Chemistry Reference
In-Depth Information
- GMP deriv-
atives (SSC), the only important interaction infl uencing the HT atropisomer popula-
tion is phosphate/NH
cis
amine H-bonding (FSC). The DHT conformer has the
5
Because no phosphate/N1H hydrogen bond is possible for 1-Me-5
′
-phosphate protruding toward the
cis
amine and able to form a phosphate/NH
cis
amine H - bond.
This conclusion is also supported by the observation that in the absence of a
cis
G (which rules out any SSC interaction), such as in the complex Me
3
dien - Pt(5
′
′
-
GMP)
-trimethyldiethylenetriamine, with all three methyl
groups on the same side of the platinum coordination plane), H-bond formation
between the phosphate and the N-H of the
cis
amine becomes the dominant
interaction.
126,127
(Me
3
dien = N,N
′
,N
′
′
5.5.5 Conformers in Oxaliplatin Adducts with Guanines Lacking a Phosphate
Group: The Role of FFC
In an early investigation of the reaction of enantiomeric dach platinum complexes
with guanine derivatives,
123
Pasini and coworkers observed that the CD spectrum
of (
R,R
) - dach - PtG
2
(G = 9 - methylguanine) was characterized by a positive Cotton
effect centred at 230 and 280 nm and a negative Cotton effect centred at 260 nm;
the corresponding Cotton effects for the (
S,S
) - dach - PtG
2
enantiomer had the oppo-
site sign (Figure 5.11). The Cotton effect was assigned to coupling between p
-
p *
electronic transitions centred on the guanine bases and was taken as a clear indica-
tion of transmission of chirality from the dach ligand to the coordinated
cis
guanines
mediated by the amine protons.
123
The details of the mode of transmission of chiral-
ity given at that time were not convincing and have remained obscure until very
recently.
128
cis
- A
2
PtG
2
complexes, with G = a guanine derivative lacking a phosphate sub-
stituent, can have only FFC interactions. However, whereas each N in retro-model
carrier ligands had substituents of very different bulk on the two sides of the
(
R
,
R
)DACH-Pt(9-EtG)
2
(
S
,
S
)DACH-Pt(9-EtG)
2
2
1
0
200
250
300
350
λ
/ nm
Figure 5.11
CD spectra of (
R
,
R
)-dach-Pt(9-EtG)
2
(broken line) and (
S
,
S
)-dach-Pt(9-EtG)
2
(solid line) at pH 7 and room temperature
