Transesterification boron esters

Covalent polymers with reversible properties arising from dynamic covalent bonds such as disulfide exchange reaction [47 9], transesterification [50,51], transetherification [52], and boronate ester formation [53] were reported without respect to DCC. These studies should involve DCLs in... 

If a diastereomeric mixture of (S,S)-2,5-dimethyl-3.4-hexanediol esters 2 and 3 reacts with methylmagnesium bromide, the result is kinetic resolution, as verified for R1 = Bn. The diastereomeric mixture was prepared from the racemic ethylene glycol a-chloro boronic ester via transesterification with chiral diol. The products isolated were (S,S)-2,5-dimethyl-3,4-hexanediol [(/ )-2-phcnyl-l-methylethyl]boronate, diastereomeric ratio >95 5 as indicated by the rotation of the (/ )- -phenyl-2-propanol derived by deboronation with hydrogen peroxide, and (S,S)-2,5-dimethyl-3.4-hexanediol methylboronate. Phenylacetaldehyde was identified by 1H NMR4. 

Micalizio and Schreiber [47] developed a key reaction, the transesterification of unsaturated boronic esters with allylic esters or propargylic alcohols. This reaction transiently provided mixed organoboronic esters that could be trapped by using... 

Scheme 3.2 Transesterification of a pinacol boronic ester. (Adapted with permission from ref. 26 2012 American Chemical Society.)...

Along with their work on the nickel-catalyzed Suzuki-Miyaura coupling of neopentylglycol boronic esters, Percec and coworkers examined the speed of transesterification of a cyclic arylboronic ester with a diol under anhydrous conditions. In both assays in Scheme 3.2, some transesterification had taken place after 10 h (equilibrium was not reached). This result shows that transesterification of cyclic boronic esters is always possible, even starting from pinacol esters. This should be kept in mind when considering reactions in the presence of alcohol solvents and water. 

Sdteme 7. Cleavage of an amido boronic ester by transesterification. 

Treatment of diphenyl chlorophosphate with primary or secondary alcohols in the presence of the Lewis acid, tris(pentafluorophenyl)boron B(QF5)3 was highly efficient for the preparation of phosphate triesters. The proposed mechanism envisioned the initial formation of a highly reactive boronate ester RO(BCgF5)2 that coordinated with the chlorophosphate, facilitating an intramolecular transesterification to yield the triesters." °... 

This q clopropyl boronic ester was converted into fois-cyclopropanes through a series of transformations shown in Scheme 3.79 [120], The iodo fois-cyclopropyl boronic acid ester 145, however gave a complex product mixture on attempted Suzuki coupling with phenylboronic acid. On the other hand, the less bulky hydroxy protected fois-cyclopropyldioxaborinane 146 obtained by transesterification underwent smooth cross-coupling with iodobenzene giving the phenyl substituted fois-cyclopropane product 147 in 79% yield (Scheme 3.79). 

For the most part boric acid esters are quantitated by hydrolysis in hot water followed by determination of the amount of boron by the mannitol titration (see Boron compounds, boric oxide, boric acid and borates). Separation of and measuring mixtures of borate esters can be difficult. Any water present causes hydrolysis and in mixtures, as a result of transesterification, it is possible to have a number of borate esters present. For some borate esters, such as triethanolamine borate, hydrolysis is sufftciendy slow that quantitation by hydrolysis and titration cannot be done. In these cases, a sodium carbonate fusion is necessary. 

The rates of transesterification of triglycerides to methyl esters, efficiently catalyzed by boron carbide (B4C), were, on the other hand, faster under microwave conditions, probably because of superheating of the boron carbide catalyst, which is known to be a very strong absorber of microwaves [40], Scheme 10.3. Yields of methyl ester of up to 98% were achieved. 

The boronic acid ester B was synthesized by transesterification of the corresponding pinacolester A with (lR,2R)-l,2-dicyclohexyl-l,2-dihydroxyethane. Stereoselective chlorination of B was carried out with (dichloromethyl) lithium and zinc chloride. Reaction of the obtained chloroboronic ester C with lithio 1-decyne followed by oxidation of the intermediate D with alkaline hydrogen peroxide afforded the propargylic alcohol E. Treatment with acid to saponify the tert-butyl ester moiety and to achieve ring closure, produced lactone F. Finally, Lindlar-hydrogenation provided japonilure 70 in an excellent yield and high enantiomeric purity. 

Acylation of a simple thiol with an alkyl carboxylate is not a very suitable method for preparation of S-alkyl thiocarboxylates. Transesterification is, however, possible if either the thiol or the carboxylic ester is activated. The enhanced reactivity of boron, aluminum and silicon thiolates has been utilized for the synthesis of a large variety of thiocarboxylic S-esters, including hydroxy derivatives (from lactones). a,P-Unsaturated thiol esters, e.g. cinnamoyl or 2-butenoyl derivatives, are also accessible. Michael addition, an undesirable side reaction of thiols, is completely avoided if alkyl trimethylsilyl sulfides ortris(arylthio)boranes are applied. ... 

Amides. Esters are converted into carboxylic acids by boron tribromide (4,42). Japanese chemists have modified this reaction to transesterification and to a synthesis of amides. The ester in CH2CI2 or C Hj is treated with... 

E)- and (Z)-(Fluoroalkenyl)boronate derivatives 712 and 714 have been prepared stereospecifically by the reaction of (E)- or (Z)-(2-fluoroalkenyl)iodonium salts 711 and 713 with [bis(4-fluorophenoxy)]alkylboranes, followed by transesterification to pinacol esters (Scheme 3.285). The mechanism of this reaction involves... 

In order to prepare the cyclohexenaldehyde 8, 3-hydroxy-2-pyrone 14 and ethyl 4-hydroxy-2-methyl-2-butenoate 15 are subjected to a Diels-Alder reaction in the presence of phenylboronic acid which arranges both reactants to the mixed boro-nate ester 19 as a template to enable a more efficient intramolecular Diels-Alder reaction with optimal control of the regiochemical course of the reaction. Refluxing in benzene affords the tricyclic boronate 20 as primary product. This liberates the intermediate cycloadduct 21 upon transesterification with 2,2-dimethylpropane-l,3-diol which, on its part, relaxes to the lactone 22. Excessive i-butyldimethyl-silyltriflate (TBSTf) in dichloromethane with 2,6-lutidine and 4-7V,A-dimethyl-aminopyridine (DMAP) as acylation catalysts protects both OH goups so that the primary alcohol 23 is obtained by subsequent reduction with lithiumaluminum-hydride in ether. 

These esters may be made from the free acids or by transesterification of the methyl esters using 2% sulphuric acid in 2-chloroethanol at 60 °C for two hours. The mixture is poured into water and the esters are extracted into petroleum ether [34]. These esters may also be made using the commercially available 10% boron trichloride in 2-chloroethanol (see Section 2.2.5). 

Transesterification of boronic acid esters s. 44, 52 2-Amino-1,3-dioxanes from 1,3-diols s. 44, 941... 

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