Boron acid derivatives compounds

The medicinal importance of 2-aryltryptamines led Chu and co-workers to develop an efficient route to these compounds (130) via a Pd-catalyzed cross-coupling of protected 2-bromotryptamines 128 with arylboronic acids 129 [137]. Several Suzuki conditions were explored and only a partial listing of the arylboronic acids is shown here. In addition, boronic acids derived from naphthalene, isoquinoline, and indole were successfully coupled with 128. The C-2 bromination of the protected tryptamines was conveniently performed using pyridinium hydrobromide perbromide (70-100%). 2-Phenyl-5-(and 7-)azaindoles have been prepared via a Suzuki coupling of the corresponding 2-iodoazaindoles [19]. 

Attempts at preparing substituted cytosine 5-boronic acid derivatives 363 were unsuccessful, as it was found that these compounds underwent a very fast deboronation to produce 364 <1998J(P2)841>. 

A new synthetic approach to polycyclic aromatic compounds has been developed based on double Suzuki coupling of polycyclic aromatic hydrocarbon bis(boronic acid) derivatives with o-bromoaryl aldehydes to furnish aryl dialdehydes. These are then converted to larger polycyclic aromatic ring systems by either (a) conversion to diolefins by Wittig reaction followed by photocyclization, or (b) reductive cyclization with trifluoromethanesulfonic acid and 1,3-propanediol (Eq. (12)) [30]. 

An interesting compound, cyclopentadienyl(manganese tricarbonyl) copper, was obtained from the boronic acid derivative on treatment with copper(II) acetate in aqueous 2% sodium hydroxide (209). 

Phenylene bis(lead triacetate) reagents, generated in situ from the corresponding bis(boronic acid) derivatives and lead tetraacetate, react with the a-methyl Meldrum s acid derivative to afford the meta-or para-phenylene bis(Meldrum s acid) derivatives in ca 45% yield. 1 Similarly to malonic acid compounds, the unsubstituted Meldrum s acid was very slow to react and the only observed product was the a,a-diarylated product in 7-17%. 

There are interesting studies, although formally lying beyond the scope of this review, on extraction and membrane transport of saccharides assisted by the formation of covalent bonds. For this purpose Shinbo and co-workers [204] introduced phenylboronic acid. It forms esters with vid-nal-diol compounds, and the resulting anion may be transported across a nonpolar membrane in the presence of, e.g., trioctylmethylammonium counterion. Even ribonucleosides were successfully transported using this technique, with a remarkable and easily understandable 200-fold preference over deoxynucleosides [205]. Some other reports have appeared on the use of boronic acid derivatives in binding [206,207] and transport [208,209] of saccharides and related compounds. 

Organoborane compounds, especially boronic acid derivatives, are less reactive and have also been utilized for the trans-selecdwe coupling reaction of l,l-dibromoalkenes.f t Use of UOH and Ba(OH)2 as the activator of boronic acids is important to carry out the selective coupling reaction (Scheme 5). 

A detailed stndy of the combination of flnorescein boronic acid with diol-appended quenchers a-c and comparison with the fluorescence outputs of nonboron or nondiol-containing systems (i.e., fluorescein or methyl red were employed directly) revealed that the boronate ester formation results in enhanced quenching in each case, and that compound c is the best overall quencher. Nncle-osides were also shown to bind to the same fluorescein boronic acid derivative. While the quenching ability of each nucleoside tested was different, the same ratiometric quenching enhancement was observed in each case, sng-gesting similar binding affinities. 

This versatile reaction has found application in a wide variety of chemical disciplines from the synthesis of biologically active compounds to materials for electronic devices (Figure 13.1). This is perhaps due in part to the wide variety of boronic acid derivatives that are commercially available or easily prepared. Not surprisingly there have been many review articles based around this Nobel Prize winning methodology. The present review is by no means comprehensive but serves to update the reader on recent advances in the area. 

Boron compounds, inducting boronic acid derivatives, can be conveniently analyzed by NMR spectroscopy [352]. Of the two isotopes, B is the most abundant (80%) and possesses properties that are more attractive towards NMR. These attributes include its lower resonance frequency, spin state (3/2) and its quadrupole moment, a wide range of chemical shifts, and its higher magnetic receptivity (16% of H). When analyzing boronic adds in non-hydroxylic solvents by NMR spectroscopy, it is often necessary to add a small amount of deuterated water (e.g. one or two drops) to the sam-... 

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