Boration Methodology

With hydride reagents such as dibal, Z-alkenes can be selectively obtained from alkynes in the phenolic lipid series (ref. 162), and related series of boron reagents greatly supplement the chemical methods of selective reduction and alkyiation. This selectivity has been achieved by the use of less reactive dialkylboranes such as bis(3-methyl-2-butyl)borane (di-isoamylborane), bis(2,3-dimethyl-2-butyl)borane (thexylborane), 9-boraUcyclo[3.kl]nonane (9-BBN) and dicyclohexylborane. Some applications in the polyethenoid field have been summarised (refs. 135,163) and the synthesis of alkenyl compounds generally reviewed (ref. 164). By the use of dibromoborane dimethylsulphide, an internal alkyne can be reduced selectively (ref. 165) as for example in the following way (R = n-alkyl). 

With bis(di-isoamyl) borane reduction of a terminal alkyne is selectively effected as shown (ref. 166). 

Selective alkylations can be achieved by the use of an intermediate involving bis (cyclohexyl) borane, as follows. 

In the triene series for the synthesis of the stereoisomers of (15 3)-anacardic acid, namely the 8(Z),11(Z),14 8(E),11(E),14 8(Z),11(E),14 and 8(Z),11(E),14 compounds by the alkylation of the ArC., intermediate (ethyl 2-methoxy-6-methybenzoate) with a 0 4 component these boration methods have been of value as an addition to selective catalytic hydrogenation and the use of terminal trimethylsilylation (ref. 167). At this stage for synthetic purposes the selective reductive use of boration methods has been mainly exploited. The chief use of combined addition/alkylation procedures is for obtaining 8(E), and 11 (E) isomers. For this, the sequence of synthons, for the side chain has to follow the different series, Ar9- ArCi2- ArCi5. 

The following scheme summarises some of the methods used to construct the C14 side chain component. The required Cg synthons were prepared by route (a) which led to compound C. Catalytic reduction of this with Pd-BaS04 in the presence of quinoline gave compound A and chemical reduction afforded compound B. The usage of A,B and C is depicted in route (b). 

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With 3,5-dimethoxybenzoyl chloride used in the monoacylation of cyclohexanone to produce finally the same (MeO ArC synthon, an alternative synthesis of cardol 8(Z)-monene was developed [138]. Fig( 4)-49, Persoonol dimethyl ether (5-[(Z)-undec-3-enyl]resorcinol dimethyl ether) was synthesised from 3,5-dimethoxybenzyl alcohol [149], This same reactant has been employed for a range of 5-substituted resorcinols although its application to unsaturated phenolic lipids was not discussed [228]. Acetylenic methods have proved valuable for deriving phenolic lipidic double bonds in the Z-configurations and with advances in boration methodology [229] the corresponding E-isomers are accessible. 

B. R. Ainley, K. H. Nimerick, andR. J. Card. High-temperature, borate-crosslinked fracturing fluids A comparison of delay methodology. In Proceedings Volume, volume , pages 517-520. SPE Prod Oper Symp (Oklahoma City, 3/21-3/23), 1993. 

The reaction of silylborane with 1-halo-l-lithio-l-alkenes yields 1-boryl-l-silyl-l-alkenes via borate formation followed by 1,2-migration of silyl group (Equation (90)).76,240 The mechanism seems to be closely related to that proposed for the silaboration of isocyanide (Figure 2). Vinyl-substituted carbenoids, l-chloro-l-lithio-2-alkenes, react with silylpinacolborane to give l-boryl-l-silyl-2-alkanes in good yield (Equation (91)).241 This methodology is applied to the synthesis of l-boryl-l-silyl-2-cyclobutene.2 2 Similar reactions are carried out with other carbenoid... 

Ishikura et al. reported the total synthesis of ellipticine (228) starting from N-Boc indole (1256) and the vinyl bromide 1258 (719-721). This methodology involves a palladium-catalyzed, tandem cyclization-cross-coupling reaction of the indolyl borate 1257 with the vinyl bromide 1258 as the key step. Using a literature procedure, the vinyl bromide 1258 was prepared as an E/Z mixture starting from CIS- and trans-crotyl alcohol. The indolyl borate 1257 was generated in situ from... 

The reaction of potassium dienoxy borates with A-fluorobis(phenylsulfonyl)amine (la) gives y-fluoro enones in good yield. The potassium dienoxy borates are prepared by treating potassium enolates derived from unsaturated ketones with 2-phenyl-1,3,2-benzodioxaborole. This methodology offers a convenient alternative to the traditional fluorination of dienol acetates, ethers, or enamines.145 An example is given by the formation of 13.145... 

Iron-catalyzed Suzuki-Miyaura coupling reactions were also reported by Nakamura and colleagues (entry 27) [67]. Alkyl halides 1 and mixed pinacol aryl(butyl)borates, generated in situ from arylboronates and butyllithium, were used as the reagents and 10 mol% of the iron complexes 16a or 16b as the catalysts. The addition of 20 mol% of MgBr2 was essential for the success of the reaction. Products 3 were isolated in 65-99% yield. The methodology tolerates ester and nitrile functions. The reaction starts probably by initial boron-iron transmetalation to generate a diaryliron(II) complex. 

The intramolecular allylboration of an aldehyde function leads selectively to cir-disubstituted cyclic ethers. It has been shown that both the reactive aldehyde and the allylboronate moiety can be initially generated in situ in a masked form and then liberated simultaneously by hydrolysis of the precursor functions <1997JA7499>. This methodology was successfully applied to the one-pot synthesis of the oxocene 82, a precursor of (-l-)-laurencin (Scheme 13). A DIBAL reduction of the Weinreb amide 80, metalation with r f-butyllithium, borylation with the pinacol borate ester, and, finally, liberation of both the aldehyde and the allylboronate function by aqueous pH 7 buffer solution generated the reactive 81, which cyclized in 38% overall yield to the oxocene 82. Only the all-cis-diastereomer is formed, which means that the cyclization proceeds under high asymmetric induction from the resident stereogenic center present in 80. 

A brief overview of some of the most important synthetic routes to organoboranes, that is, (1) transmetallation reactions, (2) the boration of unsaturated compounds, and (3) C-H activation wih be given. General aspects of these methodologies will be covered and illustrated with selected recent hterature examples. While a large number of transformations are known that convert one boron compound to another, the diversity of these methods does not allow for fiiU treatment in this context. Detailed information is given in several excellent comprehensive reviews that have been published. ... 

Details of a CZE methodology for the separation of 14 aromatic sulfonates in river water and a CZE-MS evaluation of 22 compounds in influent and effluent samples of a wastewater treatment plant " are organized in Table 31.8. With the idea that phytoremediation could be used as an alternative means of wastewater treatment of recalcitrant compounds, a simple borate buffer-based... 

CZE separation of synthetic dyes has been approached by simple (borate and citrate) and volatile buffers (ammonium acetate) modified by solvents as well as nonaqueous systems (ammonium acetate/acetic acid in MeOH). Environmental applications of CZE methodologies include the analysis of spent dyebaths and wastewater samples and the monitoring of groundwater migration, where eosin was used as a fluorescent tracer (details in Table 31.8). ... 

Tetracyclines, nonsteroidal anti-inflammatory, antidepressants, " and mixtures of acidic drugs as well as veterinary drug residues have been determined by CZE (citric acid/citrate, borate, borate in MeOH, phosphate/MeOH, ammonium formate/formic acid/ACN, ammonium acetate, ammonium acetate/acetic acid/MeOH) and EKC (SDS/pH 2.5 phosphate buffer/ACN) methodologies in water samples from influent and effluent of sewage treatment plants as well as wastewater, river, surface and groundwater as compiled in Table 31.9. Estrogens have been receiving increased attention due to the already mentioned possible interference with the reproductive... 

In the strategy described in Fig. 16b, the linear precursor is not built on the polymer, but the final cyclization reaction using the Suzuki-Miyaura coupling takes place on a polymer. The aryl boronic acid is captured on a Dowex ammonium hydroxide resin, leading to the polymer ionically bonded borate, which is subsequently treated in the appropriate conditions to give the final macrocyle. This methodology is called resin capture-release.. ... 

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