Dolbier HW Solutions 613

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 a and b methyl glycosides. 25.12 The hemiacetal opens to give an intermediate containing a free aldehyde function. Cyclization of this intermediate can produce either the a or the b con f 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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This note was uploaded on 10/03/2011 for the course CHM 2210 taught by Professor Reynolds during the Fall '01 term at University of Florida.

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