Carbonyl/carboxylate acceptor

A bound divalent metal ion, usually Mn2+, is required in the transcarboxylation step. A possible function is to assist in enolization of the carboxyl acceptor. However, measurement of the effect of the bound Mn2+ on 13C relaxation times in the substrate for pyruvate carboxylase indicated a distance of 0.7 ran between the carbonyl carbon and the Mn2+, too great for direct coordination of the metal to the carbonyl oxygen.68 Another possibility is that the metal binds to the carbonyl of biotin as indicated in Eq. 14-11. Pyruvate carboxylase utilizes two divalent metal ions and at least one monovalent cation.683... 

To distinguish between Form n and Form III we mrn to the binary graph sets. In principle it is possible to combine the three hydrogen bonds pairwise in three different ways. One of the combinations, 0 (4) for Form n is shown in Fig. 2.20(a), which reflects the fact that both hydrogen bonds in the chain are to the same carbonyl oxygen acceptor. For Form III (Fig. 2.20(b)) the chain C (6) contains both oxygens of the carboxyl group hence there are two acceptors and two additional atoms in each chain link . Also, note that an equal number of atoms in the chain formed by ca in both Forms II and III, but the former has one acceptor and the latter has two acceptors. The... 

In an extensive study of published crystal structures the average values of 0 were, as expected, found to be 180°, while % and co depended on the hybridization of the hydrogen bond acceptor [157]. For example, % was found to be 135° for carbonyl, carboxyl and sulfonamide acceptors, 109.5° for sp and 120° for sp hydroxyl acceptors, 126° for imidazole, and 120° for pyrimidine-type acceptors. These angular... 

The transition state involves the carbonyl oxygen of one carboxyl group—the one that stays behind—acting as a proton acceptor toward the hydroxyl group of the carboxyl that IS lost Carbon-carbon bond cleavage leads to the enol form of acetic acid along with a molecule of carbon dioxide... 

The final decarboxylation of mevalonate 5-diphosphate appears unusual because decarboxylations of acids do not typically occur except in /3-keto acids and malonic acids, in which the carboxylate group is two atoms away from an additional carbonyl group (Section 22.7). The function of this second carbonyl group is to act as an electron acceptor and stabilize the charge resulting from loss of CC>2- In fact, though, the decarboxylation of a /3-kelo acid and the decarboxylation of mevalonate 5-diphosphate are closely related. 

Glycosyl esters with remote functionality constitute a relatively new class of O-carbonyl glycosyl donors, which fulfill the prospect of mild and chemoselective activation protocols (Scheme 3.22). For example, Kobayashi and coworkers have developed a 2-pyridine carboxylate glycosyl donor 134 (Y = 2-pyridyl), which is activated by the coordination of metal Lewis acid (El+) to the Lewis basic pyridine nitrogen atom and ester carbonyl oxygen atom [324]. In the event, 2-pyridyl (carbonyl) donor 134 and the monosaccharide acceptor were treated with copper(II) triflate (2.2 equiv) in diethyl ether at —50 °C, providing the disaccharide 136 in 70% (a P,... 

This initial attack of the ozone molecule leads first to the formation of ortho- and para-hydroxylated by-products. These hydroxylated compounds are highly susceptible to further ozonation. The compounds lead to the formation of quinoid and, due to the opening of the aromatic cycle, to the formation of aliphatic products with carbonyl and carboxyl functions. The nucleophilic reaction is found locally on molecular sites showing an electronic deficit and, more frequently, on carbons carrying electron acceptor groups. In summary, the molecular ozone reactions are extremely selective and limited to unsaturated aromatic and aliphatic compounds as well as to specific functional groups. 

Treatment of aldehydes or ketones with acceptor-substituted carbene complexes leads to formation of enol ethers [1271-1274], oxiranes [1048], or 1,3-dioxolanes [989,1275] by O-alkylation of the carbonyl compound. Carboxylic acid derivatives... 

Vicinally donor-acceptor-substituted cyclopropanol carboxylic esters have been proven to be versatile synthetic building blocks in organic synthesis [11]. They readily undergo a retroaldol reaction, thus creating a stable enolate that at the same time can be considered as a homoenolate in relation to the newly formed carbonyl function. Shimada et al. applied this strategy to the preparation of y-substituted lactones starting from cyclopropane 21 (Scheme 3) [12]. 

Addition to carbonyl group by a nucleophile is commonplace. However, a-lactones undergo alcoholysis with formation of a-alkoxy carboxylic acids [144]. The acceptor role of the carbonyl is restored in the bistrifluoromethyl-a-lactone [145]. It may also be considered that the a-carbon is reluctant to become an acceptor because of the fluorine atoms. 

Biochemical reactions often involve addition to C = C bonds that are not conjugated with a true carbonyl group but with die poorer electron acceptor - COO. While held on an enzyme a carboxylate group may be protonated, making it a better electron acceptor. Nevertheless, there has been some doubt as to whether the carbanion mechanism of Eq. 13-6 holds for these enzymes. Some experimental data suggested a quite different mechanism, one that has been established for the nonenzymatic hydration of alkenes. An example is the hydration of ethylene by hot water with dilute sulfuric acid as a catalyst (Eq. 13-11), an industrial method of preparation of ethanol. The electrons of the double bond form the point of attack by a proton, and the resulting carbocation readily abstracts a hydroxyl... 

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