Benzylidene acetals stable

Benzyl ethers.2 Benzyl ethers of sugar alcohols can be obtained in 82-98% isolated yield by reaction with 1 and catalytic amounts of triflic acid under these conditions isopropylidene and benzylidene acetal groups are stable, as well as ester and imide groups. The reagent thus is comparable to benzyl triflate (6,44). 

David Crich and coworkers found that preactivation of 4,6-O-benzylidene-protected mannosyl sulfoxide donors at low temperature, followed by addition of a glycosyl acceptor, leads to the stereoselective formation of p-mannosides (Scheme 4.12). The necessity for preactivation at low temperature to obtain good p selectivity led to the notion that a stable intermediate is first formed, which then reacts in a stereoselective manner with the nucleophile to generate p-mannosides. In addition to preactivation, the 4,6-0-benzylidene acetal was found to be indispensable for high... 

The influence of the 4,6-O-benzylidene acetal in p-mannosylations was believed to be due to its torsional disarming effect. In an earlier work, Bert Fraser-Reid and coworkers found that a 4,6-O-benzylidene acetal locks the pyranose ring in a stable chair conformation [41], Therefore, it disfavors the formation of an intermediate oxocarbenium ion, which requires rehybridization and flattening of the sugar ring, usually leading to ahalf-chair conformation. This torsional effect, combined with the strong endo-mom nc effect in mannose, favors the a-triflate intermediate. 

The simultaneous protection of the 3 and S hydroxy groups of nucleosides and the 4,6- or 3,4-hydroxyls of hexopyranoses is a common problem in organic synthesis. In the case of hexopyranoses, we have already seen that benzylidene acetals and, in certain circumstances, isopropyiidene acetals can be used to good effect. An alternative silicon-based group would offer a wider repertoire of conditions for mild deprotection and such a group was devised by Markiewicz the 1,1,3,3-tetraisopropyldisiloxanylidene group (abbreviated TIPDS),22 229 TIPDS groups are stable to water, 0.3 M p-toluenesulfonic acid in dioxane, 10% tri-fluo mace tic acid in chloroform, 5 M ammonium hydroxide in dioxane-H20 (4 1), and tertiary amines in pyridine,... 

Oxidative debenzylatlon using dimethyldioxirane2 4 proceeds well with benzyl ethers of primary and secondary alcohols and the method is compatible with silyl ethers. Isopropylidene acetals are stable but benzylidene acetals are cleaved. The deprotection of p-bromo, p-cyano and 2-naphthylmethyl ethers can also be accomplished. 

Acetals, however, are easily regioselectively introduced (Schemes 3.11 and 3.12) [9]. Benzylidene acetals are preferentially formed as six-membered dioxane-type acetals (i.e., hexopyranosides form 4,6-0-benzylidene derivatives), whereas isopropylidene acetals are more stable as five-membered dioxolane acetals formed on vicinal cw-diols. However, the selectivity is not complete and methyl mannoside, for example, produces mixtures of the mono- and di-acetal compounds. The use of methyl propenyl ether yields mainly the kinetic product (the 4,6-0-isopropylidene acetal). Complementary to these traditional acetals is a rather recently developed acetal — the BDA-acetal [21]. In contrast to other acetals, this exclusively forms acetals between... 

Relative to their deprotection, it should be emphasized, that the selective removal of one acetal in the presence of the same (or different) type of acetal, at distinct positions in the same molecule, is possible and has been observed quite often [138]. Several well-established observations can be summarized as follows (i) 1,3-dioxanes are hydrolyzed more easily than the corresponding 1,3-dioxolane, (ii) implication of the anomeric center renders the acetal function more stable, (iii) cw-fused 1,3-dioxolanes in a furanose or pyranose ring are more stable than the ones that involve a side chain, and (iv) trows-fused benzylidene acetals of hexopyra-noses are hydrolyzed faster than the corresponding cw-fused acetals [123]. Some examples are shown in Scheme 19. 

The BPS group is more stable toward acidic hydrolysis than TBS or the TIPS groups. It survives DIBAL (diisobutylaluminum hydride) reductions, cleavage of THP with aqueous AcOH, and cleavage of isopropylidene and benzylidene acetals with aqueous CF3CO2H. BPS-Cl has been used to selectively protect primary OH groups in inositol (hexahydroxycyclohexane). ... 

Aq. AcOH, 35°C, >70% yield. In the following illustration, methoxy-substituted benzylidene acetals could not be hydrolyzed, which implies that the mesitylene acetal is more stable, but this was... 

The relative abundance of these h-fragments is high for benzylidene acetals, owing to the formation of the (stable) benzyl radical. These h-fragments are also formed by components having a 1,3,6-trioxabicyclo-[4.4.0]decane system, such as 4,6-O-benzyIidenehexopyranosides (see Scheme 28). The hi-ion is more abundant than the ha-ion, because of the... 

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