Intramolecular boronate ester

Unlike the binding of a normal arylboronic acid sensor with tartarie aeid, which is a fast process, the binding of [R]-8 or (5)-8 with mandelic acid is a slow process, with rate constants on the order of 10 s This is reasonable since a covalent bond has to be broken in order to bind the analyte to the che-mosensor. In the presence of analytes such as mandelic acid, the intramolecular boronate is broken and an intramolecular boronate ester is formed. This process can be easily followed by monitoring the fluorescence enhancement of the system. 

The mechanism involves the dissociation of the coordinated borane 15 to generate a monoborane intermediate 16. Coordination of the alkene would generate the alkene borane complex. A /3-borylalkylhydride with B-H stabilization is certainly an important resonance structure of 17. An intramolecular reaction would extrude the alkyl boronate ester product and coordination of HBcat would regenerate the monoborane intermediate. 

The synthesis by K. C. Nicolaou and co-workers is summarized in Scheme 13.43. Diels-Alder reactions are prominent in forming the early intermediates. The formation of the A ring in steps A and B involves use of z-cli loroacrylonitrile as a ketene synthon. In step C, the pyrone ring serves as diene. This reaction is facilitated by phenylboronic acid, which brings the diene and dienophile together as a boronate ester, permitting an intramolecular reaction. 

The cross-coupling of alkynylzinc halides or fluorinated alkenylzinc halides with fluori-nated alkenyl iodides allows the preparation of a range of fluorinated dienes or enynes - Functionalized allylic boronic esters can be prepared by the cross-coupling of (dialkylbo-ryl)methylzinc iodide 428 with functionalized alkenyl iodides. The intramolecular reaction provides cyclized products, such as 429 (Scheme 109) ° °. In some cases, reduction reactions or halogen-zinc exchange reactions are observed. 

Directed asymmetric reduction of a ketone has been brought about by the use of an intramolecular homochiral boronate ester250. The latter was readily introduced at a hydroxyl group in the molecule and has allowed the production of the enantiomeric alcohol, from the ketone by use of BH3-complex as the reductant (equation 64). The boronate ester may be readily removed by treatment with hydrogen peroxide-sodium hydroxide, using standard methodology. Other similar reductions have also been reported251-253. 

Cyclic boric acid esters derived from triethanolamine (Figure 9.11) or diethanolamine can be stabilized toward hydrolysis by an intramolecular, boron-nitrogen coordination bonding. The blockage of the empty-orbital on the boron atom can alleviate hydrolysis. This effect has been used to prepare... 

These equilibria can be even more rapid if the interaction with nitrogen bases occurs intramolecularly (see Table 4.4 entry k) [46,47,56,57]. In those cases in which a basic nitrogen atom is located at a favourable distance from the boronate ester, a rate acceleration of 10 -10 fold compared to the unsubstituted phenylboro-nate can be observed [56,57]. 

In its most general form, the boronic acid Mannich or Petasis reaction18 involves the reaction of boronic acid 22, a carbonyl compound 8, and an amine 23 to produce secondary amines 24. If one uses a-keto acid 25 for the carbonyl component then the corresponding product from the Petasis reaction was a-amino acid 26. The key mechanistic step was proposed to occur intramolecularly with alkyl migration from intermediate boronate ester 28 formed from aminol 27. 

A boron-tethered (C-B-O) intramolecular Diels-Alder (IMDA) approach has been used to prepare cyclic alkenyl boronic esters 140 (Scheme 19). Thus, reaction of 2equiv of the dienyl alcohol 138 with 137 in THF, in the presence of molecular sieves, provides the corresponding IMDA precursors 139. The IMDS reaction was then accomplished at 190 °C in a toluene solution, with 5 mol% of 2,6-di-fer7-butyl-4-methylphenol as a free radical inhibitor. Transformation of the carbon-boron bond in 140, using standard organoborane reactions, can then afford a variety of functionalized cyclohexene derivatives <1999JA450>. 

For examples see a) Wiederrecht, G.P., M.R Niemczyk, W.A. Svec, and M.R. Wasielewski (1996). Ultrafast photoinduced electron transfer in a chlorophyll-based triad VibrationaUy hot ion pair intermediates and dynamic solvent effects. J. Am. Chem. Soc. 118(1), 81-88 and b) Shiratori, H., T. Ohno, K. Nozaki, I. Yamazaki, Y. Nishimura, and A. Osuka (1998). Coordination control of intramolecular electron transfer in boronate ester-bridged donor-acceptor molecules. Chem. Commun. (15), 1539-1540. 

Kobayashi investigated a self-assembled boronic ester cavitand capsule for photochemical reaction of 2,6-diacetoxyanthracene. Tsuda found a self-assembled helical anthracene nanofiber in a vortex. Chou examined rotational behaviors and fluorescence energy transfer of N-l- and N-2-anthryl succinimide derivatives. " Reversible photoinduced twisting of molecular crystal microribbons via [4 - - 4] photocycloaddition of 9-anthra-cenecarboxylic acid. Reversible single walled carbon nanotubes of 1,3-bis(9-anthracenylmethyl)imidazolium chloride was examined as a functionalized anthracene salts.Karatsu reported the intramolecular photodimerization of 9-substituted anthracene derivatives (253) tethered by oligosilanes giving [4 + 4] and [2 - - 4]cycloadducts (254), (255), and (256). ... 

Since the early success of Decicco et al. [19] in extending the boronic acid cross-coupling to an intramolecular system, more examples of this approach have appeared in the literature (Scheme 5.5). Snapper and Hoveyda reported a total synthesis of the anti-HIV natural product chloropeptin 1 in which the crucial biaryl ether moiety was constructed via the Cu-mediated reaction (Scheme 5.5) [20]. Thus, treatment of the boronic ester 10 with NalO liberates the boronic acid, which then cyclizes under Cu(OAc)2 to give the biaryl ether 11, a precursor to the final target. In this case, the addition of 10 equiv. of methanol was critical for efficient intramolecular cross[Pg.210]

Free radical additions to alkenylboronic esters provided the first access to (a-haloalkyl)boronic esters [8]. Intramolecular cyclizations involving radical additions to alkenylboronic esters have been reported and, also, a (l-iodo-S-hexenyl)boronic ester cyclizes to the cyclopentyl derivative under radical conditions [83]. Reaction of pina-col (l-iodopentyl)boronate with tributyltin hydride and butyl vinyl ether yielded the addition product pinacol (l-butoxy-3-heptyl)boronate (71%), but addition of the bory-lalkyl radical to methyl acrylate was inefficient and yielded mainly the simple reduction product, pinacol pentylboronate [84]. 

Alkenyl boronic esters can also be used to trap nucleophilic carbon-centered radicals, which can be generated by the tin hydride method or by decomposition of an organomercurial derivative. The influence of the olefin and boron substituents on the reactivity and regioselectivity has been determined. Vinyl-9-BBN displayed significantly better reactivity than the boronic ester while the directing effect of the boronic ester group is weaker than that of an ester function. The Barton radical chain procedure furnished very stable adducts due to an intramolecular complexation between boron and nitrogen (Scheme 9.12) [29]. Radical additions of various xanthates also occurred smoothly in the presence of lauroyl peroxide [30]. Intermolecular ver-... 

As expected, the reactivity of 1,3-dienylboronates was increased considerably in intramolecular versions. Tethering of dienylboronate precursors to an unactivated dienophilic component allowed in situ formation of mixed boronic esters, followed by intramolecular Diels-Alder reaction and oxidation to the corresponding cyclohexene diols (Scheme 9.34). The reactions were highly regioselective and the diastereos-electivity varied with the nature of the substituents on the dienophile [75]. 

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