Borates, enol

Among the preformed enol derivatives used in this way have been enolates of magnesium, lithium, titanium, zirconium, and tin, ° silyl enol ethers, enol borinates,and enol borates, R CH=CR"—OB(OR)2. The nucleophilicity of silyl enol ethers has been examined. In general, metallic Z enolates give the syn (or erythro) pair, and this reaction is highly useful for the diastereoselective synthesis of these products. The ( ) isomers generally react nonstereoselectively. However, anti (or threo) stereoselectivity has been achieved in a number of cases, with titanium enolates, with magnesium enolates, with certain enol bor-inates, and with lithium enolates at — 78°C. ... 

L-Amino acid oxidase has been used to measure L-phenylalanine and involves the addition of a sodium arsenate-borate buffer, which promotes the conversion of the oxidation product, phenylpyruvic acid, to its enol form, which then forms a borate complex having an absorption maximum at 308 nm. Tyrosine and tryptophan react similarly but their enol-borate complexes have different absorption maxima at 330 and 350 nm respectively. Thus by taking absorbance readings at these wavelengths the specificity of the assay is improved. The assay for L-alanine may also be made almost completely specific by converting the L-pyruvate formed in the oxidation reaction to L-lactate by the addition of lactate dehydrogenase (EC 1.1.1.27) and monitoring the oxidation of NADH at 340 nm. 

An alternate approach comprises replacing the pendant sugar by either a carbo-cyclic or a heterocyclic ring. The enantioselective synthesis starts by formation of the imide (45-3) by reaction of the aion from the chiral auxiliary (45-2), derived from S-phenylalaninol and the pentene ester (45-1). Treatment of the product with triethyl amine and the trifalate from dibutylboronic acid leads to the transient enol borate (45-4). Aldol addition of that enol to acrolein proceeds stereospecifically to the alcohol (45-5) due to the transfer of chirality from the chiral auxiliary. 

Among the preformed enol derivatives used in this way have been enolates of magnesium, lithium,526 titanium,527 rhodium,528 zirconium,522 and tin,529 silyl enol ethers,530 enol bori-nates,531 and enol borates R CH=CR"—OB(OR)2.532 In general, metallic Z enolates give... 

Tandem aldol addition/allyImetal addition Protected 1,3-diols can be prepared by the aldol condensation of the enol borate of an aldehyde with another aldehyde. These products can react with an ally lie Grignard reagent to provide homoallylic 1,3-diols. 

The mechanism outlined in Scheme 61 is probably responsible for the observed transformation. Adduct radicals (57) react with the triethylborane to form the enol boronate (58) and release an ethyl radical. The enol borate is then trapped in a standard aldol reaction. The ethyl radical partitions between... 

The 1,2-dehydro ketone 4 can be reduced selectively directly to 2, but in this case a proton source is necessary to decompose an intermediate enol borate formed by 1,4-addition of hydride. Potassium dihydrogen phosphate serves this purpose and also functions as a buffer. 

Boron enolates generated from a-heterosubstituted thioacetates by treatment with 105 undergo highly enantioselective and diastereoselective condensations. On the other hand, chiral esters 106 and 107, and amides 108 behave differently. V-Acyl derivatives of the bicyclic isoxazolidine 109 ° readily undergo syn-selective aldol reactions via enol borates. 

Offenhauer and Nelsen discovered that boric acid is an excellent catalyst for aldol condensations treatment of heptanal with boric acid in refluxing toluene under a Dean-Stark trap gives the condensation product in quantitative yield (equation 10). The authors suggested an intermediate enol borate this was probably the flrst example of a boron enolate aldol reaction. 

L13. Lin, E. C. C., Pitt, B. M., Given, M., and Knox, W. E., The assay of aromatic amino acid transaminations and keto acid oxidation by the enol borate-tauto-merase method. ]. Biol. Chem. 233, 668-673 (1958). 

By having a carbanionoid center, diazo compounds react with boranes easily. For example, diazoketones yield -oxoboranes which rearrange in situ to generate enol borates (26-28). The use of organoazides instead of diazo compounds leads to secondary amines (29, 30). 

Asym. aldol-type condensation with enol borates 5j/f-p-Hydroxyketones under thermodynamic control... 

Diols may be prepared by a procedure using a tandem aldol addition-metal allyl addition procedure with enol borates (Scheme 24). ... 

A new -arylethylamine synthesis by aryl aldehyde homologation has also been developed. A procedure has been found for the specific ortho alkylation of aromatic amines.A review of the use of quaternary ammonium compounds in organic synthesis has recently appeared. Mannich bases may be prepared regiospeclfically using the reaction of enol borates with dimethyl (methylene)-ammonium iodide. A very complete review of... 

More recent studies of the utility of the complexed cations in synthesis have paid particular attention to stereoselectivity in their reaction with silyl enolates and enol borates, e.g. eq 52 in this case the same products result, but with only 2.5 1 stereoselection when using the triethylsilyl enol ether and Boron Trijluoride Etherate catalysis. 

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