Metal enolates 2- ester

The decarboxylation of allyl /3-keto carboxylates generates 7r-allylpalladium enolates. Aldol condensation and Michael addition are typical reactions for metal enolates. Actually Pd enolates undergo intramolecular aldol condensation and Michael addition. When an aldehyde group is present in the allyl fi-keto ester 738, intramolecular aldol condensation takes place yielding the cyclic aldol 739 as a main product[463]. At the same time, the diketone 740 is formed as a minor product by /3-eIimination. This is Pd-catalyzed aldol condensation under neutral conditions. The reaction proceeds even in the presence of water, showing that the Pd enolate is not decomposed with water. The spiro-aldol 742 is obtained from 741. Allyl acetates with other EWGs such as allyl malonate, cyanoacetate 743, and sulfonylacetate undergo similar aldol-type cycliza-tions[464]. 

Enolate Initiators. In principle, ester enolate anions should represent the ideal initiators for anionic polymeri2ation of alkyl methacrylates. Although general procedures have been developed for the preparation of a variety of alkaU metal enolate salts, many of these compounds are unstable except at low temperatures (67,102,103). Usehil initiating systems for acrylate polymeri2ation have been prepared from complexes of ester enolates with alkak metal alkoxides (104,105). 

Addition Reactions of Metal Enolates of Non-stabilized Esters, Amides, and Ketones to Epoxides... 

In spite of their intrinsic synthetic potential, addition reactions of metal enolates of non-stabilized esters, amides, and ketones to epoxides are not widely used in the synthesis of complex molecules. Following the seminal work of Danishefsky [64], who introduced the use of Et2AlCl as an efficient catalyst for the reaction, Taylor obtained valuable spiro lactones through the addition reaction of the lithium eno-late of tert-butyl acetate to spiro-epoxides, upon treatment of the corresponding y-... 

A direct application of the ring-opening reaction of an epoxide by a metal enolate amide for the synthesis of a complex molecule can be found in the synthesis of the trisubstituted cyclopentane core of brefeldin A (Scheme 8.35) [68a]. For this purpose, treatment of epoxy amide 137 with excess KH in THF gave a smooth cyclization to amide 138, which was subsequently converted into the natural product. No base/solvent combination that would effect cyclization of the corresponding aldehyde or ester could be found. 

In the Michael addition of achiral enolates and achiral Michael acceptors the basic general problem of simple diastereoselection (see Section D.1.5.1.3.2.), as described in Section 1.5.2.3.2. is applicable. Thus, the intermolecular 1,4-addition of achiral metal enolates to enones, a.jS-unsat-urated esters, and thioamides, results in the formation of racemic syn-1,2 and/or anti-3,4 adducts. 

Stereoselective aldol condensation. The stereoselectivity of the reaction of 1 with the ester 2 can be controlled by the choice of the metal enolate. The products are intermediates to 1-methylcarbapenems. 

The preferential -configuration of the enol esters, derived from p-dicarbonyl compounds under phase-transfer conditions, contrasts with the formation of the Z-enol esters when the reaction is carried out by classical procedures using alkali metal alkoxides. In the latter case, the U form of the intermediate enolate anion is stabilized by chelation with the alkali metal cation, thereby promoting the exclusive formation of the Z-enol ester (9) (Scheme 3.5), whereas the formation of the ion-pair with the quaternary ammonium cation allows the carbanion to adopt the thermodynamically more stable sickle or W forms, (7) and (8), which lead to the E-enol esters (10) [54],... 

Extensive investigations have been directed toward the development of chiral ester enolates that might exhibit practical levels of aldol asymmetric induction. Much of the early work in this area has been reviewed (111). In general, metal enolates derived from chiral acetate and propionate esters exhibit low levels of aldol asymmetric induction that rarely exceed 50% enantiomeric excess. The added problems associated with the low levels of aldol diastereoselection found with most substituted ester enolates (cf. Table 11) further detract from their utility as effective chiral enolates for the aldol process. Recent studies have examined the potential applications of the chiral propionates 121 to 125 in the aldol condensation (eq. [94]), and the observed erythro-threo diastereoselection and diastere-oface selection for these enolates are summarized in Table 31. For the six lithium enolates the threo diastereoselection was found to be... 

Aldol reactions of silyl enolates are promoted by a catalytic amount of transition metals through transmetallation generating transition metal enolates. In 1995, Shibasaki and Sodeoka reported an enantioselective aldol reaction of enol silyl ethers to aldehydes using a Pd-BINAP complex in wet DMF. Later, this finding was extended to a catalytic enantioselective Mannich-type reaction to a-imino esters by Sodeoka s group [Eq. (13.21)]. Detailed mechanistic studies revealed that the binuclear p-hydroxo complex 34 is the active catalyst, and the reaction proceeds through a palladium enolate. The transmetallation step would be facilitated by the hydroxo ligand transfer onto the silicon atom of enol silyl ethers ... 

Although enol esters have a similar structure to enamides, they have proven more difficult substrates for asymmetric hydrogenation, which is evident from the significantly fewer number of examples. One possible explanation is the weaker coordinating ability of the enol ester to the metal center, as compared to the corresponding enamide. Some rhodium complexes associated with chiral phosphorous ligands such as DIPAMP [100, 101] and DuPhos [102] are effective for asymmetric hydrogenation of a-(acyloxy)acrylates. 

Sodium borohydride (160) was found to serve as a hydrogen donor in the asymmetric reduction of the presence of an a,pi-unsaturated ester or amide 162 catalyzed by a cobalt-Semicorrin 161 complex, which gave the corresponding saturated carbonyl compound 163 with 94-97% ee [93]. The [i-hydrogen in the products was confirmed to come from sodium borohydride, indicating the formation of a metal enolate intermediate via conjugate addition of cobalt-hydride species (Scheme 2.17). 

Lin, M.-H. RajaBabu, T. V. Metal-catalyzed acyl transfer reactions of enol esters role of Y5(OiPr)i30 and (thd)2Y(OiPr) as transesterification catalysts. Org. Lett. 2000, 2, 997-1000. 

The addition of an alkaline earth metal enolate A to a carbonyl compound is always an exer-gonic process irrespective of whether the enolate is derived from a ketone, an ester, or an amide and whether the carbonyl compound is an aldehyde or a ketone (Figure 13.44, top). One of the reasons for this exergonicity hes in the fact that the alkaline earth metal ion is part of a chelate in the alkoxide B of the aldol addition product. The driving forces for the additions of alkaline earth metal enolates of esters and amides to carbonyl compounds are further increased because the aldol adducts B are resonance-stabilized, whereas the enolates are not. 

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