P-hydroxy aldehyde

The p hydroxy aldehyde products of aldol addition undergo dehydration on heat mg to yield a f3 unsaturated aldehydes... 

Step 2 The alkoxide ion abstracts a proton from water to give the product of aldol addition, a p-hydroxy aldehyde. ... 

A reaction of great synthetic value for carbon-carbon bond formation. Nucleophilic addition of an enolate ion to a carbonyl group, followed by dehydration of the p-hydroxy aldehyde, yields an a,p-unsaturated aldehyde. 

The product is a P-hydroxy aldehyde (called an aldol) or ketone, which in some cases is dehydrated during the course of the reaction. Even if the dehydration is not spontaneous, it can usually be done easily, since the new double bond is in conjugation with the C=0 bond so that this is a method of preparing a,P-unsaturated aldehydes and ketones as well as P-hydroxy aldehydes and ketones. The entire reaction is an equilibrium (including the dehydration step), and a,P-unsaturated and P-hydroxy aldehydes and ketones can be cleaved by treatment with OH (the retrograde aldol reaction). There is evidence that an SET mechanism can intervene when the substrate is an aromatic ketone. ... 

Johnson, J.R. Cuny, G.D. Buchwald. S.L. Angew. Chem. Int Ed. Engl., 1995, 34, 1760 Also via p-Hydroxy aldehydes Section 324 (Alcohols - Aldehyde). 

The proline-mediated intramolecular aldol condensation of dialdehyde substrates was also reported by List in 2003, affording enantioselective synthesis of cyclic p-hydroxy aldehydes via a 6-e ofexo-aldolization reaction (Scheme 11.7d). 

Polyols. Rychnovsky4 has used the BF3-catalyzed coupling of oxiranes with an alkyllithium of Ganem (12, 68) to obtain polyol chains. Thus the tetraalkyltin 1 reagent prepared from a P-hydroxy aldehyde reacts with the oxirane 2 in the presence of BF3 etherate (2.5 equiv.) to give the protected decanehexol 3. 

P-hydroxy aldehyde e -unsaturated aldehyde XIV-I XIV-I two aldehydes two aldehydes NaOH NaOH+Ij... 

P-Chelation can also result in significant diastereoselectivity. Thus the reaction of an achiral P-hydroxy aldehyde catalyzed by TiCl, results in. vvn- and rwo-adducts in the ratio 94 6 (equation III). Finally, a chiral p-alkoxyaldchydc when complexed with TiCl, can... 

Enantioselective cross aldol reactions. A 3-acylthiazolidine-2-thione can be used as an equivalent of an aldehyde or carboxylic arid in cross aldol reactions to provide p-hydroxy aldehydes or carboxylic acids. The tin enolates formed from Sn(OTf), react with aromatic and aliphatic aldehydes with high svn-selectivity (equation I),... 

A15.1.1.6 Aldol Condensation. An aldehyde is partially converted to its enolate anion by bases such as hydroxide ion (2(R—CH2—CO—H) + OH + H2O RCH2—CH(OH)—CH(R)—COH + OH ). The product (p-hydroxy aldehyde) formed in the reaction above undergoes dehydration upon heating, to yield an a,P-unsaturated aldehyde (R—CH2—CH=C(R)—COH) plus water. The process by which two molecules of an aldehyde combine to form an a,P-unsaturated aldehyde and a molecule of water (under heat and basic conditions) is called an aldol condensation. 

In the aldol reaction, two molecules of an aldehyde or ketone react with each other in the presence of base to form a 3-hydroxy carbonyl compound. For example, treatment of acetaldehyde with aqueous OH forms 3-hydroxybutanal, a P-hydroxy aldehyde. 

Protonation of the alkoxide in Step [3] forms the p-hydroxy aldehyde. 

What four p-hydroxy aldehydes are formed by a crossed aldol reaction of CH3CH2CH2CHO and C6H5CH2CHO ... 

Lewis Acid-Catalyzed Rearrangement of 2,3-Epoxy-1-ols. Epoxides with adjacent hydroxyl groups, as in 2,3-epoxy-1-ols, undergo rearrangement on treatment with Bp3 Et20 to afford aldol-like products (e.g., P-hydroxy-aldehydes). ... 

The P-hydroxy aldehydes and P-hydroxy ketones formed in aldol reactions are readily dehydrated to yield conjugated systems. These p-elimination reactions can be effected by either acid or base while heating during the condensation. 

Aldol condensation (Sections 18.9 8.10) When an aldol addition is carried out so that the P-hydroxy aldehyde or ketone dehydrates under the conditions of its formation, the product is described as arising by an aldol condensation. 

With aldehydes, this rapid and reversible reaction leads to the formation of a P-hydroxy aldehyde or aldol (aid for aldehyde and ol for alcohol). When using ketones, P-hydroxy ketones are formed. The aldol products can undergo loss of water on heating under basic or acidic conditions to form conjugated enones (containing both C=0 and C=C bonds) in condensation reactions. Conjugation stabilises the enone product and this makes it relatively easy to form. 

Aldehydes may be dimerized, either to p-hydroxy aldehydes or to a,P-unsaturated aldehydes, by the use of the anion-exchange resins Amberlite and Dowex. ... 

Nakane and Hutchinson have further shown that the aldol step in this cyclization is stereoselective as well as regioselective. Treatment of (39) with Hunig s base, acetic anhydride and 4-(A, A -dimethyl-amino)pyridine (DMAP), followed by sodium borohydride reduction of the intermediate P-hydroxy aldehyde, gives diol (40) in 48% yield no diastereomeric diols were detected (equation 102). 2i... 

On a related front, the reactions of carbonyl compounds with metaliated derivatives of 2-methylthia-zoline furnish adducts (85). Although the initial nucleophilic addition occurs smoothly with a wide variety of aldehydes and ketones, the intermediate P-hydroxythiazolines (85) suffer thermal reversion upon attempted purification by distillation. Moreover, attempted cleavage of the corresponding P-hydroxythia-zolidines, which are readily produced from (85) upon dissolving metal reduction (Al-Hg), leads to the formation of p-hydroxy aldehydes only in simple systems numerous complications arising from dimerization, dehydration and retroaldol processes of the products usually intervene. Consequently it is necessary to protect the initial 1,2-adducts (85 = H) as the corresponding 0-methoxymethyl ether... 

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