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attachment of them to the properly protected baccatin core structures and subse-
quent deprotection to furnish the target compounds. The side chains were usually
used in enantiopure forms, which are prepared by asymmetric synthesis or resolu-
tion. The b-lactam side chains were commericially available for the semisynthesis
of paclitaxel, and those in acid forms usually for the synthesis of another semisyn-
thetic taxane, Taxotere 1b (generic name docetaxel), both from 10-deacetyl baccatin
III 2 (10-DAB), a natural taxane that is abundant in the regenerated resources—
twigs and needles of yew trees. The illustrative schemes for the semisynthesis
of taxanes from 10-DAB are shown below. The latter finding
5
on the usefulness
of the side chain in oxazoline form to the synthesis of paclitaxel can be categorized
into the ''acid approach,'' in contrast to the ''b-lactam approach'' (Scheme 3-1).
For the semisynthesis of paclitaxel and its analogs, selective modification of
10-DAB is an important issue to be addressed, that is, the 7-OH and 10-OH should
R
2
O
R
1
O
NH
O
OH
10
2'
13
B
Ph
3'
O
A
7
C
2
OH
4
HO
D
H
O
BzO
AcO
1a
R
1
=Bz, R
2
=Ac
1b
R
1
=Boc, R
2
=H
1c
R
1
=tigloyl, R
2
=Ac
AcO
HO
O
O
OH
OTES
HO
HO
HO
H
O
BzO AcO
HO
H
O
AcO
BzO
2
AcO
Bz
O
NH
O
OTES
route 1
Ph
O
Bz
O
OTES
N
HO
H
O
Ph
base
OTES
AcO
BzO
deprotection
1a
AcO
O
O
OTES
Ph
route 2
O
NO
COOH
Bz
Ph
HO
H
O
R
1
R
2
O
AcO
BzO
N
Bz
R
2
DCC
R
1
Scheme 3-1. Illustrative synthesis of Taxol (paclitaxel).
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