Benzylidene and Related Acetals

The photolysable 0-nitrobenzylidene acetal has-been used as an intermediate in the synthesis of a trisaccharide. Reaction of methyl et-D-mannopyranoside with i/-methoxystyrene and catalytic pyridinium tosj -late in DMSO gave a 7 3 mixture of the 4,6-0-acetals (13) and (U). 

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P. Calinaud and J. Gelas, Synthesis of isopropylidene, benzylidene, and related acetals, in S. Hanessian (Ed.), Preparative Carbohydrate Chemistry, Marcel Dekker, New York, 1997, pp. 3-32. 

Synthesis of Isopropylidene, Benzylidene, and Related Acetals. Calinaud, P. Gelas, J. In Preparative Carbohydrate Chemistry Hanessian, S.. Ed. Marcel Dekker New York, 1997 pp 3-33. 

Isopropylidene, Benzylidene, and Related Acetals Isopropylidene or benzylidene acetals are formed either by direct condensation of the diol with the appropriate carbonyl compound (acetone or benzaldehyde, respectively) or by transacetalation with the corresponding dimethoxy acetal. Both processes are carried out in acidic conditions [ 123]. 

The conformational flexibility postulated for the 2,4 3,5-diacetals of mannitol would disappear in acetals prepared from higher aldehydes and therefore possessing substituents at the acetal carbon atoms, but the status of such acetals is at present obscure., Two isomers of l,6-dichloro-l,6-dideoxy-2,3,4,5-di-O-ethylidene-D-mannitol are known, but the structures have not been determined. Two stereoisomers with the 2,4 3,5-structure and related to the H-inside and 0-inside conformations LVI and LVII, respectively, are clearly possible, but the structures 2,3 4,5 and 2,5 3,4 must also be considered. There is considerable scope for more work on 2,3,4,5-diacetals of mannitol, and confirmation of the structure XLVII advanced by Ness, Hann and Hudson for their mono-O-benzylidene-mono-O-methylene derivative is a necessary step in the assessment of the relative stability of the different structural types. 

Two groups have independently reported very similar routes (Scheme 15) to D-erythro-sphinqosine (87) and related compounds from 2,4-0-isopropylidene-D-threose (88) or the corresponding benzylidene acetal these materials are easily accessible by periodate... 

Model studies have established that carbohydrates containing vicinal cis-related and 4,6-A-benzyloxycarbonyl-aminol units can form 0,iV-benzylidene acetals analogous to the familiar diol derivatives, and accordingly penta-N-benzyloxycarbonyl paromomycin afforded the corresponding 4 6 -0 4 -0,6 -A-dibenzylidene derivative on treatment with benzaldehyde and zinc chloride. C N.m.r. spectral data for amikacin and related compounds have been reported, and the mass spectra of a number of partially A-acetylated derivatives of kanamycin A have been studied. ... 

Benzaldehyde reacts (J59) with D-glucose and with methyl a- and )8-d-glucopyranosides to form the 4,6-0-benzylidene derivatives. In the 0-ben-zylidene and related compounds, the acetal carbon is asymmetric and should give rise to two isomers. Evidence has been evinced that such isomers exist and that some are relatively unstable 160). 

Formation of an 11-methoxybenzyl ether occurs in satisfactory yield during the preparation of a 17,21 -benzylidene derivative [e.ff., (26)]. The usefulness of this ether and of related mixed acetals in removing the inhibitory effect of the 11 j5-hydroxyl group on the 20-ketone reduction has been reported. ... 

Acetalation. As polyhydroxy compounds, carbohydrates react with aldehydes and ketones to form cyclic acetals (1,13). Examples are the reaction of D-glucose with acetone and a protic or Lewis acid catalyst to form l,2 5,6-di-0-isopropylidene-a-D-glucofuranose [582-52-5] and its reaction with benzaldehyde to form 4,6-O-benzylidene-D-glucopyranose [25152-90-3]. The 4,6-0-(l-carboxyethylidine) group (related to pyruvic acid) occurs naturally in some polysaccharides. 

Isopropylidene (acetonides) and benzylidene derivatives are the most commonly used acetals for the simultaneous protection of 1,2- and 1,3-diols in carbohydrate and nucleoside chemistry [123]. Cyclohexylidene acetals are occasionally used, most often as an alternative to benzylidene acetals. Protection using cyclohexane- 1,2-diacetals or the related butane-2,3-diacetals represents a new approach which has proved its value in complex oligosaccharide synthesis [124]. 

Kim and co-workers carried out a closely related synthesis of a /<-(l -4)-mannote-traose 300, using their method for the in situ conversion of hemiacetals to glycosyl triflates by means of reaction with phthalic anhydride ( phthalan ) followed by trifluoromethanesulfonic anhydride. In this instance cleavage of the benzylidene acetal was effected with trifluoroacetic acid, and regioselective reprotection of the diol employed benzoyl chloride (Scheme 50).145... 

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