Mannose, cyclic acetals

The major mechanistic and structural aspect of the acetalation process is its orientation toward derivatives obtained either under thermodynamically controlled conditions or under kinetically controlled conditions. We will not discuss here all structural factors concerning the relative stabilities of acyclic and cyclic acetals of polyols and monosaccharides, because such a discussion has been extensively reviewed and adequately commented on [8,10,12 -14]. However, it is important to focus here on the main consequences of these relative stabilities in relation to the various experimental conditions to orientate the choice of specific conditions, particularly for the most important monosaccharides (D-glucose, D-mannose, and D-galactose). 

Mannose would be expected to give furanoid 1,2-, 2,3-, 3,5-, or 5,6-cyclic acetals. Only products containing a 2,3- or 5,6- ring have so far been recorded. For the pyranoid, CA conformation, 1,2-, 2,3-, or 4,6-acetal rings would be anticipated. The mannopyranosides afford acetals having... 

Reactions carried out on disaccharide derivatives included the conversion of peracetates to l-phenylthio- 3-derivatives by use of phase-transfer methods and the epimerization at C-2 by calcium -amine systems (CaCl2.2H20, EtjN, for example). By this means maltose, lactose, isomaltose and melibiose were converted into the corresponding glucosyl-D-mannoses. An extensive range of variously substituted maltosyl fluorides have been made from maltose derivatives as substrates for cyclodextrin transferase. Only the 6 -methyl ether and 6 -acetate were transformed into cyclic products. In the course of the work the thio-derivatives (83) and (84) were made. ... 

In a reinvestigation of the synthesis of aldononitrile peracetates from sugars by oximation and acetylation, it was shown that (a) peracetylated acyclic oximes are present in the product mixtures, but are converted to the nitriles by elimination of acetic acid in the heated injection port during GLC analysis and (b) less of the peracetylated A-hydroxyglycosylamine byproduct is formed using 1-methylimi-dazole as a catalyst. The fate of 2,3 5,6-di-0-isopropylidene-D-mannose oxime, which can exist as its cyclic AT-hydroxy-a-D-mannofuranosylamine tautomer, on... 

Five-membered cyclic dibutylstannylene acetals formed on vicinal cw-axial-equatorial pairs of hydroxyl groups selectively enhance the nucleophilicity of the equatorial oxygen in 0-alkylation reactions [179, 180], On the basis of this rule Schuerch surmised that alkylation of the tin complex 103, having the anomeric oxygen locked in the equatorial position, should lead to P-mannosides [181]. This assumption was proved by treating the mannose diol 102 with Bu2SnO (—> 103) followed by in situ exposure to alkyl halides whereby P-mannopyranosides 104 were formed stereospecifically (Scheme 30) [181],... 

---