Boron ester enolate

Diboration of a,/3-unsaturated esters is catalyzed by the platinum(0)/diimine catalyst, giving a,/3-diboryl esters, that is, 3,4-addition products (Equation (13)). Although the a,/3-diboryl ester products are hydrolytically more stable than the corresponding 1,4-addition products bearing a boron ester enolate moiety, they gradually undergo hydrolysis... 

Most [3,3]-sigmatropic rearrangements take place thermally, and the Cope, oxy-Cope and Claisen rearrangements are among the most important rearrangements in this class. Important variants of the Claisen rearrangement include the Johnson modification via orthoesters, the Eschenmoser modification via ketene N,O-acetals, the Ireland modification via ketene silylacetals and the Corey modification via boron ester enolates [696], The aza-Claisen rearrangement has also seen... 

Deoxy-D-erythro-pentose (2 1) has been elaborated from 2,3-0-isopropylldene-D-glyceraldehyde (25) either by a stereoselective dipolar cycloaddition reaction (Scheme 6, route a)" or by a stereoselective aldol condensation involving a boronated ester enolate reagent (route b). The DL-analogue of (24) has been synthesized from 1,3 2,4-di- -benzylidene-erythrltol by mono-bromination (Hanessian s procedure with NBS) to give a l-bromo-3- -benzoate,... 

The enolates of other carbonyl compounds can be used in mixed aldol reactions. Extensive use has been made of the enolates of esters, thiol esters, amides, and imides, including several that serve as chiral auxiliaries. The methods for formation of these enolates are similar to those for ketones. Lithium, boron, titanium, and tin derivatives have all been widely used. The silyl ethers of ester enolates, which are called silyl ketene acetals, show reactivity that is analogous to silyl enol ethers and are covalent equivalents of ester enolates. The silyl thioketene acetal derivatives of thiol esters are also useful. The reactions of these enolate equivalents are discussed in Section 2.1.4. 

Although this general principle of asymmetric induction has not been demonstrated for boron enolates, the related addition reactions of allylboranes to aldehydes (eq. [115]) (131) have been examined in this context. The reaction of chiral diol 175 with either triallyl-borane or tri- -methallylborane afforded the boronic esters 176 (Ri = H, Me) in yields exceeding 95% (132a). The addition reactions of 176 to representative aldehydes are summarized in Table 40. In all cases reported, the sense of asymmetric induction from the chiral... 

If the pKa of the corresponding acid R1 - H from the stabilized carbanion is smaller than 35, the migration of R1 fails in (dichloromethyl)borate complexes. Failure to convert pinanediol [(phenylthio)methyl]boronate to an a-chloro boronic ester has been reported15. Reaction of (dichloromethyl)lithium with an acetylenic boronic ester resulted in loss of the acetylenic group to form the (dichloromethyl)boronate, and various attempts to react (dichloromethyl)boronic esters with lithium enolates have failed17. Dissociation of the carbanion is suspected as the cause, but in most cases the products have not been rigorously identified. 

The most advanced synthetic methods involve chiral directors that have C2 symmetry. These are discussed first for x-chloro boronic esters (Section 1.1.2.1.2.1.) and then for the bromo analogs, which are better in reactions involving enolates (Section 1.1.2.1.2.2.). The first syntheses of secondary alcohols utilized pinanediol as chiral director (Section 1.1.2.1.2.3.). The method is marginally successful for some tertiary alcohols (Section 1.1.2.1.2.4.). 

Carboxylic Ester Enolates with a-Bromo Boronic Esters... 

Ester enolates replace bromide from a-bromo boronic esters with remarkable diastereoselcctiv-ity. (Dibromomethyl)lithium is generated by addition of lithium diisopropylamide to dibro-momethane in the presence of a boronic ester at — 78 "C to produce an a-bromo boronic ester. Reaction of the a-bromo boronic ester with lithium 1-tert-butoxy-Tpropen-l-olate yields a product that is almost exclusively the threo-isomer (d.r. = 15 1 to 60 1), as shown by conversion to the / -hydroxy carboxylic ester24. It is worth noting the facility with which a-bromo boronic esters racemize in the presence of halide ions72. 

The one-pot condensation of an ester enolate with an imine is a very powerful synthetic procedure toward azetidin-2-ones (Equation 183). Various types of esters and imines can be utilized. Although in the vast majority the reactions have been mediated by lithium, various other metals mediate the reaction as well. Some examples include zinc, aluminium, tin, boron, indium, and titanium <1996MI119>. Theoretical studies on these reactions have been reviewed <1998JCC1826>. 

Boronic esters are not as reactive as triorganylboranes towards nucleophiles however, rearrangement does take place if the boronic ester is treated with an a-halocarbanion (equation 54). In this case, the I-chloroethylboronate (36) can be obtained in 95% de due to the diastereofacial influence of the chiral pinanediol. Similarly, chloro- or bromo-alkylboronic esters react with Grignards reagents, alkyllithiums and enolates leading to rearranged products. ... 

Masamune has documented the addition of optically active ester enolates that afford lanfi-aldol adducts in superb yields and impressive stereoselectivity (Eq. (8.3)) [4]. The generation of a boryl enolate from 8 follows from groundbreaking studies of ester enolization by Masamune employing dialkyl boryl tri-flates and amines [5]. Careful selection of di-n-alkyl boron triflate (di-n-butyl versus dicyclopentyl or dicyclohexyl) and base (triethyl amine versus Hiinigs base) leads to the formation of enolates that participate in the <2u//-selective propionate aldol additions. Under optimal conditions, 8 is treated with 1-2 equiv of di-c-hex-yl boron triflate and triethyl amine at -78 °C followed by addition of aldehyde the products 9 and 10 are isolated in up to 99 1 antv.syn diastereomeric ratio. The asymmetric aldol process can be successfully carried out with a broad range of substrates including aliphatic, aromatic, unsaturated, and functionalized aldehydes. 

This process works best when the tetrazine has electron-withdrawing snbstitnents, but 1,2,4,5-tetrazine itself will react with a range of simple aUcynes, enamines and enol ethers, nnder quite moderate condi-tions. A wide range of substitnents can be incorporated via the acetylene, including nitro and trimethyl-silyl, affording the means to access other snbstitnted pyridazines. The preparation of a boronic ester is another example. 

Reaction C in Fig. 4 is an aldol condensation between an achiral aldehyde and an ester enol borinate featuring a bidentate chiral substituent at the boron atom [24]. Upon enolate-boron/aldehyde-oxygen co-ordination, two chair-like TS can be formed, both featuring the aldehyde phenyl group in a pseudo-equatorial position. Preferential attack on the aldehyde Si face is determined by the spatial arrangement of the metal ligand. The almost exclusive formation of the anti diastereoisomers arises from control of the enolate geometry. 

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