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This observation suggested that the cationic p-orbital must be under
the influence of an adequately α-positioned electron-donating atom. A
closer examination of the structure of
27
revealed that the carbonyl
oxygen atom of the galloyl-derived II-group of its NHTP unit seems
properly located to get involved through one of its lone pairs in such
electronic interactions. The possibility of such an intramolecular
electronic “quench” of the α-face of the cationic p-orbital was further
supported by the observation of a significant recovery of its electron-
deficiency after having replaced
in silico
the supposedly influential
carbonyl group of
27
by a methylene (-CH
2
-) unit to give the cation
27'
(see Fig. 9.9) (Quideau
et al.
, 2005).
Thus, in
27
, the
exo
β-face of the cationic C-1 center is not
only more accessible but also exhibits a larger electron-deficiency, hence
the diastereoselectivity. Even though this proposal is only supported
by the results of computer-aided molecular modeling, it nevertheless
provides us with an illustrating, if not compelling, understanding of
stereoelectronic factors that can control the remarkable and non-
enzymatic diastereofacial selection observed during nucleophilic
substitution reactions at the C-1 position of vescalagin (
1
).
9.2.2 Hemisynthesis of flavano-ellagitannins - The acutissimins
story
The acutissimins A (
14
) and B (
15
) (Fig. 9.4), two regioisomeric
flavano-ellagitannins that have been isolated from the bark and/or leaves
of various oak and chestnut species (
Fagaceae
)
such as Japanese
Quercus acutissima
(Kunugi),
Quercus miyagii
(Okinawa-urajirogashi),
Quercus stenophylla
(Urajirogashi),
Quercus mongolica
var.
grosseserrata
(Mizunara) and
Castanea crenata
(Kuri), European
Quercus petraea
(
Q
.
sessiliflora
) and
Castanea sativa
, and Algerian
Quercus suber
and
Quercus coccifera
(Ishimaru
et al.
, 1987, Nonaka
et
al.
, 1990, König
et al.
, 1994, Lampire
et al.
, 1998, Ito
et al.
, 2002), are
examples of complex tannins that can derive from an acid-catalyzed
nucleophilic substitution, as described above, between vescalagin (
1
) and
(+)-catechin. The flavanol unit, (+)-catechin, ends up connected to the C-
