立体选择性还原的硼试剂及其在手性药物合成中的应用

硼试剂在药物化学合成中广泛应用. 综述了可参与立体选择性还原反应的手性硼试剂、烷基硼试剂和硼氢化钠衍生物在手性药物合成中的应用, 并讨论这些试剂用于工业化生产的潜力.     

负热电离质谱测定硼同位素的涂样试剂比较

目前人们并未注意到利用负热电离质谱方法测定硼同位素所采用涂样试剂中BO^-的同质异位素CNO^-离子的存在和影响。对硼同位素测定采用的不同涂样试剂进行比较，结果发现在去硼海水和硝酸盐溶液中存在BO2^-的同质异位素CNO^-离子，干扰离子不仅来自有机物，而且可能来自硝酸根。在硼同位素测定中，检查空白中的43峰和43／42比值是必要的。实验表明MgCl2 NaOH混合溶液是负热电离质谱测定硼同位素的较为理想的涂样试剂。     

Palladium-catalyzed coupling of thiol esters with aryl and primary and secondary alkyl organoindium reagents

Thiol esters and organoindium reagents undergo palladium-catalyzed cross-coupling under mild conditions to give ketones in moderate to excellent yields. Aryl and primary/secondary alkyl organoindium reagents can be used as coupling partners. This method has two advantages over the cross-coupling of thiol esters with boron and tin reagents: (1) no added copper reagent is required to mediate the reaction and (2) for the case of alkyl transfer, no added base is required to activate organoindium reagents for cross-coupling as is required for the coupling of alkyl boron reagents with thiol esters.     

Recent advances in the activation of boron and silicon reagents for stereocontrolled allylation reactions

Despite the popularity of boron and silicon allylation reagents in stereocontrolled synthesis, they suffer from a number of inherent limitations that have slowed down their development as synthetic tools for nucleophilic additions to carbonyl compounds and imine derivatives. These limitations are the low reactivity and diastereoselectivity of allyl trialkylsilane reagents, and the lack of catalytic systems for the activation and substoichiometric control of enantioselectivity in the additions of allyl boron reagents. To develop more efficient and general methods for the control of absolute stereochemistry in the resulting homoallylic alcohols, new approaches aimed at solving the problem of activation of allylic boron and silicon reagents are needed. This Minireview describes a number of recent approaches that have been devised to address this problem.     

Derivatives of anthrarufin,chrysazin and quinizarin as colorimetric reagents for boron

The spectrophotometric characteristics of the color reactions of six new reagents for boron are presented. These are derivatives of the dihydroxyanthraquinones, anthrarufin, chrysazin, and quinizarin, with amino, nitro, cyano, or halogen substituents. The spectral absorption characteristics of these reagents and their borate complexes are much more favorable for the colorimetric determination of boron than is the case with quinalizarin, which is commonly used for this purpose. With the exception of one of the six, all are more sensitive than quinalizarin. Two of them, tribromoanthrarufin and dicyanoquinizarin, are especially sensitive (spectrophotometric sensitivity: 0.0010 μg/cm²).     

Determination of trace quantities of boron with two new colorimetric reagents

Two new color reactions with boron are reported. Sensitive methods have been developed for their use. A new method for the decomposition of fruit tree leaves and alfalfa, and the subsequent separation of the boron is described.The reagents are 5-benzamido-6'-chloro-1, 1 '-bis (anthraquinonyl) amine(I) and 5-p-toluidino-1,1'-bis (anthraquinonyl) amine(II). Concentrated (96%) sulfuric acid is used as the solvent. The formation of the boron complex of the reagents was found to be dependent on the following variables: time of heating, temperature, sulfuric acid content and the presence of diverse ions.Solutions of the two complexes follow Beer's law over the boron range indicated: Compound I, 0 to 0.5 γ at wavelength of 635 mμ and Compound II, 0 to 0.4 γ at wavelength 720 mμ. The sensitivities are 1 : 300,000,000 and 1 : 200,000,000, respectively. The standard deviation was calculated for the determination of boron with each compound. Results showed 0.002 absorbance unit, i.e., 0.001 γ of boron for Compound I and 0.005 absorbance unit, i.e., 0.002 γ of boron for Compound II.The method of decomposing the leaves and alfalfa samples is based upon destroying all the organic matter with a mixture of 96% sulfuric acid and 30% hydrogen peroxide within, the distilling flask. Boron is then distilled off as the volatile trimethyl ester. The standard deviation of the method is 0.58%.     

Photometric determination of boron with oxazine derivatives

Eight oxazine derivatives were examined for their ability to form extractable complexes with boron fluoride. Brilliant cresyl blue, Capri blue and Nile blue form such complexes and can be used as sensitive photometric reagents for boron. Capri blue can even be more sensitive to boron than methylene blue.     

Palladium(0)-catalyzed cis-selective alkylative and arylative cyclization of alkynyl enones with organoboron reagents

A palladium(0)-tricyclohexylphosphine catalyzes cis-selective alkylative and arylative cyclization of alkyne-containing electron-deficient alkenes with organoboron reagents to provide five- or six-membered rings with exo tri- or tetra-substituted alkenes. The opposite stereoselectivity to that for the alkyne-aldehyde cyclization using the same reagents would result from palladacycle-forming oxidative addition of the substrates to the Pd⁰ catalyst followed by transmetalation with the boron reagents, protonation, and reductive elimination. The functional group compatibility, availability, stability, and non-toxicity of the reagents, and the fact that no additives are needed make the process more practical than the Ni⁰-catalyzed cyclization with organozinc reagents.     

A general,highly stereoselective synthesis of amines

The stereochemical course of cycloalkanone imine reductions by a variety of boron hydride reagents is described; very high stereoselectivity with substituted alkali metal borohydrides is reported.     

Disclosing the Structure/Activity Correlation in Trivalent Boron‐Containing Compounds: A Tendency Map

Most trivalent boron reagents are electrophiles owing to the vacancy for two electrons to fill the outer orbital of boron; however, interestingly, trivalent boron compounds can change their electrophilic character to a nucleophilic character by only changing the nature of the substituents on the boron atoms. With the help of computational tools, we have analyzed the structural‐ and electronic properties of boryl fragments that were either bonded to main‐group metals or coordinated to transition‐metals/rare‐earth‐metals and we have designed a map that might help to identify certain trends. This trend map will be useful for selecting an appropriate trivalent boron compound, depending on the sought reactivity.     

Reactions of Aromatic Aldehydes with Boron Halides

Recently, during studies directed toward the total synthesis of the antibiotic frustulosin 1^1,2 we encountered some problems in the removal of methoxyl groups during the preparation of the phenolic aldehyde 2. Our efforts to effect this transformation led us to investigate the reactions of the boron trihalides with aromatic aldehydes. Although the methoxyl group has many desirable properties for hydroxyl protection its stability to both acids and bases often presents difficulties at the time of removal. Of the many reagents and methods devised³ for the removal of methoxyl and other alkoxyl groups boron tribromide can be considered as one of the most widely applicable reagents for this purpose. However, as a fairly powerful Lewis acid, complications can arise when acid.     

Catalyst‐Free Enantiospecific Olefination with In Situ Generated Organocerium Species

Described is the in situ formation of triorganocerium reagents and their application in catalyst‐free Zweifel olefinations. These unique cerium species were generated through novel exchange reactions of organohalides with n‐Bu₃Ce reagents. The adequate electronegativity of cerium allowed for compensating the disadvantages of both usually functional‐group‐sensitive organolithium species and less reactive organomagnesium reagents. Exchange reactions were performed on aryl and alkenyl bromides, enabling enantiospecific transformations of chiral boron pinacol esters. Finally, these new organocerium species were engaged in selective 1,2‐additions onto enolisable and sterically hindered ketones.     

Air handling in clean laboratory environments: the reason for anomalously high boron background levels

The isotopic analysis of boron from material that carries only trace amounts of boron requires low boron blank levels. Large efforts are taken to keep blank levels low by purifying reagents. We performed exposure experiments and determined procedure blanks before and after the air-handling system was modified from silicate glass filters to polyethylene/polystyrole and active carbon filters. Our investigations demonstrate that the air-handling system may be the major source of boron blank in many laboratories, as the widely used silicate glass filters, which contain boron as a major component, in the form of borax and boric acid, release significant amounts of boron to the air.     

Catechol boron halides: mild and selective reagents for cleavage of common protecting groups

Catechol boron halides (1,X=Cl, Br) cleave certain ether, ester and carbamate protecting groups under mild conditions. The scope and selectivity of these readily available reagents has been examined.     

Hollow and Threadlike Nanostructures of Boron Carbide

An original low-temperature method is proposed for the synthesis of boron carbide from simple materials. It made it possible to obtain a powdered product in the form of hollow and threadlike nanostructures. The boron carbide nanotubes are shorter than bundles of nanotubes. The boron carbide nanostructures are grown from vapor-phase reagents at a high-temperature nanocenter, the enhanced reactivity of which is maintained as a result of the exothermic effect of the reaction.     

Methods for boron analysis in boron neutron capture therapy (BNCT). A review

The possibilities and limitations of the numerous individual methods used for boron analysis for BNCT in clinical and biological samples are reported in detail. The main interferences of the analytical methods are described. Sample pretreatment techniques are discussed. The methods reviewed for boron analysis for BNCT concern atomic spectrometry, radioanalytical methods, and imaging techniques. An error analysis of boron determinations in biomaterials is performed and typical boron distribution patterns in small mammalians are discussed. Received: 4 December 1998 / Revised: 21 January 1999 / Accepted: 31 January 1999     

Application of cycloaddition reactions to the syntheses of novel boron compounds

This review covers the application of cycloaddition reactions in forming the boron-containing compounds such as symmetric star-shaped boron-enriched dendritic molecules, nano-structured boron materials and aromatic boronic esters. The resulting boron compounds are potentially important reagents for both materials science and medical applications such as in boron neutron capture therapy (BNCT) in cancer treatment and as drug delivery agents and synthetic intermediates for carbon-carbon cross-coupling reactions. In addition, the use of boron cage compounds in a number of cycloaddition reactions to synthesize unique aromatic species will be reviewed briefly.     

Enantioselective Radical Addition/Cross‐Coupling of Organozinc Reagents,Alkyl Iodides,and Alkenyl Boron Reagents

A hybrid transition‐metal/radical process is described that results in the addition of organozinc reagents and alkyl halides across alkenyl boron reagents in an enantioselective catalytic fashion. The reaction can be accomplished both intermolecularly and intramolecularly, providing useful product yields and high enantioselectivities in both manifolds.     

Oxidation-resistant thermosets derived from thermal copolymerization of acetylenic monomers containing boron and silicon

Polymerization of acetylenic monomers containing silicon and boron produces materials that are resistant to oxidation at elevated temperature. In addition to homopolymerization, these inorganic–organic hybrid monomers can be copolymerized with an otherwise all carbon containing system. Inorganic elements incorporated by this method enhance the thermooxidative stability of the copolymer. This stability is a function of the amount of boron in the final product. We show that a thermoset containing an initial 5% by weight of boron can be charred to a material that when heated in air at 1000 °C loses less than 10% of its mass. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 4356–4359, 1999     

Enantioselective Radical Addition/Cross‐Coupling of Organozinc Reagents,Alkyl Iodides,and Alkenyl Boron Reagents

A hybrid transition‐metal/radical process is described that results in the addition of organozinc reagents and alkyl halides across alkenyl boron reagents in an enantioselective catalytic fashion. The reaction can be accomplished both intermolecularly and intramolecularly, providing useful product yields and high enantioselectivities in both manifolds.