Preparation of the methyl D-galactopyranosides

Abstract

Directions are given for a simple, reproducible preparation of methyl a-and methyl P-D-galactopyranoside by the Fischer method.1 The initial separation affords the two compounds, 97-9894 pure, in yields of 41 and 15% respectively. raised to about 58 and 21% by re-equilibration of the residual syrup that contains methyl galactofuranosides.The preparation of the methyl galactopyranosides has been described many times. The yields obtained by the Fischer method and the details of the separation of the pure isomers vary widely, and at least some of the published procedures have proved to be capricious and unreliable in this and other laboratories. For example, Dale and Hudson* reported a clean separation of .x and 9 isomers when the crude, syrupy mixture of methyl galactosides was dissolved in isopropyl alcohol, the a isomer separating first. as the monohydrate. In other hands,3 this caused the two pyranosides to crystallize as a mixture, and when the method w-as tried in this laboratory the P isomer separated first, but both isomers were very impure.The conflicting results were probably caused by different amounts of water in solvents, although it is possible that both isomers may exist in more than one crystalline form. The a isomer seems to crystallize only as a monohydrate. I t showed no tendency to crystallize in anhydrous form from anhydrous solvents, and no unequivocal statement that this has been achieved could be found in the literature. The monohydrate could usually be crystallized from organic solvents containing water, but the results were sometimes unpredictable. The hydrated a form did crystallize excellently from concentrated solutions in water alone,lJ and showed a remarkable power to exclude isomers and coloured impurities. The P isomer, which does not form a hydrate, shox-ed the opposite behaviour-it mas very soluble in water, but crystallized readily from anhydrous ethanol. The use of water and then ethanol as solvent provided reproducible conditions under which the two pyranosides could be isolated from the crude mixture of methyl galactosides obtained by the Fischer method. The (3 form could be isolated first,5 by using ethanol before water. The yield of P form was then lower, but the method is more convenient than the four-stage preparation6 of the (3 form through tetra-0-acety~-a-D-gda~topyranosyl

Trivalent copper, superoxide, and galactose oxidase

“…Methyl -D-and /3-D-galactopyranosides were prepared according to the procedure given by Frahn and Mills. 27 The same procedure was employed for the preparation of methyl -D-and /3-D-[l-14C]galactopyranosides from D-[l-14C]galactose (International Chemical and Nuclear Corp.). Methyl a-D-ga/acto-hexodialdo-1,5-pyranoside (MGDP) and methyl -D-[ 1 -14C] -galacto-hexodialdo-1,5-pyranoside were prepared from the corresponding galactopyranosides by oxidation with O2 catalyzed by galactose oxidase.…”

Isolation of ethyl .gamma.-D-galactopyranoside and pinitol from hexane-ethanol extracted soybean flakes

“…Synthesis of Methyl, Ethyl, and Isopropyl a-o-Galactosides. In a modification of the methods of Frahm and Mills (1965) and of Fischer and Beensch (1894), 10 g of D-galactose in 100 ml of the appropriate alcohol containing 4% dry HC1 was heated for 72 hr at 30°C on a magnetic stirrer and then refluxed for 4 hr. The reaction mixture was neutralized with BaC03, filtered through activated carbon black, and concentrated to a syrup on a rotary evaporator at 40°C.…”

Crystal structure of methyl α-D-galactopyranoside monohydrate

“…In a systematic study of the Fischer method of preparation of the methyl D-galactopyranosides Frahn & Mills (1965) report that the ~ isomer crystallizes only as the monohydrate, with a melting point which is dependent on its previous treatment and the method of measurement. Thus a large sample of the pure material, obtained by crystallization from water, when sealed in a glass capillary melted over the range 107-113 °C.…”