Chemistry Reference
In-Depth Information
Origins of chirality
(proposed)
Circularly polarized light
Chiral inorganic crystals (quartz)
Chiral organic crystals of achiral compounds
Absolute asymmetric synthesis
Biological homochirality
HO
Base
?
O
R
H
Chiral compound
with low ee
CO
2
H
H
2
N
HO
OH (H)
Asymmetric autocatalysis
with amplification of ee
L
-Amino acid
D
-Sugar
Scheme 12.37.
Proposed origins of chirality and the pathway to the biological homogeneity.
tioenrichments of organic compounds have been induced by the irradiation with CPL.
For example, asymmetric photodecomposition of
rac
- leucine by
r
- CPL (213 nm) pro-
duces L-leucine with only 2% ee [99]. Hexahelicene with less than 2% ee is formed by
asymmetric photosynthesis using CPL [100]. Irradiation of racemic alkylidenecyclohexa-
none with CPL induces a small enantiomeric imbalance (< 2% ee) [101] . These low
enantiomeric enrichments induced by CPL have not been correlated with the homochi-
rality of organic compounds. We considered that chiral organic compounds with low ee
induced by CPL could act as a chiral trigger in the asymmetric autocatalysis to afford
highly enantioenriched pyrimidyl alkanol with an absolute confi guration corresponding
to that of the handedness of the CPL.
Indeed, in the presence of L-leucine with only 2% ee as a chiral initiator, the reaction
of 2 - methylpyrimidine - 5 - carbaldehyde
30
with
i
- Pr
2
Zn produced (
R
) - pyrimidyl alkanol
31
with an enhanced ee of 21% [79,102,103]. In contrast, when D-leucine with 2% ee was
used as a chiral initiator, (
S
) -
31
with an increased ee of 26% was obtained. As described
in the preceding section, the ee of the obtained pyrimidyl alkanol can be amplifi ed sig-
nifi cantly by consecutive asymmetric autocatalysis to achieve homochirality. When
2 - (
tert
- butylethynyl)pyrimidine - 5 - carbaldehyde
32
, instead of the 2-methylpyrimidine
derivative
30
, was subjected to the autocatalytic reaction in the presence of chiral leucine
with extremely low ee, highly enantioenriched pyrimidyl alkanol
33
with the absolute
confi guration corresponding to that of chiral leucine was also obtained. But it should be
noted that the resulting alkanol
33
showed the opposite enantioselectivity to that of
alkanol
31
with high enantioenrichment.
Next, we found that (
P
)-hexahelicene with 0.13% ee, which is lower than that induced
by CPL [100,104], also acts as a chiral initiator for asymmetric autocatalysis. Thus, the
chirality of CPL has been correlated with that of alkanol
33
with high ee by using hexa-
helicene as the chiral source of asymmetric autocatalysis. And then, we performed the
irradiation of CPL to the
rac
-alkylidenecyclohexanones, and the resulting compounds
were subjected to the asymmetric autocatalysis as the chiral trigger. As a result, enan-
tioenriched (
S
) - and (
R
)-pyrimidyl alkanols with the absolute confi gurations correlated
to the chirality of CPL were obtained. The alkylidenecyclohexanone acted as the practi-
cal mediator between the CPL and highly enantiomeric organic compound in conjunc-
tion with asymmetric autocatalysis. Thus, low enantioenrichments in compounds induced
