Carbono anomérico

What is an anomeric carbon?
Often times, a carbohydrate exists in a cyclic or an acyclic form. During cyclization, the carbonyl carbon transforms into a new stereocenter.
Cyclization causes the formation of 2 new diasteriomers. They differ in the position of the attachment of a certain group (discussed later) to the new stereocenter. The new stereocenter is referred to as the anomeric carbon.

Anomeric Carbon

Anomeric Carbon

Anomeric Carbon

How do I locate the anomeric carbon?
Step 1: Locate the oxygen inside the ring
Oxygen

Step 2: Look at the carbons on either side of the oxygen. One will be attached to a CH2OH group. Do not focus on this one. The carbon on the other side is the anomeric carbon!
Carbon (attached to CH2OH) – not the focus!
Anomeric Carbon

α and β: What’s the difference?
The anomeric carbon is a stereocenter. An important feature is the direction of the OH group attached to the anomeric carbon.
Depending on the direction of the OH group, the anomeric carbon is either α or β. α: equatorial DOWN or axial DOWN β: equatorial UP or axial UP
Let’s take a look at some examples.

The OH group is axial DOWN.

The OH group is equatorial UP.

Thus, the anomeric carbon is α.

Thus, the anomeric carbon is β.

This sugar is known as:

This sugar is known as:

α-D-glucopyranose

β-D-glucopyranose

Acetal Functional Groups
When 2 monosaccharides join, they are linked by an acetal functional group. Once this happens, the anomeric carbon becomes fixed in either the α or β position
Acetal Functional Group
C-O-C-O-C

Anomeric carbon:
Fixed as α because substituent is axial DOWN

Hemiacetal Functional Groups
When the hydrogen is still attached, and the oxygen is not bonded C-O-C, this is called a hemiacetal functional group. Because the OH group can open and close to either form, in a mixture of these molecules, some will exist as α and some will exist as β.
Hemiacetal Functional Group
C-O-C-O-H