Hemiacetals conversion

Steps 4-5 Conversion of hemiacetal to carbocation These steps are analogous to the formation of carbocations m acid catalyzed reactions of alcohols... 

The second step in acetal and ketal hydrolysis is conversion of the hemiacetal or hemiketal to the carbonyl compound. The mechanism of this step is similar to that of the first step. Usually, the second step is faster than the initial one. Hammett a p plots and solvent isotope effects both indicate that the transition state has less cationic character than... 

A rather complex microwave-assisted ring-opening of chiral difluorinated epoxy-cyclooctenones has been studied by Percy and coworkers (Scheme 6.131) [265]. The epoxide resisted conventional hydrolysis, but reacted smoothly in basic aqueous media (ammonia or N-methylimidazole) under microwave irradiation at 100 °C for 10 min to afford unique hemiacetals and hemiaminals in good yields. Other nitrogen nucleophiles, such as sodium azide or imidazole, failed to trigger the reaction. The reaction with sodium hydroxide led to much poorer conversion of the starting material. 

In aqueous solution, both glucose (hemiacetal) and frnctose (hemiketal) exist as equilibrium mixtures of cyclic and open-chain carbonyl forms. Sucrose, however, is a single stable substance (acetal and ketal), and conversion back to glucose and frnctose requires more rigorous hydrolytic conditions, such as heating with aqueous acid. 

Conversion of the glucohexaosyl derivative 7 into the desired key intermediate 6 is shown in Scheme 6. Treatment of 7 with (a) NaOMe— MeOH, (b) (C1CH2C0)20—pyridine afforded in 70% yield the monoohloroacetyl derivative 21, which was deallylated with PdCl2— NaOAc—aq.AcOH to give the hemiacetal 22 in 60% yield. The transformation of 22 into fluoride 23 was achieved in 73% yield in 2 steps (a) SOClj—DMF and AgF—CH CN. Compound 23 was deacylated... 

Below this GA concentration, the relative content of each monomeric structure was found to be virtually constant regardless of the GA concentration, and also pH (pH2- 8), in contrast to the temperature which has marked effect on it On the other hand, above this concentration, considerable amount of polymerized structures exist in GA solution, which are closely related with cyclic hemiacetal structure, that is, again not related with a, 0- unsaturated structure. The conversion velocity between the monomeric and polymerized structures is quite slow, compared to that between the monomeric structures. The chemistry of GA in concentrated aqueous solution will be discussed in detail elsewhere. 

The product of the DAHP synthase, 3-deoxy-D-arabino-heptulosonate 7-phosphate, is shown in its cyclic hemiacetal form at the beginning of Eq. 25-2. Its conversion to 3-dehydroquinate is a multistep process that is catalyzed by a single enzyme, 14/15 which is the product of E. coli gene am B. The elimination of... 

Unsaturated 1,5-diones are in the correct oxidation state for direct conversion to pyrylium salts and in fact they cyclize spontaneously under strongly acidic conditions. The reaction is thought to involve three steps initial enolization is followed by ring closure to the hemiacetal and subsequent rapid dehydration to the pyrylium salt (Scheme 247). 

The position of equilibrium in acetal and hemiacetal formation is rather sensitive to steric hindrance. Large groups in either the aldehyde or the alcohol tend to make the reaction less favorable. Table 15-3 shows some typical conversions in acetal formation when 1 mole of aldehyde is allowed to come to equilibrium with 5 moles of alcohol. For ketones, the equilibria are still less favorable than for aldehydes, and to obtain reasonable conversion the water must be removed as it is formed. 

Since rapid conversion of hemiacetal to acetal requires more acidic conditions than does formation of the hemiacetal, it is possible to measure the rate of hemiacetal production without complication from the second stage of the reaction.52 As might be expected, the hemiacetal formation displays characteristics similar to those of hydration general acid and general base catalysis are observed.53... 

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