Chirality in Biology
593
alkaloid, nornicotine, and the lupinane
alkaloids, lupanine and sparteine. The im-
portant alkaloid, reticuline, is involved in
a number of biosynthetic processes. (
S
)-
(
+
)-Reticuline occurs in
Anona reticulata
and other plants and is an intermediate for
biosynthesis of, among others, berberine,
chelidonine, laudanosine, and narcotine.
Papaver somniferum
contains both enan-
tiomers depending on the age of the plants,
and (
R
)-(
)-reticuline is the major precur-
sor in morphine alkaloid biosynthesis.
Among terpenes, it is almost the rule
rather than the exception for materials
to occur in enantiomeric and/or racemic
forms. Common materials such as bor-
neol, camphor, carvone, citronellol, fen-
chone, limonene, menthol, pinene, and
terpineol occur as both enantiomers; race-
mates are known for borneol, camphor,
and limonene. Some biosynthetic infor-
mation is available. PuriFed enzymes from
sage (
Salvia ofFcinalis
) and tansy (
Tanace-
tum vulgare
) convert achiral geranyl bis-
phosphate to a linalyl bisphosphate, LPP,
probably as an enzyme-bound intermedi-
ate. The sage system forms (3
R
)-(
)-LPP
and tansy, (3
S
)-(
+
)-LPP. Subsequently,
(3
R
)-(
)-LPP is converted to (1
R
,4
R
)-(
+
)-
bornyl bisphosphate and the (3
S
)-(
+
)-LPP
to the (1
S
,4
S
) enantiomer. Hydrolysis of
the bisphosphates yields the two enan-
tiomeric borneols. Two further enzymes
from sage form enantiomeric pinenes.
The (
+
)-pinene cyclase converts achiral
geranyl bisphosphate to (
+
)-
α
-pinene and
(
+
)-camphene, and the (
)-pinene cyclase
forms mainly (
)-
α
-pinene, (
)-
β
-pinene
and (
)-camphene.
Since racemization may occur during
isolation of chiral natural products, precau-
tions are necessary. Hyoscyamine readily
undergoes racemization in the tropic acid
component and partial racemization oc-
curs during isolation. ±or pharmaceutical
use, racemization is completed by al-
kali treatment yielding the widely used
atropine (racemic hyoscyamine contain-
ing (
±
)-tropic acid). The mydriatic ac-
tion resides only in the (
)-tropic acid
component.
A few
natural
products
are
‘‘quasi-
racemates,’’
a
mixture
of
equimolar
amounts of two substances with slightly
different structures and of opposite chiral-
ity. An early example was a benzoquinone
from
Dalbergia nigra
(±ig. 9a, b). Nar-
cisamine, from King Alfred daffodils, was
a quasi-racemate of (
)-demethylgalantha-
Fig. 9
Naturally occurring
quasi-racemates. (a),
(
b
)
=
benzoquinone from
Dalbergia
nigra
;(c),
(
d
)
=
narcissamine from King
Alfred daffodils.
H
CH
3
O
O
O
H
CH
3
O
O
O
CH
3
O
CH
3
O
N
H
O
OH
CH
3
O
N
H
O
OH
(a)
(b)
(c)
(d)
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