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N
OMe
N
N
a
OMe
b
OMe
(-)-HA
O
COOMe
COOMe
O
O
OMe
O
2
12
3
-allyl)Pdcl, TMG,
chiral ligand, toluene; b) triflic acid, dioxane.
Reagents:
a)
11
, (η
R
1
R
2
N
13
: R
1
=Me, R
2
=(CH
2
)
4
OH
14
: R
1
=Et, R
2
=(CH
2
)
4
OH
15
: R
1
=cyclopentyl, R
2
=(CH
2
)
5
OH
PPh
2
Fe
PPh
2
Scheme 4-8. The preparation of (
þ
)-12 by Kaneko et al.
15 in hand, the chiral nonracemic product 12 was obtained in the desired configura-
tion for the synthesis of natural (-)-HA.
106
Lee et al. developed a new method for the construction of the skeleton of HA via
the Mn(III)-mediated oxidative radical cyclization of allylic compound 16 derived
from 2. The thermodynamically unstable exo double bond product 17 could be
easily isomerized to the endo olefin 18 by treating it with triflic acid as reported
in the literature (Scheme 4-9).
107
R
1
R
2
N
OMe
R
1
R
2
a,b
c
N
OMe
N
O
OMe
COOMe
O
COOMe
O
COOMe
2
16
17
R
2
R
1
d
N
OMe
O
COOMe
18
Reagents
: NaH/DMF; b) allylic bromides; c) Mn(OAc)
3
, Cu(OAc)
2
, AcOH; d) triflic acid
Scheme 4-9. The new method constructing the skeleton of HA by Lee et al.
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