Benzoate groups

BzCl or BZ2O, Pyr, 0°. Benzoyl chloride is the most common reagent for the introduction of the benzoate group. Reaction conditions vaiy depending on the nature of the alcohol to be protected. Cosolvents such as CH2CI2 are often used with pyridine. Benzoylation in a polyhydroxylated system is much more selective than acetylation. A primary alcohol is selectively... 

HBF4, MeOH, 23°, 48 h, 83% yield. This system cleaves acetate groups in the presence of benzoate groups. HCl in methanol can also be used. ... 

Garcia-Rubio et al.b3 529 examined S and MMA polymerizations initiated by RPO and have shown that I V can be used to distinguish and quantitatively determine aliphatic and aromatic benzoate groups in MMA and S polymerizations. 

The synthesis of this derivative of KDG was accomplished following the sequence depicted in figure 6. Methylation of the intermediate 8 with methyl Inflate afforded compotmd 21. Subsequent removal of the enol benzoate group at C-2 and of the silyl ether at C-6 provided the target compound 23 in 11% overall yield based on 8. This low yield could be due to the unstability of compotmds 8, 21 and 22 in a basic medium. However 24 was obtained in quantity sufficient for testing. 

In Step H-7 the addition of phenyllithium to the cyclic carbonate group neatly generates the C(2) benzoate group. A similar reaction was used in several other Taxol syntheses. 

The reaction goes faster in more polar solvents (a range of 106 in the rate constant) and parallels carbonium ion rearrangements in that respect. The effect of substituents in the para position of the benzoate group also suggests that the rate-determining step is the formation of an initial ion pair. The reaction is faster with the nitro than with the methoxyl substituent.819 820 The Hammett p value is 1.34. The activation parameters are not known for any but the unsubstituted member of the series however, and hence it is not known to what extent the relative rates depend upon the temperature. 

Figure 1-17. Exciton chirality of acyclic allylic benzoates and the sign of the predicted benzoate Cotton effects. The thick line denotes the electric transition moment of the benzoate group. Reprinted with permission by Am. Chem. Soc., Ref. 61. 

Cleavage of benzyl ethers3 Anhydrous FeCl3 in CH2C12 cleaves carbohydrate benzyl and p-phenylbenzyl ethers at 25° without effect on methyl ethers or acetate and benzoate groups. Yields are usually >70%. 

Scheme 10 Migration in 1-glycosyl radicals with a labeled benzoate group...

Second-order rate constants A for the replacement of a ring-substituted benzoate group by hydroxide ion in a number of complexes (base hydrolysis) have been carefully determined at 25°C in 40% aqueous methanol ... 

Taxol (Paclitaxel) 137, a natural product derived from the bark of the Pacific yew, Taxus brevifolia [213-215], and the hemisynthetic analogue Docetaxel (Taxotere) 138, two recent and promising antitumour agents, have been the matter of extensive in vivo and in vitro animal metabolic studies. The major metabolites of taxol excreted in rat bile [216] were identified as a C-4 hydroxylated derivative on the phenyl group of the acyl side chain at C-13 (139), another aromatic hydroxylation product at the mefa-position on the benzoate group at C-2 (140) and a C-13 deacylated metabolite (baccatin III, 142) the structure of six minor metabolites could not be determined. The major human liver microsomal metabolite, apparently different from those formed in rat [217], has been identified as the 6a-hydroxytaxol (141) [218, 219]. A very similar metabolic pattern was... 

Fig. 4. The structure of the cation of 4B, [Fe402(BICOH)2(BICO)2 (02CPh)4]showing the 40% probability thermal ellipsoids. Only the first atom of the phenyl rings is shown. BICOH stands for bis(N-methylimldazol-2-yl)carbinol. BICO is the alkoxide anion, (a) The Fe402 core iron coordination spheres and bridging benzoate groups. Continued on next page.

An efficient and selective Cu-assisted ortho-hydroxylation procedure for the conversion of benzoate to salicylate has been described, involving trimethylamine N-oxide (TMAO) as the oxidant [191,192]. The reaction was assumed to proceed via oxidation of a Cu carboxylate complex by TMAO to produce the active species (postulated to be a Cu hydroxo complex, but with only circumstantial evidence), followed by oxygen transfer to the benzoate group (Scheme 14). Using a set of different amide derivatives of benzoic acid, Comba and co-workers gained additional mechanistic hints in support of a reactive Cu-oxo or Cu-hydroxo intermediate that is stabilized by a five-membered chelate [193]. A pre-equilibrium involving Cu the ligand, and TMAO was proposed, but details of the reaction are far from clear. 

In another study,125 methyl 5-0-benzoyl-2,3-dideoxy-/8 D-g(t/cero-pent-2-enofuranoside (81) was treated with bromine in methanol in the presence of silver acetate and barium carbonate the two monobenzoates (shown as 84) of methyl 2-bromo-2-deoxy-j8-D-xylofurano-side were obtained. A rationalization for this reaction involves formation of an intermediate bromonium ion (82), produced by attack of Br on the less-hindered side of the double bond, which then undergoes trans-attack by the benzoate group at C-5 to give the benzoxonium intermediate (83) the monobenzoates (84) are obtained on processing of the reaction mixture with water. 

Only relatively few displacements of chloro groups in carbohydrate derivatives have been reported. Treatment of 6-chloro-6-deoxy-l,2 3,5-di-O-isopropylidene-a-D-glucofuranose (22) with anhydrous hydrazine for 2 days at reflux temperature yielded the corresponding 6-deoxy-6-hydrazino derivative.47 The chloro group in methyl 6-chloro-6-deoxy-a-D-glucopyranoside could be displaced by a benzoate group to afford methyl 6-O-benzoyl-a-D-glucopyranoside, in 74% yield, by... 

The axial-directing effect of the C-l substituent depends on the total stereochemistry and mode of substitution and falls roughly in the order of polarity of the substituents. The axial-directing effects of the C-l substituent is augmented when acetates at the other ring positions are replaced by benzoates possibly there are attractive syn-diaxial interactions between benzoate groups. 

P2i2i2i Z = 4 Dx = 1.528 R = 0.095 for 1,067 intensities. The conformation of the unsaturated pyranose is °HS. The attachment of the iodine atom and of the ethoxyl group is quasi-axial, and that of the benzoate group is quasi-equatorial. The benzoate group is planar, and is approximately perpendicular to the pyranose ring, as defined by the torsion angles 0-5-C-5-C-6-0-6, C-5-C-6-0-6-C=0 of +74°,... 

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