Bases. hemiketals

Aldehydes and ketones undergo reversible addition reactions with alcohols. The product of addition of one mole of alcohol to an aldehyde or ketone is referred to as a hemiacetal or hemiketal, respectively. Dehydration followed by addition of a second molecule of alcohol gives an acetal or ketal. This second phase of the process can be catalyzed only by acids, since a necessary step is elimination of hydroxide (as water) from the tetrahedral intermediate. There is no low-energy mechanism for base assistance of this... 

Anthocyanins are usually represented as the red flavylium cations (Figure 5.1, left). However, this form is predominant only in very acidic solvents (pH < 2) such as those used for HPLC analysis. In mildly acidic media, the flavylium cations undergo proton transfer and hydration reactions, respectively, generating the quinonoidal base and the hemiketal syn carbinol) form (Figure 5.1, right) that can tautomerize to the chalcone. Thus, at wine pH, malvidin 3-glucoside occurs mostly as the colorless hemiketal (75%), the red flavylium cation, yellow chalcone, and blue quinonoidal base being only minor species. 

The hydration and protonation reactions of the methylmethine-linked malvidin 3-glucoside dimer can be summarized as follows, with AH ", AOH, and A representing the flavylium, hemiketal, and quinonoidal base forms, respectively ... 

Although the crystalline forms of a- and /3-D-glucose are quite stable, in solution each form slowly changes into an equilibrium mixture of both. The process can be observed as a decrease in the optical rotation of the a anomer (+112°) or an increase for the /3 anomer (+18.7°) to the equilibrium value of 52.5°. The phenomenon is known as mutarotation and commonly is observed for reducing sugars. Both acids and bases catalyze mutarotation the mechanism, Equation 20-1, is virtually the same as described for acid- and base-catalyzed hemiacetal and hemiketal equilibria of aldehydes and ketones (see Section 15-4E) ... 

Anthocyanin pigments undergo reversible structural transformations with a change in pH manifested by strikingly different absorbance spectra (Fig. FI.2.1). The colored oxonium form predominates at pH 1.0 and the colorless hemiketal form at pH 4.5 (Fig. FI.2.2). The pH-differential method is based on this reaction, and permits accurate and rapid measurement of the total anthocyanins, even in the presence of polymerized degraded pigments and other interfering compounds. 

A versatile synthesis of cyclopropanones and closely related derivatives is provided by the diazoalkane-ketene reaction as shown in Scheme 2. Using this method, the parent ketone 2>3> and alkyl-substituted cyclopropanones 1()) have been prepared in yields of 60—90% based upon the concentration of diazoalkaneb) (Table 2). The reaction is rapid at Dry Ice-acetone temperatures and is accompanied by evolution of nitrogen. Although most cyclopropanones are not isolable, dilute solutions of 3 (0.5—0.8 M) may be stored at — 78 °C for several days or at room temperature in the presence of suitable stabilizing agents.15) The hydrate and hemiketal derivatives are readily prepared by the addition of water or alcohols to the solutions of. .2>8>5)... 

Cyclopropanone ethyl hemiketal is contaminated with 8-9% of the cyclopropanone methyl hemiketal as a result of exchange with the solvent.2 The calculated number of mmoles of total cyclopropanone hemiketal is based upon this composition. 

A variant of the mechanism shown in Figure 10 is one in which the aetive site base (instead of the the 04 atom) abstracts a methyl proton from the hemiketal adduct (Itoh et al, 1998) but this possibility is not consistent with the position of the methanol in the active site as seen in the X-ray strueture (Section 7.4.3). This orientation of methanol lends further support to the hemiketal mechanism. The hydroxyl oxygen is close to C5 (3.2 ) while the methyl group is considerably further away (3.9 ). This would favour covalent bond formation between C5 and the nearby hydrox over hydride ion transfer from the more distant methyl group of the substrate (Xia etal, 1999). 

Copigmentation is driven by hydrophobic vertical stacking between the anthocyanin and the copigment to form tt-tt complexes from which water is excluded. The flavylium cation as well as the quinonoidal base are planar hydrophobic structures and can be involved in such complexes whereas the hemiketal form cannot. The association thus results in displacement of the anthocyanin hydration equilibrium from the colorless hemiketal to the red flavylium form that can be easily measured by spectrophotometry. 

The anthocyanins are structurally dependent on the conditions and composition of the media where they are dissolved and suffer interactions among them and with other compounds that influence their structural equilibria and modify their color. Anthocyanins are usually represented as their red flavylium cation, but in aqueous media this form undergoes rapid proton transfer reactions, leading to blue quinonoidal bases, and hydration, generating colorless hemiketals in equilibrium with chalcone structures. The proportion of each form is determined by the pH... 

Fig. 9D.1 Equilibrium distribution at 25 °C among structural forms of malvidin 3-glucoside as a function of pH. AH+, flavyUum cation A, quinonoidal base B, hemiketal C, chalcone (from Brouillard 1982, with permission from Elsevier)...

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