Biology Reference
In-Depth Information
stimulated investigations to what extent also higher substituted
galloylglucoses could act as acyl donors, provided they still possessed
the energetically indispensable 1-
O
-acetyl moiety. Such compounds
were actually found to exert galloyl donor potentials, however, the
reactivity of higher substituted analogues was drastically reduced, most
likely because of increasing steric hindrance due to excessive bulkiness.
Thus, 1-mono- and, at a lower degree, 1,6-diesters proved to be the
predominating galloyl donors (Denzel and Gross, 1991).
Side
reactions
Side
reactions
Main pathway
1-
O
-Galloyl-
β
-D-glucose (
2
)
+ 1,6-Di-
O
-galloyl-
β−
D-glucose (
14
)
1,6-Di-
O
-galloyl-
β
-D-glucose (
14
)
1,3,6-Tri-
O
-galloyl-
β
-D-glucose
1,2,6-Tri-
O
-galloyl-
β
-D-glucose (
15
)
1,2,4,6-Tetra-
O
-galloyl-
β
-D-glucose
1,2,3,6-Tetra-
O
-galloyl-
β
-D-glucose (
16
)
Fig. 3.6 Overview of the main pathway and side-reactions in the biosynthetic route from
mono- to pentagalloylated β-
D
-glucopyranoses.
1,2,3,4,6-Penta-
O
-galloyl-
β
-D-glucose (
3
)
Some abortive side-reactions are shown in Fig. 3.6. Concerning the
acylation of 1,2,6-trigalloylglucopyranose (
15
), its 1,3,6-isomer was
transformed efficiently to the same product, 1,2,3,6-
tetragalloylglucopyranose (
16
). However, it was impossible to assign any
importance to this alternative
in vivo
due to negligible supply of the
precursor from the preceding step. The reverse situation applies to
1,2,4,6-tetragalloylglucopyranose, which is formed as a by-product in the
synthesis of the 1,2,3,6-isomer (
16
), but is not accepted as substrate for
the subsequent conversion to pentagalloylglucopyranose (
3
). Thus, in
spite of these
in vitro
detectable side-reactions, the main pathway as
depicted in Figs. 3.5 and 3.6 must be considered as the exclusive route in
