302
Chapter 17 · Vital Amines - Vitamins
17
enantiomer l-ascorbic acid is formed. Its
IUPAC name, in which the stereocenters are
named exactly, is: (5R)-5-[(1S)-1,2-dihydoxyethyl)-3,4-dihydroxy-5-hydrofuran-2-one.
Reichstein was awarded the Nobel Prize for
Physiology or Medicine in 1950 for this synthesis. In the meantime, there are a number of
alternative syntheses of l-ascorbic acid. A process in which the oxidation of l-sorbose leads
biotechnologically directly to 2-keto-l-gulonic
acid is particularly economical, thus making the
protective group chemistry superfluous. In 2017,
approximately 95% of the vitamin C production
was located in China. 150,000 tons of vitamin C
were synthetically produced worldwide.
The best way to obtain vitamin C, however,
remains a nutrition rich in fruit and vegetables.
Rose hip, sea buckthorn berries and blackcurrants contain a particularly high amount of vitamin C. Citrus fruits, i.e. lemons, oranges and
grapefruits, are also important for the supply of
vitamin C. Many cabbages such as green cabbage, Brussels sprouts, white cabbage and pickled
cabbage are also important sources of vitamin C.
The physiological effect of vitamin C is
based on its chemical structure: The two enolic
hydroxyl groups at the C-atoms 3 and 4 can
be oxidized relatively easily to a diketone, the
L-dehydroascorbic acid. Conversely, this means
that L-ascorbic acid itself is a reducing agent.
Vitamin C thus protects the body cells from oxidative stress and acts as a scavenger for free radicals. The reducing effect of vitamin C is used in
numerous applications, e.g. in the conservation
of food and the prolonged durability of meat
products, flour, beer or fats.
tion of the terminal aldehyde group into an
alcohol group in the presence of a nickel
catalyst results in the sugar alcohol d-sorbitol. This hydrogenation takes place at high
temperatures and hydrogen pressures.
2. d-Sorbitol is then dehydrogenated microbiologically to l-sorbose. This is done on
an industrial scale with the easily accessible
aerobic bacterium acetobacter suboxidans.
Dehydrogenation takes place under very
mild conditions: at a temperature of 37 °C, an
atmospheric pressure of 2 bar and a pH value
of 4–6. After a reaction time of 24 h, a yield of
>98% is achieved. The biomass is completely
separated by a double filtration and the
l-sorbose is purified by crystallization.
3. In the third step, the four hydroxyl groups
of the l-sorbose are double ketalized with
acetone and thus protected from a subsequent reaction. The diacetone l-sorbose now
contains only one primary hydroxyl group,
which can be used for a further reaction.
4. In the fourth step, the diacetone-l-sorbose
is oxidized to diacetone-2-keto-l-gulonic
acid. This can be done conventionally with
alkaline potassium permanganate solution.
In industry, however, more economical
catalytic or electrochemical oxidations are
preferred for this step.
5. In the fifth step, the two acetone protective
groups are hydrolytically removed by heating
in water: Free 2-keto-l-gulonic acid is formed.
6. This is finally converted in the presence of
concentrated hydrochloric acid under water
cleavage into a γ-lactone, which is in its enol
form. Ascorbic acid thus formed contains
two asymmetric carbon atoms. In Reichstein
synthesis, only the physiologically active
BOX: A “Good Hot Lemon”
To protect themselves from
colds, many people drink a glass
of hot lemon juice to activate
their immune system with
vitamin C. You can also buy
bags of ready-made beverage
powder in the shops, which then
only needs to be dissolved with
boiling water. Here, you should
realize that most vitamins -
especially vitamin C - are among
the most sensitive ingredients
of food. Incorrect storage,
processing and preparation of
food, but especially excessive
heating, can inactivate or almost
completely destroy vitamins. The
“cooking losses” of ascorbic acid
are estimated at 70%, of folic
acid even at 90%!
Another important point is
the good water solubility of
vitamins B and C: If, for example,
vitamin-rich vegetables are
cooked in a good English way over
a long period of time, vitamins
B and C that have not yet been
destroyed dissolve in the cooking
water and a vitamin-depleted
residual vegetable remains for
consumption. Those who value
vitamins should therefore not only
eat the vegetables, but also sip the
cooking water. Bon appétit!
Chapter 17 · Vital Amines - Vitamins
17
enantiomer l-ascorbic acid is formed. Its
IUPAC name, in which the stereocenters are
named exactly, is: (5R)-5-[(1S)-1,2-dihydoxyethyl)-3,4-dihydroxy-5-hydrofuran-2-one.
Reichstein was awarded the Nobel Prize for
Physiology or Medicine in 1950 for this synthesis. In the meantime, there are a number of
alternative syntheses of l-ascorbic acid. A process in which the oxidation of l-sorbose leads
biotechnologically directly to 2-keto-l-gulonic
acid is particularly economical, thus making the
protective group chemistry superfluous. In 2017,
approximately 95% of the vitamin C production
was located in China. 150,000 tons of vitamin C
were synthetically produced worldwide.
The best way to obtain vitamin C, however,
remains a nutrition rich in fruit and vegetables.
Rose hip, sea buckthorn berries and blackcurrants contain a particularly high amount of vitamin C. Citrus fruits, i.e. lemons, oranges and
grapefruits, are also important for the supply of
vitamin C. Many cabbages such as green cabbage, Brussels sprouts, white cabbage and pickled
cabbage are also important sources of vitamin C.
The physiological effect of vitamin C is
based on its chemical structure: The two enolic
hydroxyl groups at the C-atoms 3 and 4 can
be oxidized relatively easily to a diketone, the
L-dehydroascorbic acid. Conversely, this means
that L-ascorbic acid itself is a reducing agent.
Vitamin C thus protects the body cells from oxidative stress and acts as a scavenger for free radicals. The reducing effect of vitamin C is used in
numerous applications, e.g. in the conservation
of food and the prolonged durability of meat
products, flour, beer or fats.
tion of the terminal aldehyde group into an
alcohol group in the presence of a nickel
catalyst results in the sugar alcohol d-sorbitol. This hydrogenation takes place at high
temperatures and hydrogen pressures.
2. d-Sorbitol is then dehydrogenated microbiologically to l-sorbose. This is done on
an industrial scale with the easily accessible
aerobic bacterium acetobacter suboxidans.
Dehydrogenation takes place under very
mild conditions: at a temperature of 37 °C, an
atmospheric pressure of 2 bar and a pH value
of 4–6. After a reaction time of 24 h, a yield of
>98% is achieved. The biomass is completely
separated by a double filtration and the
l-sorbose is purified by crystallization.
3. In the third step, the four hydroxyl groups
of the l-sorbose are double ketalized with
acetone and thus protected from a subsequent reaction. The diacetone l-sorbose now
contains only one primary hydroxyl group,
which can be used for a further reaction.
4. In the fourth step, the diacetone-l-sorbose
is oxidized to diacetone-2-keto-l-gulonic
acid. This can be done conventionally with
alkaline potassium permanganate solution.
In industry, however, more economical
catalytic or electrochemical oxidations are
preferred for this step.
5. In the fifth step, the two acetone protective
groups are hydrolytically removed by heating
in water: Free 2-keto-l-gulonic acid is formed.
6. This is finally converted in the presence of
concentrated hydrochloric acid under water
cleavage into a γ-lactone, which is in its enol
form. Ascorbic acid thus formed contains
two asymmetric carbon atoms. In Reichstein
synthesis, only the physiologically active
BOX: A “Good Hot Lemon”
To protect themselves from
colds, many people drink a glass
of hot lemon juice to activate
their immune system with
vitamin C. You can also buy
bags of ready-made beverage
powder in the shops, which then
only needs to be dissolved with
boiling water. Here, you should
realize that most vitamins -
especially vitamin C - are among
the most sensitive ingredients
of food. Incorrect storage,
processing and preparation of
food, but especially excessive
heating, can inactivate or almost
completely destroy vitamins. The
“cooking losses” of ascorbic acid
are estimated at 70%, of folic
acid even at 90%!
Another important point is
the good water solubility of
vitamins B and C: If, for example,
vitamin-rich vegetables are
cooked in a good English way over
a long period of time, vitamins
B and C that have not yet been
destroyed dissolve in the cooking
water and a vitamin-depleted
residual vegetable remains for
consumption. Those who value
vitamins should therefore not only
eat the vegetables, but also sip the
cooking water. Bon appétit!