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Hemiacetals, hydrolysis isolation

The last step is hydrolysis of the unstable hemiacetal. Alkoxycarbocation intermediates (73, R=alkyl) have been isolated in superacid solution at low temperatures, and their structures proved by The protonated hydroperoxides... [Pg.1419]

In an extension beyond hetaryl onium salt promoted hemiacetal activation, Ishido and coworkers have reported the carbodiimide activation of hemiacetals [141]. In the method (Scheme 3.13), the hemiacetal donor 1 is treated with a carbodiimide electrophile 83 and copper(I) chloride to provide glycosyl isourea intermediate 85. Highly susceptible to hydrolysis, the isourea 85 was not isolated but could be detected by 13C NMR and IR spectroscopy [142,143], Accordingly, the reaction between intermediate 85 and the glycosyl acceptor (NuH) provides glycoside product 3, along with urea by-product 84. [Pg.131]

Cyclohexylpropionic acid was deprotonated with 2.2 equivalent of lithium diisopropylamide and the resulting dianion was condensed with trifluoroacetaldehyde which was generated in situ from its ethyl hemiacetal. The P-hydroxy acid 1 was isolated as a racemic mixture of two diastereomers. Silylation with tert-butyldimethylsilyl triflate was followed by ester hydrolysis to give the acid 2. A Curtius rearrangement with diphenylphosphoryl azide in the presence of benzyl alcohol afforded the protected P-amino alcohol 3 which was used in the preparation of the trifluoromethyl alcohol I. Oxidation using the Dess-Martin periodinane reagent (9) yielded the trifluoromethyl ketone II as a mixture of diastereomers. The signal for the carbonyl carbon in the 13C-NMR spectrum of this ketone appeared at 94.5 ppm and this is consistent with the hydrated form of the trifluoromethyl ketone. [Pg.165]

The preparation of the title lactone has been described by a multistep synthesis from holarrhimine.5 The method described in detail above is essentially an application of the hypoiodite reaction published by Ch. Meystre and co-workers.6 These authors also describe the isolation of the intermediate hemiacetal in pure form. Saturated lactones epimeric at C-20 have also been obtained by chromic acid oxidation of 18,20-dihydroxy compounds7 which were in turn prepared by treatment of 20-hydroxypregnanes with lead tetraacetate, acetolysis of the resulting 18,20j3-oxides, and hydrolysis. Saturated lactones of the... [Pg.61]

An acetal (e.g., 3 Figure 13.1) can be prepared by the equilibrium reaction of two equivalents of an alcohol and one equivalent of a compound possessing an aldehyde (or ketone to generate a ketal). The hemiacetal intennediate 2 is hydrolytically labile to both base and acid, but the resulting acetal product 3 is only labile to hydrolysis at acidic pH values. When polyacetals are prepared by acid catalysis, it is important to remove or neutrahze any residual add to ensure the polymer is stable enough to isolate and for storage. [Pg.220]

On the basis of literature precedents (58) it was hoped that hydrolysis of (149) would produce a nitrocyclitol directly. This turned out to be the case although the intervening hemiacetal (150) could be isolated by judicious crystallisation. However the nitrocyclitol was not the desired substance (151) but the stereoisomer (153 a). [Pg.44]

Notice that acetal formation is a reversible process that involves a series of equilibria (eq. 9.17). How can these reactions be driven in the forward direction One way is to use a large excess of alcohol. Another way is to remove water, a product of the forward reaction, as it is formed. The reverse of acetal formation, called acetal hydrolysis, cannot proceed without water. On the other hand, an acetal can be hydrolyzed to its aldehyde or ketone and alcohol components by treatment with excess water in the presence of an acid catalyst. The hemiacetal intermediate in both the forward and reverse processes usually cannot be isolated when R and R" are simple alkyl or aryl groups. [Pg.264]

Each step is reversible the entire sequence, starting from the carbonyl compound and ending with the acetal, is an equilibrium process. In the presence of the acid catalyst, the equilibrium may be shifted in either direction toward acetal by using excess alcohol or removing water toward starting aldehyde or ketone by adding excess water, a process called acetal hydrolysis. However, in contrast with hydrates and hemiacetals, acetals may be isolated as pure substances by neutralizing the acid catalyst used in their formation. Reversal of acetal formation cannot occur in the absence of acid. Therefore, acetals may be prepared and put to use synthetically, as described in the next section. [Pg.756]

Enzymatic hydroxylation of a carbon adjacent to an oxygen or nitrogen usually results in dealkylation by spontaneous hydrolysis of the initial hemiacetal or hemiaminal product. These conversions are commonly employed for two synthetic purposes cleavage of methyl ethers and oxidative deamination of amines. The latter is particularly useful in amino-add chemistry. These reactions can be catalyzed by P-450 monooxygenases or by flavin-containing monooxygenases (which are typically metal-free enzymes). As in the previous example, these hydroxylations require two dectrons that must be supplied by NADH or NAD PH, and most synthetic applications have rdied on whole miaobial cells rather than the isolated enzymes (Figure 1.7). [Pg.19]


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See also in sourсe #XX -- [ Pg.253 ]




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