The Fischer presentation of D-glucose is given below. The correct structure(s) of beta-L-glucopyrano

Q: Why is $\beta$-D-glucose considered the most stable sugar?

In its standard $^4C_1$ chair conformation, $\beta$-D-glucose is the only aldohexose where all of its bulky substituents (the four $-OH$ groups and the $-CH_2OH$ group) can simultaneously occupy the roomy equatorial positions, minimizing steric strain.

Q: What is the difference between $^4C_1$ and $^1C_4$ chairs?

They are the two possible chair conformations for a pyranose ring. A 'ring flip' converts one into the other. During a ring flip, all equatorial bonds become axial, and all axial bonds become equatorial.

Q: Why do equatorial bonds become axial when we draw the enantiomer in the same chair?

An enantiomer is a mirror image. If a group is 'up and equatorial' in the original molecule, its mirror image will have the group 'up and axial' if forced into the exact same spatial chair skeleton. To keep them equatorial, the enantiomer must adopt the flipped chair conformation.

Q: How do I quickly identify axial and equatorial bonds in these 2D drawings?

In standard 2D chair representations, axial bonds are drawn as straight vertical lines (pointing straight up or straight down), while equatorial bonds are drawn slanted, roughly parallel to the ring bonds.

Q: Is Option C also a valid structure for something?

Yes! Option C is actually just $\beta$-D-glucopyranose. It is the exact same molecule as Option B, just rotated 180 degrees so that the ring oxygen appears on the top left instead of the top right.

Question

The Fischer presentation of D-glucose is given below.

The correct structure(s) of $\beta$-L-glucopyranose is (are) :-

Sigma Percentile · Accepted Answer

### The Setup: D-Glucose and its Mirror Image

To crack this beautiful stereochemistry problem, we must first deeply understand the relationship between D and L sugars. We are given the Fischer projection of D-glucose. By definition, L-glucose is the exact mirror image, or **enantiomer**, of D-glucose.

If we were to draw the Fischer projection of L-glucose, every single chiral center would be inverted. The hydroxyl ($-OH$) groups that sit on the right side in D-glucose will flip to the left side in L-glucose, and vice versa.

### The Elegance of $\beta$-D-Glucopyranose

Before we jump to the L-isomer, let's recall why $\beta$-D-glucopyranose is so special. It is the most abundant and stable aldohexose in nature. Why? Because when it folds into its most stable chair conformation (known as the $^4C_1$ chair), **every single bulky substituent**—that is, the $-OH$ groups on C1, C2, C3, C4, and the $-CH_2OH$ group on C5—occupies an **equatorial position**.

Equatorial positions point outward along the equator of the ring, minimizing steric hindrance and making the molecule incredibly stable.

### The Chair Flip: Entering the L-Universe

Since L-glucose is the enantiomer of D-glucose, $\beta$-L-glucopyranose must be the exact mirror image of $\beta$-D-glucopyranose. Consequently, in its most stable chair conformation (which would be the $^1C_4$ chair), it too would have all its substituents in equatorial positions.

However, there is a massive catch in this question! If you look closely at the options provided, all the cyclic structures are drawn using the exact same $^4C_1$ chair skeleton (with the oxygen atom at the top right, C1 on the right, and C4 on the left).

What happens when you force an enantiomer into the chair skeleton of its mirror image? **Every equatorial bond becomes an axial bond, and every axial bond becomes an equatorial bond.**

### Cracking the Options

Because $\beta$-D-glucopyranose has all its substituents in equatorial positions in the $^4C_1$ chair, its enantiomer, $\beta$-L-glucopyranose, when drawn in that exact same $^4C_1$ chair, must have **all its substituents in axial positions**.

Let's scan the options for a structure where every bulky group is axial (pointing straight up or straight down):

- **Option (A):** This is $\alpha$-D-glucopyranose.
- **Option (B):** All substituents are equatorial. This is $\beta$-D-glucopyranose.
- **Option (C):** This is also $\beta$-D-glucopyranose, just viewed from a different angle (rotated 180 degrees).
- **Option (D):** Look closely at the bonds. The $-OH$ groups on C1, C2, C3, C4, and the $-CH_2OH$ group on C5 are all on straight vertical bonds. They are **all axial**.

Thus, Option (D) perfectly represents $\beta$-L-glucopyranose drawn in the $^4C_1$ chair conformation.

Animated Solution for Chemistry - Organic Chemistry: The Fischer presentation of D-glucose is given below. The correct structure(s) of $β$ -L-glucopyranose is (are) :-

Select Answer:

Visualized Solution

$D -glucose Analysis$

$Enantiomeric Relationship$

$L-glucose Fischer Projection$

$Stability of β -D-glucopyranose$

$Deducing β -L-glucopyranose$

$The Chair Flip Catch$

$Final Structure$

$Matching Options$

The Sigma Insight: Biomolecules

The Setup

The Elegance of $β$ -D-Glucopyranose

The Chair Flip

Cracking the Options

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Animated Solution for Chemistry - Organic Chemistry: .math-text-wrapper .katex { font-size: 1.05em !important; } .math-text-wrapper .katex-display { margin: 1rem 0 !important; } The Fischer presentation of D-glucose is given below. The correct structure(s) of β-L-glucopyranose is (are) :-