Chemistry Reference
In-Depth Information
TABLE 8B.16. Azlactones,
α
- Amino Ketones, and
α
-Amino Esters as Pronucleophiles in Allylic
Alkylations according to Scheme 8B.30
SiMe
3
BH
3
BH
3
Ph
o
-An
HN
NH
OC
PPh
2
P
P
Fe
o
-An
Ph
CO
PPh
2
Si
Ph
2
P
Fe
Ph
2
SiMe
3
(
R
)-
TMS-BINAP
(
L23b
)
(
S
,
S
)-
L27
L28
O
O
O
O
O
O
P(OMe)
2
CO
2
Me
Ph
N
Bn
Ph
O
t
-Bu
O
Ph
N
O
M
e
N
NHAc
Ph
Me
NHAc
Ph
43% ee (42%)
L * = L27
Ph
88% ee (78%)
91% ee (92%)
75% ee (95%)
L * = L28
93% ee (68%)
L * = BINAP
(L23b)
L * = DPPBA - 1
(L1a)
L * = TMS - BINAP
(L23b)
R = Ph, R
1
= H
[53,133]
R = Ph, R
1
= H
[134]
R = Ph, R
1
= H
[135]
R = R
1
= H [136]
R
1
= H [137]
X = OCO
2
Et
ees up to 99% and excellent diastereoselectivities were reached using the ligand
DPPBA -
1
(
L1a
) [53,133]. Another example was cited in Scheme 8B.15 in conjunction with Trost's
synthesis of sphingofungins. This early work has been extensively reviewed [14]. Good
results have somewhat later also been achieved with
BINAP
(
L23a
) and
TMS - BINAP
(
L23b
) as ligands (Table 8B.16 ).
The pronucleophile
tert
- butyl
N
- (diphenylmethylene) - glycinate is often used for the
synthesis of amino acids. Its use as pronucleophile in standard allylic alkylations con-
trolled by phosphine ligands furnished initially only low to moderate selectivities (Table
8B.16 ).
Better results were obtained once again by stereocontrol via the nucleophile using
chiral phase-transfer catalysts of the Corey-Lygo type (Scheme 8B.32). In a fi rst attempt
with catalyst
L29a
[138], the best ee achieved was 61%, even though double stereocon-
trol with the phase-transfer catalyst and
BINAP
as chiral phosphine was tried.
Superior results were reached by Takemoto et al. with the very similar catalyst
L29b
[139]. Up to 96% ee and excellent yield were obtained under optimized conditions in
the allylation with cinnamyl acetate. The racemic branched acetate furnished similar
enantioselectivity but a low yield, because the branched substitution product was mainly
formed [140]. The yield of the linear product could be increased by using
L29b
in com-
bination with the chiral monodentate ligand (
R
) -
MeO - MOP
(
L20
). Formation of the
branched product was anticipated under these conditions according to Section 8B.2.2.3.1;
however, the linear product was mainly formed. In addition to those described in Scheme
8B.32, several other allylic acetates were successfully used as substrates.
