CARBOHYDRATES 707 25.10 Reaction of a carbohydrate with an alcohol in the presence of an acid catalyst gives mixed acetals at CHO OH CHOH CHOH H CHOH OCH3 H-OH D-Galactose Methanol 25.11 Acid-catalyzed addition of methanol to the glycal proceeds by regioselective protonation of the dou ble bond in the direction that leads to the more stable carbocation. Here again the more stable car- bocation is the one stabilized by the ring oxygen. HO HOCH, O HOCH HOCH2 HO-\ Capture on either face of the carbocation by methanol yields the a and B methyl glycosides. 25.12 The hemiacetal opens to give an intermediate containing a free aldehyde function Cyclization of his intermediate can produce either the a or the B configuration at this center. The axial and equa torial orientations of the anomeric hydroxyl can best be seen by drawing maltose with the pyranose rings in chair conformations HOCH HOCH, CH,OH CHOH O CH=O hemiacetal (equatorial) Key intermediate forme cleavage of hemiac HO- OH CHOH a-Configuration of hemiacetal(axial) Back Forward Main Menu TOC Study Guide Toc Student OLC MHHE Website25.10 Reaction of a carbohydrate with an alcohol in the presence of an acid catalyst gives mixed acetals at the anomeric position. 25.11 Acid-catalyzed addition of methanol to the glycal proceeds by regioselective protonation of the dou￾ble bond in the direction that leads to the more stable carbocation. Here again, the more stable car￾bocation is the one stabilized by the ring oxygen. Capture on either face of the carbocation by methanol yields the and methyl glycosides. 25.12 The hemiacetal opens to give an intermediate containing a free aldehyde function. Cyclization of this intermediate can produce either the or the configuration at this center. The axial and equa￾torial orientations of the anomeric hydroxyl can best be seen by drawing maltose with the pyranose rings in chair conformations. HO CH2OH OH HO O -Configuration of hemiacetal (equatorial) OH HO HO O HOCH2 O Key intermediate formed by cleavage of hemiacetal CH O CH2OH HO HO OH OH HO HO O HOCH2 O -Configuration of hemiacetal (axial) CH2OH HO HO HO O OH HO HO O HOCH2 O HOCH2 HO H O HO HO HOCH2 H HO O H HO HOCH2 H O HO   CHO CH2OH H H H H HO HO OH OH d-Galactose Methanol Methyl -d-galactopyranoside Methyl -d-galactopyranoside  CH3OH  HCl OH CH2OH H OH HO OCH3 O OH H CH2OH OCH3 OH HO O CARBOHYDRATES 707 Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website