What is Diastereoselective reaction?
A diastereoselective reaction is one in which one diastereomer is formed in preference to another (or in which a subset of all possible diastereomers dominates the product mixture), establishing a preferred relative stereochemistry.
What is Zimmerman Traxler model?
Zimmerman–Traxler model In 1957, Howard Zimmerman and Marjorie D. Traxler proposed that some aldol reactions have “six-membered transition states having a chair conformation.” This is now known as the Zimmerman–Traxler model. E-enolates give rise to anti products, whereas Z-enolates give rise to syn products.
What is Zimmerman Traxler transition state?
Zimmerman-Traxler’s seminal publication on this model, however, fails to show the most stable transition state to rationalize the reaction outcome. The transition state used in the paper had a clear unfavorable 1,3-pseudo-diaxial interaction (shown in red circles).
Are aldol reactions stereoselective?
A simple stereoselective process for the synthesis of highly substituted γ-lactones was developed based on aldol reactions between the enolate of dioxanes derived from tartaric acid and aldehydes. A range of aromatic and aliphatic aldehydes were reacted, in most cases achieving good yields and stereoselectivity.
What is the difference between Diastereoselective and enantioselective reactions?
A stereoselective process is one in which one stereoisomer predominates over another when two or more may be formed. If the products are enantiomers, the reaction is enantioselective; if they are diastereoisomers, the reaction is diastereoselective.
What is regioselectivity and stereoselectivity?
Regioselectivity is when the two possible products in the reaction are regioisomers (also called constitutional isomers) Stereoselectivity is when the two possible products in the reaction are stereoisomers. Chemoselectivity is when the reactants will prefer one functional group over another in the substrate.
What happens when aldols readily lose water?
Beta hydroxy aldehydes, also known as aldols, are known to form beta-unsaturated carbonyl compound when they lose water molecules. These products are a part of Aldol condensation. Was this answer helpful?
Who discovered aldol reaction?
The aldol reaction, first discovered by Wurtz in 1872,1 is one of the most powerful transformations in organic chemistry. The process unites two carbonyl partners to give β-hydroxyketones with up to two new stereocenters (Scheme 1).
How many aldol products are formed?
Complete answer: There are four possible products of aldol condensation between acetaldehyde and propanal. The aldol of acetaldehyde with itself will be one of the products. Similarly, one will be propanal with propanal.
What is Stereoselectivity and stereospecificity?
A stereospecific mechanism specifies the stereochemical outcome of a given reactant, whereas a stereoselective reaction selects products from those made available by the same, non-specific mechanism acting on a given reactant.
How do you know if a reaction is stereoselective or regioselective?
How many types of aldols are formed in the following reaction?
There are 2 possible enolate ions formed from 2-butanone which attacks on propionaldehyde, to form 4-hydroxy-3-methylhexan-2-one which has 2 chiral carbons, and 5-hydroxyheptan-3-one which has 1 chiral carbon. So there are 6 total possible products.
Why is the presence of alpha hydrogen necessary for aldol condensation?
(A) Presence of Alpha hydrogen in aldehydes and ketones is essential for aldol condensation. The alpha hydrogen atoms are acidic in nature due to the presence of electron withdrawing carbonyl group. These can be easily removed by a base and the carbanion formed is resonance stabilised.
Why are hemiacetals reducing sugars?
This means that the cyclic hemiacetal form of a sugar will produce an equilibrium amount of the open-chain aldehyde form, which will then reduce the copper(II) to copper (I) and give a positive test. A hemiacetal form is thus a reducing sugar.