(a) Heating p-altrose with dilute acid produces a...

(a) Heating p-altrose with dilute acid produces a nonreducing anbydro sugar (C $_{6} \mathrm{H}_{10} \mathrm{O}_{5}$ ). Methylation of the anhydro sugar followed by acid hydrolysis yiclds 2,3,4 -tri- $Q$ -methyl-p-altrose. The formation of the anhydro sugar takes place through a chair conformation of $\beta$ -D-altropyranose in which the $-\mathrm{CH}_{2} \mathrm{OH}$ group is axial. What is the structure of the anhydro sugar, and how is it formed? (b) p-Glucose also forms an anhydro sugar but the conditions required are much more drastic than for the corresponding reaction of D-altrose. Explain.

Key Concepts

Anhydro Sugar Formation

An anhydro sugar is produced when a sugar molecule undergoes an intramolecular dehydration reaction, forming a cyclic acetal (or ketal) between a hydroxyl group and the anomeric center. The resulting structure lacks a free reducing end, rendering it nonreducing. This process is generally acid-catalyzed, which facilitates protonation and subsequent elimination of water during the transformation.

Chair Conformation and Stereochemical Effects

The chair conformation of a pyranose sugar is crucial in determining its reactivity, as the orientation of substituents (axial vs. equatorial) affects the feasibility of intramolecular reactions. When a substituent such as the –CH2OH group is in an axial position, it is situated suitably for nucleophilic attack on the anomeric carbon, thereby promoting the formation of a cyclic acetal. This stereochemical arrangement explains why certain sugars can form anhydro derivatives more easily than others.

Intramolecular Acetal Formation Mechanism

The formation of an internal acetal in sugars involves a nucleophilic hydroxyl group attacking a protonated carbonyl (or hemiacetal) carbon, leading to ring closure through the elimination of water. This mechanism, often under acid-catalyzed conditions, transforms the reducing sugar into a nonreducing anhydro sugar by locking the anomeric center within a stable cyclic structure.

Comparative Reactivity of Sugars

The differential reactivity of sugars such as p-altrose and p-glucose in forming anhydro sugars is a direct consequence of their conformational differences. While p-altrose, in a chair conformation with an axial –CH2OH group, is predisposed to form the cyclic acetal under mild acidic conditions, p-glucose, with its –CH2OH group typically in an equatorial position, has a less favorable stereoelectronic alignment for the intramolecular cyclization. Consequently, p-glucose requires more drastic conditions for anhydro sugar formation.

Transcript

00:03 This is the answer to chapter 21, problem number 20, from the smith organic chemistry textbook.

00:11 And this problem asks us to draw the product of a series of reactions.

00:18 So we're given this starting material here, which i've drawn, which the problem identifies as molecule a.

00:28 And so there's molecule a.

00:30 And we're told that if we treat molecule a successively with first this vitigree agent that i've drawn here, and then if we hydrogenate that product, that we get a product c, and we're asked to identify product c.

00:52 And so the best way to do this is to just take this a step at a time.

00:55 So first we'll identify what product b is, and then we will hydrogenate that product, and that will give us c.

01:02 So the first thing to do is to look at the reaction of a with this vitig reagent.

01:07 So the six -membered aromatic ring portion of this molecule, of course, is not going to be affected by the vidig reaction.

01:22 So there we go.

01:24 Six -membered aromatic ring.

01:26 The methyl group at the top is also unaffected.

01:29 And so remember the product of a vidig reaction i like to think of it as just sort of like adding the double so replacing the oxygen in the carbonyl with with the carboneneal with the carbon that's double bound to the phosphorus and then drawing the rest of the chain in basically and so maybe that's not maybe i didn't do a great job describing that but so so if we were to redraw a, right, so we would have this, and then, of course, in a, we have an oxygen here.

02:09 But because this is the product of a vidig, we can just put our carbon there.

02:17 So here's, let me go ahead and label these in red.

02:21 So this carbon that i've drawn here is this carbon here...

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