Diastereoselectivity in the synthesis of unnatural α-amino acid esters by phase transfer catalysis

Article abstract of DOI:10.1515/znb-2004-0312

Two unnatural α-amino acid esters were prepared in good yields via phase transfer catalyzed Michael addition of ethyl N-acetylaminocyanoacetate to chalcone and benzalketone. For both α-enones, a progressive increase in product diastereomeric excess (d.e.) was observed during the course of reaction, even in the absence of quaternary ammonium salt. However, for a fixed reaction time, higher d.e. values were obtained under phase transfer catalytic condition. Analogous reactions were performed using S-aryl thiocinnamates as Michael acceptors, affording a 2-pyrrolidinone in good yield but low d.e. These results were interpreted on the basis of the reversibility of the Michael reaction.

Full text of DOI:10.1515/znb-2004-0312

Diastereoselectivity in the Synthesis of Unnatural α-Amino Acid Esters
by Phase Transfer Catalysis
Francisco Furtado Tavares Lins, L a´ zaro Pereira da Cunha,
Yvone Br ´ı gido Mem o´ ria Pouliquen, Telma Leda Gomes Lemos,
and Marcos Carlos de Mattos
Departamento de Qu ´ı mica Org aˆ nica e Inorg aˆ nica, Universidade Federal do Cear a´ ,
P.O. Box 12200, 60451-970, Fortaleza, Cear a´ , Brazil
Reprint requests to Prof. Dr. M. C. de Mattos. Fax: +55-85-2889787. E-mail: mcdmatto@ufc.br
Z. Naturforsch. 59b, 305 – 309 (2004); received April 4, 2003
Two unnatural α-amino acid esters were prepared in good yields via phase transfer catalyzed
Michael addition of ethyl N-acetylaminocyanoacetate to chalcone and benzalketone. For both α-
enones, a progressive increase in product diastereomeric excess (d.e.) was observed during the course
of reaction, even in the absence of quaternary ammonium salt. However, for a fixed reaction time,
higher d.e. values were obtained under phase transfer catalytic condition. Analogous reactions were
performed using S-aryl thiocinnamates as Michael acceptors, affording a 2-pyrrolidinone in good
yield but low d.e. These results were interpreted on the basis of the reversibility of the Michael reac-
tion.
Key words: Phase Transfer Catalysis, Unnatural α-Amino Acid, Diastereoselectivity
Introduction
The Michael addition is one the most valuable meth-
ods for carbon-carbon and carbon-heteroatom bond
formation [7]. Stereochemical and mechanistic aspects
of these classical reactions have been object of intense
investigation, mainly focused on the direct addition of
enolates or stabilized carbanions [8]. Regio-, enantio-
and diastereoselectivity of the phase transfer catalyzed
version of this type of reaction is well documented [9],
as well as its applicability to the synthesis of unnatural
α-amino acids [10].
α-Amino acids have been the focus of great inter-
est in all areas of both the physical and life sciences
for over 150 years. It is well known that α-amino
acids are vital to life itself as the “building blocks”
of peptides, proteins, and many other natural products.
Beyond this fundamental role, amino acids are used
extensively as food additives, agrochemicals, as en-
zyme inhibitors, antibacterial agents, neuroactive com-
pounds, pharmaceutical starting materials and fungi-
cides [1 – 4]. Amino acids have also been used in or-
ganic synthesis as synthetic targets, as a source of
chiral raw materials, and as constituents for reagents
and/or catalysts in asymmetric synthesis. The impor-
tance of amino acids has prompted the development
of methods for their racemic and asymmetric synthe-
sis [1].
With a surge of interest focused on small peptides
and peptidomimetics, new methods are needed to pre-
pare an array of unnatural amino acids designed to im-
prove binding potency, chemical and biological stabil-
ity, and pharmacokinetic characteristics when substi-
tuted into peptide-based compounds [5]. The incorpo-
ration into peptides results in significant influence on
the conformational preferences, which eventually pro-
vides useful information for the elucidation of enzy-
matic mechanism [5 – 6].
Results and Discussion
Herein, we wish to report on the Michael addition
of ethyl N-acetylaminocyanoacetate to α-enones and
to S-aryl thiocinnamates, promoted by quaternary am-
monium salts, as a valuable method for the synthesis of
three novel unnatural α-amino acid esters (3a, 3b and
5, Scheme).
The present report will be divided into two parts:
the first one deals with variation of several reaction pa-
rameters, aiming at optimization of product yield; the
second one is dedicated to a study on the diastereose-
lectivity of the above mentioned addition reactions.
Part one: optimization of reaction conditions
1,4-Addition of 1 to chalcone 2a: In order to ver-
ify the influence of the addition of a quaternary
0
932–0776 / 04 / 0300–0305 $ 06.00 ꢀc 2004 Verlag der Zeitschrift f u¨ r Naturforschung, T u¨ bingen · http://znaturforsch.com
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F. F. T. Lins et al. · Diastereoselectivity in the Synthesis of Unnatural α-Amino Acid Esters
Table 1. Michael addition of 1 with enone 2a, influence of Table 2. TEBAC catalyzed addition of malonate 1 to chal-
phase transfer catalyst. cone 2a at different reaction times.
∗
Entry
Time (min)
Yield of 3a (%)
d. e. (%)
35
46
72
81
Entry
Time (h)
Catalyst
none
TEBAC
none
TEBAC
none
TEBAC
Yield of 3a (%)
1
2
3
4
5
5
15
30
60
120
49
70
69
71
67
1
2
3
4
5
6
2
2
4
4
6
6
26
67
48
65
71
73
85
∗
Isolated product, column chromatography on silica using Table 3. Michael addition of 1 with chalcone 2a, during
dichloromethane/acetone (9:1 v/v).
15 min, in absence of solvent, using different tetraalkylam-
monium salts as catalysts.
Entry
Catalyst
Yield of 3a (%)
1
2
3
4
5
TEBAC
63
70
61
61
51
TBAHa
b
cetrimide
CBTAc
d
aliquat
a
tetrabutylammonium hydrogen sulfate; b hexadecyltrimethylam-
monium bromide; c benzyltributylammonium chloride; methyltri-
d
octylammonium choride.
Table 4. Michael addition of 1 to thiocinnamates.
Entry
R
Yield of 5 (%)a
ammonium salt on the rate of reaction, ethyl N-
1
2
3
Ph
p-MePh
p-MeOPh
53
59
80
acetylaminocyanoacetate 1 was allowed to react with
◦
chalcone 2a in toluene at 60 C, in presence or absence
a
Isolated product, column chromatography on silica using hex-
ane/acetone (8:2 v/v).
of triethylbenzylammoniumchloride (TEBAC) and us-
ing, as base, a catalytic amount of KOH (Table 1).
The analysis of data of Table 1 shows that equilib- KOH as base. In this case, the mixture could not be
rium product concentration could be reached after 2 stirred in the absence of solvent (toluene).
hours under the influence of TEBAC (Table 1, entries
1,4-Addition of 1 to thiocinnamates: The addition
2
, 4 and 6), as compared to 6 hours for the biphasic un- reactions of 1 to thiocinnamates 4a, 4b and 4c afforded
catalyzed process (Table 1, entry 5). A more detailed pyrrolidinone 5. The formation of the heterocyclic ring
monitoring of the catalyzed reaction (Table 2) demon- can be attributed to an intramolecular cyclization of the
strated that no more than 15 min. were necessary to former Michael adduct. Optimization of experimental
afford 3a in 70% yield (Table 2, entry 2).
conditions led us to use toluene as solvent (again the
mixture could not be stirred in absence of solvent). Af-
It is worth noting that the same reaction performed
◦
at room temperature yielded, after 15 min., 43% of 3a. ter vigorous stirring for 15 min. at 60 C, compound
◦
5
was obtained in yields ranging from 53 to 80%, de-
Moreover, at 60 C, but in the absence of solvent, 63%
pending on the thiocinnamate S-aryl group (Table 4). It
of 3a was formed for the same reaction time. Using
this environmentally friendly system, the performance is noteworthy that, for all three thiocinnamates, blank
of several ammonium salts was compared (Table 3). experiments, performed in the absence of catalyst, led
to complete recovery of starting materials.
Slightly better results were obtained for TBAH as com-
pared to the other three catalysts (Table 3, entries 1 –
ꢀ
R
Part two: A study on the diastereoselectivity
4
), whereas Aliquat 336 showed to be less efficient.
1
,4-Addition of 1 to benzalketone 3b: Likewise, af-
1,4-Addition of 1 to chalcone 2a: During the mon-
ter examination of various parameters, adduct 3b could itoring of the Michael addition of 1 to chalcone (Ta-
be prepared in 80% yield, as follows: The mixture was ble 2), we observed a progressive increase of product
◦
stirred vigorously by a mechanical stirrer, at 60 C d.e. during the course of reaction. In order to further
for 2 hours, using triethylbenzylammonium chloride investigate this effect, we decided to collect data until
(TEBAC) as phase catalyst and a catalytic amount of the d.e. value was constant (Table 5).
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F. F. T. Lins et al. · Diastereoselectivity in the Synthesis of Unnatural α-Amino Acid Esters
307
Table 5. Values of d. e. for the Michael addition of 1 to chal- admit that the cyclization of the Michael adduct may
cone 2a.
occur as soon as it was produced. Thus, no retro-
Michael should occur in this case, explaining the low
d. e. values.
d. e. (%) for 3a
Entry
Time (h)
Absence of
Presence of
TEBAC
85
TEBAC
34
In summary, we have demonstrated that after careful
examination of various parameters, unnatural α-amino
acid esters could be obtained in good yields by Michael
addition of ethyl N-acetylaminocyanoacetate to α-
enones and thiocinnamates under PTC conditions. In
addition, based on the reversibility of the Michael re-
action, we have devised an explanation for diastere-
oselectivity profiles in the formation of α-enones and
thiocinnamates Michael adducts. Further studies on
the aforementioned reactions, using asymmetric phase
transfer catalysis, are under investigation.
1
2
3
2
4
6
52
100
90
100
Except for a reaction time of 6 hours (entry 3),
when for both reactions (in the presence or absence
of TEBAC) only one diastereoisomer was formed, the
reaction afforded a mixture of two stereoisomers (un-
known configuration) with one as the major product,
which we did not manage to separate by column chro-
matography. As can be seen, d.e. increases with in-
creasing reaction time, and it seems reasonable to sug-
gest that the retro-Michael reaction of the minor di-
astereomer, leading to starting materials, allows for the
Experimental Section
conversion to the thermodynamicallymore stable com- General
ponent. This process seems to be more efficient for the
catalyzed reaction (Table 5, entries 1 and 2), as ex-
pected for a lower energy transition state.
It should be mentioned that when the reaction was
conducted in the absence of solvent for 15 min., d. e.
NMR spectra were recorded on a Bruker DRX 500 spec-
trometer operating at 500 MHz for proton and 125 MHz for
C spectra. IR spectra were run on a Perkin-Elmer 1000 FT-
IR spectrometer using KBr pellets. Melting points were de-
termined on a Mettler FP5 apparatus and are uncorrected.
13
was higher (75%) as compared to the analogous reac- Microanalyses were performed on a Perkin Elmer 2400 B
tion in toluene (Table 2, entry 2). In order to account CHN elemental analyzer. Gravity column chromatography
for these results, two points should be taken into con- was performed on Merck Kieselgel 60 (70 – 230 mesh).
sideration: (i) in a solvent free system, chalcone pro- Thiocinnamates 4a,4b,4c were prepared according to liter-
ature procedures [11].
duces a “polar medium”; (ii) the intermediate for the
Michael reaction has anionic character. Invoking the
Hammond postulate, we propose that stabilization of General procedure for the Michael addition of ethyl N-acet-
ylaminocyanoacetate 1 to α-enones 2a, 2b and thiocinna-
mates 4a, 4b and 4c
the transition state, favored in polar medium, is respon-
sible for the observed increase of d.e. value in solvent-
free conditions.
A
mixture of ethyl N-acetylaminocyanoacetate
1
,4-Addition of 1 to benzalketone 2b: A simi-
(
1.5 mmol), α-enone (1.5 mmol) or thiocinnamate
lar stereochemical behavior was observed when the (1.5 mmol), KOH (0.15 mmol) and tetraalkylammonium
Michael addition was performed using benzalketone salt (0.15 mmol), in toluene or in absence of solvent,
◦
instead of chalcone. Thus, for two hours of reac- was stirred vigorously by a mechanical stirrer at 60 C in
tion time we could observe that 3b was produced reaction time that varied from 5 min. to 6 h. The reaction
mixture was diluted with dichloromethane (30 ml), the
with a higher d.e. value (30%) in presence of cata-
organic extract was treated with water, and then dried
over anhydrous sodium sulfate. After removal of solvent,
under reduced pressure, the crude product was purified by
column chromatography using (A) dichlorometane/acetone
lyst (TEBAC), as compared to the uncatalysed reaction
(20%). Likewise, for a longer reaction time (4 h), only
one diastereoisomer was obtained in the presence or
absence of catalyst.
(
9:1 v/v) or (B) chloroform/ethyl acetate (1:1 v/v) or (C)
hexane/acetone (8:2 v/v) to yield, in sequence, products
a, 3b and 5. The diastereomeric excess (d.e.) values
1
,4-Addition of 1 to thiocinnamates 4a, 4b, 4c:
Under similar conditions, pyrrolidinone 5 was ob-
tained as a mixture of two diastereoisomers in low
d.e. values (≤ 9%). The major diastereoisomer could
3
for 3a obtained after column chromatography are sum-
marized in Table 5. A similar stereochemical behavior
be separated by column chromatography on silica us- was observed for 3b, while 5 was obtained with low
ing hexane/acetone (8:2 v/v). It seems reasonable to d. e. values (≤ 9%).
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F. F. T. Lins et al. · Diastereoselectivity in the Synthesis of Unnatural α-Amino Acid Esters
1
Ethyl 2-acetylamino-2-cyano-3,5-diphenyl-5-oxopentanoate
3a)
H, all arom. H), 8.10/8.10 (s, 1 H, NH). – 13C{ H} NMR
(
(CDCl ), major/minor isomer: δ = 13.34/13.83 (MeCH ),
2.42/22.42 (MeCONH), 30.43/30.47 (MeCO), 44.80/44.80
CH), 47.31/44.91 (CH COMe), 62.98/63.72 (CH O),
3.15/60.65 (Cq), 116.098/116.19 (C=N), 128.79/128.96,
28.82/129.24, 128.90/129.48, 136.50/135.92 (all arom.
C), 165.79/165.55 (COOEt), 170.35/169.77 (CONH),
09.34/205.91 (COMe). – C H N 0 (316.3): calcd. C
17 20 2 4
4.56, H 6.33, N 8.86; found C, 64.34, H 6.24, N 9.12.
3
2
2
IR (KBr): ν = 3357, 2366, 1756 (C=O), 1688 (C=O),
(
−
451, 1239 cm . – H NMR (CDCl ), major/minor isomer:
3
1
1
2
2
1
6
1
δ = 0.82/1.22 (t, J = 7.0 Hz, 3 H, MeCH O), 2.05/1.96 (s,
2
3
3
H, MeCO), 3.71/4.16 (dq, J = 10.6, 7.2 Hz, 1 H, CH O),
2
.88 – 3.93/4.28 (m, 1 H, CH O/dq, J = 10.6, 7.2 Hz, 1
2
2
6
H, CH O), 3.88/3.75 – 3.93 (dd, J = 18.0, 3.0 Hz, 1 H,
2
CH COPh/m, 2 H, CH COPh), 3.84 – 3.93/4.28 (m, 1 H,
2
2
CH O/dq, J = 10.6, 7.2, 1 H, CH O), 3.96/4.02 – 4.08
2
2
N-Acetyl-5-cyano-5-ethoxycarbonyl-4-phenyl-2-pyrrol-
idinone (5)
(dd, J = 8.0, 3.0 Hz, 1 H, CH/m, 1 H, CH), 4.05 (dd,
J = 18.0, 8.0 Hz, 1 H, CH COPh), 7.24 – 7.99/6.39 – 7.99
2
13
1
(
m, 10 H, all arom. H), 8.26/8.26 (s, 1 H, NH). – C{ H}
IR (KBr): ν = 2367, 1765 (C=O), 1703 (C=O),
−1
1
NMR (CDCl ), major/minor isomer: δ = 13.28/13.71 1249 cm . – H NMR (CDCl ), major/minor isomer:
3
3
(
MeCH ), 22.42/22.42 (Me), 42.67/40.06 (CH COPh), δ = 0.81/1.35 (t, J = 7.0 Hz, 3 H, MeCH ), 2.65/2.64
2
2
2
4
1
1
1
1
1
5
4.90/45.13 (CH), 62.90/60.63 (CH O), 63.33/63.69 (Cq), (s, 3 H, MeCO), 2.97/3.05 (dd, J = 17.0, 8.0 Hz, 1 H,
2
16.14/116.24 (C=N), 128.41/128.11, 128.73/128.71, CH CO ), 3.37/3.30 (dd, J = 17.0, 13.0 Hz, 1 H, CH CO
28.81/128.79, 128.86/128.93, 128.88/129.12, 134.44/ ), 3.72 – 3.84 (m, 1 H, CH O), 3.91 – 4.05 (m, 1 H, CH O),
2
2
2
2
33.83, 135.53/135.91, 136.85/136.02 (all arom. C), 4.22/4.43 (dd, J = 13.0, 8.0 Hz, 1 H, CH), 7.34 – 7.42
1
3
1
65.81/165.53 (COOEt), 170.18/169.57 (CONH), 200.04/ (m, 5 H, all arom. H). – C{ H} NMR (CDCl ), ma-
3
96.91 (COPh). – C H N O (378.4): calcd. C 69.84, H jor/minor isomer: δ = 13.48/14.11 (MeCH ), 24.99/24.99
22
22
2
4
2
.82, N 7.41; found C 69.92, H 6.21, N 7.32.
(MeCO), 34.99/36.69 (CH CO), 46.13/46.30 (CH),
2
6
3.91/64.38 (CH O), 65.64/67.38 (Cq), 116.00/113.83
2
Ethyl 2-acetylamino-2-cyano-3-phenyl-5-oxohexanoate (3b) (C=N), 128.31/128.43, 129.28/129.33, 129.67/129.84,
1
1
31.54/132.43 (all arom. C), 163.85/165.17 (COOEt),
70.37/170.16 (COMe), 171.45/171.20 (COCH2).
IR (KBr): ν = 3303, 2362, 1752 (C=O), 1717 (C=O),
–
−
665 (C=O), 1665, 1456, 1248 cm . – H NMR (CDCl ),
3
1
1
1
C H N O (330.3): calcd. C 64.00, H 5.33, N 9.33; found
C 64.04, H 5.56, N 9.46.
1
6
16
2
4
major/minor isomer: δ = 0.80/1.25 (t, J = 7.0 Hz, 3 H,
MeCH ), 2.03/1.90 (s, 3 H, MeCONH), 2.24/2.14 (s, 3 H,
2
MeCO), 3.29/3.34 (dd, J = 19.0, 3.0 Hz, 1 H, CH COMe/dd,
2
Ackowledgements
J = 18.0, 6.0 Hz, 1 H, CH COMe), 3.56/3.17 (dd, J = 19.0,
2
8
.5 Hz, 1 H, CH COMe/dd, J = 18.0, 7.0 Hz, 1 H,
2
We are grateful to CAPES for fellowship to F.F.T.L., to
CH COMe), 3.70/3.81(dd, J = 8.5, 3.0 Hz, 1 H, CH/dd, UFC for a fellowship to L.P.C. and to FUNCAP for finan-
2
J = 7.0, 6.0 Hz, 1 H, CH ), 3.64 – 3.68/4.19 (m, 1 H, cial support. We also thank Dr. Liliana Marzorati for valu-
CH O/dq, J = 12.0, 7.0 Hz,1 H, CH O ), 3.87/4.25 (dq, able discussions and for her assistance in the preparation of
2
2
J = 12.0, 7.0 Hz, 1 H, CH O), 7.24 – 7.31/7.25 – 7.38(m, 5 the manuscript.
2
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