Retrograde aldol reaction

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. ... 

Originally, the term aldol condensation referred specifically to the reaction of an aldehyde (having an a-hydrogen) with an aldehyde/ketone to form a j8-hydroxy aldehyde (the aldol). The reverse reaction is often referred to as a retrograde aldol reaction, a retro-aldol condensation (or reaction), or an aldol cleavage. March categorizes aldol condensations into five classes. The first is condensation between two identical aldehydes... 

An example of a retrograde aldol reaction (retroaldolization) is probably the pyrolytic decomposition of cellulose with formation of hydroxyacetaldehyde (see Section 7.2), Other mechanisms for pyrolysis of cellulose are also possible [3]. More paths for the same process is a common occurrence in pyrolysis, and more than one mechanism is frequently needed to explain the variety of reaction products. 

The alkoxide effects a retrograde aldol reaction, producing a ketone mid a lactone enolate. 

Strong Lewis acid (activation of the ketone partner) and forms a tightly bound cationic complex with the initially generated alkoxide (no retrograde aldol reaction), are responsible for the success of this protocol in these extremely challenging condensations (Scheme 3.15). 

Di-f-menthyl acetoxymethylenemalonate (67) is another chiral dienophile that has been utilized in asymmetric Diels-Alder reactions [51]. High-pressure-mediated addition of 67 to furan produced a mixture of labile endo and exo cycloadducts (i.e., 68) that were immediately converted into the corresponding acetonides 69. The reductive retrograde aldol reaction of the entio-product 69 resulted in the formation of (3-D-ribofuranosylmalonate 70. Analogous manipulation of the exo product gave the corresponding synthetic L-analog (Scheme 13.21) [51]. 

Another control experiment was done to determine the importance of water in this oxidative cleavage reaction. Water was found to be a necessary reagent for the reaction to occur since no p-hydroxybenzaldehyde was obtained when the sodium salt of chlorostilbene 5b was heated in neat nitrobenzene with or without solid sodium hydroxide and a crown ether phase transfer catalyst. Another set of controls was done to evaluate the formation of p-hydroxybenzaldehyde by a nonoxidative reaction, such as the loss of X-PI1-CH2 in a retrograde-type Aldol reaction. No p-hydroxybenzaldehyde was formed when the chlorostilbene 5b was heated at 155 °C for 5 hours in the presence of 2N NaOH but without the presence of nitrobenzene and atmospheric oxygen. Finally, in all of the above control experiments, no oxidized cleavage products were observed from the nonphenolic side of the alcohols 4 or stilbenes 5 (Dershem, S. M., et al., Holzforschung, in press). 

Retinal pigment epithelium (RPE), 3925, 3927-3929, 3937, 3939 3941, 3943-3947 Retinoids, 2675, 2695 Retinol, 3369 Retinopathy, 1450 Retroaldol-aldol mechanism, 2700 Retrochalcones, 1869, 1874, 1875 Retrograde amnesia, 292 Retronecine, 1058, 1064 Retro-Prince reaction, 3075, 3076 Retrorsine, 1061, 1063, 1385 Reversed phase... 

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