Benzylidene acetals cleaved

LiBF4, CH3CN, 70°, 3-8 h, 81-90% yield. This system of reagents also cleaves benzylidene acetals. Conventional reagents failed to cleave these glycosides. It is interesting to note that the /3-anomers are cleaved more rapidly than the a-anomers and that the furanoside derivatives are not cleaved. 

Pd-C, hydrazine, MeOH. In this case a 1,2-benzylidene acetal was cleaved in the presence of a 1,3-benzylidene acetal. 

Benzylidene acetals have the useful property that one of the two C—O bonds can be selectively cleaved. The direction of cleavage is dependent on steric and electronic factors as well as on the nature of the cleavage reagent. 

The acetal can also be cleaved with DDQ (CH2C12, H2O, 66% yield) to afford the monobenzoate. Treatment of a 3,4-dimethoxybenzyl ether containing a free hydroxyl with DDQ (benzene, 3 A molecular sieves, rt) affords the 3,4-dimethoxy-benzylidene acetal. ... 

I2, MeOH, reflux, 2 h, 79% HCIO4, H2O, 25°, 16 h, 87% yield. These conditions also cleave acetonides and benzylidene acetals. ... 

Me2BBr, TEA, BH3-THF, —78° warm to —20° over 1 h, 70-97% yield. These conditions cleave the benzylidene acetal, to leave the least hindered alcohol as a free hydroxyl. If diborane is omitted from the reaction mixture and the reaction is quenched with PhSH and TEA, the benzylidene group is cleaved to give an (9,5 -acetal [ROCH(SPh)Ph]. Acetonides are cleaved similarly." ... 

BH3 SMe2, CH2CI2, 0°, 1 h, then BF3, 5 min. Simple benzylidene acetals are cleaved efficiently without hydroboration of alkenes that may be present, and acetonides are converted to the hydroxy isopropyl ethers. 

With ring G in place, the construction of key intermediate 105 requires only a few functional group manipulations. To this end, benzylation of the free secondary hydroxyl group in 136, followed sequentially by hydroboration/oxidation and benzylation reactions, affords compound 137 in 75% overall yield. Acid-induced solvolysis of the benzylidene acetal in 137 in methanol furnishes a diol (138) the hydroxy groups of which can be easily differentiated. Although the action of 2.5 equivalents of tert-butyldimethylsilyl chloride on compound 138 produces a bis(silyl ether), it was found that the primary TBS ether can be cleaved selectively on treatment with a catalytic amount of CSA in MeOH at 0 °C. Finally, oxidation of the resulting primary alcohol using the Swem procedure furnishes key intermediate 105 (81 % yield from 138). 

Cyclic Acetals Benzyl ethers are obtained from benzylidene acetals on Ni but the product ethers are cleaved about as readily as the acetals.166 The hydrogenolysis of a p-bromobenzylidene derivative was achieved on 10% Pd/C in AcOEt and NaHC03.154... 

The first step is reduction of the ester group to give primary alcohol 33, which is subsequently protected as the fcrr-butyldiphenylsi-lyl ether 34 The benzylidene acetal is selectively cleaved with Lewis-acid activation under reductive conditions to generate protected secondary alcohol 8... 

Simple benzylidene acetals of carbohydrates are photochemically cleaved in a way similar to the well-used AT-bromosuccinimide process regioselective ring opening process into bromodeoxysugars. 

For sodium cyanoborohydride — hydrogen chloride reagent the direction of the reductive opening of the dioxolane acetals obeys the same rule, it depends on the stereochemistry at the asymetric, benzylidene acetal carbon [168, 169]. Methyl exo-2,3 4,6-di-0-benzylidene-a-D-mannopyranoside is cleaved in oxolane solution to give 50 % of the 3,6-di-O-benzyl derivative, which is also the major product in the reaction of methyl 3-0-benzyl-4,6-0-benzylidene-a-D-mannopyranoside. The 2-O-benzyl isomer (20) was cleaved nonselectively, indicating again the effect of the bulk of the C-3 substituent [169]. A compatibility of this reagent, as well as of borane — tri-... 

Benzylidene acetals can be oxidatively cleaved by jV-bromosuccinimide (NBS) to give 6-bromo-4-benzoyl-hexopyranosides (see Chapter 3). This reaction has also been performed on 1,2-0-benzylidene derivatives and it appears that the cleavage reactions result in the formation of a derivative that has an axial benzoyl ester.33... 

Catalytic hydrogenation offers the mildest and one of the most efficient means for cleaving benzylidene acetals to toluene and the corresponding dioL The reaction is especially useful for reactions on a small scale performed on frail substrates as exemplified by the final step in a short synthesis of Oteandolide by Evans and co-workers [Scheme 3.54J.88 Birch reduction involving sodium or lithium in liquid ammonia in the presence of a proton source such as ethanol accomplishes the clean scission of a benzylidene acetal but few functional groups are compatible with such powerfully reducing conditions and the reaction is seldom used.89... 

Similarly, benzylidene acetals can be oxidatively cleaved with /er/-butyl hydroperoxide in the presence of catalytic amounts of copper(II)124 or palla-dium(ll)l2iu26 to give a benzoate ester and a free hydroxyl group but the regioselectivity of the reaction is usually poor. Recent efforts to optimise the efficacy of Taxol derivatives for the treatment of solid tumours included an oxidative cleavage of a benzylidene acetal [Scheme 373].127... 

Benzylidene acetals also cleave on heating with excess DDQ in aqueous acetonitrile. The reaction is exemplified by the closing steps of a synthesis of Tricolor-in A, an unusual tetrasaccharide macrolactone isolated from Ipomoea tricolor, a... 

Trost first introduced the di-fe/7-butylsilylene derivative as a means for protecting 1,2- and 1,3-diols during a synthesis of PiUaromycinone derivatives.213 Di-ferf-butylsilylene derivatives are not as robust as isopropylidene or benzylidene acetals and their use is best reserved for systems requiring deprotection under very mild conditions. Di-isopropylsiiylene derivatives are occasionally used but they usually only survive in highly crowded environments.214 Di-feri-butylsily-lene derivatives survive hydroboration with 9-BBN, mild oxidation (e g the Dess-Martin, ozone), Lewis acids such as trifluoroborane e the rate and titanium tetrachloride, mild acids (pyridinium p-toluenesulfonate). camphorsulfonic acid, strong bases such as feri-butyllithium (THF, -50 °C), DDQ, and sodium meth-oxide in methanol at 0 C — conditions used to cleave acetate esters. 

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