Of partially methylated sugars

For the preparation of partially methylated sugars direct methylation is not as a rule a suitable process, for the mixture of isomers thus produced can be fractionated only with difficulty. Instead, it is customary to introduce protecting substituents prior to methylation and to remove them afterwards. As will be seen later, this is the method used in the synthesis of mono-, di-, and tri-methylglucoses. 

By analysis112 of the fragmentation patterns of partially methylated carbohydrates that have subsequently been deuteriomethylated, the disposition of the original methyl groups can be determined. This procedure may have a significant advantage over other methods of identification of partially methylated sugars, and the method has been discussed in a previous article in this Series.116... 

The methylated somatic polysaccharide has been hydrolyzed, and from the mixture of partially methylated sugars so formed, methyl 3,4,6-trimethyl-D-mannopyranoside (25%), methyl 3,4-dimethyl-D-man-nopyranoside (20%), methyl 2,3,4-trimethyl-L-rhamnopyranoside (25%) and methyl 3,5-dimethyl-n-arabofuranoside (25%) have been charac-... 

The mixture of partially methylated sugars obtained on hydrolysis of fully methylated polysaccharides is routinely analyzed in the authors laboratory, as alditol acetates, by g.l.c.-m.s. For this reason, the mode of fragmentation of these derivatives has been extensively studied " by using the deuterium-labelling technique. The results of the studies are summarized next. 

The use of alditol acetates for the separation of mixtures of partially methylated sugars not only simplifies the g.l.c. but also the mass spectra, because such compounds contain carbon to carbon bonds of only three types those between (a) carbon atoms that each carries a methoxyl group, or (b) each bears an acetoxyl group, or (c) adjacent carbon atoms having one substituent of each type. Mass spectrometry of partially methylated alditol acetates thus gives rise to relatively few primary fragments that are characteristic of the pattern of substitution. This technique was introduced to carbohydrate chemistry by Lindberg and his associates, and has been employed extensively by them in subsequent structural studies, some of which are discussed next others are listed in Table XXXI (see p. 103). 

Reverse-phase h.p.l.c. on Wates /i-Bondapak C-18 column has been used successfully for the fairly rapid separation of mono-, di-, and tri-saccharides using water as the mobile phase and, for the separation of partially methylated sugars, 1% ammonium acetate was used as mobile phase, with ethanol being added to elute the more highly methylated sugars. ... 

In the carbohydrate field, pH values outside the range 3 to 5 should be avoided whenever possible. When such factors as the presence of blocking groups render this infeasible, some reactions can still be performed at pH s of 7.5264 and 5.7.266 The partially methylated sugars,67 68 69a 60 the highly hindered glycols,261 and certain amino alcohols,9 98 100 101 l03 6 are exceptions and tend to be most readily oxidized at a pH of 7.5. 

IV. Degradation of the Permethylated Sample, and Analysis of the Partially Methylated Sugars... 

Separation of the partially methylated sugars by g.l.c. gives evidence on the methyl-substitution pattern complementary to that obtained from the mass spectrum. The electron-impact, mass spectra do not differentiate between the sugar isomers (such as the galactose,... 

The basic principles concerned with the preparation of partially methylated derivatives of D-galactose and D-glucose are identical. Since Bourne and Peat1 have already provided a full discussion of these principles in connection with the latter sugar it would serve no useful purpose merely to re-enumerate their statements. It must be pointed out, however, that in the case of D-galactose, with only two exceptions, synthetic operations have always commenced with derivatives of the (1) E. J. Bourne and S. Peat, Advances in Carbohydrate Chem., 5, 145-190 (1950). 

The main limitation of the use of partially methylated alditol acetates for the characterization of methylated sugars lies in the structural symmetry that may exist when the primary hydroxyl group (0-5 in pentoses and 0-6 in hexoses) is not etherified. This difficulty, however, can be overcome by introducing deuterium at C-1 by reduction of the sugar with sodium borodeuteride. 

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