Diels-Alder reaction boron compound

Chiral boron(III) Lewis acid catalysts have also been used for enantioselective cycloaddition reactions of carbonyl compounds [17]. The chiral acyloxylborane catalysts 9a-9d, which are also efficient catalysts for asymmetric Diels-Alder reactions [17, 18], can also catalyze highly enantioselective cycloaddition reactions of aldehydes with activated dienes. The arylboron catalysts 9b-9c which are air- and moisture-stable have been shown by Yamamoto et al. to induce excellent chiral induction in the cycloaddition reaction between, e.g., benzaldehyde and Danishefsky s dienes such as 2b with up to 95% yield and 97% ee of the cycloaddition product CIS-3b (Scheme 4.9) [17]. 

Enyne 112a having a boron moiety on the alkyne gives vinyl boron 113a [Eq. (6.83)], whose carbon-boron bond can be converted to a carbon-carbon bond. Enyne metathesis followed by Diels-Alder reaction of the resulting diene with DMAD gives tricyclic compound 114 [Eq. (6.84)]. ... 

In order to introduce functionality at key positions, an intramolecular Diels-Alder reaction with furan as the diene component was planned. Suzuki coupling of the previously mentioned 105 with furan-3-boronic acid 106 gave compound 107, which was N-tosylated to 108. Construction of the dienophile portion was performed as in the previous case, by addition of vinylmagne-sium bromide to give 109, followed by MnC>2 oxidation to give the cyclization precursor, which was immediately heated at 120 °C in toluene and afforded compound 110, which incorporates an oxygen atom at both the ketone and vinyl chloride positions of welwistatin (Scheme 24). 

Imidazole-containing compounds have been utilized as reagents for various synthetic transformations. A convenient access to substituted allyl enol carbonates was established through the reaction of ketone enolates with the complex of allyl l//-imidazole-l-carboxylates 74 and boron trifluoride etherate <07JOC9372>. Relatively mild and highly efficient Cul-catalyzed /V-arylation procedures for imidazoles with aryl and heteroaryl bromides or chlorides have been developed in the presence of ligands 75 and cesium carbonate <07JOC2737>. a, -Unsaturated 2-acyl imidazoles 76 are an alternative and practical class of dienophiles for the DNA-based catalytic asymmetric Diels-Alder reaction in... 

About 10 years later the catalytic use of boron derivatives was again investigated by Kaufmann et al. Compounds of the type RBBr2 (R = pinanyl) catalyze the Diels-Alder reaction at -78 °C with low ee (Eq. 3) [4]. Modified isopinocamphenyl dibro-moborane, which has been synthesized since then, gives better results in the reaction of cyclopentadiene and methyl acrylate (Eq. 4) [5]. 

The Diels-Alder reaction is a key reaction in organic synthesis. Its high versatility in the synthesis of six-membered ring compounds and its potential for the control of up to four stereogenic centers have attracted much attention. Lewis acid catalysis has further enhanced the scope of this reaction. Lewis acids activate the dienophile by coordination to a Lewis basic substituent (usually a carbonyl group) and direct the stereochemistry. Boron Lewis acids are often the catalysts of choice for the Diels-Alder reaction. Early (Ti(IV)) and late (Cu(II)) transition metal complexes in combination with chiral ligands have also been used with much success and the reader is referred to the relevant chapters in this book. 

The first total synthesis of ( )-lycoridine margetine (33), a member of the non-basic alkaloids which possess antimitotic activity has been achieved (Scheme 3). The homoallylic alcohol (26) served as a latent diene in a Diels-Alder reaction with ethyl acrylate to give a diastereomeric mixture of adducts which upon base equilibration and hydrolysis provided the tmns-acid (27). Modified Curtius reaction on (27) afforded the corresponding isocyanate which was cyclized to the lactam (28) in 89% yield by a new method using boron trifluoride etherate. Compound (28) was converted into an N-acetyl derivative which upon basic hydrolysis gave the acid (29). Treatment of (29) with NBS followed by reflux in pyridine solution gave the lactone... 

Early applications of chiral Lewis acid catalyzed stereoselective Diels Alder reactions used either boron- or aluminum-derived systems in carbocyclic ring formation18 1Q, or studied the effect of chiral shift reagents, such as Eu(hfc)3, in hetero-Diels-Alder cycloadditions of carbonyl compounds to dienes20 23,77, 78. The latter type of transition metal catalyzed addition is classified as heterocarboration and is described in Section 1.5.8.4. 

Recently, some efficient asymmetric Diels-Alder reactions catalyzed by chiral Lewis acids have been reported [67]. The chiral Lewis acids employed in these reactions are generally based on traditional acids such as titanium, boron, or aluminum reagents, and they are well modified to realize high enantioselectivi-ties. Although lanthanide compounds were expected to be Lewis acid reagents, only a few asymmetric reactions catalyzed by chiral lanthanide Lewis acids were reported. Pioneering work by Danishefsky et al. demonstrated that Eu(hfc)3 (an NMR shift reagent) catalyzed hetero-Diels-Alder reactions of aldehydes with si-loxydienes, but enantiomeric excesses were moderate [68]. 

Modifications of these basic materials include the formation of the V-methanesu 1 tonate 36 which has been used for the synthesis of boronates used in allyl transfer reactions39 (Section D. 1.3.3.3.) and the attachment of the compounds to a chloromethylated polystyrene (Merrifield resin)40. The immobilized amino alcohol has been used as a catalyst for the addition of zinc alkyls to carbonyl compounds (Section D. 1.3.1.4.). 2,/ -Unsaturated dihvdrooxazoles have been prepared from such amino alcohols by reaction with imino ethers41 and used as chiral dienophiles in Diels-Alder reactions (Section D. 1.6.1.1.1.2.2.1.1.). 

Both enantiomers of binaphthol have found many uses as chiral reagents and catalysts. Thus, they modify reducing agents (e.g., lithium aluminum hydride) for the reduction of ketones to chiral secondary alcohols (Section D.2.3.3.2.) or react with aluminum, titanium or boron compounds to give chiral Lewis acids for asymmetric Diels-Alder reactions (Section D. 1.6.1.1.1.3.) and ene reactions (Section D.I.6.2.). They have also been used as chiral leaving groups in the rearrangement of allyl ethers (Section D.l.1.2.2.) and for the formation of chiral esters with a-oxo acids (Section D. 1.3.1.4.1, and many other purposes. 

A significant contribution in the area of low-coordinate boron chemistry was realized by Corey with the synthesis of chiral tri-coordinate and cationic oxaborolidine complexes such as (4), for use in asymmetric catalysis. These compounds can be obtained by reaction of the desired chiral aminophenol with BBrs, and they are highly effective catalysts for enantioselective Diels-Alder reactions. Smith also reported the generation of the tri-coordinate boron cation HC(CMeNAr)2 BMe+, obtained by a Me abstraction reaction of HC(CMeNAr)2 BMe2 by B(C6F5)3. This boron cation appears to be much less Lewis acidic than its A1 counterpart. 

Chiral Catalysts Containing Group 3 (Sc and Y) Metals Including Lanthanides and Actinides. Although lanthanide compounds were expected to be Lewis acid reagents and efficient asymmetric Diels-Alder reactions catalyzed by Lewis acids (containing aluminum and boron) were reported, only few asymmetric Diels-Alder reactions are known. 

The chemists have long believed that the trivalent boron atom with its empty p orbital should behave in a fashion to that of the traditional organic electron-withdrawing group [1]. This idea is attractive when applied to the activation of Diels-Alder dienophiles, because the intermediate boron compounds can be transformed to an array of different functional moieties, none of which can usually be obtained by a direct Diels-Alder reaction. Matteson [2] and Evans [3] have reported the reactions of vinylboronic esters with dienes under strenuous reaction conditions but with low yields of the products. Similarly, vinyldichlo-roborane gives low yield [4] on reaction with cyclopentadiene. 

Chiral alkyldihaloboranes are one of the most powerful chiral Lewis acids. However, in general, since alkyldihaloboranes readily decompose into alkanes or alkenes by protonolysis or (3-hydride elimination, it is difficult to recover them as alkylboronic acids quantitatively. Aryldichloroboranes are relatively more stable and can be reused as the corresponding boronic acids. Ishihara and Yamamoto et al. have developed chiral aryldichloroboranes 21 bearing binaphthyl skeletons with axial chirality as asymmetric catalysts for the Diels-Alder reaction of dienes and a,(3-unsaturated esters (Equation 31) [29]. (J )-2-Dihydroxyboryl-l,T-binaphthyl (20) can be synthesized from (R)-binaphthol in several steps [25]. The synthesis of racemic 20a has also been reported by Kaufmann et al. [30]. Compound 20 has been converted into 21 by two dif-... 

Toure and Hall described a three-component reaction involving the coupling of an aza Diels-Alder reaction with an in situ vinylboration that affords piperidines and their fused derivatives. Thus, treatment of azadiene 126 with the chiral sulfinimide dienophile 127 in the presence of benzaldehyde afforded compound 128 via the initial generation of boronate 129 (Scheme 3.38) [86], This method was later employed as the basis for the synthesis of the alkaloid (-) methyl dihydropalustramate [87],... 

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