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エナミンの光化学E/Zイソメリゼーションによる脱セミ化

  • 0Center of Basic Molecular Science, Department of Chemistry, Tsinghua University, Beijing 100084, China.
Clinical Neuroscience (new York, N.y.) +

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2022年2月24日

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Stereochemical Effects of Enolization 01:12

2.2K

The chiral α-carbon of the carbonyl compound is the stereocenter of the molecule. As shown in the figure below, when such a carbonyl compound undergoes racemization under an acidic or basic condition, an achiral enol is formed.

Under an acidic medium, an oxygen atom of the carbonyl group is protonated with simultaneous removal of the α-hydrogen, giving an enol.
Alternatively, in the basic medium, the removal of α-hydrogen generates a resonance-stabilized enolate. Next, the...

Photochemical Electrocyclic Reactions: Stereochemistry 01:26

1.9K

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.
Selection Rules: Photochemical Activation

Dehydration of Aldols to Enals: Base-Catalyzed Aldol Condensation 01:14

5.8K

This lesson delves into the aldol condensation catalyzed by bases, where aldols undergo dehydration to enals. As shown in Figure 1, the β-hydroxy aldehyde formed in a base-catalyzed aldol addition reaction dehydrates on heating to yield an unsaturated carbonyl product, which is commonly referred to as an enal.

Figure 1. The dehydration of a β-hydroxy aldehyde to an enal.
This process follows a two-step mechanism, as depicted in Figure 2. Here, the base first deprotonates the mildly acidic...

Synthesis of α-Substituted Carbonyl Compounds: The Stork Enamine Reaction 01:26

3.5K

α-Substituted ketones or aldehydes can be synthesized from enamines by the Stork enamine reaction, named after its pioneer Gilbert Stork. Enamines are useful synthetic intermediates where the lone pair on nitrogen is in conjugation with the C=C bond. They resemble enolate ions, as the resonance forms of both species have a nucleophilic α carbon.

However, enamines are neutral and less reactive than enolates, which bear a net negative charge. Consequently, enamines are effective Michael donors...

Reactivity of Enols 01:18

3.3K

Enols are a class of compounds where a hydroxyl group is attached to a carbon–carbon double bond, which implies that it is a vinyl alcohol. A carbonyl compound with an α hydrogen undergoes keto–enol tautomerism and remains in equilibrium with its tautomer, the enol form. Usually, the keto tautomer is present in a higher concentration than the enol tautomer due to the higher bond energy of C=O compared to C=C. Moreover, the direction of the keto–enol equilibrium is...

Aldehydes and Ketones with Amines: Enamine Formation Mechanism 01:14

6.0K

Enamine formation involves the addition of carbonyl compounds to a secondary amine through a series of reactions. The mechanism begins with the generation of carbinolamine, a nucleophilic attack followed by several proton transfer reactions. The hydroxyl group of the carbinolamine is converted into water to make a better leaving group that can push the reaction forward by eliminating a water molecule. In enamine formation, the last step involves the abstraction of a proton from the α carbon to...