Introduction
l-ascorbic acid, also known as Vitamin
C, has an important role in physiology. As an essential vitamin,
it has to be taken regulary by mammals which cannot produce it themselves.
It serves to prevent deseases and has proven to act beneficially
in the human body. Moreover, it is used in large amounts in the
food industry as a nutritional additive and as a radical scavenger
to e.g. prevent the oxidative degradation of lipids in food. The
oxidation of l-ascorbic acid (or its monoanion 1) proceeds via monodehydro-l-ascorbic
acid (radical anion 2), which disproportionates
to l-ascorbate (1)
and dehydro-l-ascorbic acid (3) (Scheme
[¹]
).
[¹]
Scheme 1
l
-ascorbic
acid can be synthesized in large amounts from d-glucose
by a combination of chemical and microbiological steps in an overall
yield of 66%.
[²]
Despite its physiological significance, the chemistry of l-ascorbic acid remained unexplored
for a long time. This is probably due to its many reaction possibilities
and the resulting lack of chemo- and/or regioselectivity.
Procedures for selective functionalization of l-ascorbic
acid had to be developed before any further utilization was possible.
For example, highly regioselective O-alkylation was described recently:
5,6-Ο-Isopropylidene protected l-ascorbic
acid 4 reacts with many electrophiles to
yield 2-O-substituted products 5 or 3-O-substituted
products 6 in excellent yields and regioselectivities,
depending on the reaction conditions (Scheme
[²]
).
[³]
Scheme 2
