Enediolate ion

In most plants photosynthesis is also strongly inhibited by 02. This observation led to the discovery that 02 competes directly for C02 at the active site of rubisco in a process called photorespiration. Chloroplasts inhibited by oxygen produce glycolate in large amounts2823 as a result of the reaction of the intermediate enediolate ion formed in step b of Eq. 13-48 with 02... 

In ammoniacal solutions of copper salts, the oxidation products are likely to contain nitrogen thus, hexoses give oxalic acid, imidazoles, hydrogen cyanide, and urea. Kinetic studies have been reported for the reaction of Cu(II) in the presence of ammonia with maltose, lactose, melibiose, and cellobiose.190 For the oxidation by tetraamminecopper(II) in ammoniacal and buffered media the rate of reaction is first order in disaccharide concentration, order one-half in ammonia concentration, but it is independent of Cu(II) concentration. The reaction rate is decreased by the addition of ammonium chloride, because of the common ion effect. These kinetics suggested mechanisms involving an intermediate enediolate ion, with the rate of reaction being equal to the rate of enolization.191 A similar mechanism has been proposed for the oxidation of D-fructose by a copper-pyridine complex in an excess of pyridine.192... 

Scheme I illustrates the interesting position of n-fructose in the sugar series, and the widely differing products obtained in the past few years by treatment with acid or alkali. Only intermediates and end products are given the reaction conditions are described in the text. Emphasis is placed on D-fructose because of its abundant occurrence in Nature and its rapid conversion into other isomers. The 1,2-enediol (c) and its ionized form (f) have central positions, indispensable to explaining hexose interconversions in alkaline media. The enediolate ion was mentioned for the first time during work on the saccharinic acids, and, in 1944, IsbeU used this same mechanism during development of his electron-displacement theory. ...

A second system is essentially the same, but with two more electrons—the enediolate ion 2.33 has the re molecular orbitals of butadiene, lowered by the presence of the two electronegative atoms, but with two electrons in ip3. However one thinks of it, it is a re system higher in energy than the separated components. We have seen therefore that both the diketone and the enediolate are destabilised systems, but that the... 

A mechanism involving an intermediate enediolate ion with the rate of reaction being equal to the rate of enolizatiori was proposed. The kinetics and mechanism of thallium(III) oxidation of cellobiose in acid medium have been determined the acid-catalyzed reaction was first order in each reactant, and the active oxidant was thallium(III) diacetate. The sugar products were identified as D-gluconic acid and glucose. ... 

The reductive coupling of carbonyl compounds with active metals (Na, Mg, Al) yields pinacols. An electron transfer from the metal surface to the carbonyl oxygen (ketyl formation), a soft-soft interaction, is undoubtedly involved. The conversion of esters to acyloins (22, 23) on the surface of metallic sodium is well known. Here the enediolate products can be trapped in situ by Me3SiCl (24). The chlorosilane does not interfere with the coupling, yet it effectively removes the alkoxide ions and neutralizes the enediolate ions immediately on formation. The elimination of RO is imperative, for otherwise Claisen or Dieckmann condensations would compete with the normal course of reaction. These complicating processes require a hard base (e.g. RO ) to abstract a proton from the starting esters, whereas the desired coupling is accomplished by a soft base which is the electrons on the metal surface. 

The pyranoid conformations of the amino-deoxyoctoses (6) and (7) have been calculated using modified Karplus equations. Enediol anion formation in inososes has been studied by u.v. and n.m.r. spectroscopy. The results suggested that the interpretation of n.m.r. data by Dufaye (Appl. Polym. Symp., 1976, 28, 955) was erroneous and that only small concentrations of enediolate ions are formed. 

This procedure is representative of a new general method for the preparation of noncyclic acyloins by thiazol ium-catalyzed dimerization of aldehydes in the presence of weak bases (Table I). The advantages of this method over the classical reductive coupling of esters or the modern variation in which the intermediate enediolate is trapped by silylation, are the simplicity of the procedure, the inexpensive materials used, and the purity of the products obtained. For volatile aldehydes such as acetaldehyde and propionaldehyde the reaction Is conducted without solvent in a small, heated autoclave. With the exception of furoin the preparation of benzoins from aromatic aldehydes is best carried out with a different thiazolium catalyst bearing an N-methyl or N-ethyl substituent, instead of the N-benzyl group. Benzoins have usually been prepared by cyanide-catalyzed condensation of aromatic and heterocyclic aldehydes.Unsymnetrical acyloins may be obtained by thiazol1um-catalyzed cross-condensation of two different aldehydes. -1 The thiazolium ion-catalyzed cyclization of 1,5-dialdehydes to cyclic acyloins has been reported. 

The mechanism of HMF formation from D-fructose and sucrose was reviewed by Antal et a/.48 Several arguments were advanced for favoring a mechanism involving furanose rings and a fructose oxocarbonium ion over an open-chain [3-elimination mechanism that proceeds via an enediol intermediate to a... 

The criteria for choosing inhibitors in this study were the ability to compete with diethanolamine for the nitrite and lack of toxicity. An attempt was made to cover as broad a group as possible within the limits of feasibility. Ascorbic acid in its water soluble form and its oil soluble form, the palmitate, represent the enediols, Sorbate is a diene fatty acid which has been shown to inhibit nitrosation (10), Since the pK of sorbic acid is 4,76, at the pH of these experiments, both water soluble sorbate ion and oil soluble sorbic acid are present in significant amounts. Sodium bisulfite is a strong inorganic reducing agent which has an acceptable lack of toxicity at the concentration... 

The first two reactions of the sequence are similar to reactions that occur in acidic medium. The 1,2- and 2,3-enediols, and the unsaturated elimination-products derived from them, are present both in acidic and basic solutions. In general, however, reactions in basic solution are much faster than in acidic solution, because of the greater catalytic effect of the hydroxyl ion on the transformation reactions Mechanistic differences between the media become operative in steps c and d. In acid, further dehydration, if it is possible, occurs rapidly, before equilibrium of the deoxy-enediol with the dicarbonyl compound has been established,17 and the products are furans. In alkaline solution, the rapid formation of the tautomeric dicarbonyl compound permits the benzilic acid rearrangement42 to proceed. 

As shown in Fig. 2, two mechanisms involving an intermediate oxidation may be written for the epimerization at C-4". In the first one (A), the oxidation results in an a-D-xj/lo-hexopyranosyl-4-ulose derivative (96), which is then attacked by a hydride ion from the opposite side of the carbonyl group a change in conformation of the enzyme-intermediate complex seems necessary for such a process. The mechanism depicted under (B) postulates oxidation at C-3", and the resulting hexopyranosyl-3-ulose derivatives (54 and 97) then achieve equilibrium through the common enediol intermediate (98) before undergoing reduction at C-3". Compound 98 may also be formed from the hexopyranosyl-4-ulose ester 96, and in such a manner, both of the pathways may be linked. 

Central to the proposed mechanism for plant rubisco is a carbamoylated Lys side chain with a bound Mg2+ ion. The Mg2+ ion brings together and orients the reactants at the active site (Fig. 20-6) and polarizes the C02, opening it to nucleophilic attack by the five-carbon enediolate reaction intermediate formed on the enzyme (Fig. 20-7). The resulting six-carbon intermediate breaks down to yield two molecules of 3-phosphoglycerate. 

Vife have extensively studied (10) the photoinduced catalytic system (Figure 2) which is based upon the photochemical generation of cyanide ions from various cyanometa I lates, especially from octa-cyanomolybdate(IV) aod octacyanotungstate(IV) ions. Upon photolysis both compounds form cyanide ions with relatively high quantum yields. Cyanide Sons may act as a catalyst for the dimerizat ion of appropriate heterocyclic carb-2-aldehydes to enediols. Since this photocatalytic system has proved to operate also in solid layers, it may be used for an unconventional photographic process (11,12). 

Stacey and Turton61 objected to Isbell s mechanism on two counts first, that he did not specify that a proton acceptor must be used to promote the reaction and second, that the orthoacetate intermediate would not be applicable in the conversion which they demonstrated (by absorption spectra data) to take place on treatment with dilute, aqueous sodium hydroxide. (The presence of the proton acceptor seems implicit in Isbell s general description of the process of enolization.) The mechanism of Stacey and Turton is shown in Formulas XXIV to XXVIII it calls for the donation of electrons by pyridine to the incipient, ionic proton at C2 and elimination of acetic acid between C2 and C3 with the formation of the partially acetylated enediol-pyridinium complex. The pyridinium ion is removed by acetic acid. Electronic readjustment results in the elimination of acetic acid from positions 4 and 5. The final step, conversion of XXVII to XXVIII, was not explained. Stacey and Turton considered that with sodium hydroxide the reaction proceeds after deacetylation by a similar mechanism except that hydroxyl groups take the place of acetyl groups. Neither mechanism requires a free hydroxyl group at Cl, a condition considered by Maurer to be essential to kojic acid formation. 

The solution12 is to carry out the reaction in the presence of M SiCl. This does two things. The more obvious is that the enediol dianion 51 is trapped as the silyl enol ether 56, a useful intermediate, but the more important thing is the removal of the basic ethoxide ions as the neutral silyl ether EtOSiMe3. 

Another base-catalyzed side reaction is the enediol rearrangement, which moves the carbonyl group up and down the chain, as shown in Mechanism 23-3. If the enolate ion (formed by removal of a proton on C2) reprotonates on the Cl oxygen, an enediol intermediate results. Removal of a proton from the C2 oxygen and reprotonation on Cl gives fructose, a ketose. 

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