Boronic esters, reaction+acids

For oxidation of the boronic ester or acid, 1 g of PIB was dissolved in 15 ml of THF, followed by the addition of 4 ml of 10 w% NaOH solution. The reaction was started by the addition of 1.02 ml of 30% hydrogen peroxide while maintaining the temperature below 50°C. The reaction mixture was stirred for 30 min, then 30 ml of Hex and 20 ml of saturated aqueous potassium carbonate were added. The mixture was washed with distilled water until the odor of THF could not be detected. The solution was dried over anhydrous sodium sulfate overnight, followed by evaporation of Hex on a rotavapor. 

Addition compounds of alkali-metal hydrides. 25. Rapid reaction of boronic-esters and acids with lithium aluminium hydride. A novel and quantitative synthesis of lithium monoorganylborohydrides, B.Singaram, T.E.Cole and H.C.Brown, Organometalllcs, 1984, 3, 774. 

Inorganic Esters. Boric acid and borax form cycHc esters with poly(vinyl alcohol) (85—100). The reaction is markedly sensitive to pH, boric acid concentration, and the cation-to-boron ratio. An insoluble gel is formed at pH above 4.5—5.0 ... 

Reaction of methyl a-L-rhamnopyranoside with triphenylboroxole gave a syrupy boronate ester which was characterized as a crystalline phenyl-carbamate. Removal of the phenylboronic acid residue gave a product identified as methyl a-L-rhamnopyranoside 4-N-phenylcarbamate, since it was identical with that resulting from removal of the ketal group from methyl 2,3-O-isopr opylidene-a-L-rhamnopyranoside 4-N-phenylcarbamate (12). This establishes the structure of the original ester as methyl a-L-rhamnopyranoside 2,3-phenylboronate (24). 

Iron chelators can also be used to selectively bind iron in areas where oxidative stress is observed, thereby preventing the iron from taking part in Fenton reactions without interfering with normal iron homeostasis. Charkoudian et al. have developed boronic acid and boronic ester masked prochelators, which do not bind metals unless exposed to hydrogen peroxide (237,238). The binding of these chelators to iron(III) prevents redox cycling. Similar studies of these systems have been performed by a separate group (239,240). 

Severin and coworkers reported (146) the reaction of tris(2-aminoethyl)amine and 4-formylphenylboronic acid with penta-erythritol to give, via multicomponent assembly, the boronic acid based macrobicyclic cage 35 (Fig. 25). The cage has the form of an ellipsoid with a diameter of 20.5 A and binds two Cud) ions in a fashion similar to the smaller tren-based cryptands. The reversible formation of boronic esters has also been employed to build other hollow structures such as nanotubes (147) and porous covalent organic frameworks (148,149). 

The allyl boronate esters (R,R)- and (S,S)-1 are prepared by reaction of allylboronic acid, CH2=CHCH2B(OH)2 with l- and D-diisopropyl tartrate.1... 

The synthesis in Scheme 13.31 also relies on me.so-2,4-dimethylglutaric acid as the starting material. Both the resolved aldehyde employed in Scheme 13.30 and a resolved half-amide were successfully used as intermediates. The configuration at C-2 and C-3 was controlled by addition of a butenylborane to an aldehyde (see Section 9.1.2). The boronate ester was used in enantiomerically pure form so that stereoselectivity was enhanced by double stereodifferentiation. The allylic additions carried out by the butenylboronates do not appear to have been quite as highly stereoselective as the aldol condensations used in Scheme 13.30, because a minor diastereomer was formed in the boronate addition reactions. 

The synthesis by K. C. Nicolaou and co-workers is summarized in Scheme 13.43. Diels-Alder reactions are prominent in forming the early intermediates. The formation of the A ring in steps A and B involves use of z-cli loroacrylonitrile as a ketene synthon. In step C, the pyrone ring serves as diene. This reaction is facilitated by phenylboronic acid, which brings the diene and dienophile together as a boronate ester, permitting an intramolecular reaction. 

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. 

For the synthesis of amino acids, the reaction of an a-haloalkyl boronic ester 4 with sodium azide and a phase-transfer catalyst in dichloromethane/water requires a large excess of azide in order to form the a-azidoalkyl boronic ester 5 with only 1-2% epimer34. With the exception of R1 = benzyl, where epimerization of 4 is relatively rapid, bromoalkyl boronic esters are preferred. Chloroalkyl boronic esters react so slowly that the azide and dichloromethane may generate hazardously explosive diazidomethane65,66. Chain extension of 5 to 6 proceeds normally. Sodium chlorite, which is known to oxidize aldehydes to carboxylic acids67-69, also oxidizes a-chloroalkyl boronic esters to carboxylic acids34. The azido acid is hydrogenated to the amino acid. 

The procedure described4 is a modification of the method of Khotinsky and Melamed,8 who first reported the preparation of boronic acids from Grignard reagents and borate esters. Benzeneboronic acid and the corresponding anhydride also have been prepared by reaction of phenylmugnesium bromide with boron... 

Treatment of 2-lithiofurans with ethyl borate and hydrolysis of the resultant boronic ester yields 2(3//)-furanones (69AK(29)229). 2,5-Dimethyl-3(2i/)-furanone (358) has been prepared by a Curtius reaction on the ester (359) and acid hydrolysis of the intermediate urethane (360) (Scheme 97). This type of reaction has been used to synthesize muscarine and its stereoisomers (61QR153). 

Boronic acids can be reversibly esterified with resin-bound diols (Figure 3.15). The resulting boronic esters are stable under the standard conditions of amide bond formation, but can be cleaved by treatment with water under acidic or neutral conditions to yield boronic acids. Treatment of the resin-bound boronic esters with alcohols yields the corresponding boronic esters [197]. Resin-bound boronic esters are suitable intermediates for the Suzuki reaction [198], Treatment with H202 leads to the formation of alcohols (Entry 8, Table 3.36), while treatment of resin-bound aryl boronates with silver ammonium nitrate leads to the conversion of the C-B bond into a C-H bond (Entry 14, Table 3.46). 

Polystyrene-derived phenylboronic acids have been used for the attachment of diols (carbohydrates) as boronic esters [667]. Cleavage was effected by treatment with acetone/water or THF/water. This high lability towards water and alcohols severely limits the range of reactions that can be performed without premature cleavage of this linker. Arylboronic acids esterified with resin-bound diols can be oxidatively cleaved to yield phenols (Entry 8, Table 3.36). Alcohols have also been prepared by nucleophilic allylation of aldehydes with polystyrene-bound, enantiomerically enriched allyl-silanes [668], as well as by Pummerer reaction followed by reduction of resin-bound sulfoxides [669]. 

In further studies, Matteson et a/.10,11 reported relative rates of substitution of various boronic esters by mercuric chloride details are given in Table 3. The solvent used was a mixture of ethanol (88 %), water (8 %), and glycerol (4 %), buffered with sodium acetate and acetic acid, and reactions were run in the presence of added sodium chloride. Under these conditions, the kinetics of reaction of benzylboronic ester with mercuric chloride were found to obey the rate law... 

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