Of methyl a-D-galactopyranoside

Although there were early reports on the selective acylation72,73 and p-toluenesulfonylation73 of methyl a-D-glucopyranoside, and the p-toluenesulfonylation of methyl a-D-galactopyranoside,74 to yield... 

The results of a study of the di- and tri-molar methanesulfonylation of methyl a-D-galactopyranoside suggested, amongst the secondary hydroxyl groups, a reactivity order of HO-2 > HO-3 > HO-4, and, from the latter reaction, the 2,3,6-triester could be isolated in 30% yield a similar reaction of methyl a-D-mannopyranoside gave the... 

Table VIII lists the increases in molecular rotation caused by adding an a-D-galactopyranose unit to various saccharides. The average value, about 39,000, is nearly identical with the molecular rotation of methyl a-D-galactopyranoside (38,000) or with the molecular rotation per monosaccharide unit in the galactan68 of von Lippmann (38,600).

Conformational analysis of BS I by c.d. spectroscopy indicated that the protein contained 30-40% of /3 structure in its native conformation.623 The c.d. spectrum was relatively insensitive to alteration in pH, removal of bound metal, and addition of methyl a-D-galactopyranoside. However, addition of dodecyl sodium sulfate or 2,2,2-trifluoroethanol resulted in the formation of some a-helical structure, and was accompanied by the loss of polysaccharide-precipitating capacity. Urea (8 M) irreversibly denatured the lectin. 

The 4,6-0-benzylidene acetal of methyl a-D-galactopyranoside was prepared over 50 years ago by using henzaldehyde and sodium sulfate at 150°. The discrepancies between the physical constants which appeared in the literature for this acetal were resolved by Bell and Greville, who established that the compound has m.p. 168-170° and [ajn 4-142° (CHCU). The suspicion of diastereoisomerism was therefore dismissed. The 4,6-0-benzylidene acetal of methyl a-D-galactopyranoside has been described. Some interconversions of these benzylidene acetals into other sugar derivatives are described later (see p. 260). 

Reaction of methyl a-D-galactopyranoside 102.1 with 6,6 -bis(3,4-dihydro-2//-pyran) (bis-DHP, 1023) in refluxing chloroform (thermodynamic conditions) gives the dispoke (dispiroketal) derivative 102.2 in 64% yield. - The preference for rram-diequatonal protection is a consequence of steric interactions and multiple stabilising anomeric effects of the two acetal functions in the dispoke derivative. The pure bis-DHP reagent 1023 is a low melting solid (mp 49-50 C) that is stable at -10 °C in the absence of acid and moisture. It is prepared in 55% yield by the oxidative dimerisation of 6-lithio-3,4-dihydro-2//-pyr-an. As can be seen from Scheme 3.103, the preference for frum-diequatorial protection is excellent in the fiico, manno, and fyxo series but it is poor in the case of the arabino and rhamno series. 

Takagi. S. Jeffrey, G.A. Neutron diffraction refinement of the crystal structures of methyl a-D-galactopyranoside monohydrate and methyl p-D-galactopyranoside. Acta Crystallogr. Sect. B 1979. 35, 902-906. 

Physical studies on glycosides have included the X-ray diffraction analysis of methyl a-D-galactopyranosides and methyl p-D-glucopyranosides with fatty acid esters at C-6 which showed bilayer structures with zig-zag alkyl chains. The conformational behaviour of several non-ionisable lactose analogues have been examined by NMR and molecular mechanics methods, and the energy profile of methyl p-D-arabinofuranoside as a function of ring conformations has been determined by gas phase computations. ... 

Selective tosylation of methyl a-D-galactopyranoside with toluene-p-sulphonyl chloride (2 molar equivalents) in pyridine gave the 2,6-ditosylate (26.6%), the 2,3,6-tritosyIate (17.2%), and the 3,6-ditosylate (2.2%). Sucrose with 4 molar equivalents of toluene-p-sulphonyl chloride gave sucrose 6, l, 6 -tritosylate (33%) and 2,6,1,6-tetratosylate (32%) the latter was previously reported by Long but was here characterized for the first time. ... 

Per-O-acetyl aldononitriles and methylated aldononitrile acetates have been separated by g.I.c. on a mixed liquid phase. Chemical ionization m.s. showed the presence of peaks at M + 1 in each instance. The retention times of a number of partially methylated alditol acetates have been recorded on a variety of stationary phases. Using a combination of silica gel chromatography and g.I.c., fifteen methyl ethers of methyl a-D-galactopyranoside have been isolated from the partial methylation products of D-galactose. ... 

The 2-, 3-, 4-, and 6-monosulphates of methyl a-D-galactopyranoside and the 2-and 3-monosulphates of methyl a- and j8-D-glucopyranosiduronic acid have been prepared by standard methods. Sulphates or mixtures of sulphates and cyclic sulphates were produced when benzyl 2,3-anhydro-4-0-triflylpentopyranosides were treated with t-BUOSO3H. An example is presented in Scheme 15. The conversion of cyclic sulphates to epoxides is covered in Chapter 5. 

Low yields of uronic acid usually result from the action of selective oxidants on glucosides and non-reducing acetals of the aldoses in which all the hydroxyl groups are free. However, a 56% yield of D-galacturonoside has been claimed for the oxidation of methyl a-D-galactopyranoside with nitrogen dioxide . 

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