Chemistry Reference
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displacement of the primary triflate
55
with the corresponding thiolate (Scheme
12.11) [29, 30]. Reaction of thioethers
56a-c
with chloro(diphenyl)phosphine in
dry THF in the presence of pyridine and DMAP, or the biaryl-phosphorochloridite
in the presence of pyridine, provided the desired thioether-phosphinite ligands
57a-c
[31] or the thioether-phosphite ligands
58a-e,
respectively [29, 32].
D-
xylo
57a
R
1
=
i
Pr (72% ee)
57b
R
1
= Me
57c
R
1
= Ph
R
1
S
OPPh
2
O
c)
O
O
D-
xylo
71-89%
HO
OH
TfO
OH
R
1
SOH
b)
a)
O
O
O
D-
xylo
R
2
70-72%
O
O
65%
O
O
O
O
2
R
2
55
d)
O
56a
R=
i
Pr
56b
R= Me
56c
R= Ph (61% ee)
56d
R=
R
2
43-91%
R
1
SO
P
O
R
2
O
AcO
O
AcO
AcO
O
O
OAc
58a
R
1
= Me, R
2
=
t
Bu
58b
R
1
=
i
Pr, R
2
=
t
Bu
58c
R
1
= Ph, R
2
=
t
Bu
58d
R
1
=
i
Pr, R
2
= H
58e
R
1
= Ph, R
2
= H (41% ee)
(a) Tf
2
O, pyridine, CH
2
Cl
2.
(b) NaH, R
1
SH, THF. (c). PPh
2
Cl, pyridine, DMAP. (d) (RO)
2
PCl, pyridine, toluene.
Scheme 12.11
Synthesis of bidentate P,S- and O,S-
xylo
-ligands.
12.2.4
Monodentate P or S-Ligands
The use of a highly modular sugar-based monophosphite ligand library in conju-
gate additions has been reported by Diéguez, Pàmies and Woodward [33] (ligands
59, 61, 66, 69, 71
) using previously described methodologies for their synthesis
(see references therein). All ligands were synthesized by reaction of the corre-
sponding sugar alcohols with PCl
3
and subsequent addition of the biaryl alcohols
(a
-
f)
in the presence of triethylamine. The sugar alcohols, in turn, were easily
prepared on a large scale from d-(
+
)-glucose through the isopropylidene derivatives
17
and
22,
d-(
)-galactose
64,
and d-(-)-fructose
67
(Scheme 12.12). All ligands
were stable during purification on neutral silica under an argon atmosphere and
isolated in moderate yields as white solids.
Ligand
63
[35] was obtained by reaction of tetrakis(acetonitrile)copper(I) hex-
afluorophosphate [36] with the lithium salt of 1,2:5,6-di-
O
-isopropylidene-3-thio-
α
+
-d-glucofuranose [37]. This thiol was obtained by inversion at C3 of the alcohol
60
via triflate displacement by potassium thiocyanate and reduction with NaBH
4
.
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