Lead acetate Mannose

The serologically active, type B heteropolymer, containing xylose, mannose, and glucuronic acid, could be isolated by fractional precipitation with ethanol in the presence of acetic acid and sodium acetate, and also by precipitation with aqueous lead acetate. In the latter procedure, a galactose-containing substance remained in the supernatant liquor. An increase in the galactose component was effected by fractional precipitation of the type A heteropolymer with Type XIV antipneumococcal serum. Precipitation with cetyltrimethylammo-nium bromide did not effect any fractionation. " ... 

A further quantity of the same substance was prepared by these workers and purified by repeated precipitation with lead acetate and ammonia. The product had [q ]d + 67.7° which changed to —10° after hydrolysis with dilute sulfuric acid for four hours. The amount of reducing sugar present was 75.6%. Mannose, D-arabinose and an unidentified sugar acid were obtained from the hydrolyzate. The substance did not produce a skin reaction, nor did it stimulate antibody formation. It reacted with immune serum in high dilution (1 2,000,000) in the precipitin test. It was claimed that this product was closely related to the polysaccharide isolated by Laidlaw and Dudley. ... 

C3-hydroxy benzylation, through alkylation of the intermediate 2,3-0-dibutylstannylene of mannose acetal 24, gives 25 (Scheme 5). Hydroboration of the exocyclic 5-alkene derived from this material provides 26, in which the C5 stereochemistry is inverted. Benzylation and acetate methanolysis leads to the suitable glycosyl acceptor 27. 

Fig. 1.—Rates of Consumption97 of Lead Tetraacetate by the Methyl a-D-Pyrano-sides of Mannose (1), Galactose (2), and Glucose (3), in Glacial Acetic Acid (Temp. 20°).

The influence of the 4,6-O-benzylidene acetal in p-mannosylations was believed to be due to its torsional disarming effect. In an earlier work, Bert Fraser-Reid and coworkers found that a 4,6-O-benzylidene acetal locks the pyranose ring in a stable chair conformation [41], Therefore, it disfavors the formation of an intermediate oxocarbenium ion, which requires rehybridization and flattening of the sugar ring, usually leading to ahalf-chair conformation. This torsional effect, combined with the strong endo-mom nc effect in mannose, favors the a-triflate intermediate. 

Replacement of one of the carbon atoms in the sugar moiety by oxygen, in effect converting that ring to an acetal, leads to yet another false substrate for viral reverse transcriptase. Glycosdation of the sdylated purine (79) with chiral dioxolane (80), prepared in several steps from anhydro-mannose, in the presence of ammonium nitrate affords the coupling product as a mixture of anomers. The mixture of products is then separated on a chromatographic column. The desired diastereomer (81) is the reacted... 

Displacements Catalyzed by Adds.—The acid-catalyzed mercaptala-tion reaction with ethanethiol normally leads to the aldose diethyl dithio-acetal, but, in anhydrous reaction media, further ethylthio substituents may be introduced into the molecule, usually at C-2. Brigl and coworkers found that ethanethiol and hydrogen chloride convert 3,4,5,6-tetra-O-benzoyl-D-glucose diethyl dithioacetal into 3,4,5,6-tetra-O-benz-oyl-2-)S-ethyl-2-thio-D-glucose (mannose) diethyl dithioacetal. The diethyl... 

Even in highly polar solvents, the relatively low reactivity of hydroxy alde-hydes may lead to marked underoxidation of polyhydric alcohols. Thus, 4, 6-(9-ethylidene-D-mannose (and -o-galactose) and o-glucofuranose 5,6-carbonate each consume 2 mol of lead tetraacetate per mole in acetic acid when the concentration is 10 mM, but, in more-dilute solution (100 pM), take up only 1 mol at a significant rate. This behavior was attributed to the minimal reactivity of the aldehydo-pentose formic esters that are produced by initial cleavage of the ot-hydroxy hemiacetal group. 

The oxidation of one mole of o-mannose by lead tetraacetate in acetic acid, which involves rapid reduction of 2.8 molar proportions of oxidant, is consistent with a principal reaction pathway in which the pyranose is degraded stepwise 4-(9-formyl-D-arabinose (76 77) is produced first, further oxidized to... 

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],... 

---