Boronate derivatives

Synthesis of some alkaloids using allyl boron derivatives 99KGS1015. 

Allylboronates prepared from simple diols display appreciable reactivity, but eyelie boronate derivatives prepared from 1,2- or 1,3-diols display considerably less. The commonly employed pinacol esters are among the least reactive members of this class. 2-Allyl-3-methyl-l,3,2-oxaza-... 

Sekiguchi, H., and Clarisse, C., Polymerization of e-caprolac-tone with boron derivatives as catalysts, Makromol. Chem., ... 

These reactions proceed with retention of double-bond configuration in both the boron derivative and the alkenyl halide. The oxidative addition by the alkenyl halide, transfer... 

The stereoselectivity of the P-carboethoxyallylic boronate derived from the endo-phenyl auxiliary A (p. 803) toward R- and. S -glyccraldchydc acetonide has been investigated. One enantiomer gives the anti product in 98 2 ratio, whereas the other favors the syn product by a 65 35 ratio. Based on the proposed transition structure for this boronate, determine which combination leads to the higher stereoselectivity and which to the lower. Propose the favored transition structure in each case. 

The starting aldehyde could have been present on the primeval Earth the question of the necessary boron derivatives is open, although it seems possible that weathering of the borate minerals referred to above could have set free boron salts, which then stabilized the ribose. 

The reaction of triallylborane with silicon triyne 123 is interesting. A113B attacks both internal and external triple bonds giving rise to silole 124 and two heterocycles with bridgehead boron 125 and 126 in a 1 3 3 ratio as a result of competitive sequential reactions (Scheme 52). When 1,1-allylboration of the internal C C bond followed by intramolecular 1,1-vinyIboration takes place, the silole 124 is formed, while in another case 1,1-allylboration followed by a series of intramolecular 1,2-allylboration reactions leads to boron derivatives 125 and 126 <2002JOM(657)146>. 

Sessler, J. L. Allen, W. E. Krai, V. A. Preparation of highly boronated derivatives of expanded porphyrins (texaphyrins) for potential use in boron neutron capture therapy and related applications. US Patent 5,955,586, 1999 Chem. Abstr. 1999, 131, 222611. 

Recently, a rhodium-catalyzed tandem cyclization has been reported with an arylboronic ester bearing a pendant Michael-type acceptor olefin and acetylenic65 or olefinic66 derivatives. This transformation proceeds in a water-containing medium as solvent and proton source. This catalyst system is optimized with electron-rich and bulky ligands to stabilize the rhodium intermediate and decrease the protonolysis of boron derivatives in a protic solvent. 

Stable aryl boronates derived from tartaric acid catalyze the reaction of cyclo-pentadiene with vinyl aldehyde with high selectivity. Chiral acyloxy borane (CAB), derived from tartaric acid, has proved to be a very powerful catalyst for the enantioselective Diels-Alder reaction and hetero Diels-Alder reaction. Scheme 5 23 presents an example of a CAB 73 (R = H) catalyzed Diels-Alder reaction of a-bromo-a,/i-cnal 74 with cyclopentadiene. The reaction product is another important intermediate for prostaglandin synthesis. In the presence of... 

U.S. 6,353,072 Polymer preparation from boron derivative functional group-containing monomers... 

Qy-boronate derivatives have the ability to form covalent bonds with d,v-diol sugars at alkaline pH. Unlike lectins, which typically bind only the terminal sugar in a complex carbohydrate, boronates can bind any physically accessible CM-diol on a glycoprotein.23-25 When hydrophobic boronate derivatives are used, the hydrophobicity of each complexation site is enhanced. To the extent that the protein components of a sample are differentially glycosylated, this can provide a tool for exploiting those differences. 

A wide variety of substituted boronates are available commercially. For phenyl supports, use a phenyl or biphenyl (napthalene) derivative. For nonphenyl supports, evaluate both weakly and strongly hydrophobic alkyl derivatives (straight chain or branched). Raise sample pH to at least 8 and add the boronate derivative of choice to a concentration of at least 1 mM. The sample need not be buffer-exchanged prior to injection. Column buffers should be at least pH 8. 

Mineral fillers and additives aluminium trihydrate (ATH), magnesium hydroxide and boron derivates are the best known but tin derivates, ammonium salts, molybdenum derivates and magnesium sulphate heptahydrate are used to varying extents and nanofillers are developing. 

Even esters can be ortholithiated, provided the electrophile is present in situ during the hthiation. Neopentyl ester 160 gives, on treatment with LDA in the presence of triisopropylborate, the boron derivative 161, which can be deprotected and used in Suzuki coupling reactions (Scheme 79) . Less hindered esters can also be successfully orthohthiated in the presence of a boron electrophile (Scheme 80) . ... 

In addition to the predominant allyl and crotyl reagents, a large nnmber of allylic borane 1 and boronate derivatives 2 (Eq. 1) with varions snbstitnents (R -R" ) have been reported. Interested readers can refer to the comprehensive Tabnlar Snrvey at the end of this monograph, which covers the literatnre np to the end of 2005. Several reviews on allylic boron componnds and other allylmetal reagents and their additions to carbonyl compounds and imines have been written prior to this one," " and these sonrces may be consnlted if a more in-depth historical perspective is desired. 

The immediate products of additions between carbonyl substrates and allylic boranes 1 or boronate derivatives 2 are borinate or borate esters, respectively. To cleave the covalent B-O bond in these intermediates (structme 6, Scheme 1) and to obtain the desired free alcohol, a hydrolytic or oxidative work-up is required. This issue is discussed in detail in the section Work-Up Conditions . In the interest of simplifying chemical equations, specific work-up conditions are not inclnded in most of the examples highlighted in this chapter. 

If the presence of sensitive functional groups poses problems of chemose-lectivity in the use of hard allylic metal reagents, allylboronate derivatives also can be accessed by a milder transmetalation of allylic tin species with boron halides.This approach has been used by Corey in the synthesis of chiral bis(sulfonamido)boron reagents such as the medially 1 reagent 15 (Eq. 19) (see section Chiral Boronate Derivatives ). ... 

The boron-oxygen mesomeric effect described in the previous section explains the lower reactivity of allylic boronates towards carbonyl compounds compared to that of allylic boranes. The use of Lewis acids, however, allows boronate derivatives, including hindered ones, to react at temperatures comparable to the analogous boranes. As described above (see section Mechanism and Stereochemistry ), the most reactive allylic boronates are those with the most electrophilic boron centers.The nucleophilicity of the y-position of an allylic boron reagent (the position that forms the new C-C bond with the aldehyde) is also important to the reactivity of the reagent. For example, allylic boronates with... 

The addition of allylic boron reagents to carbonyl compounds first leads to homoallylic alcohol derivatives 36 or 37 that contain a covalent B-O bond (Eqs. 46 and 47). These adducts must be cleaved at the end of the reaction to isolate the free alcohol product from the reaction mixture. To cleave the covalent B-0 bond in these intermediates, a hydrolytic or oxidative work-up is required. For additions of allylic boranes, an oxidative work-up of the borinic ester intermediate 36 (R = alkyl) with basic hydrogen peroxide is preferred. For additions of allylic boronate derivatives, a simpler hydrolysis (acidic or basic) or triethanolamine exchange is generally performed as a means to cleave the borate intermediate 37 (Y = O-alkyl). The facility with which the borate ester is hydrolyzed depends primarily on the size of the substituents, but this operation is usually straightforward. For sensitive carbonyl substrates, the choice of allylic derivative, borane or boronate, may thus be dictated by the particular work-up conditions required. 

Chiral Boronate Derivatives. A large number of chiral auxiliary reagents based on allylic boronates has been reported. This section provides a brief overview of the historically important ones, but it focuses mainly on the most popular systems and the emerging ones (Fig. 4). 

Figure 4. Common allylic boronate derivatives used as chiral auxiliary reagents in enan-tioselective carbonyl additions. (Only one stereoisomer is shown for simplicity.)...

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