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galloyl-2,3-
O
-(
R
)-hexahydroxydiphenoyl-
D
-glucopyranose (
13
). Several
physical and chemical analyses then provided the proofs of this
assumption. The regiochemistry of every single reaction step was always
unambiguously ensured by the chosen synthesis strategy. We could
determine in two different ways that the HHDP unit at the 2,3-positions
of the glucopyranose core had the (
R
)-configuration. As mentioned
before, the basic hydrolysis of (
R
)-
17
with anhydrous KOH (Gassman
and Schenk, 1977) yielded diphenic acid (
R
)-
16
with a negative specific
rotation. Furthermore, CD spectroscopy was used to verify the absolute
configurations (Berova
et al.
, 1994, Snatzke, 1981 and 1982). It is well
known that CD spectroscopy is a suitable empirical method for the
determination of the absolute stereochemistry of ellagitannins without
the need for chemical degradation reactions (Okuda
et al.
, 1982c/d,
Khanbabaee and Lötzerich, 1998).
Fig. 5.5
CD Spectra of (
S
)-configured ellagitannins, pterocaryanin C (
14
) and praecoxin
B (
15
), and (
R
)-configured 1-
O
-galloyl-4,6-di-
O
-galloyl-2,3-
O
-hexahydroxydiphenoyl-β-
D
-glucopyranose (mahtabin A,
12
) and 4,6-di-
O
-galloyl-2,3-
O
-hexahydroxydiphenoyl-
D
-
glucopyranose (pariin M,
13
).
The Cotton effects near 220 and 250 nm can be correlated with the
absolute configuration of the HHDP unit, using the sign of the Cotton
effect between 220 and 235 nm as a diagnostic criterion. A positive value
in this region unambiguously indicates the presence of a (
S
)-configured
biaryl unit, whereas a negative value indicates the presence of the (
R
)-
