Chemistry Reference
In-Depth Information
12.4.4. Chiral Crystals of Achiral Organic Compounds
Some achiral organic compounds form chiral crystals, with each crystal exhibiting one
of the two possible enantiomorphs [89-94]. These chiral crystals composed of an achiral
organic compound may serve as an effi cient chiral seed in a prebiotic world; therefore,
a study of asymmetric autocatalysis using these chiral organic crystals is an interesting
subject.
Cytosine
34
, a constituent of DNA and RNA, is a base of cytidine and deoxycytidine,
and is an essentially fl at achiral molecule. It is conceivable that cytosine
34
was formed
under prebiotic conditions [109] and already existed before the RNA world emerged.
Thus, the investigation of the enantioselective reaction utilizing the crystal chirality of
achiral cytosine is an important experimental approach to understanding of the origin
of biological homochirality (Fig. 12.5 ).
At fi rst, we discovered that achiral cytosine
34
, when crystallized from methanol with
stirring without adding any seed crystal, affords powder-like crystals that exhibit either
plus or minus Cotton effect in solid-state CD spectra [110] at ca. 310 nm. The stochastic
behavior of the formation of [CD(+)310]- and [CD(-)310]-crystals of cytosine
34
was
observed.
Next, the chiral crystals that are spontaneously formed with stirring are used as
chiral triggers for asymmetric autocatalysis (Scheme 12.40 ) [110] . When pyrimidine - 5 -
carbaldehyde
32
and
i
- Pr
2
Zn reacted in the presence of a [CD(+)310]-crystal of
cytosine
34
, enantioenriched (
R
) - pyrimidyl alkanol
33
was obtained after the subsequent
autocatalytic amplifi cation of ee. On the other hand, spontaneously obtained
[CD(-)310]-cytosine crystal induced the production of enantioenriched (
S
) - alkanol
33
. These results clearly exhibit the correlation between the chirality of the crystal
of cytosine and the absolute confi guration of the resulting alkanol.
This sequence of reactions represents one of the chemical processes in which the
scenario for the evolution of chirality from the achiral nucleobase cytosine was achieved
in real chemical reactions. The sequential process of asymmetric induction in the organic
product with an asymmetric carbon atom and the amplifi cation of chirality through
via
prebiotic synthesis
Accessible
NH
2
N
Enantiomorphous crystal of cytosine
34
Achiral
cytosine
from cyanoacetylene
cyanate, urea, etc.
H
O
34
Generation of chirality
Chiral crystal of cytosine
Chiral transfer
Organic compound
with tiny chirality
Amplification of chirality
Left-handed
M
Right-handed
P
H-bond
-crystal
Helical arrangement of cytosine in crystal (
-crystal
P
2
1
2
1
2
1
)
Enantiopure organic compound
Figure 12.5.
Proposed scenario for the evolution of chirality in nature using achiral cytosine as an
origin of chirality.
