Diols boronates

Reduction of (3-hydroxy ketones through chelated TSs favors. yy -l,3-diols. Boron chelates have been exploited to achieve this stereoselectivity.129 One procedure involves in situ generation of diethylmethoxyboron, which then forms a chelate with the (3-hydroxyketone. Reduction with NaBH4 leads to the vyn-diol.130... 

After reaction of an (S,S )-diol boronic ester 1 with (dichloromethyl)lithium yields a 99 1 ratio of (a/ )-a-chloro boronic ester 2 and its (aS)-diastereomer 3 (Section 1.1.2.1.2.1.), further... 

Addition of anhydrous zinc chloride and warming to 20 C causes rearrangement of the borate complex 3 to the x-chloro boronic ester 4. If an (S,5)-diol boronic ester 1 or 2 as illustrated is used, the product is generally — 99 % (a/ )-a-chloro boronic ester 4 based on NMR analyses. The enantiomeric (/ ,/ )-diol boronic ester yields the (aS)-a-ehloro boronic ester4. 

The effect of borax on the strength of the hemicellulose adhesive was subsequently investigated. A factorial experiment was done employing five concentrations of borax and five concentrations of sodium hydroxide. The sodium hydroxide variable was included, since formation of the diol-boron complex is pH sensitive 12). 

Fig. 8. Structure of the boronate derivatized polymer (A) and the formation of the cis-diol boronate complex (B),...

The formation of a diol boronate complex, defined by formally liberates two equivalents of water, but this stoichiometric factor is usually ignored as a constant in dilute aqueous solution. In a formal sense, phenylboronic acid could also bind diols to form a trigonal complex (K jg), and this species would itself act as an acid according to Kj. The acidification of solutions containing phenylboronic acid and diols is always discussed in terms of the trigonal complex being a stronger acid than the parent phenylboronic acid, i.e. K > [17]. Since all the equilibria are coupled this re-... 

However, these four coupled equilibria from Scheme 12.1 are not the full story. Boronic acids readily form stable complexes with buffer conjugate bases (phosphate, citrate and imidazole) [20], In fact, both binary boronate-X complexes are formed with Lewis bases (X), as well as ternary boronate-X-saccharide complexes. In some cases, these previously unrecognized species persist into acidic solution and tmder some stoichiometric conditions they can be the dominant components of the solution. These complexes suppress the boronate and boronic acid concentrations, leading to a decrease in the measured apparent formation constants (f pp). As a consequence, the scope of the simple diol-boronate recognition system is greatly expanded over the simple picture of Scheme 12.1. 

Scheme 7 The equilibria for boronate ester formation couple to generate a thermodynamic cycle. The formation of the diol boronate anion complex is defined as Ktet and the formation of the diol boronic acid complex is defined as K,rig, where it is observed that > K ig. The acidity constant of the unbourid complex is defined as Ka and the acidity constant of the bound complex is defined as Kf, where it is observed that pKa > pKj.

Considering Scheme 7 we define the formation of the diol boronate anion complex as Aitet and the formation of the diol boronic acid complex as ATtrig, where it is observed that ATtet > ATtrig. For instance, the logarithm of these constants for phenylboronic acid binding fructose in 0.5 M NaCl water is log ATtet=3.8 whereas log ATtrig < 1.4. This difference in the value of the binding... 

Although boronates are quite susceptible to hydrolysis, they have been found useful for the protection of carbohydrates. It should be noted that as the steric demands of the diol increase, the rate of hydrolysis decreases. For example, pin-acol boronates are rather difficult to hydrolyze in fact, they can be isolated from aqueous systems with no hydrolysis. 

The affinity method may be biospecific, for example as an antibody-antigen interaction, or chemical as in the chelation of boronate by ci5-diols, or of unknown origin as in the binding of certain dyes to albumin and other proteins. 

A suspension of lithium aluminum deuteride (1.6 g) in dry tetrahydrofuran (60 ml) is added dropwise to a stirred and cooled (with ice-salt bath) solution of 5a-androst-l4-ene-3j3,17j3-diol (179, 1.6 g) and boron trifluoride-etherate (13.3 g) in dry tetrahydrofuran (60 ml). The addition is carried out in a dry nitrogen atmosphere, over a period of 30 min. After an additional 30 min of cooling the stirring is continued at room temperature for 2 hr. The cooling is resumed in a dry ice-acetone bath and the excess deuteriodiborane is destroyed by the cautious addition of propionic acid. The tetrahydrofuran is then evaporated and the residue is dissolved in propionic acid and heated under reflux in a nitrogen atmosphere for 8 hr. After cooling, water is added and the product extracted with ether. The ether... 

Propanediol, acetone. This method removes the boronate by exchange. 2-Methylpentane-2,5-diol in acetic acid cleaves a phenyl boronate (85% yield). 

Treatment of the boronate with Bui, AgO affords the monoalkylated diol in a manner similar to stannylene-directed monoalkylation and acylation. ... 

Boronic esters are easily prepared from a diol and the boronic acid with removal of water, either chemically or azeotropically. (See Chapter 2 on the protection of diols.) Sterically hindered boronic esters, such as those of pinacol, can be prepared in the presence of water. Boronic esters of simple unhindered diols are quite sensitive to water and hydrolyze readily. On the other hand, very hindered esters, such as the pinacol and pinanediol derivatives, are exceedingly difficult to hydrolyze and often require rather harsh conditions to achieve cleavage. 

The inhibitive efficiency of boric acid polyesters differs greatly. The highest efficiency is exhibited by polyesters of boric acid, aromatic diols and triols. This derives from the fact that in this case the radicals are accepted not only by boron, but also by the aromatic nucleus. Among the aromatic polyesters, most efficient is ester of boric acid and pyrocatechin due to the Frank-Rabinovich cage effect. The efficiency of inhibi-... 

The synthesis of the polyol glycoside subunit 7 commences with an asymmetric aldol condensation between the boron enolate derived from imide 21 and a-(benzyloxy)acetaldehyde (24) to give syn adduct 39 in 87 % yield and in greater than 99 % diastereomeric purity (see Scheme 8a). Treatment of the Weinreb amide,20 derived in one step through transamination of 39, with 2-lithiopropene furnishes enone 23 in an overall yield of 92 %. To accomplish the formation of the syn 1,3-diol, enone 23 is reduced in a chemo- and... 

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-... 

The cyclohexyloxy(dimethyl)silyl unit in 8 serves as a hydroxy surrogate and is converted into an alcohol via the Tamao oxidation after the allylboration reaction. The allylsilane products of asymmetric allylboration reactions of the dimethylphenylsilyl reagent 7 are readily converted into optically active 2-butene-l, 4-diols via epoxidation with dimethyl dioxirane followed by acid-catalyzed Peterson elimination of the intermediate epoxysilane. Although several chiral (Z)-y-alkoxyallylboron reagents were described in Section 1.3.3.3.3.1.4., relatively few applications in double asymmetric reactions with chiral aldehydes have been reported. One notable example involves the matched double asymmetric reaction of the diisopinocampheyl [(Z)-methoxy-2-propenyl]boron reagent with a chiral x/ -dialkoxyaldehyde87. 

An extremely attractive feature of the route outlined at the beginning of this section for the transformation of boronates 3 or 4 to a-substituted allylboron compounds 5 is that reagents with very high enantiomeric purity (> 90% ee) may be prepared when precursors such as 3 and 4, and therefore also ate complex 1, contain a suitable diol chiral auxiliary17. The following syntheses of (S)-68, lib9, and 1310 illustrate this feature. 

Studies have established that the partition between transition states 3 and 4 depends on the nature of the diol unit bound to boron and on the steric and electronic effects of the a-sub-stituent X23. The data shown below demonstrate that the reactions of2-(l-methyl-2-propenyl)-4,4,5,5-tetramethyl-l,3,2-dioxaborolane proceed with a moderate preference for transition state 3 with the C2 methyl group in an axial position. Selectivity diminishes with 2-(l-methyl-2-propenyl)-l,3,2-dioxaborolane and reverses with dimethyl (l-methyl-2-propenyl)boronale, suggesting that steric interactions (gauche interactions in the case of the tetramethyl-1,3,2-diox-aborolane) between X and the diol unit on boron are capable of destabilizing transition state 4 relative to 3. 

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