Dehydration acid-base catalyzed

For the acid-base catalyzed hydration and dehydration of organic bases, such as pteridine, the equations for and k are ... 

The reactions (forward and reverse) are acid-base-catalyzed and complicated in basic solutions at higher concentrations of reactants by continued condensation to form polymers of the general formula H—[CH2—C(R)(OH)]n—OH. Depending on R the final diol may revert to the aldehyde or ketone form. In acid solution dehydration of the tertiary alcohol leads to the formation of a-0 unsaturated ketones [for example, R—C(CH3) = CH—CO(R)], which may condense further. 

Dehydration is one of the important acid-base-catalyzed reactions of carbohydrates. In alkaline media, a deoxyaldosulose is formed from a carbohydrate molecule after one molecule of water has been split oflF this product then usually undergoes an intramolecular, oxidoreductive disproportionation to give the corresponding saccharinic acid in acidic media (with pentoses and hexoses), up to three molecules of water are split oE per molecule, with formation of 2-furaldehyde or 5-(hydroxymethyl )-2-furaldehyde. In many cases, polarography makes possible the monitoring of carbohydrate dehydration and the determination of its final products. 

The reaction is further complicated by thermodynamic equilibrium limitations, as indicated in Table I. The condensation/dehydration of acetone to MO is limited to about 20% conversion at 120 C (16). However, there is no equilibrium limitation to the overall acetone-to-MIBK reaction. This, coupled with the possibility of numerous thermodynamically favorable side reactions that are also acid/base-catalyzed (Fig. 1), suggests the need to balance the acid/base and hydrogenation properties of the selected catalyst. 

The mechanism involves an aldol reaction with dehydration, followed by a Michael addition, and then an intramolecular aldol with dehydration (all base catalyzed). Loss of the two ester groups begins with an acid-catalyzed hydrolysis, followed by decarboxylation of each of the resulting carboxylic adds. 

Scheme 10.18 shows the standard electron pushing for an acid-catalyzed dehydration reaction. Base-catalyzed reactions typically occur only when conjugated dienes are formed, and we do not cover these reactions here. Alcohols show specific-acid-catalyzed dehydrations, and thus the mechanism given starts with a reversible protonation of the alcohol. 

Conjugation of the newly formed double bond with the carbonyl group stabilizes the a p unsaturated aldehyde provides the driving force for the dehydration and controls Its regioselectivity Dehydration can be effected by heating the aldol with acid or base Normally if the a p unsaturated aldehyde is the desired product all that is done is to carry out the base catalyzed aldol addition reaction at elevated temperature Under these conditions once the aldol addition product is formed it rapidly loses water to form the a p unsaturated aldehyde... 

Reactions 16-38-16-49 are base-catalyzed condensations (though some of them are also catalyzed to acids). In 16-38-16-47, a base removes a C—H proton to give a carbanion, which then adds to a C=0. The oxygen acquires a proton, and the resulting alcohol may or may not be dehydrated, depending on whether an a hydrogen is present and on whether the new double bond would be in conjugation with double bonds already present ... 

Table III summarizes the parameters that affect Brrfnsted acid-catalyzed surface reactions. The range of reaction conditions investigated varies widely, from extreme dehydration at high temperatures in studies on the use of clay minerals as industrial catalysts, to fully saturated at ambient temperatures. Table IV lists reactions that have been shown or suggested to be promoted by Br nsted acidity of clay mineral surfaces along with representative examples. Studies have been concerned with the hydrolysis of organophosphate pesticides (70-72), triazines (73), or chemicals which specifically probe neutral, acid-, and base-catalyzed hydrolysis (74). Other reactions have been studied in the context of diagenesis or catagenesis of biological markers (22-24) or of chemical synthesis using clays as the catalysts (34, 36). Mechanistic interpretations of such reactions can be found in the comprehensive review by Solomon and Hawthorne (37).

Indeed, it was shown that aposcopolamine was not formed by direct dehydration of scopolamine, but via the conjugate scopolamine O-sulfate generated by a sulfotransferase [127]. This explains the species differences observed, and indicates a mechanism of heterolytic C-0 bond cleavage made possible by the electron-withdrawing capacity of the sulfate moiety. The reaction is also facilitated by the acidity of the departing proton carried by the vicinal, stereogenic C-atom. This acidity also accounts for the facile base-catalyzed racemization of scopolamine and hyoscyamine [128]. 

These results establish that the base-catalyzed dehydration is slow relative to the reverse of the addition phase for the branched-chain isomer. The reason for selective formation of the straight-chain product under conditions of base catalysis is then apparent. In base, the straight-chain ketol is the only intermediate which is dehydrated. The branched-chain ketol reverts to starting material. Under acid conditions, both intermediates are dehydrated however, the branched-chain ketol is formed most rapidly, because of the preference for acid-catalyzed enolization to give the more substituted enol (see Section 7.3 of Part A). 

Sorbitol. Sorbitol is the sugar alcohol obtained by reduction of glucose and it can be dehydrated to either isosorbide or to 1,4- and 2,5-sorbitan in acid or base catalyzed processes, respectively. Using sulfonic acid functionalized MCM-41 type materials lauric acid esters of isosorbide can be achieved quite selectively starting from sorbitol (>95% selectivity towards isosorbide dilaurate at 33% lauric acid conversion) in a dehydration-esterification... 

From the base-catalyzed degradation of D-fructose (pH 8.0), Shaw and coworkers147 identified 18 compounds, none of which was (a) isomeric with the starting material, or (b) a simple dehydration product. Among the products, the hydroxy-2-butanones and 1-hydroxy-2-propanone (acetol) were shown to participate in forming the carbo-cyclic products identified, but the mechanism of their formation was not elucidated. Several furan derivatives were isolated, but no lactic acid was isolated. In a similar study but with weak acid,41 most of the products were formed by a combination of enolization and dehydration steps, with little fragmentation. 

The carbon alpha to the carbonyl of aldehydes and ketones can act as a nucleophile in reactions with other electrophilic compounds or intermolecu-larly with itself. The nucleophilic character is imparted via the keto-enol tau-tomerism. A classic example of this reactivity is seen in the aldol condensation (41), as shown in Figure 23. Note that the aldol condensation is potentially reversible (retro-aldol), and compounds containing a carbonyl with a hydroxyl at the (3-position will often undergo the retro-aldol reaction. The aldol condensation reaction is catalyzed by both acids and bases. Aldol products undergo a reversible dehydration reaction (Fig. 23) that is acid or base catalyzed. The dehydration proceeds through an enol intermediate to form the a,(3-unsaturated carbonyl containing compound. 

A general methodology for the construction of quaternary carbon atoms at the carbonyl carbon of ketones has been successfully exploited for the facile synthesis of ( )-lycoramine (299) (Scheme 30) (165). Thus, the O-allylated o-vanillin 322 was allowed to react with vinyl magnesium bromide followed by Jones oxidation, and the acid-catalyzed addition of benzyl IV-methylcarbamate to the intermediate a,(3-unsaturated ketone furnished 323. Wadsworth-Emmons olefination of 323 with the anion derived from diethyl[(benzylideneami-no)methyl]phosphonate (BAMP) provided the 2-azadiene 324. The subsequent regioselective addition of n-butyllithium to 324 delivered a metalloenamine that suffered alkylation with 2-(2-bromoethyl)-2-methyl-l,3-dioxolane to give, after acid-catalyzed hydrolysis of the imine and ketal moieties, the 8-keto aldehyde 325. Base-catalyzed cycloaldolization and dehydration of 325 then provided the 4,4-disubstituted cyclohexenone 326. The entire sequence of reactions involved in the conversion of 323 to 326 proceeded in very good overall yield and in one pot. 

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