Boric acid hydroxy compounds

Most reported boric acid esters are trialkoxy or triaryloxy boranes. The esters range from colorless low boiling Hquids to soHds that possess high melting points. Boric acid esters usually have an odor similar to the hydroxy compound from which they are derived. A more complete description of the physical... 

Compounds with hydroxy groups, such as flavonoids, sugars, or hydroxy acids, often react with 1% diphenyl boric acid (3-aminoethylester in methanol or ethanol. Various colors are formed by many natural products Neu reagent). 

Boron heterocycles bearing a hydroxy group on the boron atom are acidic. Boric, boronic and borinic acids are Lewis acids towards hydroxide ion. It was suggested that six-membered 5-hydroxy compounds behaved as Brdnsted acids. The experimental evidence for this was the similarity between the UV spectra of the hydroxy compounds in neutral and basic solution. The rationale for this fact was the maintained aromaticity which was supposed to favour the formation of, for example, ion (163) over ion (164). This suggestion was at first supported by nB NMR spectroscopy, which differentiates between anions of Lewis-acidic... 

The N—O bond of isoxazolines can easily be cleaved via reduction. It is for this reason that isoxazolines are interesting synthetic intermediates. y-Amino alcohols are formed by reduction with LiAlH4 (for an example, see Figure 15.46, left). Hydrogenolysis of isoxazolines catalyzed by Raney nickel yields /j-hydroxy imines, which undergo hydrolysis to / -hydroxy-carbonyl compounds in the presence of boric acid (Figure 15.46, right). 

A general method for the cleavage of 4,5-dihydroisoxazoles is hydrogenation over Raney nickel in the presence of an acid or a Lewis acid, such as boric acid ° or boron trichloride. The products are normally p-hydroxy ketones but in the case of compound (35), which was prepared as an intermediate in the construction of the AB-ring system of forskolin, reduction over Raney nickel gave an isolable imine (36). ... 

The reduction by Na(Hg) of several o-hydroxynaphthalenecarboxylic acids in slightly acid solution containing boric acid has been investigated. 1-Hydroxynaphthalene-2-carboxylic acid [109] is, like salicyclic acid, reduced to the aldehyde (62% yield), isolated as the Schiff base, whereas 2-hydroxynaphthalene-3-carboxylic acid [110] under similar conditions yields mainly 2-hydroxy-1,2,3,4-tetrahydronaphtha-lene-3-carboxylic acid (75-80%) and 5% of the aldehyde. When hydrochloric acid rather than CO2 was used to neutralize the base formed during the reduction, the 2-oxo-l,2,3,4-tetrahydronaphthalene-3-carboxylic acid [111] was isolated (60%). On heating, the compound, a y6-keto acid, decarboxylated. 

A loss of phenolic hydroxyl group occurs during the reduction of 1-hydroxynaphtha-lene-4-carboxylic acid with Na(Hg) in the boric acid medium from which 1,4-dihydro-naphthoic acid is isolated [112]. The primary reduction probably yields 1,4-dihydro-1-hydroxy-4-naphthoic acid, which loses water to 1-naphthoic acid this compound is under these conditions reduced mainly to the 1,4-dihydro derivative, whereas 2-naphthoic acid predominantly gives the 1,2-dihydro compound. 

Milborrow [102] first isolated 8 -OH-ABA, but since then this compound has remained elusive due to its spontaneous rearrangement to PA. However, 8 -OH-ABA has been identified by GC-MS in methylated and trimethylsilylated extracts from immature fruits of Vigna [103], maize seedlings [104], and tree ferns [105] that were analyzed for gibberellins. Also, the 3-hydroxy-3-methylglutaryl conjugate of 8 -OH-ABA, which upon alkaline hydrolysis yielded PA, has been isolated from the fruits of Robinia pseudoacacia [106,107] (Fig.4). Recently, it has been found that 8 -OH-ABA forms a complex with boric acid that dissociates in water. The liberated 8 -OH-ABA is relatively stable in acidic solutions, but cyclizes to PA under neutral or basic conditions [108]. These findings may explain why isolation of 8 -OH-ABA has been so difficult to reproduce. 

H. Schafer, Z. Anorg. Chem. 1949, 259, 86-91. Boric acid and hydroxy compounds. IV. New salts of dipyrocatechol boric acid. 

Isopropoxy-4/f-l,3,2-benzodioxaborin (18) has been synthesized and found to react with a variety of hydroxy compounds to replace the isopropoxy group on boron. Aryl boric acids, RB(OH)2, reportedly condense with diols of the type HO(CH a)nOH to yield oligomeric rings (19) of varying size. The reaction of allenyl borates (20) with hydrogen bromide to yield alkenes and alkynes has been published. ... 

In animals and humans, at intracellular pH, nearly all natural boron exists as boric acid, which behaves as a Lewis acid, and forms molecular additive compounds with amino- and hydroxy acids, carbohydrates, nucleotides, and vitamins through electron donor-acceptor interactions. In most of the commercial dietary boron supplements now available, boron is chelated with amino acids or with hydroxy acids (i.e., glycine, aspartic acid, or citric acid) in combination with vitamins. However, little is known about the molecular structure of these boron chelates. ... 

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