Ester phenylboronic

Our initial studies were directed towards the synthesis and characterization of phenylboronate esters derived from methyl 6-deoxy-/ -D-allopyranoside, methyl a-L-rhamnopyranoside, methyl -L-fucopyranoside, and methyl 6-deoxy-/ -D-glucopyranoside. Previous work (14) in this laboratory indicated that the reaction of triphenylboroxole and methyl... 

Solid czs-1,2-diols and pyrocatechols react quantitatively with phenyl-boronic acid to form the phenylboronic esters upon stoichiometric milling. The water of reaction is taken up by the crystals and can be removed in a vacuum [90]. Thus, the aliphatic diols 186 and 189 give the borolic esters 188 and 190 with 100% yield (Scheme 25). Equally successful is the reaction of pyrocatechol (191) and 187. The reaction mixture is heated to 80 °C after milling at 0 °C. 

The opposite directions of stereoselection in the reactions of the phenylboronic ester and the ethylboronic ester were unexpected. The small difference in size between a methyl group and a chlorine atom may be the cause of this inconsistency. Alkyl chains longer than ethyl result in very small diastereoselectivities in the reaction with (l,l-dichloroethyl)lithium47. 

Keywords phenylboronic acid, cyclization, phenylboronic ester... 

Instead of attachment of the phenol entity, one can also link the phenylboronic acids to solid supports as phenylboronic esters [112, 113]. 

For this reason, the phenylboronic ester of levoglucosan is unstable,y4 and iso-propylidene or benzylidene derivatives of levoglucosan have not yet been prepared. 

Keywords phenylboronic acid, phenylboronic esters, mannitol, inositol, regio-specificity, stereospecificity, quantitative, solid-solid reaction, ball mill... 

This conclusion concerning monocrotalic acid, which is at variance with the original proposal of Adams et al., is confirmed by a study of the n.m.r. spectra of the phenylboronate esters of monocrotaline and trichodesmine, which shows conclusively that these esters assume identical or nearly identical conformations. This is only possible if the configurations at C-2 and C-3 are the same in monocrotaline and trichodesmine. It is now believed that the original synthesis of... 

Before references] Brooks and Watson5 have found the phenylboronate esters of 1,2- and 1,3-diols useful for characterization by gas chromatography and mass spectroscopy. The cij-diol system in the sesquiterpenoid (1) was confirmed by for-... 

Phenylboronate esters were used as protective derivatives in the oxidation of glycosides with DMS0-Ac20.6 The particular advantage is the ease of removal from sensitive ketoglycosides and, of course, the stability to the oxidizing agent. 

Few medicines based on boron are known, in general boric acid or a boronic acid serve to esterify an a-diol or an o-diphenol. This is the case for the emetic antimony borotartrates of the ancient pharmacopoeias, for the injectable catecholamine solutions, for tolboxane, which is close to meprobamate and which was commercially available as a tranquillizer some decades ago, and also for the phenylboronic esters of chloramphenicol. Boro-mycine was the first natural product containing boron. It is a complex between boric acid and a polyhydroxylated tetradentate macrocycle. Some boronic analogues of amino acids were prepared as chymotrypsine and elastase inhibitors. The most important medical use of derivatives of boron derivatives is the treatment of some tumours by neutron capture therapy, " the problem here being to ensure a sufficient concentration of the product in the tumour being treated. 

Cis- and trans 3,4-thiolandiols (463 and 464) were obtained by reaction of sodium sulfide with l,4-dichloro-2,3-butanediol. Oxidation of both cyclic hydroxysulfldes gave the corresponding sulfoxides, 465 (a mixture of two stereoisomers) and 466. After esterification the sulfoxides were subjected to Pummerer rearrangement. From 465 (both stereoisomers) all-a s triester sulfide (467) was obtained. However, cyclic phenylboronate ester gave under the same conditions a trans 1-0-acetate (468). The trans diester sulfoxide 466 yielded both of the possible rearranged products, 470 and 471, in roughly equal proportions. 

When trifluoroacetic anhydride was used for the Pummerer rearrangement of phenylboronate ester 465, the 1-O-trifluoroacetyl derivative 469 was formed rapidly and could be used in situ for further reactions. The 1-O-trifluoroacetyl group could be replaced by bromide which, in turn, was exploited for glycoside synthesis. 

C-methyl and 2-C-methyl triacetates (480 and 481), with the latter distinctly prevailing. The same cycle of reactions applied to 479 gave the 5-thiopentose system (482) in low yield. Preparation of the 1,3-phenylboronate ester (483) followed by oxidation to a mixture of stereoisomeric sulfoxides and Pummerer rearrangement proved to be a better way the yield of both regioisomeric 2-C-methyl and 4-C-methyl compounds (484 and 485) was 20%. Compound 485... 

CgH5B[N(CH3)2]2 reacts with 2-(plperldln-2-yl)ethanol In refluxing benzene to yield 2-phenyl-1,3,2-oxazaborabicyclo[4.4.0]decane [8]. C6H5B[N(CH3)2]2 is used in the characterization of diols as phenylboronate esters [9]. 

Roy and Brown examined the rates and equilibrium compositions resulting from the reaction of the cyclic, unhindered phenylboronic ester 2-phenyl-l,3,2-dioxaborolane (A), with several 1,2- and 1,3- diols of various structural types (Scheme 3.1). The equilibrations are rapid (<0.1 h) in CDCI3 at 25 °C. From the equilibrium ratios in Scheme 3.1 it is obvious that six-membered dioxaborinanes are more stable thermodynamically than five-membered dioxaborolanes, and also that introduction of methyl substituents at the a position from oxygen stabilizes the cyclic esters. The very fast and complete displacement by diethanolamine is also worth noticing. Unfortunately, hexyleneglycol was not introduced in this study. 

Many reactions of these boronic esters will involve in the first step a nucleophilic addition onto the boron atom, leading to a tetrahedral boronate anion. With a borolane, this would involve a decrease of the ring strain, and would be kineti-cally more favorable than with the corresponding borinane. Nevertheless, steric hindrance by the a methyl substituents will also be important. This is consistent with the work of Bowie and Musgrave, who compared the rates of hydrolysis of phenylboronic esters exposed to a water-saturated atmosphere, and found that unsubstituted phenylborolane was hydrolyzed faster than unsubstituted phenylborinane. Under the same conditions, phenylboronic esters of pinacol, neopentylglycol and hexyleneglycol were unaffected for several days. 

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