Biology Reference
In-Depth Information
5.6.2
The Cyclic Homo- and Heterodimer Equilibrium
Upon
standing,
however,
the
homochiral
cyclic
dimer
2B
spontaneously, albeit slowly, equilibrated with its heterochiral
diastereomer
at room temperature, until their ratio reached 1:1
after 24 h (Scheme 5.13).
2K
O
C
l
Cl
Cl
O
C
l
O
N
O
O
O
O
O
O
O
O
N
O
N
O
O
O
O
O
K
= 1
S
S
O
N
O
O
S
Cl
Cl
Cl
Cl
S
S
S
S
O
CH
2
Cl
2
RT
O
O
Cl
C
l
Cl
O
Cl
S
O
O
N
O
O
O
O
O
N
N
O
O
O
O
2
B
(R,R) or (S,S)
O
2K
(R,S)
O
O
N
O
O
Cl
Cl
Cl
Cl
Scheme 5.13
The
homochiral
cyclic
dimer
2B
,
however,
slowly
interconverts to the meso heterochiral cyclic dimer
2K
(R,S)
at RT, until reaching a 50:50 mixture.
1
H NMR first revealed the equilibrium between the cyclic homo-
and heterochiral dimers, which are diastereomers. On the NMR
time scale,
2B
exists as a pair of enantiomers (R,R and S,S) with D
2
symmetry.
Thus, both diastereomers have a single peak representing all eight
aromatic protons, although the peaks are slightly different for the
two compounds. Similarly, the TEG chain protons also experience
slightly different chemical shifts between the diastereomeric pair;
the protons directly adjacent to the disulfide bonds are conveniently
well resolved from other peaks and particularly sensitive to
isomerization. Immediately following the reaction quenching,
symmetry, while
2K
is a meso compound (R,S) with C
2h
was
purified and characterized by NMR, showing mainly one aromatic
peak and a single triplet for the disulfide-adjacent protons (Fig. 5.20,
top). However, upon allowing the purified
2B
2B
solution to equilibrate
in CH
for >24 h, two aromatic peaks with equal integration were
apparent and the disulfide-adjacent protons displayed two equal-
sized overlapping triplets concomitantly (Fig. 5.20, bottom). The
first reaction product generates the higher field aromatic peak, and
has a lower chromatographic
Cl
2
2
0
−
0
0
−
1
R
and a lower
A
/
A
absorbance
f
ratio, consistent with greater
overlap. Modeling shows that the
homostructure has a larger overlap than the hetero one; thus, we
assign the product as
π−π
2B
.
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