JS-D-Glucopyranose

Trimethylsilylation is adversely affected by moisture, and therefore, hydrolyzates should be evaporated to dryness as completely as possible. If trimethylsilylation is catalyzed by trifluoroacetic acid, instead of chlorotrimethylsilane, moderate proportions of water may be tolerated,117,127-129 but, even under these conditions, extra peaks may be obtained from partly trimethylsilylated derivatives.130 Catalysis with trifluoroacetic acid is useful when aqueous aliquots from a reaction are to be trimethylsilylated.131 A further advantage of this method, which has been used in the determination of 1,6-anhydro-jS-D-glucopyranose in corn syrup,132 for cycloamyloses,133 and for a series of malto-oligosaccharides,134 is that ammonium trifluoroacetate is soluble in pyridine. 

It should be noted that the per(trimethylsilyl) derivative of 1,6-anhydro-jS-D-glucopyranose has the same retention time as that of a-D-xylopyranose when they are chromatographed on a column of 3%... 

The addition of silanes across alkenes has been investigated both experimentally145 and theoretically.146 The effect of optically active thiol catalysts to catalyse radical hydrosilylation (polarity reversal catalysis) has been studied. The use of 2,3,4,6-tetra-O-acetyl-thio-jS-D-glucopyranose as the chiral thiol (used to reduce the intermediate carbon-based radicals) furnished the hydrosilylated alkenes in low to moderate enantiomeric excesses.145 In addition to this work a theoretical study on the reactions of SiH3 with ethene and propene has been undertaken using PMP2(6-31G ) and QCISD(T)(6-31G ) methods. Results indicated that the alkene-addition pathway is favoured over the alternative possible mode of reaction (H-abstraction). This is contrary to that previously suggested for the reaction of SiF with propene.146... 

Mannose differs from glucose in configuration at C-2. All hydroxyl groups are equatorial in jS-D-glucopyranose the hydroxyl at C-2 is axial in /3-D-mannopyranose. 

It is of interest to contrast the above-described mechanism (for the alkali-catalyzed transformation of phenyl /S-D-glu copyranoside to 1,6-anhydro-jS-D-glucopyranose) to the mechanism proposed by Lemieux and Brice38 for the acid-catalyzed transformation of 1,2,3,4-tetra-O-acetyl-jS-D-glucose (XXXI) to tri-0-acetyl-l,6-anhyd ro-/S-D-glu copy ran ose (XXXIII). Evidence was obtained showing that, in the presence of stan-... 

Fig. 2-11. Chair conformations of jS-D-glucopyranose. Form 1, in which the OH and CH2OH substituents are equatorial, is favored.

The glycosidic bond to an anomeric carbon can be either a or p. Maltose, the disaccharide obtained by enzyme-catalyzed hydrolysis of starch, consists of two a-n-glucopyranose units joined by a 1- 4-Q-glycoside bond. Cellobiose, the disaccharide obtained by partial hydrolysis of cellulose, consists of two jS-D-glucopyranose units joined by a l->4-/ -glycoside bond. 

Both anomers of n-glucopyranose can be crystallized and purified. Pure fv-n-glucopyranose has a melting point of 146 °C and a specific rotation, (all), of -112.2 pure jS-D-glucopyranose has a melting point of 148 to 155 °C and a specific rotation of +18.7. When a sample of either pure anomer is dissolved in water, however, the optical rotation slowly changes and ultimately reaches a constant value of -52.6. That is, the specific rotation of the -anomer solution decreases from +112.2 to +52.6, and the specific rotation of the j3-anomer solution increases from +18.7 to - -52.6. Called mutarotation, this change in optical rotation is due to the slow conversion of the pure anomers into a 37 63 equilibrium mixture. 

The selectivity of alkylation of 1,6-anhydrohexopyranoses is lower than drat of acylations. The relative rate-constants for the three hydroxyl groups in 1,6-anhydro-jS-D-glucopyranose (6) during methyla-tion with dimethyl sulfate in 19% sodium hydroxide solution689 are k2 ks k4 = 2.5 1 1.8, so that the formation of pure monomethyl or dimethyl ethers of 6 by this method is of little preparative value (compare Refs. 224 and 671). l,6-Anhydro-/3-D-galactopyranose (12) affords mostly the 2,4-di-O-methyl derivative,690 and 2-acetamido-l,6-anhy-dro-2-deoxy-/3-D-galactopyranose is mainly methylated at 0-4 by the... 

Lemieux and Stevens have introduced a set of empirical rules to enable the chemical shift of any proton relative to jS-D-glucopyranose tetra-acetate (1) to be calculated. The rules are (t values p.p.m. 

Fig. 3. The 100 Me./sec. n.m.r. spectrum of 2-acetamido-l,3,4,6-tetra-O-acetyl-2-deoxy-jS-D-glucopyranose in deuterochloroform. (From Inch et...

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