Preparation of enolates

As in the case of aldol addition, the scope of conjugate addition reactions can be extended by the use of techniques for regio- and stereospecific preparation of enolates and enolate equivalents. If the reaction is carried out with a stoichiometrically formed enolate in the absence of a proton source, the initial product is the enolate of the adduct. The replacement of a tt bond by a ct bond ensures a favorable AH. 

All of the glassware used in the preparation of enol ether (3) was dried for at least 10 min at 200°C, assembled hot, and allowed to cool under an atmosphere of argon. 

The reduction of a,/ -unsaturated ketones, e.g., 7, by lithium in ammonia is another possibility for generation of enolates19. This method has been mainly used in the preparation of enolates for stereo- and regioselective alkylations in the steroid and terpenoid fields (see... 

The magnesium amides of choice for the preparation of magnesium enolates via met-allation are the Hauser bases, such as 39 and 40, or (bis)amidomagnesium reagents, such as 46 and 47. The reaction has been successfully applied to the preparation of enolates derived from cyclic, acyclic and a-siloxyketones, benzyUc ketones, aldehydes, carboxylic esters and amides, even with the less hindered Hauser bases. 

Enol carbonates and carbamates. Olofson and Cuomo have extended the regio-ipccilic alkylation of ketones (6, 44) to a regiospeeific preparation of enol carbonates and carbamates. The enol carbonates are prepared by reaction of silyl... 

Preparation of enamines, 409 Preparation of enol acetates, 411 Preparation of enol ethers, 409 Preparation of a-halo ketones, 267 Preparation of 3/3-hydroxyandrosta-5,15-dien-17-one, 302... 

Some of the reactions of PO3- parallel enzymatic reactions promoted by adenosine triphosphate (ATP). Pyruvate kinase catalyzes the equilibration of ATP and pyruvate with adenosine diphosphate (ADP) and phosphoenol pyruvate (11,12). In a formal sense, this reaction resembles the preparations of enol phosphate (eqs. 6 and 7). Cytidine triphosphate synthetase catalyzes the reaction of uridine triphosphate with ammonia to yield cytidine triphosphate (13). In a formal sense, this reaction resembles the replacement of the ester carbonyl group of ethyl acetate by the nitrogen of aniline (eq. 8). 

Reduction of enones. Reduction of CH3Cu, prepared in situ from Cul and CH3Li in THF, with DIBAH in the presence of HMPA results in a form of copper hydride that effects efficient and selective 1,4-reduction of enals, enones, and enoates, and 1,6-reduction of dienones and dienoates. The reagent does not reduce isolated carbonyl groups or double bonds.2 It can also be used for regiospecific preparation of enol silyl ethers from an enone.2... 

Scheme 4. Preparation of enol esters and dienyl esters from terminal alkynes and carboxylic acids.

Table 13.4 also shows that the deprotonation of isopropanol with LiHMDS is less than half as exothermic as the deprotonations with LDA or LTMP. Hence, LiHMDS is a much weaker base than the other two amides. This is due to the ability of the SiMe3 groups of LiHMDS to stabilize the negative charge in the a-position at the N atom. The mechanism of this stabilization might be the same as in the case of the isoelectronic triphenylphosphonium center in P ylides (Figure 11.1), that is, a combination of an inductive effect and anomeric effect. Because of its relatively low basicity, LiHMDS is employed for the preparation of enolates primarily when it is important to achieve high chemoselectivity. 

D. Cahard, P. Duhamel, Alkoxide-Mediated Preparation of Enolates from Silyl Enol Ethers and Enol Acetates - From Discovery to Synthetic Applications, Eur. J. Org. Chem. 2001, 1023-1031. 

Fig. 10.10. Regioselective generation of ketone enolates II the effects of different substituents in the ft and [V positions. (For the regioselective preparation of enolates C and F, see Figures 10.16 and 10.17, respectively.)...

Enol and enolate reactions at oxygen preparation of enol ethers... 

The preparation of enols of simple aldehydes and ketones has been achieved by rDA reactions under FVP conditions. Equation (17) illustrates the formation of enols by this method. The percentage of enol reported shows that the enols of aldehydes appear to be isolated more readily than those of the ketones. The last two entries are preparations of the simplest enediols, ( )- and (Z)-ethylene-l,2-diols. A very similar process was used to unmask the protected ethylene cycloadduct (23) and produce 2-hydroxybu-tadiene (24), an elusive enol form of the more stable unsaturated ketone, as given by equation (18). ... 

Transition metal catalyzed hydrosilylation of a, -unsaturated ketones and aldehydes (78) proceeds in a 1,4- and/or 1,2-fashion to give enol silyl ethers (79) (or the saturated carbonyl compounds 80 after acidic work-up) and/or allyl silyl ethers (81) (or allyl alcohols 82), respectively (equation 54). Monohydrosilanes combined with a Pt or Rh catalyst prefer 1,4-addition. This reaction is an alternative meth for the preparation of enol silyl ethers (79). Diphenylsilane with [RhCl(PPh3)3] catalyst reduces the car-... 

Suitable derivatives to render most steroids more stable and volatile for gas chromatography and to improve their mass spectrometric properties have been developed. The trimethylsilyl ethers [206] or the methoxime-trimethylsilyl ethers of hydroxy keto steroids [207] have been widely used and their mass spectra extensively studied [204], A comparison of the GC-MS behaviour of several derivatives of some of the adrenocortical hormones has been made [208], including methoxime-trimethylsilyl ethers, dimethylsiliconides, methyl boronates, oxetanones and acetonides. The recently described improved preparation of enol-trimethylsilyl ethers of corticoids [209] should also prove of value. 

Polymers and copolymers of vinylic carbonates and carbamates may find interesting applications as aroma and flavours releasing agents. For example, isopropenyl menthyl carbonate has been patented (Ref. 148) as an useful monomer for the manufacture of a smoking composition comprising an admixture of tobacco and a menthol-release agent. Recently, Harwood et. al (Ref. 149) have published a new preparation of enol carbonates including especially isopropenyl menthyl carbonate by selective O-acylation of ketones sodium... 

Although carboxylic acids and their derivatives are somewhat weaker carbon acids than aldehydes and ketones, it is generally possible to quantitatively convert them to the corresponding metal enolates with dialkylamide bases, the most popular of which is LDA. - - Thus, monoanions of saturated esters, lactones, nitriles, /VA -dialkylamides and V-alkyllactams and dianions of carboxylic acids and V-unsub-stituted amides and lactams are easily prepared in aprotic solvents such as THF and C-alkylated with a variety of simple and functionalized SN2-reactive alkylating agents at room temperature or below. When more-hindered systems are involved, the basicity of the metal dialkylamide and the reactivity of the metal enolate can be enhanced by the addition of HMPA. Of course, many of the indirect methods used for the generation of aldehyde and ketone enolates are also applicable to the preparation of enolates of carboxylic acid derivatives (Section 1.1.2.1). O-Alkylations or dialkylations at carbon generally are of minimal importance with metal enolates of carboxylic acid derivatives. 

Preparation of enol acetates. Barton et al. used perchloric acid as catalyst for... 

Preparation of enol ethers. Condensation of a 3-keto-5o -steroid (1) with triethyl orthoformate in ethanol containing a trace of hydrogen chloride affords the diethyl ketal (2) in good yield when refluxed in xylene the ketal loses a molecule of ethanol and affords the A -enol ether (3). A A -3-ketosteroid (4) when condensed with... 

Cyclopropanes II via Nucleophilic Attack on Tt-Allylpalladium Complexes General Procedure Preparation of Enolates ... 

Preparation of enol acetates [1, 800, after citation of ref. 18] It has been shown in at least two cases that the perchloric acid catalyzed acetic anhydride enol acetylation is thermodynamically controlled.183 Thus the 3-keto-5/3-steroid, 17/3-acetoxy-50-androstane-3-one (1), on reaction with acetic anhydride-perchloric acid gives the two enol acetates (2) and (3) in the ratio 93.5 to 6.5. The calculated ratio derived from the relative stability of (2) and (3) is 96 4. If a mixture of 65% of (2)... 

Etiolates of prednisone-BMD. Two groups1-2 have independently studied the preparation of enolates of prednisone-BMD (1), (17,20 20,21-bismethylene-dioxyprednisone). Treatment of (1) with the more usual bases (triphenylmethyl-... 

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