JEE Challenger
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ChemHardAdv 2018 • Paper 2

Identify Correct Structure of Beta L Glucopyranose

The Fischer projection for D-glucose is depicted below:

Select the correct structure(s) representing β\beta-L-glucopyranose.

Question Diagram 1

Options

A
Option A Diagram 1
B
Option B Diagram 1
C
Option C Diagram 1
D
Option D Diagram 1
Correct

Topics & Concepts

BiomoleculesCarbohydratesIsomerism

Step-by-Step Solution

To determine the correct structure representing β\beta-L-glucopyranose, we proceed step-by-step using stereochemical principles.

Step 1: Fischer Projection of D-Glucose

The given Fischer projection for D-glucose is:

CHO∣H−C−OH(C2)∣HO−C−H(C3)∣H−C−OH(C4)∣H−C−OH(C5)∣CH2OH(C6)\begin{array}{c} \text{CHO} \\ | \\ \text{H} - \text{C} - \text{OH} \quad (\text{C2}) \\ | \\ \text{HO} - \text{C} - \text{H} \quad (\text{C3}) \\ | \\ \text{H} - \text{C} - \text{OH} \quad (\text{C4}) \\ | \\ \text{H} - \text{C} - \text{OH} \quad (\text{C5}) \\ | \\ \text{CH}_2\text{OH} \quad (\text{C6}) \end{array}

Step 2: Fischer Projection of L-Glucose

L-glucose is the non-superimposable mirror image (enantiomer) of D-glucose. Therefore, the configuration at every chiral center (C2, C3, C4, C5\text{C2, C3, C4, C5}) is inverted:

CHO∣HO−C−H(C2)∣H−C−OH(C3)∣HO−C−H(C4)∣HO−C−H(C5)∣CH2OH(C6)\begin{array}{c} \text{CHO} \\ | \\ \text{HO} - \text{C} - \text{H} \quad (\text{C2}) \\ | \\ \text{H} - \text{C} - \text{OH} \quad (\text{C3}) \\ | \\ \text{HO} - \text{C} - \text{H} \quad (\text{C4}) \\ | \\ \text{HO} - \text{C} - \text{H} \quad (\text{C5}) \\ | \\ \text{CH}_2\text{OH} \quad (\text{C6}) \end{array}

Step 3: Determining the Stereochemistry of β\beta-L-Glucopyranose

  1. β\beta-D-Glucopyranose Structure: In the standard Haworth projection (with the pyranose ring oxygen placed at the top-right position):

    • C1\text{C1}: −OH-\text{OH} is UP
    • C2\text{C2}: −OH-\text{OH} is DOWN
    • C3\text{C3}: −OH-\text{OH} is UP
    • C4\text{C4}: −OH-\text{OH} is DOWN
    • C5\text{C5}: −CH2OH-\text{CH}_2\text{OH} is UP

  2. β\beta-L-Glucopyranose Structure: Since β\beta-L-glucopyranose is the exact enantiomer of β\beta-D-glucopyranose, every stereocenter is inverted. Thus, all UP substituents become DOWN, and all DOWN substituents become UP:

    • C1\text{C1}: −OH-\text{OH} is DOWN, −H-\text{H} is UP
    • C2\text{C2}: −OH-\text{OH} is UP, −H-\text{H} is DOWN
    • C3\text{C3}: −OH-\text{OH} is DOWN, −H-\text{H} is UP
    • C4\text{C4}: −OH-\text{OH} is UP, −H-\text{H} is DOWN
    • C5\text{C5}: −CH2OH-\text{CH}_2\text{OH} is DOWN, −H-\text{H} is UP

(Note: In an L-sugar, the β\beta-anomer has the anomeric −OH-\text{OH} group at C1\text{C1} and the −CH2OH-\text{CH}_2\text{OH} group at C5\text{C5} in a cis relationship, meaning both are pointing DOWN).

Step 4: Comparison with the Given Options

  • Option (A): C1\text{C1} has −OH-\text{OH} UP   ⟹  \implies represents α\alpha-L-glucopyranose.
  • Option (B): Incorrect configuration at C2\text{C2}.
  • Option (C): C1\text{C1} (−OH-\text{OH} UP), C2\text{C2} (−OH-\text{OH} DOWN), C3\text{C3} (−OH-\text{OH} UP), C4\text{C4} (−OH-\text{OH} DOWN), C5\text{C5} (−CH2OH-\text{CH}_2\text{OH} UP)   ⟹  \implies represents β\beta-D-glucopyranose.
  • Option (D): C1\text{C1} (−OH-\text{OH} DOWN), C2\text{C2} (−OH-\text{OH} UP), C3\text{C3} (−OH-\text{OH} DOWN), C4\text{C4} (−OH-\text{OH} UP), C5\text{C5} (−CH2OH-\text{CH}_2\text{OH} DOWN)   ⟹  \implies represents β\beta-L-glucopyranose.

Hence, the correct structure representing β\beta-L-glucopyranose is given by D.

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