Chemistry Reference
In-Depth Information
n
-C
3
H
7
C
2
H
5
C
n
-C
4
H
9
n
-C
6
H
13
N
(
R
)-
35
S
t
-Bu
N
OH
(
S
)-
33
N
Discrimination of cryptochirality
by asymmetric autocatalysis
t
-Bu
CHO
N
N
32
+
R
t
-Bu
C
2
H
5
n
-C
3
H
7
N
OH
(
R
)-
33
Zn
C
2
n
-C
6
H
13
n
-C
4
H
9
(
S
)-
35
Scheme 12.42.
Chiral discrimination of cryptochiral quaternary hydrocarbon.
asymmetric autocatalysis in the presence of the (
S
) - quaternary hydrocarbon
35
pro-
duced (
R
) - alkanol
33
with 94% ee in 91% yield (Scheme 12.42). These stereochemical
correlations were found to be reproducible. The present chiral discrimination may
involve the CH- π interactions between the CH group of the chiral hydrocarbon and the
π - electrons of the pyrimidine - 5 - carbaldehyde
32
.
In addition, various chiral hydrocarbons with such as saturated tertiary hydrocarbons
[120] , 1,1 ′ - binaphthyls [121] , helicenes [122] , olefi ns [123], allenes [124], and [2.2]para-
cyclophanes [125] also serve as chiral initiators in this asymmetric autocatalysis.
12.5.3. Recognition of Chirality Generated by the Isotope Substitution
Isotopically chiral compounds form unique category of chiral compounds, which are not
superimposable with its mirror image due to the substitution of isotopes. The chirality
of hydrogen isotope enantiomers is mainly due to the very small difference between the
lengths of carbon-deuterium and carbon-hydrogen bonds [126]. Thus, unlike other
enantiomers whose chirality results from the difference in the number of protons in the
atomic nucleus, these isotopic enantiomers are considered to show only very small dif-
ferences in asymmetric reactions and recognition [127].
We investigated highly enantioselective asymmetric autocatalysis of a chiral com-
pound induced by the isotopic enantiomer of a primary alcohol-α -
d
(Scheme 12.43 )
[128]. The correlation between the absolute confi gurations of the obtained pyrimidyl
alkanol and the isotopic chiral compound is reproducible; thus, the small isotope chiral-
ity can be recognized by asymmetric autocatalysis.
When aldehyde
32
was reacted with
i
- Pr
2
Zn in the presence of chiral (
S
) - benzyl
alcohol - α -
d
36
, (
R
) - alkanol
33
with 96% ee was obtained with a yield of 95%. On the
other hand, in the presence of (
R
) - deuterated alcohol ( >95% ee), (
S
) -
33
with 95% ee
was obtained in 98% yield. Thus, (
S
) - and (
R
) - benzyl alcohol - α -
d
36
acted as chiral
inducers to give (
R
) - and (
S
) - pyrimidyl alkanols
33
with high ee after consecutive asym-
metric autocatalysis, respectively.
Furthermore, chiral tolyl methanol-α -
d
, 2,2 - naphthyl methanol - α - d and
3 - phenylpropanol - α -
d
acted as a chiral initiator in the enantioselective addition of
i
- Pr
2
Zn to aldehyde
32
, and pyrimidyl alkanol (
R
) -
33
with high ee was synthesized,
