When the following aldohexose exists in its D-configuration, the total number of stereoisomers in its pyranose form is
Given is D configuration for pyranose. Cyclization will happen by OH of fifth carbon.
Configuration at C-5 is D(glucose) this will not change. New chiral generates at C-1 and total number of chiral center which may have D or L continue are 3. Hence total number of stereoisomers= 23 = 8
This question involves determining the total number of stereoisomers for the pyranose form of a given D-aldohexose. Let's break down the concepts step by step.
Step 1: Understanding the Aldohexose Structure
An aldohexose is a six-carbon sugar with an aldehyde functional group. The given structure has four chiral centers (asymmetric carbons) in its open-chain form. In the D-configuration, the highest numbered chiral carbon (C5) has the hydroxyl group on the right in Fischer projection.
Step 2: Pyranose Form Formation
The pyranose form is a cyclic hemiacetal formed when the aldehyde group reacts with the hydroxyl group on C5, creating a six-membered ring. This cyclization generates a new chiral center at the anomeric carbon (C1).
Step 3: Counting Stereoisomers
In the open-chain form, there are 4 chiral centers, so the number of stereoisomers is . However, in the pyranose form, the anomeric carbon (C1) can have two configurations: α and β. Since the ring formation does not eliminate any existing chiral centers (C2, C3, C4, C5 remain chiral), the total stereoisomers in pyranose form are due to the 5 chiral centers (C1, C2, C3, C4, C5). Thus, the number is .
Final Answer: The total number of stereoisomers in the pyranose form is 32.
Carbohydrate Stereochemistry: Aldohexoses have multiple chiral centers leading to numerous stereoisomers. The D/L configuration specifies the orientation around the penultimate carbon. Cyclization to pyranose forms introduces an additional chiral center at the anomeric carbon, doubling the number of stereoisomers compared to the open-chain form.
Number of stereoisomers = , where n is the number of chiral centers.