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Me
N
OMe
N
OMe
HO
N
a
b
OMe
O
H
O
H
O
OMe
O
R
O
COOR
2a
8
9
N
N
OMe
OMe
c
(-)-HA
COOR
O
COOH
10
Me
R =
Me
Ph
Me
O
-
Reagents and conditions:
a) RH, PhH, reflux, 3 days (91%); b).methacrolein, TMG, CH
2
Cl
2
,
rt (90%); c). MsCl, Et
3
N, DMAP, CH
2
Cl
2
, rt; NaOAc, HOAc, 110
°
C.
Scheme 4-6. Route to optical pure (
)-HA by Yamada et al.
was evidently improved with ligand 14 to afford 12 in 81% ee. It was obvious that
fine-tuning of the size of the N-substituent of the ligand with an appropriate chain
length had a dramatic effect on the enantioselectivity of the reaction. Enantioselec-
tivity of 90.3% ee for 12 was achieved with (R,S)-ferrocenylphosphine ligand 15
possessing a cyclopentyl group at the nitrogen. With the most efficient chiral ligand
Me
HO
N
OMe
N
methacrolein
MsCl/Et
3
N/cat. DMAP
OMe
0.1 eq quimine, CCl
4
rt, 10 days
CH
2
Cl
2
O
O
COOMe
H
OMe
O
2a
(5
S
,9
R
)-
4
(90%)
Me
MsO
N
OMe
(-)-HA
O
COOMe
(93-98%)
Scheme 4-7. The route to (5S,9R)-4 by Chen and Yang.
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