2.3 Optimisation of the Synthetic Sequence
33
Table 2.7: Screening of the Schönecker oxidation. All reactions were carried
out on a 0.1 mmol scale (1.0 equiv of picolylimine 31a, 1.3 equiv of
[Cu(MeCN)4][PF6], 2.0 equiv of sodium l-ascorbate and O2 atmosphere) at 50 °C.
N
H
H
O
H
Cl
O
N
O
H
H
O
H
Cl
O
HO
Solvent, 50 °C
(v) [Cu(MeCN) 4 ][PF 6 ]
(1.3 equiv),
Na L-ascorbate
(2.0 equiv),
CO 3
2– (0–2 equiv),
O 2 atmosphere
31a
8a
Entry
Solvent
V
/ml Additive
n(Additive)
/equiv
Note
1
MeOH/AcMe
a 0.5
—
—
Hydrolysed
2
AcMe
0.5
—
—
Hydrolysed
3
MeOH/AcMe
a 0.8
K 2 CO 3
2.0
Hydrolysed
4
MeOH/AcMe
a 0.8
CaCO 3
2.0
Hydrolysed
5
MeCN
0.8
CaCO 3
2.0
Hydrolysed
6
DMF
0.8
CaCO 3
2.0
Hydrolysed
7
DMSO
0.8
CaCO 3
2.0
Hydrolysed
8
THF
1.6
CaCO 3
2.0
Hydrolysed
a 1:1 mixture.
ketone 8a was not detected with mass spectroscopy in any of these reactions.
Thus, the preparation of β-hydroxy ketone 8a via Schönecker oxidation was
not successful.
2.3.3 Deacetylation and Deoximation
Deacetylation and deoximation were carried out in a one-pot reaction as
elaborated by Berger (Section 2.2.3), giving β-hydroxy ketone 8b as a single
diastereomer in 55 % yield.