1]- and [4+2]-Annulations of Azoalkenes Under Mild Conditions

Published on: July 17, 2020

Area of Science:

  • Organic Chemistry
  • Medicinal Chemistry
  • Synthetic Chemistry

Background:

  • Polycyclic structures are prevalent in natural products like steroid hormones and alkaloids.
  • Developing efficient synthetic routes to these complex architectures remains a key challenge in organic chemistry.

Purpose of the Study:

  • To establish a general and concise synthetic strategy for architecturally stimulating polycyclic compounds.
  • To explore the utility of combining Ugi multicomponent reactions with Pictet-Spengler reactions for novel compound synthesis.

Main Methods:

  • A two-step synthetic procedure was employed, beginning with a Ugi multicomponent reaction (MCR).
  • The intermediate products underwent a subsequent Pictet-Spengler reaction, starting from phenylethylamine-derived isocyanides.
  • Ten novel compounds were synthesized and characterized with full experimental details.

Main Results:

  • The synthetic approach yielded ten compounds with moderate to excellent efficiency.
  • Several reactions demonstrated a high degree of stereoselectivity, producing specific stereoisomers.
  • The resulting polycyclic structures bear resemblance to known steroid hormones and alkaloid natural products.

Conclusions:

  • The described two-step sequence provides a versatile and efficient method for constructing complex polycyclic frameworks.
  • The synthesized compounds, particularly after reduction to tertiary amines, show promise as potential biological probes for further investigation.