Ketoses are a special group of sugars. D-ribulose derivatives play a vital role in photosynthesis. A — Physical Chemistry — Thermodynamics Chemistry Question
Ketoses
Ketoses are a special group of sugars. D-ribulose derivatives play a vital role in photosynthesis. An α-methyl glycoside of D-ribulose (A) can be prepared from D-ribulose on treatment with methanol and an acid catalyst. Heating A in acetone with HCl leads to B, a propylidene derivative. Acetone forms acetals with vicinal diols, if the orientation of the two OH groups is suitable.
[VISUAL]
Although acetonide formation is a versatile method for the temporary protection of OH groups that are close enough, in many cases it gives multiple products (or the product composition is highly dependent on the reaction conditions). In general, this is the case with sugars with 6-membered rings.
It has been shown that no acetonide can be formed when the neighbouring OH groups are both axial. However, both diequatorial and axial-equatorial vicinal diols react with acetone/HCl.
During the synthesis of B two possible products can form. Draw their structures. Which is the main product?
Model Answer
The formation of the 1,3 isomer is less likely (trans anellation of a 5 and 6-membered ring). The main product is the 3,4-acetonide.
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Draw the structures of C - E.
Model Answer
[VISUAL]
Is it possible to predict the conformation around carbon atom C1 of E?
Model Answer
No. D has an unblocked glycosidic OH group, so during the synthesis of E both α and β isomers can be formed. The isomeric composition depends on the reaction conditions.
Draw the two chair conformers of 1-O-methyl-6-O-acetyl-β-D-galactose<1.5> (F). Designate the OH groups as axial (a) or equatorial (e). Mark the more stable conformer.
Model Answer
[VISUAL]
How many acetonide isomers can form from this compound? How many different chair conformers of these acetonides exist?
Model Answer
The 3,4-acetonide isomer has two different chair conformers, the 2,3 isomer has only one:
[VISUAL]
Draw the Haworth projection of L-galactose <1.5>.
Model Answer
[VISUAL]