Question

Draw the structures of the compounds named in Problem 23-20 parts (a), (c), and (d). Allose is the C3 epimer of glucose and ribose is the C2 epimer of arabinose.

Short answer

Pyranose is a six-membered cyclic hemiacetal while a furanose is a five-membered cyclic hemiacetal.

Step-by-step solution

Pyranose and furanose

Pyranose is a six-membered cyclic hemiacetal while a furanose is a five-membered cyclic hemiacetal.

Epimers

Sugars which have the common configuration like the arrangement of H - and - OH around carbon atoms are same except the carbon present at α -position or carbon number 2 are known as epimers. For example, glucose and mannose are epimers of each other.

Anomers

The diastereomers resulting from cyclisation are known as anomers. They differ only in the configuration around first carbon (C1) which is referred to as the anomeric carbon (hemiacetal carbon atom).

Identifying α and β anomers of pyranoses and furanoses

For α -anomer, hydroxy group (-OH) on the anomeric carbon is down (axial position) while for β -anomer, hydroxy group (-OH) on the anomeric carbon is up (equatorial position). Again, for α -anomer, the anomeric hydroxy (-OH) group is placed trans to the terminal -CH₂OH group while for β -anomer, the anomeric hydroxy (-OH) group is placed cis to the terminal -CH₂OH group.

Structure of the sugars

In chair conformation of methyl α -D-galactopyranoside, the anomeric carbon is C1. The methyl group (-CH₃) on the anomeric carbon is down (axial position).

In chair conformation of α -D-galactopyranoside, the anomeric carbon is C1. The hydroxy group (-OH) on the anomeric carbon is down (axial position).

In Haworth projection of β-D-ibofuransoide, the anomeric carbon is C1. The ethyl group (-CH₂CH₃)on the anomeric carbon is up which is cis to the terminal -CH₂CH group.