Are aldol reactions stereoselective?
Are aldol reactions stereoselective?
Stereoselective Aldol Condensation using Boron Enolates The next major advance in controlling aldol stereochemistry came with the discovery that boron enolates could usually be formed with high stereocontrol even with unhindered ketones, and equally important, the aldol reactions were often highly stereopecific.
What are the limitations of aldol condensation?
The aldol condensation is not limited to aldehydes. Ketones may also be used. In fact, it is possible to use two different carbonyl compounds to form a crossed aldol product. One limitation of the crossed aldol reaction is the fact that multiple products can form due to the presence of multiple different α-hydrogen.
What is the disadvantage of aldol reaction?
The reaction conditions used for the aldol condensation of 3-pentanone described above are rather harsh, and this is a disadvantage if elimination of water from the aldol product occurs, or if self-condensation occurs under these equilibrating conditions.
What is asymmetric aldol reaction?
Asymmetric aldol reactions are a powerful method for the construction of carbon – carbon bonds in an enantioselective fashion. New methods have improved the reactivity, selectivity and substrate scope of the direct aldol reaction and enabled the synthesis of complex molecular targets (357 references).
Why mixed aldol condensation of aliphatic aldehyde is not useful?
Between an aldehyde and a ketone Also known as the Claisen-Schmidt reaction, it is typically practicable due to the fact that aldehydes are stronger enolates compared to ketones. It is essential to use preformed enolates while using unsymmetrical ketones to insure regioselectivity.
How are mixed aldol reactions avoided?
Mixed Aldol Reaction and Condensations The product in such cases is always a dimer of the reactant carbonyl compound. To avoid complex reaction mixtures the reactants should be chosen to favor one particular donor/acceptor reaction. Successful mixed aldol reactions usually use one of two combination of factors.
What is a mixed aldol reaction?
A mixed Aldol reaction means that the enolate and the carbonyl are not from the same aldehyde or ketone. The most useful mixed Aldols are those where: Only one reactent can form an enolate. Recall that in general, aldehydes are more reactive (electrophilic) than ketones and will usually be the electrophile.
What is aldol example?
Aldol structural units are found in many important molecules, whether naturally occurring or synthetic. For example, the aldol reaction has been used in the large-scale production of the commodity chemical pentaerythritol and the synthesis of the heart disease drug Lipitor (atorvastatin, calcium salt).
What is Mukaiyama aldol reaction?
A directed cross-aldol reaction of silyl enol ethers with carbonyl compounds, such as aldehydes and ketones, promoted by a Lewis acid, a reaction which is now widely known as the Mukaiyama aldol reaction. It was first reported in 1973, and this year marks the 40th anniversary.
What are the applications of Mukaiyama reaction?
The Mukaiyama reaction is extremely useful on a lab scale and has been used in numerous complex molecule syntheses. The reaction usually requires only a catalytic amount of Lewis acid activator. Lewis bases such as fluoride anion can also work as the activator of silyl enol ethers.
What is evevans aldol reaction?
EVANS Asymmetric Aldol. Also known as Evans–Mukaiyama. Asymmetric Mukaiyama aldol reaction by addition of silyl enol derivatives 2 to aldehydes 3 catalyzed by chiral bisoxazoline (Box) catalysts like 1, 5. Compare with Chan–Brassard.
What is the action of trimethylsilyl on aldol?
The trimethylsilyl group is thought of as a sterically demanding hydrogen equivalent that activates the enol and traps the aldol hydroxyl. The open transition state is preferred, but the outcome of the reaction ( syn/anti) depends on the size of substituents and on the Lewis acid.
