Question

Question:How many different aldoheptoses are there? How many are d-sugars? Draw all d-aldoheptoses having the Rconfiguration at C2 and C3.

Short answer

Answer

32 aldoheptoses out of which 16 are d-sugars.

Step-by-step solution

Aldoheptose and number of stereoisomers

As the name suggests, aldoheptoses consist of an aldehyde group and 7 carbons in their chain.

In the case of D-aldoses, the number of stereoisomers is given as , where n is the number of stereogenic centers.

Stereoisomers for D-aldoheptose

D-aldoheptose consists of 7 carbons, out of which C-1 carbon and C-6 carbons are not considered as stereogenic centers.

Thus, there are 5 stereogenic centers for D-aldoheptose. The number of stereoisomers formed would be stereoisomers.

Since the 32 stereoisomers are actually 16 pairs of enantiomers, the number of D-sugars would be 16, and the rest 16 would have the L-configuration.

Step 3: Draw all d-aldoheptoses having the R configuration at C2 and C3

The diagram below shows the R configuration of C-2 and C-3 carbon of one D-aldopentose:

D-aldoheptose with R at C-2 and C-3

Thus, to keep the R configuration of C-2 constant, the orientation of H and OH of C-3 carbon remains the same. D-aldoheptoses show R at both C-2 and C-3, as given below:

Stereoisomers of D-aldoheptose having R configuration at C-2 and C-3