Structure enols

Examples of the remaining potential 3,4-dihydroxy heterocycles are presently restricted to furan and thiophene. Although the parent 3,4-dihydroxyfuran apparently exists as the dioxo tautomer (86), derivatives bearing 2-alkyl or 2,5-dialkyl substituents prefer the keto-enol structure (87) (71T3839, 73HCA1882). The thiophene analogues also prefer the tautomeric structure (87), except in the case of the 2,5-diethoxycarbonyl derivative which has the fully aromatic structure (88) (71T3839). 

A more detailed representation of the reaction requires more intimate knowledge of the enolate structure. Studies of ketone enolates in solution indicate that both tetrameric and dimeric clusters can exist Tetrahydrofliran, a solvent in which many synthetic reactions are performed, favors tetrameric structures for the lithium enolate of isobutyr-ophenone, for example. ... 

Due to the nonaromatic character of the oxepin system the oxepinones do not usually form stable enol structures. By O-acylation or O-alkylation, however, the enol forms can be stabilized as enol esters and ethers, respectively. A large number of substituted 1-benzoxepins have been synthesized by this route. Acetylation of l-benzoxepin-3(2//)-ones 1 and l-benzoxepin-5(2/T)-ones 3 was readily achieved with acetic anhydride in the presence of an appropriate base such as pyridine, triethylamine or sodium acetate.t5,t6 t72 176... 

Carboxylic acids can be alkylated in the a position by conversion of their salts to dianions [which actually have the enolate structures RCH=C(0")2 ] by treatment with a strong base such as LDA. The use of Li as the counterion is important, because it increases the solubility of the dianionic salt. The reaction has been applied to primary alkyl, allylic, and benzylic halides, and to carboxylic acids of the form RCH2COOH and RR"CHCOOH. This method, which is an example of the alkylation of a dianion at its more nucleophilic position (see p. 458),... 

Fig. 1.3. Potassium enolate of methyl t-butyl ketone open circles are oxygen and small circles are potassium, (a) left panel shows only the enolate structures (b) right panel shows only the solvating THF molecules. The actual structure is the superposition of both panels. Reproduced from J. Am. Chem. Soc., 108, 462 (1986), by permission of the American Chemical Society.

The first element of stereocontrol in aldol addition reactions of ketone enolates is the enolate structure. Most enolates can exist as two stereoisomers. In Section 1.1.2, we discussed the factors that influence enolate composition. The enolate formed from 2,2-dimethyl-3-pentanone under kinetically controlled conditions is the Z-isomer.5 When it reacts with benzaldehyde only the syn aldol is formed.4 The product stereochemistry is correctly predicted if the TS has a conformation with the phenyl substituent in an equatorial position. 

Experiments attempting the interaction of D-glucose with ethyl O-pro-pionylacetoacetate have proved fruitless. A positive result for this compound, in which the enolic structure has been fixed by substitution on the oxygen, would have been favorable to theories based on involvement of the enolic form of the ketonic compound. 

In the enol transition state the structure that is the more stable by virtue of having the negative charge on oxygen rather than on carbon also has the least separation of charge. In the ketone transition state the enolate structure that should otherwise be important in stabilizing the... 

The keto/enol ratio in organotin enolates is dependent on the enol structure and on the environmental conditions, a good ligand for the tin shifting the equilibrium toward the enolate, and increasing its reactivity (Equation (142)).392... 

In the free state and in their reactions the simple aldehydes and ketones are in general known only in the aldo- and keto-forms. Erlen-meyer suggested the rule that the isomeric enol-structure, which might be formed at first in the production of acetaldehyde from glycol, for example, should in no case be capable of existence. 

Schulenberg (117) describes the material 55a, which is obtained from the amide 55c on reaction with a deficiency of sodium methoxide the white prisms of the product melt at 110 to 122°C (variable). These crystals give fairly stable solutions, enabling measurement of the NMR spectrum and observation that the material gives a positive FeCl3 reaction, in accord with the enol structure. After recrystallization a mixture of crystals of 55a and pale yellow prisms melting at... 

One of the first careful studies of the influence of chirality proximal to ketone enolates is illustrated in eq. [95] (113). Condensation of the enolate 126 (M = Li) with propanal (THF, -100 C) afforded a modest bias for the (5,i )-diastereomeric aldol adduct 127 (127 128 = 57 43). The influence of the metal center in this condensation has recently been examined. The boryl enolate 126 [M = B(n-C4H9)2l afforded a ratio 127 128 = 64 36 in pentane (-78°C) (6a, 113). Similar studies designed to probe the dependence of diastereoface selection on metal enolate structure have been carried out with metal enolates 129 (eq. [96], Table 32). 

Example The NR mass spectrum of acetone closely resembles its 70 eV El mass spectrum (Chap. 6.2.1), thereby demonstrating that the molecular ion basically retains the structure of the neutral (Fig. 2.29). [144] However, the isomeric CsHeO" ions formed by McLafferty rearrangement of 2-hexanone molecular ion are expected to have enol structure (Chap. 6.7.1), and thus the corresponding NR mass spectrum is easily distinguished from that of acetone. 

Discussing the stereochemical outcome of the Claisen rearrangements, two aspects had to be considered. On the one hand, the relative configuration of the new stereogenic centers was found to be exclusively syn in 201 and 202, pointing out the passing of a chair-like transition state c-a and c-jS, respectively, including a Z-acylammonium enolate structure (complete simple diastereo-selectivity/internal asymmetric induction). 

The CPop intermediate is the j5-cuprio ketone intermediate widely debated in mechanistic discussions of conjugate addition (cf. Scheme 10.3). On the basis of recent theoretical analysis, two limiting structures for CPop may now be considered these are shown in the bottom box in Scheme 10.5. The reason for the exceptional stability of CPop as a trialkylcopper(III) species can be readily understood in terms of the j5-cuprio(III) enolate structure, with the internal enolate anion acting as a strong stabilizing ligand for the Cu state [82]. 

Organolithium 482 has been represented by the extended enolate structure 487 (Scheme 191), though the chirality of some analogues of 482 argues in favour of the localized structure 482 °". ... 

An interesting variant of reactions with double bonds is the action of [ F]F2 or other electrophilic [ F]reagents on enol structures. 

In cyclopropane carboxylates the ring strain influences the acidity of the a-carbon, thus the enolates are more difficult to prepare and once made, are more reactive than in the higher-inem-bered rings. These enolates probably do not have an enolate structure, but rather are a-metal-lated species. 

The opportunity for chelation in the various enolate intermediates offers a possible explanation for the observed diastereoselectivities. In the dianions derived from l-acyl-2-pyrrolidinemethanols strong chelation of both of the lithium cations should lead to a rigid enolate structure 9. It is reasonable to assume that the pyrrolidine ring is locked in one conformation. Since, according to models, it is difficult to attribute the observed high diastereoselectivity to steric hindrance, it is probable that the lone pair on the nitrogen directs the facial selectivity of electrophilic attack (see Section 1.1.1.3.3.1.) to one side of the enolate a-carbon. 

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