Sugars benzylidene deriv

In other reports, the lipase-catalyzed acylation of benzylidene derivatives of sugars, useful intermediates in the synthesis of oligosaccharides, has been described [42]. For example, the esterification of 4,6-O-benzylidene-a-D-glucopy-ranoside (13) with vinyl acetate by action of Pseudomonas cepacia lipase gave quantitatively the 2-O-acetate 13a. [42b] As a third case, it deserves to be mentioned the extensive work of Russo and coworkers for the chemo-enzymatic synthesis of milk oligosaccharides [43]. 

The reaction of benzylidene acetals of sugars with N-bromosuccini-mide has since known great popularity, and, especially for 4,6-O-ben-zylidene derivatives, is now a well established, laboratory procedure.140 It has been extended to 2, 3 -0-benzylidene derivatives of nucleosides.141,142... 

The cyclic acetals of sugars constitute an important class of compounds, and their value as synthetic intermediates is well recognized.100 The reaction of methyl /3-maltoside with benzaldehyde in the presence of zinc chloride has been reported to give the crystalline methyl 4, 6 -0-benzylidene-/3-maltoside.52 Similar benzylidenation of l,6-anhydro-/8-maltose affords the corresponding 4, 6 -0-benzylidene derivative in 63% yield.82... 

A number of benzylidene derivatives of simple sugars have been converted into epoxides in similar fashion (Eqa. 254 and 255). Tin- reaotiotis are of considerable interest beoauso they involve structure that axe oonformationaJIy related to da and tams-decaiins.145 I nr example, methyl 3-0-benzcyl-4,6-0-benzyLclene-2-0-to yl-a- j- luoi side and methyl 2-0-benzoyl-4,8-0-benzyiideQe-3 0-toeyl- -D -gl i irosi i le... 

In the synthesis of the glycolipid asialo GMI described by Ogawa and co-workers [107] the trisaccharide derivative (215) (used in the synthesis of asialo GM2) was converted into the 4,6-O-benzylidene derivative (233) and this was condensed with aceto-bromogalactose in the presence of mercury(II) cyanide and molecular sieves to give the P-linked acetylated galactose derivative in 97 % yield. The product was deprotected and acetylated to give the peracetyl derivative (234) of the tetrasaccharide which was converted into the free sugar (235) with hydrazine hydrate. Compound (235) was converted into asialo GMI via the imidate as described above for the synthesis of asialo GM2 from the imidate (219). 

The formation of a cyclohexyloxyl radical via 6-exo-trig manner is promoted by the rigid bicyclic structure derived from the sugar benzylidene group (eq. 3.57a). Moreover, eq. 3.57b indicates that radical ring closure to a formyl group is preferable to that onto an olefinic group. 

Methyl-L-arabinose has been isolated from the products of hydrolysis of methylated damson gum37 and the araban found associated with pectin.84 88 This sugar has been synthesized by the conversion of methyl /3-L-arabopyranoside to the 3,4-benzylidene derivative (XXIX) which on methylation gave methyl 2-methyl-3,4-benzylidene-/J-L-arabopyrano-side (XXX) from which the desired 2-methyl ether was obtained by hydrolysis.40... 

Since acid hydrolysis of methyl a-D-altroside leads principally to D-altrosan, it became necessary to resort to other devices in order to complete the transformation to the free sugar. The pioneer work of Robertson and his collaborators had established a clear route from D-glucose to many D-altrose derivatives. The final steps, as described by Richtmyer and Hudson, are the acetolysis of methyl o-o-altroside (XXXVIII), or more simply of its benzylidene derivative (XXXVII), followed by catalytic deacetylation of the pentaacetyl-a-D-altrose thus produced. In this way crystalline D-altrose becomes readily available. 

Formation of cyclic acetals by sugar hydroxyls generally retards nucleophilic displacements at the anomeric centre of the same sugar residue. In the case of 4,6-benzylidene derivatives, the mechanism of deactivation appears to be that the dipole of the C6-06 bond is constrained with its positive end directed towards the anomeric centre.In the case of the 1,2-diketals, the deactivation arises from the increased difficulty of forming half-chair or boat conformations in a six-membered ring, which is part of a traw -fused decalin structure. 

The reaction was also applied to the 4,6-O-benzylidene derivative of methyl a-D-glucopyranosides. The method is useful for preparation of partially esterified sugars. 

Sugar Acid Derivatives.- 2-0-Acetyl-3,4-0-(R)-benzylidene-D-ribono-... 

The method of choice for separating the epimeric acids usually is unique for each individual starting sugar and is based on fractional crystallizations of the lactones, metallic or alkaloidal salts, the phenylhydrazides, amides, double salts, benzylidene derivatives, etc. 

Although the sugars condense with nitromethane in aqueous alkali (148a), the reaction usually is carried out in methanol solution (or suspension) with sodium methoxide acting as the basic catalyst. In the most favorable examples of the condensation reaction, the sodium salts of the C-nitroalco-hols precipitate from this solution and can be isolated simply by filtration. As in the cyanohydrin condensation, two epimeric products are formed, usually in unequal amounts. After removal of the sodium by ion exchange, the epimeric (7-nitroalcohols frequently can be separated by fractional crystallization. For some syntheses, partially substituted sugars, such as the benzylidene derivatives, may be used to advantage. 

The amino sugars form hydrazones, oximes, glycosyl halides, glycosides, and benzylidene derivatives analogous to those of the sugars. 

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