Biology Reference
In-Depth Information
McMurry, 1976, Haslam, 1980). Cleavage of methyl ester
45
with LiI in
absolute pyridine produced a well-defined, polar product
46
. Spraying
the thin layer chromatogram with a bromocresol green solution
immediately showed a yellow colour, which is indicative for the free
carboxylic function in
46
(Khanbabaee and Lötzerich, 1999). The
removal of the protective groups in the final step of this synthetic
sequence with Pd/C-H
2
would lead to lagerstannin C (
47
) (Fig. 5.9).
5.2.2.5
Tellimagrandin I and tellimagrandin II
For the construction of the tellimagrandin framework, both the groups of
Meyers and Lipshutz used the double esterification of enantiopure (
S
)
-
hexamethoxydiphenic acid (
S
)
-
50
with suitable diol derivatives of
D
-
glucopyranose to form the corresponding macrocyclic products (
i.e.
,
method
A
, see Fig. 5.1). The two syntheses only differ in the way the
enantiopure (
S
)
-
hexamethoxydiphenic acid (
S
)
-
50
was constructed.
Lipshutz used a cuprate-catalyzed biaryl coupling to form (
S
)
-
50
,
whereas Meyers used an Ullmann coupling of two identical aromatics
containing oxazolines as chiral auxiliaries. Only the syntheses of the
fully methyl ether-protected tellimagrandins
53
and
61
were described.
Cleavage of the methyl ethers that would have completed the total
syntheses to the respective natural products tellimagrandin I (
67
) and
tellimagrandin II (
73
) was not executed (Lipshutz
et al.
, 1994b, Nelson
and Meyers, 1994). Since methyl ethers cannot be easily removed
(Quideau and Feldman 1996, Khanbabaee and Lötzerich, 1997a), benzyl
ether-protected gallic acid derivatives could have constituted more
appropriate coupling units, as benzyl ethers can be easily removed by
hydrogenolysis at the final step of the synthesis. However, coupling of
benzyl ether-protected gallic acid derivatives into the corresponding
hexabenzyloxydiphenoyl unit has not yet been described; the possibility
of such a coupling may be impaired by the higher steric demand of the
benzyl groups relatively to that of smaller alkyl groups like methyl
groups.
