Aldol reaction chiral enolates

As the t-butyl group can readily be removed upon acidic or basic hydrolysis, this method can also be used for //-hydroxyl acid synthesis. In analogy with allylation reactions, the enolate added preferentially to the Re-face of the aldehydes in aldol reactions. Titanium enolate 66 tolerates elevated temperatures, while the enantioselectivity of the reaction is almost temperature independent. The reaction can be carried out even at room temperature without significant loss of stereoselectivity. We can thus conclude that this reaction has the following notable advantages High enantiomeric excess can be obtained (ee > 90%) the reaction can be carried out at relatively high temperature the chiral auxiliary is readily available and the chiral auxiliary can easily be recovered.44... 

Table 9.3 Asymmetric aldol reactions of enol silyl ethers with benzaldehyde catalyzed by chiral quaterna7 ammonium fluorides 4b or 4c.

Ghosh also took advantage of the C—2 hydroxyl moiety of aminoindanols as a handle in the aldol reaction. Chiral sulfonamide 41 was O-acylated to give ester 42. The titanium enolate of ester 42 was formed as a single isomer and added to a solution of aldehyde, precomplexed with titanium tetrachloride, to yield the anft -aldol product 43 in excellent diastereoselectivities.63 One additional advantage of the ester-derived chiral auxiliaries was their ease of removal under mild conditions. Thus, hydrolysis of 43 afforded a ft -a-methyl- 3-hydroxy acid 44 as a pure enantiomer and cis-1-/ -1 o I y I s u I f on a m i do- 2 - i n da n ol was recovered without loss of optical purity (Scheme 24.7).63... 

Mukaiyama and co-workers developed a chiral Lewis acid complex 15 consisting of tin (II) triflate and a chiral diamine. An aldol reaction of enol silyl ether 16 and octanal is promoted by 15 to give 17 in a highly diastereo-and enantioselective manner. The enantioface of the aldehyde is selectively activated by coordination with 15. This method is similar to method 3, in that an aldehyde-chiral Lewis acid complex can be regarded as a chiral electrophile. An advantage of method 4 over method 3 is the possible catalytic use of a chiral Lewis acid. In the reaction of Scheme 3.6, 20 mol% of 15 effects the aldol reaction in 76% yield with excellent selectivity.9... 

Asymmetric Aldol Reactions. Lithium enolates, derived from an ester, and LDA react with aldehydes enantioselectively in the presence of the chiral amide 2 (eq 3). When benzaldehyde is employed, the major diastereomer is the anrt-aldol with 94% ee, while the minor yn-aldol is only 43% ee. In this reaction, the lithium amide 2 coordinates to an additional lithium atom. There are four additional examples of aldehydes with the same ester enolate. 

The asymmetric aldol reaction of enol silyl ethers of thioesters with aldehydes is performed in high enantiomeric excess by employing a chiral promoter, tin(II) trifluoromethanesulfonate coordinated with chiral diamine 1 and tri-n-butyltin fluoride (eqs 20 and 21). Highly enantioselective aldol reactions of achiral ketene silyl acetals with achiral aldehydes are carried out by means of the same chiral promoter (eq 22). ... 

Davies, S. G., Edwards, A. J., Evans, G. B., Mortlock, A. A. Bifunctional chiral auxiliaries. 7. Aldol reactions of enolates derived from 1,3-diacylimidazolidine-2-thiones and 1,3-diacylimidazolidin-2-ones. Tetrahedron 1994, 50, 6621-6642. 

Boron enolates generated from a-heterosubstituted thioacetates by treatment with 105 undergo highly enantioselective and diastereoselective condensations. On the other hand, chiral esters 106 and 107, and amides 108 behave differently. V-Acyl derivatives of the bicyclic isoxazolidine 109 ° readily undergo syn-selective aldol reactions via enol borates. 

In the Evans synthesis of the polypropionate region (Scheme 9-45), the boron-mediated anti aldol reaction of -ketoimide ent-25 with a-chiral aldehyde 145 afforded 146 with 97% ds in what is expected to be a matched addition. Adduct 146 was then converted into aldehyde 147 in readiness for union with the C -Cs ketone. This coupling was achieved using the titanium-mediated syn aldol reaction of enolate 148 leading to the formation of 149 with 97% ds. 

In contrast with the above Lewis acid-catalyzed asymmetric aldol reactions, chiral Pd and Pt cationic complexes have been found to catalyze the asymmetric process by a transmetalation mechanism involving a metal enolate intermediate (Section 10.2.1.3). 

In recent years, catalytic asymmetric Mukaiyama aldol reactions have emerged as one of the most important C—C bond-forming reactions [35]. Among the various types of chiral Lewis acid catalysts used for the Mukaiyama aldol reactions, chirally modified boron derived from N-sulfonyl-fS)-tryptophan was effective for the reaction between aldehyde and silyl enol ether [36, 37]. By using polymer-supported N-sulfonyl-fS)-tryptophan synthesized by polymerization of the chiral monomer, the polymeric version of Yamamoto s oxazaborohdinone catalyst was prepared by treatment with 3,5-bis(trifluoromethyl)phenyl boron dichloride ]38]. The polymeric chiral Lewis acid catalyst 55 worked well in the asymmetric aldol reaction of benzaldehyde with silyl enol ether derived from acetophenone to give [i-hydroxyketone with up to 95% ee, as shown in Scheme 3.16. In addition to the Mukaiyama aldol reaction, a Mannich-type reaction and an allylation reaction of imine 58 were also asymmetrically catalyzed by the same polymeric catalyst ]38]. 

Galatsis group [14] reported a study on an NARC sequence involving (i) aldol reactions of enolates derived from the kinetic deprotonation of unsaturated esters, such as 25 and 28, to ketones (Fig. 9) and aldehydes (Fig. 10) followed by (ii) endo-cyclisation via intramolecular iodoetherification. As the enolates used in the study were racemic and the aldol reactions stereorandom, it would be interesting to repeat this work using a chiral auxiliary (e.g. a chiral amide). This should ensure high levels of enantio- and diastereo-selectivity. 

Oppolzer and coworkers [147, 454] have developed a class of reagents based on the enantiomeric bomane-2,10-sultam skeleton 1.133. These chiral auxiliaries are easily prepared from the enantiomeric 10-camphosulfonic adds [455]. Saturated or a,P-unsaturated TV-acylsultams 1.134, occasionally prepared from Af-silyl precursors [396], have been used very frequently. Asymmetric alkylations, animations and aldol reactions of enolates or enoxysilane derivatives of 1.134 (R = R CH2) [147, 404, 407, 456-460] are highly selective. The a,(3-unsaturated TV-acylsultams 1.134 (R = R R"C=CH) suffer highly stereoselective organocuprate 1,4-additions [147, 173], cyclopropanations [461], [4+2] and [3+2] cydoadditions [73,276,454,462], OSO4 promoted dihydroxylations [454,463] and radical addi-... 

Copper(II) complexes of two imino nitrogen atoms belonging to chiral oxazoline and sulfoximine moieties (70) are able to elicit asymmetric consequences in the Mukaiyama-aldol reaction of enol silyl ethers and a-keto esters/ ... 

In an aldol reaction, the enolate of one compound reacts with the electrophilic carbonyl carbon of the other carbonyl compound. A problem can arise when the other regioisomeric enolate can form easily or when the electrophilic carbonyl compound is enolizable. In addition, the product is enolizable and the wrong carbonyl compound could act as the electrophile therefore a mixture of products or predominantly the undesired product may result. An added complication arises when more than one chiral centre is present in the product and therefore two diastereomeric products can be formed. The course of the reaction between unlike components must be directed so that only the product required is obtained, or at least is formed predominantly. In addition, the stereochemical course of the reaction must be controlled. These difficulties have been overcome as a result of intensive study of the aldol reaction, spurred on by the presence of the (3-hydroxycarbonyl functional group in the structures of many naturally occurring compounds such as the macrolides and ionophores. 

Example 4.3 This example demonstrates a notable difference in stereoselectivity between alkylation and aldol reactions of enolates derived from chiral oxazolidi-nones. Lithium enolates of oxazolines 27 and 28 proved exceptionally efiicient in the control of the stereoselectivity of alkylation (Sect. 3.7.3, Schemes 3.12 and 3.13 ) but react with low stereoselectivity in aldol reactions. Instead, high stereochemical control of aldol reactions is achieved with boronic enolates of chiral oxazolidinones 9-13 (Scheme 4.11). 

Diastereoselective Aldol Reactions Using Chiral Enolates Although the approach that uses chiral auxiliaries or other elements of stoichiometric chiral information is not as elegant as asymmetric catalysis, it is highly practical. At present, the boron-mediated aldol reaction of enolates 112 and 115 with aldehydes giving the syn aldol products 113 and 116 constimtes one of the best C C bond forming methods (Scheme 10.26). ... 

SCHEME 10.35. Paterson et al. s diastereoselective aldol reaction with enolates possessing a-chirality. 

Within this section, the term aldol reaction includes additions of enols and enolates to carbonyl compounds. This section concentrates on aldol additions which deliver nonracemic, /i-hydroxycarbonyl compounds. The chiral information can be located ... 

Extension of this aldol reactivity to the preparation of chiral materials via condensation reactions of the chiral enolate species 2 and 3 is discussed in the following sections. 

Aldol reactions of a-substituted iron-acetyl enolates such as 1 generate a stcrcogenic center at the a-carbon, which engenders the possibility of two diastereomeric aldol adducts 2 and 3 on reaction with symmetrical ketones, and the possibility of four diastereomeric aldol adducts 4, 5, 6, and 7 on reaction with aldehydes or unsymmetrical ketones. The following sections describe the asymmetric aldol reactions of chiral enolate species such as 1. 

The achiral molybdenum enolate 2 reacts with benzophenone and benzaldehyde at —78 °C to yield the x,/i-unsaturated molybdenum-acyl complexes34, which presumably arise via elimination of hydroxide from unobserved aldolate intermediates such as 3. No examples of such aldol reactions with complexes that are chiral at molybdenum have been reported. 

In a chiral aldehyde or a chiral ketone, the carbonyl faces are diastereotopic. Thus, the addition of an enolate leads to the formation of at least one stereogenic center. An effective transfer of chirality from the stereogenic center to the diastereoface is highly desirable. In most cases of diastereoface selection of this type, the chiral aldehyde or ketone was used in the racemic form, especially in early investigations. However, from the point of view of an HPC synthesis, it is indispensable to use enantiomerically pure carbonyl compounds. Therefore, this section emphasizes those aldol reactions which are performed with enantiomerically pure aldehydes. 

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