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Dolbier HW Solutions 613 - torial orientations of the...

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25.10 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 con fi guration at this center. The axial and equa-
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Unformatted text preview: torial orientations of the anomeric hydroxyl can best be seen by drawing maltose with the pyranose rings in chair conformations. HO CH 2 OH OH HO O b-Configuration of hemiacetal (equatorial) OH HO HO O HOCH 2 O Key intermediate formed by cleavage of hemiacetal O CH CH 2 OH HO HO OH OH HO HO O HOCH 2 O a-Configuration of hemiacetal (axial) CH 2 OH HO HO HO O OH HO HO O HOCH 2 O HOCH 2 HO H O HO HO HOCH 2 H HO O H 1 HO HOCH 2 H O HO 1 1 CHO CH 2 OH H H H H HO HO OH OH d-Galactose Methanol Methyl a-d-galactopyranoside Methyl b-d-galactopyranoside CH 3 OH 1 1 HCl OH CH 2 OH H OH HO OCH 3 O OH H CH 2 OH OCH 3 OH HO O CARBOHYDRATES 707...
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