Cyclic boronate esters

An efficient process for one-carbon homologation to aldehydes is based on cyclic boronate esters.17 These can be prepared by hydroboration of an alkene with dibromobor-ane, followed by conversion of the dibromoborane to the cyclic ester. The homologation step is carried out by addition of methoxy(phenylthio)methyllithium to the boronate ester. The migration step is induced by mercuric ion. Use of enantioenriched boranes and boronates leads to products containing the groups of retained configuration.18... 

The cyclic mechanism would predict that the addition reaction would be stereo-specific with respect to the geometry of the double bond in the allylic group. This has been demonstrated to be the case. The E- and Z-2-butenyl cyclic boronate esters 1 and 2 were synthesized and allowed to react with aldehydes. The /- -boronate gave the carbinol having anti stereochemistry whereas the Z-boronate gave the syn product.34... 

Chiral cyclic boronic esters with (dichloroniethyl)lithium at —100 C form borate complexes4. Borate complexes cart also be formed by generation of (dichloromethyl)lithium from dichloro-methane and lithium diisopropylamide in the presence of a boronic ester at —78 C to — 5 C (Section 1.1.2.1.2.2,)28,19. In situ generation of (dibromomethyl)lilhium is required for preparing a-bromo boronic esters (see Sections 1.1.2.1.1.2. and 1.1.2.1.3.2.). 

An important consequence of the C2 symmetry of the cyclic boronic esters is that both faces of the trigonal boron atom are equivalent. Consequently, there is only one possible intermediate borate, which can also be generated by reaction of a (dichloromelhyl)boronic ester with an organolithium or organomagnesium reagent at — 78 C2126. 

A variation within the osmium-catalysed asymmetric dihydroxylation (AD) of alkenes has been described that yields cyclic boronic esters from alkenes in a straightforward manner. A protocol based on the Sharpless AD conditions (for enantiose-lective oxidation of prochiral olefins) has been developed that gives cyclic boronic esters, rather than free diols, with excellent enantiomeric excesses. Some of the... 

The cyclic boron esters (99) resulted from the three-component reaction between benzil (96), urea (97), and boric acid (Scheme 21). The stereoisomeric products arose as a consequence of boric acid addition to the intermediate cyclic amides (98) <89PHA110>. Carbon-13 NMR spectroscopy and mass spectrometry are consistent with structure (99). 

Most chiral organoboron Lewis acids reported to date are based on an organoborane that is attached to a chiral organic moiety such as a diol, aminoalcohol, or other readily available chiral substrates.Organoboron derivatives recently used as catalysts in enantioselective Diels-Alder reactions include the family of chiral acyloxyboranes (CAB) with (196) and (197) as representative examples and various cyclic boronic esters such as (198) and (199). An interesting system that combines the favorable Lewis acid properties of fluorinated arylboranes with a chiral Bronsted acid has been developed by Ishihara and Yamamoto. The Bronsted acid-assisted chiral Lewis acids (BLA) (200) was found to be highly effective in enantioselective cycloadditions of Q ,jS-enals with various dienes. The presence of the Bronsted acid functionality leads to significant acceleration of the reaction. 

Cyclic boronic esters in asymmetric synthesis 88ACR294 89T1859. Cyclic a-halo boronic esters in asymmetric synthesis 89CRV1535. B-Heterocycles as structure fragments of polymers 88UK1529. B,N-Heterocycles in the synthesis of boron nitride 90CRV73. B,P-Heterocycles 90AG(E)449. 

When considering the suitability of a bead production technique for imprinting, it is essential to evaluate the compatibility of the conditions used for polymerisation with those required for complex formation between functional monomers and templates. Where covalent imprinting methods are used, the covalent adducts are often highly stable and need quite harsh conditions to disrupt them. Such adducts could be used in most of the procedures described below with reasonable expectation of success. The same can be said for many metal-chelate complexes, which have stabilities approaching covalently bonded structures. The use of cyclic boronate esters is an exception. This adduct is unstable in water and hence cannot be combined efficiently with aqueous suspension polymerisation. 

The analysis of 2- and 4-hydroxyestradiol and 2- and 4-hydroxyestrones is relevant in relation to the development of breast and prostate cancer. These compounds have been analysed in mine after hydrolysis by means of P-glucuronidase/sulfatase and extraction by LLE or SPE. Prior to LC-MS analysis, the hydroxyestiadiols and hydroxyestrones were derivatized with ferrocene boronic acid to form cyclic boronate esters [32], or with p-toluenesulfonhydrazine to form p-toluenesulfonhydrazones [33], respectively. Product-ion MS-MS enables discrimination between two isomers [32]. 

Determination of the structures of the complexes by NMR spectroscopy is more experimentally difficult for boronates than for borates, since each cyclic boronate ester is a pair of stereoisomers, diastereomeric at the newly created asymmetric boron. The association of various sugars with 3-propionidobenze-neboronic acid has been measured as a function of The variation for... 

If the molecule is indeed bent out of a plane the nitro substituent should produce the potentiality for molecular asymmetry. Hence the nitro derivative was treated with p-boronobenzoic acid in benzene under a water separator to yield the cyclic boronate ester carboxylic acid (8). a derivative directly suitable for optical resolution. As a carboxylic acid. (8) gave a crystalline salt with quinine which, after four recrystallizations (aD — 37.4°), liberation of the organic acid, and hydrolysis gave optically active nitroglycol (6), X = NO,. Lead tetraacetate cleavage of the dextrorotatory (6), X = N02. gave optically active (5), X = NO,. the racemization of polarimetry in a chloroform solution. 

Resolution. The cyclic boronate ester derived from racemic BINOL and BHj SMc2 preferentially forms a diastereoisomer with L-proline, thereby (R )-BINOL can be isolated by crystallization. On decomposition of the complex (5)-BINOL is recovered. 

In order to obtain strong, molecular-ion peaks, the use of N-phenyl-glycosylamines ( anilides ) or phenylosazones has been recommended, in addition to the 3-methyl-l-naphthyl and 1-phenylflavazole derivatives mentioned in Section XXV,1 (see p. 37). Diols are possible products of the periodate degradation of polysaccharides, and the mass spectra of their cyclic boronic esters have been examined. 

First, there are a number of effective receptors that use boronic acid functions to form five- or six-membered cyclic boronate esters with two appropriately orientated hydroxy groups (at least in organic and basic aqueous media). This approach was used intensively with regard to sensing applications. However, this approach is neither biomimetic nor strictly supramolecular. because, although... 

The matter of purity is of real importance for SPC and should therefore be addressed. As boron-based functional groups, free boronic acid or one of the many cyclic boronic esters are used. The latter esters may hydrolyze to the acids during polymerization, which then enter the normal cross-coupling or follow an independent mecha-nism.f " Boronic acids always contain some water. Otherwise they are partially or completely condensed to cyclic boroxines. This water content has to be precisely... 

A striking example of the improvement in the mass spectrum of a hydroxylated vitamin D metabolite TMS derivative by formation of a cyclic boronate ester mixed derivative is shown in Section 6 (Cyclization). 

Scheme 1 The rapid and reversible formation of a cyclic boronate ester between a boronic acid and 1.2-diol.

The primary interaction of a boronic acid with a diol is covalent and involves the reversible and rapid formation of a cyclic boronate ester. An array of hydroxyl groups presented by saccharides provides an ideal architecture for these interactions and has led to the development of boronic acid-based sensors for saccharides (Scheme 1). 

Boronic acids can be used to sense diols, such as sugars, as well as amino acids. The binding mechanism is shown in Figure 7. Boronic acid reacts with 1,2- or 1,3- diols to produce stable 5- or 6-membered cyclic boronate esters. The removal of resultant water can make the reversible reaction go forward. This covalent but reversible interaction between boronic acid and c -diols provides a sensing tool for saccharides, which are difficult to detect in aqueous solutions. 

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