Boronic catechol derived

One of the clearest pieces of evidence for the reversibility of some covalent interactions came from the work of Gunther Wulff at the University of Dusseldorf, Germany [172], He used imprinted polymers of o-aminomethyl phenylboronic acids as chromatographic stationary phases for the separation of saccharides. Older studies [173] also point to the reversible nature of the boronic acid-saccharide interaction. The pioneering studies of fluorescent transduction of this phenomenon by Czarnik and Yoon [174] (Ohio State University), Aoyama et al. [175] (Kyushu University, Japan), and Shinkai et al. [176] (Kyushu University, Japan) have been reviewed previously [9], Our concern in this review is particularly with the systems that clearly involve PET. Czarnik and Yoon s 93 [177] which interacts with catechol derivatives to produce 94 also belongs here. It... 

Different approaches were investigated to overcome this problem. The first strategy reported in 1991 was based on the use of a chiral aminodiol. Donation of electrons from nitrogen to boron activates the diene and accelerates the addition to activated olefins compared to its boronic ester analogue [70]. The endo cycloadducts were exclusively produced with N-phenyl maleimide at room temperature. A low asymmetric induction was observed with dienes derived from N-substituted amino acid (12% ee) [71]. Ate complexes prepared by quatemarization of a catechol derivative with a stoichiometric amount of CsF are highly reactive in these cycloaddition reactions (Scheme 9.32) [72]. 

The Claisen rearrangement has attracted much attention as an attractive tool for the construction of new carbon-carbon bonds. Taguchi et al. reported the enantioselective and regioselective aromatic Claisen rearrangement of catechol mono allylic ether derivatives by means of Corey s chiral boron reagent (Eq. 70) [53a,54]. The mechanism of enantioselectivity is that a rigid five-membered cyclic intermediate is formed by reaction of catechol mono allylic ethers with the chiral boron reagent and this is fol-... 

These two milestone syntheses were soon followed by others, and activity in this field continued to be driven by interest in the biologically active esters of cephalotaxine. In 1986, Hanaoka et al. (27) reported the stereoselective synthesis of ( )-cephalotaxine and its analog, as shown in Scheme 4. The amide acid 52, prepared by condensation of ethyl prolinate with 3,4-dimethoxyphenylacetyl chloride, followed by hydrolysis of the ethyl ester, was cyclized to the pyrrolobenzazepine 53 by treatment with polyphos-phoric acid, followed by selective O-alkylation with 2,3-dichloropropene (54) in the presence of sodium hydride. The resulting enol ether 55 underwent Claisen rearrangement on heating to provide C-allylated compound 56, whose reduction with sodium borohydride yielded the alcohol, which on treatment with 90% sulfuric acid underwent cationic cyclization to give the tetracyclic ketone 57. Presumably, this sequence represents the intramolecular version of the Wichterle reaction. On treatment with boron tribromide, ketone 57 afforded the free catechol, which was reacted with dibromometh-ane and potassium fluoride to give methylenedioxy derivative 58, suited for the final transformations to cephalotaxine. Oxidation of ketone 58... 

A standard way to make vinyl boronic acids 237 is to hydroborate an alkyne with catechol borane38 235 and again hydrolyse the ester products 236. The empty orbital of the boron atom attacks the electron-rich terminus of the alkyne where is the larger HOMO of the Jt-bond. One detailed example39 includes Suzuki coupling to Z-styryl bromide. Another diester group commonly used in the Suzuki coupling is derived from pinacol - examples appear later. 

Furthermore, boron trichloride can selectively O-demethylate a methoxyl group adjacent to a phenolic function to afford the corresponding catechol. In this fashion, the mono-(5-methyl derivative of boldine, i.e. (31), was converted into the unstable catechol (32), whose methylenation using methylene chloride in DMSO containing sodium hydroxide provided dicentrine (33). ... 

Enantioselective aromatic Claisen rearrangement was reported from our group in 1997 [65]. The Claisen rearrangement of catechol mono allyl ether derivative 80 was catalyzed by chiral bis-sulfonamide-boron reagent 81. Although a stoichiometric amount of chiral reagent is required for this reaction, this is only one successful example of the enantioselective aromatic Claisen rearrangement. 

Chiral boron reagent (1) can facilitate Claisen rearrangement of catechol monoallylic ether derivatives (2), affording catechol adducts (3) (eq 1). Products formed by rearrangement of the allylic moiety to the para position and by abnormal Claisen rearrangement are not detected. 

Bromination of dibutyl vinylboronate at low temperature readily affords a stable di-bromo derivative [2, 3]. Similarly, ( )-alkenylboronic acids or their catechol esters yield the corresponding substituted compound by stereospecific trans addition of bromine. Treatment with base induced anti elimination of the boronic group and the bromine ion. The final result of this sequence was the replacement of boron by bromine with inversion of olefin geometry in excellent yields and 99% stereochemical purities (Scheme 9.1) [4]. 

Interestingly, it is evident that the reactivity of boric acid is better at high temperatures, and it declines at lower reaction temperatures. Subsequently, a wide variety of boron-based reagents, such as catechol and arylboronic acid-based catalysts, have been developed for direct amide formation. The enhancement of the Lewis acidity of boron has been proven to enhance catalytic activity e.g. chlorocatechol derivative 9 or 10), as in eqn (13.2). ... 

The adducts derived from catechol borane are hydrolyzed by water to vinyl-boronic acids. These materials, are useful intermediates for preparation of terminal vinyl iodides. Since the hydroboration is syn and the iodinolysis occurs with retention of alkene geometry, the iodides which result have the E configuration. 

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