Acetals of sugars

This reduction technique also applies to the benzaldehyde acetals of sugars with reduction of the benzaldehyde acetals taking place in preference to reduction of the anomeric acetal (Eq. 305).497... 

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... 

Aldonitrile acetates of sugars have been applied to their GC analysis in different polysaccharides [442] on LAC-4R-886 polyester stationary phase (190°C)and to the analysis of polyols and aldoses in urine and crystalline lenses [444] on a capillary column of borosilicate glass (60 m X0.3 mm l.D.) coated with SE-30 containing a dispersion of Silanox 101 (temperature programming at l°C/min from 150°C). These derivatives were very stable and a uniform product was formed from every individual substrate. 

The differences in reactivity of acetals of sugar sulfonates demonstrate the need for care in the choice of acetal protecting groups in preparative work. 

Numerous thiols have been found suitable for preparing dithio-acetals of sugars ethanethiol1 and a-toluenethiol27 have been used most commonly, but small, aliphatic thiols function interchangeably in this reaction. Condensation with 1,10-decanedithiol produces a dimeric species (1), assumed to possess a 26-membered ring,28... 

Sulfuric acid is generally employed at 0.1-0.5%, although values outside this range have been reported. Using a 5% concentration of sulfuric acid, Bell obtained yields of di-O-isopropylidene acetals of sugars of 53-86%. Ethanesulfonic acid and cation-exchange resins have also had some limited application. 

It is difficult to assess the relative stability of benzylidene rings to acids as, in most cases, they have been cleaved by hydrogenolysis. Robertson and Speedie reported that they were unable to effect partial hydrolysis of a di-O-benzylidenexylose. Di-O-ethylidene acetals of sugars seem to have been somewhat neglected. 

A sensitive method for the detection and separation of micro quantities of cyclic acetals of sugars by means of gas-liquid, partition chromatography has been described by Jones and coworkers. The column packing consisted of an intimate mixture of Apiezon M grease on Cromosorb W and butanediol succinate polyester on Cromosorb W, which was packed on top of a column of SE-30 methylsilicone polymer and glass beads. The column temperatme was maintained at 200°. Some cyclic acetals of sugars have been fractionated by gas-liquid chromatography as their (trimethyl-silyl) ethers. These derivatives are conveniently prepared and the method is especially suited to the less volatile monoacetals. 

The infrared spectra of carbohydrates in general have been reviewed by Tipson, and those of cyclic acetals of sugars by S. A. Barker and coworkers, and by Tipson and coworkers. ... 

But, more importantly, the synthetic applications of 1,2-diacetals proved not to be limited to the selective protection of fra s-diequatorial-l,2-diols alone. In the following sections we will illustrate how these protective groups have an effect on the next level of carbohydrate architecture 1,2-acetals of sugar derivatives can have a dramatic effect on controlling the rates of coupling reactions in oligosaccharide synthesis. 

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