The diastereoselective synthesis of difluoro-βlactam using Reformatsky-Honda reaction

Article abstract of DOI:10.3987/COM-07-S(U)29

The high diastereoselective synthesis of chiral difluoro-β-lactams (3) was attained by treatment of ethyl bromodifluoroacetate (1), 2-hydroxy-l-phenylethyl group substituted imines (2) and diethylzinc in the presence of rhodium-catalyst. The absolute conf

Full text of DOI:10.3987/COM-07-S(U)29

HETEROCYCLES, Vol. 73, 2007
203
HETEROCYCLES, Vol. 73, 2007, pp. 203 - 208. © The Japan Institute of Heterocyclic Chemistry
Received, 27th June, 2007, Accepted, 8th August, 2007, Published online, 10th August, 2007. COM-07-S(U)29
THE DIASTEREOSELECTIVE SYNTHESIS OF DIFLUORO-β-LACTAM
USING REFORMATSKY-HONDA REACTION
Atsushi Tarui, Keigo Kondo, Hiroko Taira, Kazuyuki Sato, Masaaki Omote,
Itsumaro Kumadaki, and Akira Ando*
Faculty of Pharmaceutical Sciences, Setsunan University, 45-1, Nagaotoge-cho,
Hirakata, Osaka, 573-0101, Japan; e-mail: aando@pharm.setsunan.ac.jp
Abstract – The high diastereoselective synthesis of chiral difluoro-β-lactams (3)
was attained by treatment of ethyl bromodifluoroacetate (1),
2-hydroxy-1-phenylethyl group substituted imines (2) and diethylzinc in the
presence of rhodium-catalyst. The absolute configuration of 3 was determined
by X-ray analysis of 3,5-dinitrobenzoate (6) derived from 3.
Nowadays, fluorinated compounds have received a great interest of medicinal chemists due to the unique
physical and biological properties imparted by fluorine.1,2,3 A large number of therapeutic agents
containing strategically placed fluorine atoms are currently widely used.4 On the other hand, the role of
amino acids in biological functions is well established, and the area of fluorine-containing amino acids is
rapidly expanding, taking an important place in the family of unusual amino acids. Given the biological
influence of fluorine substitution in the β-position relative to the amine and the biomedicinal and
synthetic potential reported for β-amino acids,5 the development of synthetic methodology for preparing
fluorine-containing and enantiomerically pure β-amino acids is of particular interest. Since the amino
acids are often interconverted to lactams, difluoro-β-lactams are source of difluoro-β-amino acids supply.
Furthermore, β-lactams also are important compounds, which have an inhibition activity for bacteria’s
enzyme synthesizing cell walls.6 So, we focused on importance of getting enantiopure β-lactams.
R3
R3
R3
O
RhCl(PPh3)3
Et2Zn
NH
CF2CO2Et
N
N
BrCF2CO2Et
+
+
R1
R1
F
R2
R1
1
R2
R2
F
2
3
4
Scheme 1. Reaction of 1 with 2 in the presence of RhCl(PPh3)3

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HETEROCYCLES, Vol. 73, 2007
We reported Reformatsky-type addition reaction of BrCF2CO2Et (1) to imines (2) using Et2Zn and
RhCl(PPh3)3, according to Honda’s protocol,7 to give racemic difluoro-β-lactams (3) and racemic
2,2-difluoro-3-aminocarboxylic ester (4), respectively depending on reaction condition (Scheme 1).8 In
order to expand our previous studies, we attempted the stereoselective synthsis of optically active 3.
Recently, Honda et al. reported that the reaction of Reformatsky reagent, prepared by BrCH2CO2Et and
Et2Zn, with chiral imines in the presence of RhCl(PPh3)3 gave the 3-aminocarboxylic esters
diastereoselectively.9 Thus, we applied to the diastereoselective synthesis of difluoro-β-lactams (3) by
using chiral imines carrying chiral auxiliary on the imine-nitrogen. Among a variety of chiral auxiliaries
reported in the literature, we selected commercially available phenylethylamine and phenylglycinol as
amine components, respectively.10
The diastereoselective reaction utilizing (R)-benzylidene-N-phenethylamine (2a) gave a diastereo mixture
of 3a (entry 1 in Table 1). In order to improve the diastereo ratio of 3a, we examined solvent effect.
The results are summarized in Table 1.
O
Ph
RhCl(PPh3)3
N
N
Ph
BrCF2CO2Et
+
Et2Zn, Solvent
1
F
Ph
H
Ph
0°C
F
2a
3a
Entry
Time (h)
Yield of 3a (%)a
Ratiob
Solvent
CH2Cl2
toluene
hexane
Et2O
86
89
86
62
60
1
4
1
2
3
4
5
3 : 2
1 : 1
1 : 1
3 : 2
7 : 3
1
2
THF
14
a: Isolated yield
1
b: Determined by H-NMR
Table 1. Solvent effect for diastereo ratios of 3a
Generally, non-coordinative solvents give the good results (yield and enantioselectivity) in asymmetric
dialkylzinc addition, because these solvents do not contribute to the coordination to asymmetric ligand,
electrophile, and dialkylzinc.11 However, in mentioned of an asymmetric Reformatsky reaction, Jiang et
al. have been reported that THF showed better enantioselectivity.12 In our reaction, improvement on
diastereo ratio of 3a was achieved not by using low coordinative solvents such as CH2Cl2, toluene, and
hexane but by using THF (entries 1-5). These results were similar to their reported.12
Fujii et al. reported that the reaction of 1 with chiral imine bearing a methyl ether of (R)-phenylglycinol
as chiral auxiliary gave a 2,2-difluoro-3-aminocarboxylic ester as a Reformatsky adduct on the way to
fluoroalkene dipeptide isosteres.13 So we applied their methodology to the reaction of 1 with chiral
imine same as them, but the main product 3 was in poor yield. Furthermore, Honda et al. reported the

HETEROCYCLES, Vol. 73, 2007
205
reaction by using the benzyl ether of (R)-phenylglycinol as chiral auxiliary.9 However, the removal of
the auxiliary was troublesome because the deprotection was required a high pressure in hydrogenolysis,
and gave a side product. To solve the problems, we used another imine (2b) bearing non-protected
hydroxy group. Using compound 2b prepared from (R)-phenylglycinol with benzaldehyde caused the
high diastereoselective addition of 1, and only one diastereomer was obtained (entry 1 in Table 2). It
was obviously due to the strong coordinative effect between the hydroxy group of 2b and zinc in the
addition of zinc enolate derived from 1. The results of searching the best reaction condition are shown
in Table 2.
Ph
Ph
HO
RhCl(PPh3)3
O
OH
N
N
BrCF2CO2Et
+
Et2Zn, Solvent
1
F
Ph
H
Ph
Temp.
F
2b
3b
Entry
Temp.
0 °C
Yield of 3b (%)a
Time (h)
de (%)
Solvent
CH2Cl2
MeCN
DMF
46
36
29
40
40
>99
>99
>99
>99
>99
-
1
2
3
4
5
46
3
0 °C
0 °C
2
rt
rt
rt
toluene
Et2O
0 °C
9
0 °C
0 °C
18
22
3
N.R.
41
6
7
DME
1,4-dioxane
THF
>99
>99
>99
>99
10 °C
0 °C
54
8
9
1
63
THF
THF
-30 °C
-10 °C
2.5
1
67
10
-10 °C
a: Isolated yield
Table 2. Examination of reaction of 1 with 2b
In examination of solvent, only one diastereomer of 3b was obtained in all cases (entries 1-8). And,
reaction temperature also affected the yield of 3b (entries 8-10). Among them, we found the entry 10
was the best condition.
1) Pd(OH)2
Boc
NH
CF2CO2Et
HO
conc. H2SO4
EtOH, reflux
H2 (1atm), EtOH
O
F
HO
N
NH
CF2CO2Et
5b
Scheme 2. Removal of chiral auxiliary from 3b
Next, we examined the ring opening and the cleavage of the chiral auxiliary. These reactions proceeded
2) Boc2O, NaHCO3
THF, reflux
F
3b
6b

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HETEROCYCLES, Vol. 73, 2007
easily, and following N-Boc protection gave N-Boc-2,2-difluoro-3-aminocarboxylic ester (6b) in good
yield (total yield is 65% from 3b to 6b: Scheme 2).
On the basis of the above results, experiments to probe the reaction generality are summarized in Table
3.14
Ph
Ph
H
HO
RhCl(PPh3)3
O
OH
N
N
BrCF2CO2Et
+
Et2Zn, THF
-10°C
1
F
R
R
F
2b-h
3b-h
Imine 2
Entry
Time (h) Yield of 3 (%)a de (%)
R
1
2
3
4
5
6
7
Ph-
b
c
d
e
f
1
12
24
1
67
56
46
54
74
38
29
>99
>99
>99
>99
>99
>99
>99
4-MeO-C6H4-
4-Me-C6H4-
4-Cl-C6H4-
4-MeOCO-C6H4-
4-CF3-C6H4-
Ph-CH2CH2-
1
g
h
2
1
a: Isolated yield
Table 3. Reaction of 1 with various imines
When a wide variety of chiral aromatic aldimines with electronic properties were reacted with 1, the
desired difluoro-β-lactams were afforded in moderate to good yields. The substituents on the phenyl
group did not affect the yields so much, but greatly affected the reaction rate, in which the electron
withdrawing groups were much more improved it, dramatically (entries 1-6). To expand the scope of
substrates, we further examined the aliphatic aldimine (entry 7). We obtained the desired product,
although the yield was merely 29%. In examination of generality on substrates, only the diastereomeric
pure products (3b-h) were obtained in all cases, regardless of the functional group of aldimine-carbon.
The absolute configuration of difluoro-β-lactam (3b) was determined by X-ray analysis of the
3,5-dinitrobenzoate (7b) (Figure 1).
Figure 1. The result of X-ray analysis (7b)

HETEROCYCLES, Vol. 73, 2007
207
On the basis of Honda’s transition state,9 this X-ray result supported that the transition state in
diastereomeric addition of 1 to imine (2b) was as in Figure 2.
OEt
Et
O
Zn
F
H Et
Zn
F
H
N
O
Ph
Re face attack
Figure 2. Proposal transition state of diastereoselective addition
This transition state well explained that the zinc enolate of 1 attacked from the sterically less hindered re
face of the imine, avoiding the large phenyl group of the auxiliary.9,15,16
Unfortunately, the reaction mechanism is not obvious, but the result that the reaction stopped by addition
of styrene as radical scavenger might suggested a radical process.
In conclusion, we achieved highly diastereoselective addition of 1 to the chiral imines to give
difluoro-β-lactams (3) in good yields. The diastereoselective reaction proceeded smoothly, and this
chiral auxiliary was readily cleaved from product 3 to provide N-Boc-2,2-difluoro-3-amiocarboxylic ester
(6) in good yield. Compound 3 is very important synthetic intermediate in pharmacological substances
such as the difluoro-β-amino acids and its peptide. We can provide a new route to the fluorine analog of
optically active β-amino acid. Further applications of this methodology to the synthesis of biologically
active compounds including natural products are in progress.
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