Chemistry Reference
In-Depth Information
OPiv
PivO
OPiv
PivO
O
R-CHO
4
Me
3
SiCN
Lewis acid,
solvent
O
NH
2
OPiv
N
R
PivO
PivO
OPiv
H
3
5
OPiv
PivO
OPiv
PivO
COOH
N
N
O
O
HCl/HCOOH
(R =
p-
ClC
6
H
4
)
CN
CN
(R)
(S)
Cl
or
PivO
PivO
NH
3
Cl
OPiv
OPiv
H
H
R
R
H
6
6'
7
SnCl
4
, THF or ZnCl
2
,
i
-PrOH
ZnCl
2
, CHCl
3
R =
p-
tolyl 6.5:1 dr (ZnCl
2
,
i
-PrOH)
p-
tolyl 12:1 dr (SnCl
4
, THF)
p
-NO
2
-C
6
H
4
7:1 dr (ZnCl
2
,
i
-PrOH)
o
-NO
2
-C
6
H
4
only (
R
) (SnCl
4
, THF)
p
-F-C
6
H
4
6.5:1 dr (ZnCl
2
,
i
-PrOH)
p
-F-C
6
H
4
10:1 dr (SnCl
4
, THF)
p
-Cl-C
6
H
4
11:1 dr (SnCl
4
, THF)
i
Pr
R =
Me
3
C- 1:9 dr
Me
2
CH- 1:5 dr
C
6
H
5
CH
2
CH
2
1:3 dr
p
-CH
3
C
6
H
4
1:4.5 dr
p
-F-C
6
H
4
1:3 dr
p
-Cl-C
6
H
4
1:4 dr
m
-Cl-C
6
H
4
1:6 dr
8:1 dr (SnCl
4
, THF)
i
Bu
13:1 dr (SnCl
4
, THF)
Scheme 1.3
Kunz's asymmetric Strecker reactions with a galactose-derived chiral auxiliary.
the 2-
O
-pivaloyl group. This complex is preferably attacked by the cyanide, which
is liberated in the polar medium from TMSCN, from the sterically less hindered
rear face, that is, the
Si
face of the imine [4a].
After the successful syntheses of d-amino acids via Lewis-acid-catalyzed Strecker
reactions with galactosylamine
3
as the stereodifferentiating auxiliary, Kunz has
developed the pivaloylated-arabinosylamine
10
as a new chiral auxiliary [5]. Apart
from the missing hydroxy methyl group at C5, d-arabinose is a mirror image of
d-galactose and therefore arabinosylamine can be regarded as a pseudo-enanti-
omer of d-galactosylamine
3.
To prepare pivaloylated-arabinosylamine
10,
the
peracetylated arabinopyranose is transformed into arabinopyranosyl azide
8,
as
has been described by Paulsen and coworkers [6]. After deacetylation and subse-
quent pivaloylation,
8
gives arabinopyranosyl azide
9,
which is subsequently
reduced by hydrogenation with Raney nickel to furnish the auxiliary
10
(Scheme
1.4).
1.NaOMe/MeOH
r.t., 24h
2. PivCl/Py, r.t., 24h
N
3
NH
2
N
3
O
H
2
/Raney Ni
MeOH, r.t., 24h
O
O
OPiv
OPiv
OAc
OPiv
OPiv
OAc
PivO
AcO
89%
PivO
88%
D-arabinose
8
9
10
Scheme 1.4
Synthetic pathway to arabinosylamine
10.
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