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1.1.4. Definition of
α
and
β
(Haworth) –
the anomeric
carbon:
The carbon involved in ring formation is often referred to as the
anomeric
carbon.
For
α
-D-hexoses, the –OH group at C1 points
down
, whereas it points
up
for
β
-D-hexoses (as in the figure below).
For L-hexoses the situation is
opposite
: The –OH at C1 points
up
in
α
-L-
hexoses, and
down
for
β
-L-hexoses.
1.1.5.
Fisher-‐Haworth
interconversion
rules:
A given Haworth formula can easily be transferred to the corresponding
Fisher formula:
a)
–OH groups pointing
down
in the Haworth formula should point to the
right
in the Fischer formula
b)
–OH groups pointing
up
in the Haworth formula should point to the
left
in the Fischer formula
Therefore, D-sugars are easily recognised in Haworth by having the –CH
2
OH
group (C6 in hexoses) pointing up (figure above)
Complete Fisher-Haworth conversion rules: See
http://goldbook.iupac.org/H02749.html
O
CH
2
OH
OH
OH
OH
OH
It is a D-sugar because C6 points up
It is a
β
-D-sugar because
the anomeric -OH points up