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O
CHO
OH
O
O
P
HN
D-
ribo
PCl ( 2 equiv)
H
H
2
N
O
O
(i-v)
O
HO
H
H
OH
O
Toluene, Py, 80
o
C
O
O
H
2
N
NH O
CH
2
OH
O
P
D-(+)-xylose
19
O
R
1
R
2
R
1
O
O
O
=
O
O
O
R
1
R
2
R
1
a
R
1
= R
2
=
t
Bu
b
R
1
=
t
Bu; R
2
= OMe
c
R
1
= SiMe
3
; R
2
= H
d
R
1
= R
2
= Me
e
(
R
)
ax
; R
1
= SiMe
3
f
(
S
)
ax
; R
1
= SiMe
3
Scheme 10.8
Synthesis of furanoside diphosphoramidite ligands
19:
(i) I
2
/acetone (95%
yield); (ii) H
2
SO
4
/CH
3
OH (90% yield); (iii) Tf
2
O/Py/CH
2
Cl
2
,
−
20 °C (55% yield); (iv) NaN
3
/
DMF, 80 °C (76% yield); (v) PPh
3
/THF/H
2
O (77% yield).
CH(COOMe)
2
CH(COOMe)
2
CH(COOMe)
2
Ph
Ph
Ph
19e
78% Conv (30 min)
75% (
R
) at rt
19a
51% Conv (360 min)
95% (
S
) at rt
19a
100% Conv (120 min)
65% regio, 83% (
S
) at rt
Figure 10.9
Summary of the best results obtained using ligands
19.
10.2.1.5
P-P
′
Ligands
The first successful family of P-P
carbohydrate ligands were the phosphite-
phosphoroamidite ligands
20
-
23
(Scheme 10.9). These ligands were synthesized
very efficiently from the corresponding easily accessible aminoalcohol sugar deriv-
atives, which are easily made in a few steps from the corresponding d-xylose or
d-glucose, by reaction with two equivalents of the appropriate
in-situ
formed phos-
phorochloridite [ClP(OR)
2
; (OR)
2
′
a
-
f
] in the presence of pyridine (Scheme 10.9).
Ligands
20
-
23
were successfully applied in Pd-asymmetric allylic substitution (up
to 98% ee) [19]. Interestingly, this ligand family also provides high activity (because
of the high
=
-acceptor capacity of the phosphoroamidite moiety) and high enanti-
oselectivities for different substrate types [mono- (
S7
and
S9
) and disubstituted
(
S1
and
S3
) linear and cyclic (
S4
and
S5
) substrates] (Figure 10.10). Related phos-
phine-phosphite ligands
24
(Figure 10.11) with a furanoside backbone have also
π
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