Aldehyde covalent hydrates

At the present time, the greatest importance of covalent hydration in biology seems to lie in the direction of understanding the action of enzymes. In this connection, the enzyme known as xanthine oxidase has been extensively investigated.This enzyme catalyzes the oxidation of aldehydes to acids, purines to hydroxypurines, and pteridines to hydroxypteridines. The only structural feature which these three substituents have in common is a secondary alcoholic group present in the covalently hydrated forms. Therefore it was logical to conceive of this group as the point of attack by the enzyme. 

Pyridoxal phosphate exists in an equilibrium between the aldehyde and its covalent hydrate (as in Eq. 13-1). The aldehyde has a yellow color and absorbs at 390 nm (Fig. 14-9), while the hydrate absorbs at nearly the same position as does PMP. The absorption bands of Schiff bases of PLP are shifted even further to longer wavelengths, with N-protonated forms absorbing at 415-430 nm. Forms with an unprotonated C = N group absorb at shorter wavelengths.149 240... 

Role of covalent hydration and addition of OH-ions — In aqueous solutions the reductions of some carbonyl compounds, for example of numerous aldehydes [i], and some ketones [ii] as well as of some compounds with azomethine (C=N) bonds [iii] are complicated by hydration-dehydration equilibria. In a re-... 

Covalent hydrates can undergo ring opening especially in acidic media, for example, the triazanaphthalene (196 197). 1,2,4,5-Tetrazines 198 are hydrolyzed with a base to give aldehyde hydrazones, RCH=NNHCOR. Polyaza rings suffer complete hydrolytic ring cleavage. 1,2,3-Benzotriazines are easily converted into derivatives of 2-aminobenzaldehyde. 

One of the most important characteristics of the aldol reaction is its easy reversibility under many conditions. Since this factor has such a generally profound effect, we briefly introduce the topic here. For an aldol addition reaction that is carried out under the influence of a catalytic amount of acid or base in protic medium A//° and AG° can be estimated from thermochemical data. Guthrie has estimated A// and AG for the aldol addition depicted in equation (3) to be -9.8 kcal mol and -2.4 kcal mol , respectively (1 cal = 4.18 J). ° The thermochemical values used in this estimation, and the derived values of A// and AG% refer to species at equilibrium with the covalently hydrated aldehydes. It is not expected that values for the free aldehydes would be greatly different. The equilibrium constant for equation (3) is 4(X) M- ... 

A series of ethyl 2-hydroxy-2-(trifluoromethyl)-2//-chromene-3-carboxylates (344) was obtained in high yields via the Knoevenagel condensation of salicyl-aldehydes with ethyl triflnoroacetoacetate in the presence of piperidinium acetate. The subsequent recyclization of these chromenes proceeds smoothly in refluxing chlorobenzene in the presence of p-toluenesnKonic acid affording 3-(trifluoroacetyl) coumarins (345) in good yields [202], These compounds were also prone to the facile and reversible covalent hydrate formation [120] (Scheme 110). 

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