Hemiacetal anion

The relatively modest increase in the value of the J- function upon increasing the ethanol content of a sodium hydroxide solution from 90 to 98 vol % indicates that either the nucleophilic reactivity of the ethoxide ion under these conditions does not differ substantially from that of the hydroxide ion (while solvation of the hydroxide ion and the geminal diol anion is similar to solvation of the ethoxide ion and the hemiacetal anion) or that compensation of effects takes place. 

McClelland et al.221 have suggested that the general acid-catalyzed decomposition of a hemiacetal anion, Equation (63), proceeds through an imbalanced transition state where sp3 to sp2... 

Fig. 13.41. Alkylation of an enantiomerically pure lactone enolate (B) for the preparation of enantiomerically pure a-a l kyl- - h yd roxyca r boxy li c acids and enantiomerically pure 1,2-diols, respectively. In the lactone Cthe carboxyl group may react with water or with a hydride donor. In any case, the leaving group released is a hemiacetal anion that decomposes to pivalaldehyde and the a-alkyl-a-hyd roxyca rboxylic acid D or the enantiomerically pure 1,2-diol E.

D — hydroxylated organosodium compound B — hemiacetal anion A. This sequence is completely analogous to the sequence ketone —> ketyl — hydroxylated radical A —> hydroxylated organosodium compound B —> sodium alkoxide that occurs in the reduction of a ketone with Na in /PrOH (Figure 17.53). 

An alternative procedure, based on the intramolecular displacement of a suitable leaving-group by a hemiacetal anion, was explored by Webber and coworkers53 (see Scheme 2). Thus, benzaldehyde was... 

In order to craft the lactone ring, 38 was oxidized to 40 under Swem conditions in a prelude to intramolecular 1,4-addition of the hemiacetal anion [20] formed via nucleophilic attack by methoxide ion at the aldehyde site. With the availability of acetal 41, it became necessary to consider carefully whether to elaborate the epoxy lactone segment in advance of, or subsequent to, introduction of the a,p-unsaturated ester subunit. Since the latter option was considered more workable, 41 was transformed into the enol triflate and subjected to palladium(II) catalyzed methoxycarbonylation [21]. This methodology allowed for proper homologation of 42 to 43, and subsequent conversion to 44, in totally regiocontrolled fashion. 

Unlike similar intermediates in aldol reactions, this intermediate (a hemiacetal anion) has an ethoxy leaving group. 

The effect of a substituent may be substantially modified by fast, concurrent, reversible addition of the nucleophile to an electrophilic center in the substituent. Ortho- and para-CS.0 and pam-CN groups have been found by Miller and co-workers to have a much reduced activating effect on the displacement of halogen in 2-nitrohaloben-zenes with methoxide ion [reversible formation of hemiacetal (143) and imido ester anions (144)] than with azide ion (less interaction) or thiocyanate (little, if any, interaction). Formation of 0-acyl derivatives of 0x0 derivatives or of A-oxides, hydrogen bonding to these moieties, and ionization of substituents are other examples of reversible and often relatively complete modifications under reaction conditions. If the interaction is irreversible, such as hydrolysis of a... 

At this time we do not have a firm nnderstanding of how CrCl2 and VCI3 catalyze the double bond isomerization and why other metal chlorides are less effective. We propose that CrCh" or VCh" anion plays a role in hydride transfer, facilitating donble bond isomerization. CnCh is less effective and both lactic acid and pyruvaldehyde are formed. FeCh" and MnCh" anions are ineffective in the transformation and only pyruvaldehyde is formed. The fact that only a small amount of 1,3-dihydroxyacetone is formed is consistent with the NMR observation that the compounds exist as hemiacetal dimers in ionic hquids and not as monomers. Otherwise 1,3-dihydroxyacetone would be expected as a major product (16). 

Three possible mechanistic schemes can be suggested for this process. One involves elimination of the proton attached to the p-C atom of nitronate A or A followed by elimination of the OSi group from the intermediate anion (cf. Scheme 3.93). Another mechanism is associated with a 1,4-C,O-transfer of the proton from the p-C atom of nitronate A to the oxygen atom of the N—>0 fragment followed by elimination of silanol from hemiacetal B. The third mechanism is based on the concerted elimination of silanol from the minor cis isomer of SENA. 

Jencks and his coworkers (Funderburk et al., 1978) proposed a mechanism for the breakdown of hemiacetals catalysed by hydroxide-ion which consists of the reversible formation of the anion followed by its unimolecular breakdown. A similar mechanism can be written for the breakdown of hemi-orthoesters (7), (8). For at least one hemiorthoester viz. 2-hydroxy-2-... 

There are at least three possible mechanisms for the spontaneous breakdown of hemiorthoesters, hemiacetals, and related species. Firstly, there may be a rapid and reversible ionization equilibrium followed by hydronium-ion catalysed breakdown of the anion (9) (Gravitz and Jencks, 1974). A necessary condition for this mechanism to be valid is that k2 calculated from kHi0 and Ka should fall below the diffusion controlled limit of c. 10loM 1s 1. The second mechanism (10) is similar to this but involves formation of the anion and hydronium ion in an encounter pair which react to give products faster than the diffuse apart (Capon and Ghosh, 1981). With this mechanism therefore the ionization equilibrium is not established and the rate constant for... 

Water-soluble biodegradable polycarboxylates with an acetal or ketal weak link were inventions of Monsanto scientists in the course of their search for biodegradable deteigent polymers. However, the polymers were prevented by economics from reaching commercial status. The polymers are based on the anionic or cationic polymerization of glyoxylic esters at low temperature (molecular weight is inversely proportional to the polymerization temperature) and subsequent hydrolysis to the salt form of the polyacid, which is a hemiacetal (R = H) or ketal (R = CH3) if methylglyoxylic acid is used, and stable under basic conditions. 

The two possible valence-bond structures of the enolate anion, 7a and 7b, show that the anion should act as an ambident nucleophile—a nucleophile with nucleophilic properties associated with both carbon and oxygen. The addition step in the aldol reaction therefore may be expected to take place in either of two ways The anion could attack as a carbon nucleophile to form a carbon-carbon bond, 8, leading ultimately to the aldol 9, or it might attack as an oxygen nucleophile to form a carbon-oxygen bond, thereby leading to the hemiacetal 10. By this reasoning, we should obtain a mixture of products 9 and 10. However, the aldol 9 is the only one of these two possible products that can be isolated ... 

The lithium anion of chloromethyl phenyl sulfoxide reacts with tetrahydrofuran-2-ones 1011 to afford a diastereo-meric mixture of hemiacetal adducts 1012, the potassium enolate of which is treated with /-BuLi followed by addition of a proton source leading to to-hydroxyalkyl ketenes 1013, which themselves cyclize to 6-substituted tetrahydropyran-2-ones in excellent overall yield (Scheme 263) <1998TL9215>. 

After conjugate addition of the hydroperoxide anion to the chelated a,p-unsaturated lactone, the resulting enolate counterattacks the proximal [3-hydroperoxide group to form an epoxide. The latter then ring-expands to create another enolate, which captures a proton from water. The product hemiacetal finally undergoes ring-opening and saponification to produce 9 (Scheme 6.5). 

Intermediate A of the Bouveault-Blanc reduction of Figure 17.59 is not a simple alkoxide but rather the anion of a hemiacetal. Accordingly, it decomposes into an alkoxide anion and an aldehyde. In the further course of the Bouveault-Blanc reduction, this aldehyde is reduced by Na/EtOH just as the ketone of Figure 17.53 is reduced by Na/iPrOH. 

The so-called acyloin condensation consists of the reduction of esters—and the reduction of diesters in particular—with sodium in xylene. The reaction mechanism of this condensation is shown in rows 2-4 of Figure 14.51. Only the first of these intermediates, radical anion C, occurs as an intermediate in the Bouveault-Blanc reduction as well. In xylene, of course, the radical anion C cannot be protonated. As a consequence, it persists until the second ester also has taken up an electron while forming the bis(radical anion) F. The two radical centers of F combine in the next step to give the sodium glycolate G. Compound G, the dianion of a bis(hemiacetal), is converted into the 1,2-diketone J by elimination of two equivalents of sodium alkoxide. This diketone is converted by two successive electron transfer reactions into the enediolate I, which is stable in xylene until it is converted into the enediol H during acidic aqueous workup. This enediol tautomerizes subsequently to furnish the a-hydroxyketone—or... 

This is an example of gac (general acid because of the strain in the ketone. The anion of the hemiacetal decomposes by cleavage of a C-C... 

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