BR CBT 1 orgo question 113

[feipar]

---

Members don't see this ad.

Treatment of Compound X with excess H3COSO3C6H5 results in the complete methylation of its hydroxyl groups. What is the final product after complete hydrolysis of the methylated disaccharide?

Compound X is a dissacharide of D-Mannose 1,4 alpha linked with galactose.



When hydrolysis occurs, the answer key states that the methoxy group on carbon 1 on galactose that had been methylated in the prior step is replaced by an -OH group.



The final products after hydrolysis are 2,3,4,6-tetra-O-methyl-D-mannose and 2,3,6-tri-O-methyl-D-galactose.


WHY?


appreciate any help. Thx.

 

[Rabolisk]

The first step (methylation) replaces the OH groups off of carbon 2, 3, 4, and 6 of the D-mannose with OCH3. It does the same with the carbons 1, 2, 3, and 6 of the D-galactose. Carbon 1 of the mannose and carbon 4 of the galactose have the glycosidic bond, and no OH, so they are unaffected by the methylation. Then the hydrolysis breaks the bond. It also converts the methoxy group on carbon 1 of the galactose to an OH in the same manner as it breaks the bond between the two monosaccharides. If you think of that carbon 1 of galactose as being bonded to -OR (R = CH3), which is analogous to carbon 1 of mannose being bonded to -OR (R = galactose), then you can see that the same enzyme or catalyst that catalyzed the hydrolysis of the glycosidic bond must do the same to the bond between carbon 1 and the methoxy group. The reason that these that the rest of the OCH3s are unaffected is, I think, because those carbons are not bonded to an oxygen. Does that help?

 

[feipar]

yes, it does. thx!