Preparation of pseudo-peptide building blocks with retro-thioamide bond mediated via thiocarbamoyl Meldrum's acid

Article abstract of DOI:10.1002/hlca.201100347

An easy and efficient synthesis of pseudo-tripeptide containing a thiomalonamide moiety was developed. Isothiocyanate derivatives of amino acids react smoothly with 2,2-dimethyl-1,3-dioxane-4,6-dione (Meldrum's acid) to yield new thiocarbamoyl derivatives of Meldrum's acids. Thermal decomposition of these new derivatives leads to thiocarbamoyl ketenes, which acylate amino acid esters to give pseudo-tripeptides. Copyright

Full text of DOI:10.1002/hlca.201100347

Helvetica Chimica Acta – Vol. 95 (2012)
461
Preparation of Pseudo-Peptide Building Blocks with retro-Thioamide Bond
Mediated via Thiocarbamoyl Meldrumꢀs Acid
by Karolina Janikowska, Sławomir Makowiec*, and Janusz Rachon´
Department of Organic Chemistry, Faculty of Chemistry, Gdansk University of Technology, Narutowicza
11/12, PL-80-952 Gdansk
(phone: þ 48583471724; fax: þ 48583472694; e-mail: mak@pg.gda.pl)
An easy and efficient synthesis of pseudo-tripeptide containing a thiomalonamide moiety was
developed. Isothiocyanate derivatives of amino acids react smoothly with 2,2-dimethyl-1,3-dioxane-4,6-
dione (Meldrumꢀs acid) to yield new thiocarbamoyl derivatives of Meldrumꢀs acids. Thermal
decomposition of these new derivatives leads to thiocarbamoyl ketenes, which acylate amino acid esters
to give pseudo-tripeptides.
Introduction. – It has been more than well-known that peptides play an essential
role in virtually all biochemical processes. One can, at this point, mention the
widespread use in medicine of long peptides derived from biotechnology such as, e.g.,
well-known interferons, erythropoetin, insulin, etc., to very short ones, like oxytocine,
enkephalin, or tuftsine. But all peptides with long or short chains possess the same
drawback, i.e., their rapid enzymatic degradation in living organisms [1]. As a
consequence, several modifications of the peptides backbone were undertaken. For
example, an amide bond could be replaced by a retro-inverso hydroxyethylene [2],
hydroxyethylene [3], ketomethylene [4], aminomethylene [5], retro-inverso [6] or
thioamide moiety [7].
In our research, we focused on the possibility of obtaining a scaffold of retro-
modified petides, which also contain a thioamide modification-introduced regioselectively.
One of the aproaches to retro-modification of peptide is based on the introduction
of a malonodiamide moiety in the structure of the molecule. Implementation of this
task requires the use of any malonic derivative. Most of the syntheses of malonodia-
mide type of retro-modified peptides start from mono- or diesters of malonic acid,
which are incorporated into the peptide chain by the typical condensation procedures
[8]. In another original aproach, Meldrumꢀs acid was used as an equivalent of malonic
acid; such a strategy allows applying the acylating properties of 2,2-dimethyl-1,3-
dioxane-4,6-dione (Meldrumꢀs acid) to facilitate preparation of the first amide bond,
whereas the second amide bond was obtained by the classical method [9].
Recently, we have demonstrated that carbamoyl Meldrumꢀs acid derivatives in the
presence of Me3SiCl (TMSCl) may acylate efficiently also more basic nucleophiles as
aliphatic secondary amines [10] in contrast to the work of Pak and co-workers [11]. In
such a synthetic strategy, 5-[(aryl/alkylamino)hydroxymethylene]-2,2-dimethyl-1,3-
dioxane-4,6-diones by thermal decomposition are a source of carbamoyl ketenes, which
can acylate a broad spectrum of nucleophilic reagents.
ꢁ 2012 Verlag Helvetica Chimica Acta AG, Zꢂrich

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Helvetica Chimica Acta – Vol. 95 (2012)
Here, we propose that S analogs of carbamoyl Meldrumꢀs acid, 1, could be used for
the preparation of modified peptides containig a retro-thioamide bond. Realization of
this goal requires, first, preparation of sufficiently stable compounds of type 1, where R1
is an amino acid moiety, second, generation of thiocarbamoyl ketenes from 1, and
finally acylation of C-protected amino acids with the new thiocarbamoyl ketenes
(Scheme 1).
Scheme 1
Results and Discusion. – First, we decided to check if the ꢃnon-peptideꢀ S derivative
2,2-dimethyl-5-[(phenylamino)(sulfanyl)methylidene]-1,3-dioxane-4,6-dione
(1a)
would be a good source of thiocarbamoyl ketenes, and if the acylation of nucleophiles
with this ketenes would be possible in the usual way.
In the first experiments, we heated in boiling PhCH2Me 1 equiv. of 1a in the
presence of 2 equiv. of aniline. The process was stopped when TLC analysis showed
disappearance of all starting Meldrumꢀs derivative 1a. From the reaction mixture, we
isolated 3-anilino-N-phenyl-3-thioxopropanamide (3aa) in 79% yield (Table, Entry 1).
In the next very similar experiment, we used p-toluidine as a trapping nucleophile. The
result of this experiment was surprising, as we isolated from the mixture an inseparable
mixture of two very similar thiomalonamides 3ab and 3xb (Entry 2).
The unexpected formation of a second product 3xb may occur following two paths:
first, by the vinylic substitution on the starting carbamoyl Meldrumꢀs acid derivative,
followed by decomposition to thiocarbamoyl ketene and its reaction with the
nucleophil, or, by a second path, which involves transamidation of the initially formed
thioamide 3ab (Scheme 2).
To decide which of the paths leads to the formation of thioamide 3xb, we performed
two experiments. In the first one, we heated in boiling PhCH2Me, 1 equiv. of 3aa, which
was previously prepared in an independent way, with 2 equiv. of p-toluidine (4-
methylaniline) for 1 h. The 1H-NMR spectra of this mixture indicated formation of N-
(4-methylphenyl)-3-[(4-methylphenyl)amino]-3-thioxopropanamide (3xb). The ratio
of thiomalonoamides 3ab and 3xb was estimated on the basis of integration of the
malonic CH2 group and indicated that 3xb, from classical transamidation, arose only in
9% yield, while, in the reaction of 1a with toluidine, 3xb was formed with 16% yield
within only 35 min. In the second experiment, we heated 1a with a 15-fold excess of p-
toluidine in CH2Cl2 as a low boiling solvent, in order to ensure a non thermolytic
condition, to check if the vinylic substitution would be the predominant reaction.

Helvetica Chimica Acta – Vol. 95 (2012)
463
Table. Reactions of Thiocarbamoyl Meldrumꢀs Acid with Nucleophiles
Entry
1
R1
2
HꢀNu
HꢀNHPh
Product 3 Solventa) Time [h] Yield [%]
1b)
2b)
3
4
5
6
7
8
9
10
11
12
13d)
14
15d)
16d)
a
a
b
b
b
c
c
c
c
c
c
d
c
c
c
d
PhNH
PhNH
MeNH
MeNH
a
b
b
c
d
b
e
f
3aa
A
A
A
A
A
B
B
B
C
B
B
B
B
B
B
B
0.6
0.6
1
79
–c)
HꢀNHꢀp-Tol
HꢀNHꢀp-Tol
HꢀNHꢀt-Bu
HꢀNHꢀBn
HꢀNHꢀp-Tol
HꢀGlyOMe
HꢀAlaOMe
HꢀAlaOMe
HꢀValOMe
HꢀLeuOMe
HꢀGlyOMe
HꢀAlaOMe
3ab þ 3xb
3bb
3bc
3bd
3cb
3ce
3cf
3cf
3cg
100
99
75
68
50
62
58
49
71
50
51
33
70
73
1
1
4.1
6
6.5
1.1
5.5
5.5
7
6.5
18
18
22.5
MeNH
MeOGly
MeOGly
MeOGly
MeOGly
MeOGly
MeOGly
MeOVal
MeOGly
MeOGly
MeOGly
MeOVal
f
g
h
e
f
i
i
3ch
3de
3cf
HꢀNHCH2CH2NHAc 3ci
HꢀNHCH2CH2NHAc 3ci
HꢀNHCH2CH2NHAc 3di
i
a) Solvent: A, PhCH2Me, B, ClCH2CH2Cl, C, toluene. b) 2 Equiv. of 2 were used. c) Mixture of two
products. d) TMSCl (2 equiv.) was used.
Scheme 2

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Helvetica Chimica Acta – Vol. 95 (2012)
However, we observed only unreacted starting material in the mixture. These
observations strongly indicate that 3xb is formed via both reaction paths, but higher
yields of transamidation product are obtained from the reaction where the starting
material was 1a, and suggests that vinylic substitution at 1a is a faster reaction than
classical transamidation in already formed thiomalonamide.
The complications in the experiment with 1a are connected with the properties of
aromatic amines as moderately good leaving groups. Hence, in the next experiments,
we used 2,2-dimethyl-5-[(methylamino)(sulfanyl)methylidene]-1,3-dioxane-4,6-dione
(1b; Entries 3 – 5). Application of the methylamino derivative allowed obtaining
thiomalonoamides with good yields without transamidation products; these experi-
ments confirmed that thiocarbamoyl ketenes can be generated from 1 and trapped by
nucleophiles.
In the next step, we synthesized compounds 1 with substitutent R1 other than alkyl
or aryl group, e.g., to obtain the Gly and Val derivatives 1c and 1d, which, after
thermolysis in the presence of a nucleophile, would be equivalents of retro-thioamide
dipeptides GlyY(NHCS)Gly or ValY(NHCS)Gly, respectively [12]. For our purposes,
we synthesised isothiocyanato derivatives of acetic and isovaleric acid 4c and 4d,
respectively, by the method described in [13]. These compounds were used for the
preparation of Meldrumꢀs acid derivatives by a modification of the procedure
introduced by Pak and co-workers [11]. We obtained new Meldrumꢀs acid derivatives
1c and 1d, however, it should be stressed that these new derivatives were much less
stable than ordinary Meldrumꢀs acid derivatives and decomposed when trying to purify
or standing at room temperature. Hence, for our reactions we have used freshly
prepared crude compounds 1c and 1d (Scheme 3).
First, we performed a reaction of 1 equiv. of 1c with 1 equiv. of p-toluidine. The
reaction was carried out in boiling PhCH2Me up to complete decomposition of 1c, and
we obtained a dark-brown mixture with a lot of tar. TLC Analysis indicated the
presence of a complex mixture of compounds. We repeated this experiment with
ClCH2CH2Cl as a lower-boiling solvent, which increased the reaction time but allowed
us to obtain the required thiomalonoamide 3cb in 68% yield (Entry 6).
Encouraged by the results of this experiment, we conducted a series of reactions to
obtain the desired pseudo-peptides with a thiomalonoamido residue. We reacted in
boiling ClCH2CH2Cl 1 equiv. of 1c or 1d in the presence of amino acid hydrochloride.
From the mixtures, we isolated pseudo-tripeptides in good yields (Entries 6 – 12). We
also performed one experiment in toluene as a higher-boiling solvent to check if it
would be possible to reduce the time required for completion of the reaction (Entry 9).
Indeed, the reaction time was shortened; however, the yield was slightly lower. For this
reason, in the remaining experiments we used only the less harsh conditions.
The prepared pseudo-peptides 3 are terminated with carboxylic groups on both
ends. For practical application, pseudo-peptide building blocks terminated with an
amino group on one side would be more useful [14]. For this purpose, we used
NH2CH2CH2NHAc as a trap for thiocarbamoyl ketenes. However, from the first
experiment, carried out with 1d and the N-(2-aminoethyl)acetamide (2i), we obtained
the pseudo-tripeptide in low yield (Entry 14). This case, again, confirmed the previous
observation [10] that less basic nucleophiles, i.e., esters of amino acids, provide better
yield than more basic ones, e.g., derivatives of ethylenediamine.

Helvetica Chimica Acta – Vol. 95 (2012)
465
Scheme 3
Based on our previous experiences with acylation of amines by ketenes generated
from Meldrumꢀs acid derivatives, we used TMSCl for a solution of this problem. From
the reaction mixture formed by heating 2i with 1c or 1d in ClCH2CH2Cl in the presence
of 2 equiv. of TMSCl ,we isolated retro-thiotripeptides 3ci and 3di, respectively, in high
yields. Additionally, we repeated the reaction of 1c with alanine methyl ester
hydrochloride in the presence of TMSCl, and, in this case, in accordance with our
predictions, we did not observe an increase of yield (Entry 13).
In summary, we developed a new one pot method for the preparation of retro-
thioamide-modified peptides, based on thermolysis of new Meldrumꢀs acid derivatives.
It should be mentioned that our method allows two concurrent modifications, i.e., the
regioselective introduction of a thioamide and a retro-amide moiety.
This work was financially supported by Funds for Science in 2010 – 2011 (research project N N204
088338).
Experimental Part
General. All of the org. solvents used in this study were dried on appropiate drying agents and
distilled prior to use. Commercially available reagents were from SigmaꢀAldrich. Commercially
unavailable reagents were prepared according to literature procedures: 2,2-dimethyl-5-[(phenylami-
no)(sulfanyl)methylidene]1,3-dioxane-4,6-dione (1a): [15], amino acid isothiocyanates: [13]. TLC:
Merck Kieselgel 60 F254. Flash column chromatography (CC): Zeochem ZEOprep 60/40-63. M.p:
Warsztat Elektromechaniczny W-wa; uncorrected. NMR Spectra: Varian Unity Plus 500 (1H: 500 and 13C:
125 MHz), Varian Gemini 200 (1H: 200 and 13C: 50 MHz); chemical shifts (d) in ppm rel. to internal
Me4Si; coupling constants J in Hz. HR-ESI-MS: MicroMas Quattro LCT mass spectrometer.

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Helvetica Chimica Acta – Vol. 95 (2012)
2,2-Dimethyl-5-[(methylamino)(sulfanyl)methylidene]-1,3-dioxane-4,6-dione (1b). To a soln. of
Meldrumꢀs acid (0.72 g, 5 mmol) in dry DMF (2 ml) in a glass ampoule, Et3N (1.4 ml, 10 mmol) was
added. Methyl isothiocyanate (0.365 g, 5 mmol) was added, and the ampoule was sealed. The ampoule
was placed in the bath for 24 h at 408. The mixture was poured into ice-cold aq. 2m HCl (15 ml). The
precipitate was filtered and washed with cold H2O. The precipitate was dissolved in AcOEt (30 ml) and
dried (MgSO4). The solvent was removed under reduced pressure. Crystallization from AcOEt/hexane
gave 0.400 g (37%) of 1b. M.p. 99 – 1018. 1H-NMR (200 MHz, CDCl3): 1.72 (s, 6 H); 3.16 (d, J ¼ 5.0, 3 H);
11.32 (br. s, 1 H); 13.40 (br. s, 1 H). 13C-NMR (50 MHz, CDCl3): 26.5; 32.1; 82.8; 104.7; 163.4; 170.8;
182.1. HR-ESI-MS: 240.0331 ([M þ Na]þ, C8H11NNaO4Sþ; calc. 240.0305).
Methyl N-[(2,2-Dimethyl-4,6-dioxo-1,3-dioxan-5-ylidene)(sulfanyl)methyl]glycinate; 1c). To a soln.
of Meldrumꢀs acid (0.72 g, 5 mmol) in dry DMF (2 ml) in a glass ampoule, Et3N (1.4 ml, 10 mmol) was
added. Methyl 2-(isothiocyanato)acetate [13] (4; 0.655 g, 5 mmol) was added, and the ampoule was sealed
and kept for 24 h at r.t. The mixture was poured into ice-cold aq. 2m HCl (15 ml). The precipitate was
filtered and washed thoroughly with cold H2O. The precipitate was dried in vaccum desicator over P4O10
1
and used without further purification. Yield: 1.16 g (88%). Yellow solid. H-NMR (500 MHz, CDCl3):
1.77 (s, 6 H); 3.84 (s, 3 H); 4.36 (d, J ¼ 5.2, 2 H); 11.61 (br. s, 1 H); 16.25 (br. s, 1 H). 13C-NMR (125 MHz,
CDCl3): 26.2; 46.5; 53.1; 83.9; 105.0; 163.6; 168.2; 172.9; 184.5. HR-ESI-MS: 274.0395 ([M ꢀ H]ꢀ,
C10H12NO6Sꢀ; calc. 274.0384).
Methyl N-[(2,2-Dimethyl-4,6-dioxo-1,3-dioxan-5-ylidene)(sulfanyl)methyl]valinate (1d). To a soln.
of Meldrumꢀs acid (0.72 g, 5 mmol) in dry DMF (2 ml) in a glass ampoule, Et3N (1.4 ml, 10 mmol) was
added. Methyl 2-(isothiocyanato)-3-methylbutanoate [13] (0.865 g, 5 mmol) was added, and the ampoule
was sealed and kept for 24 h at r.t. The mixture was poured into ice-cold aq. 2m HCl (15 ml). The mixture
was extracted with AcOEt (2 ꢁ 30 ml), and the org. layer was washed with brine (2 ꢁ 30 ml) and sat. aq.
NaHCO3 (2 ꢁ 30 ml). The alkaline layer was acidified with 2m HCl and extracted with AcOEt (2 ꢁ
30 ml), and the org. layer was dried (MgSO4). After filtration, the solvents were removed under
reduced pressure. The obtained yellow oil was used without further purification. Yield: 0.72 g (47%).
1H-NMR (500 MHz, CDCl3): 0.99 (d, J ¼ 6.7, 3 H); 1.02 (d, J ¼ 7.0, 3 H); 1.71 (s, 6 H); 2.30 – 2.37 (m,
1 H); 3.73 (s, 3 H); 4.60 (dd, J ¼ 7.9, 4.8, 2 H); 11.62 (d, J ¼ 7.9, 1 H); 15.93 (br. s, 1 H). 13C-NMR
(125 MHz, CDCl3): 18.3; 19.6; 26.1; 31.3; 52.7; 63.1; 83.7; 104.7; 163.1; 170.3; 183.9.
Preparation of N,Nꢀ-Disubstituted Thiomalonamides or Pseudo-Peptides, 3aa – 3dg, with retro-
Thioamide Bond. General Procedure. A soln. of 1 (2 mmol) and the corresponding amine or amino acid
hydrochloride 2 (2 mmol) in an anh. solvent (PhCH2Me (A), ClCH2CH2Cl (B), or toluene (C); 10 ml)
was stirred and heated to reflux for the time specified in the Table. After completion of the reaction, the
solvent was removed under vacuum, and the residue was purified.
N-Phenyl-3-(phenylamino)-3-thioxopropanamide (3aa). Purification by crystallization from
1
AcOEt/hexane. M.p. 141 – 1448. H-NMR (200 MHz, (D6)acetone): 4.08 (s, 2 H); 7.14 – 7.81 (m, 10 H);
8.51 (s, 1 H); 11.20 (s, 1 H). 13C-NMR (50 MHz, (D6)acetone): 55.2; 119.4; 123.1; 123.7; 126.4; 128.8;
129.0; 139.2; 139.7; 165.8; 194.9. HR-ESI-MS: 293.0731 ([M þ Na]þ, C15H14N2NaOSþ; calc. 293.0725).
3-(Methylamino)-N-(4-methylphenyl)-3-thioxopropanamide (3bb). Purification by crystallization
1
from AcOEt/hexane. M.p. 127 – 1298. H-NMR (500 MHz, CDCl3): 2.34 (s, 3 H); 3.20 (d, J ¼ 4.8, 3 H);
3.95 (s, 2 H); 7.14 (d, J ¼ 8.3, 2 H); 7.40 (d, J ¼ 8.3, 2 H); 9.07 (s, 1 H); 9.79 (s, 1 H). 13C-NMR (125 MHz,
CDCl3): 21.2; 33.4; 53.0; 120.7; 129.8; 134.7; 135.1; 166.3; 196.0. HR-ESI-MS: 245.0730 ([M þ Na]þ,
C11H14N2NaOSþ; calc. 245.0725).
N-(tert-Butyl)-3-(methylamino)-3-thioxopropanamide (3bc). Purification by CC (AcOEt/hexane
1:1). M.p. 115 – 1188. 1H-NMR (500 MHz, CDCl3): 1.38 (s, 9 H); 3.20 (d, J ¼ 4.8, 3 H); 6.51 (s, 1 H); 9.97
(s, 1 H). 13C-NMR (125 MHz, CDCl3): 28.8; 29.9; 33.2; 52.3; 167.9; 196.4. HR-ESI-MS: 211.0877 ([M þ
Na]þ, C8H16N2NaOSþ; calc. 211.0881).
N-Benzyl-3-(methylamino)-3-thioxopropanamide (3bd). Purification by crystallization from
AcOEt/hexane. M.p. 94 – 968. 1H-NMR (500 MHz, CDCl3): 3.19 (d, J ¼ 4.8, 3 H); 3.77 (s, 2 H); 4.42
(d, J ¼ 5.8, 2 H); 7.15 (s, 1 H); 7.26 – 7.37 (m, 5 H); 9.80 (s, 1 H). 13C-NMR (125 MHz, CDCl3): 33.3; 44.1;
51.8; 127.9; 128.0; 129.1; 137.4; 168.3; 195.9. HR-ESI-MS: 245.0721 ([M þ Na]þ, C11H14N2NaOSþ; calc.
245.0725).

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Methyl N-{3-[(4-Methylphenyl)amino]-3-oxopropanethioyl}glycinate (3cb). Purification by CC
(AcOEt/hexane 2 :3). M.p. 105 – 1078. 1H-NMR (500 MHz, CDCl3): 2.32 (s, 3 H); 3.80 (s, 3 H); 3.97
(s, 2 H); 4.43 (d, J ¼ 4.8, 2 H); 7.12 (d, J ¼ 7.81, 2 H); 7.40 (d, J ¼ 7.81, 2 H); 8.69 (s, 1 H); 9.86 (s, 1 H).
13C-NMR (50 MHz, CDCl3): 21.4; 47.8; 53.1; 53.2; 120.9; 129.9; 135.0; 135.2; 166.2; 169.1; 197.2. HR-ESI-
MS: 303.0789 ([M þ Na]þ, C13H16N2NaO3Sþ; calc. 303.0779).
Dimethyl 2,2’-[(1-Oxo-3-thioxopropane-1,3-diyl)diimino]diacetate (3ce). Purification by CC
(AcOEt/hexane 7:2). Orange oil. 1H-NMR (200 MHz, CDCl3): 3.76 (s, 3 H); 3.79 (s, 3 H); 3.86 (s,
2 H); 4.07 (d, J ¼ 5.5, 2 H); 4.43 (d, J ¼ 4.8, 2 H); 7.28 (s, 1 H); 9.92 (s, 1 H). 13C-NMR (50 MHz, CDCl3):
41.3; 47.2; 50.7; 52.5; 168.1; 168.6; 169.8; 196.0. HR-ESI-MS: 285.0518 ([M þ Na]þ, C9H14N2NaO5Sþ; calc.
285.0521).
Methyl N-{3-[(2-Methoxy-2-oxoethyl)amino]-3-thioxopropanoyl}alaninate (3cf). Purification by CC
(AcOEt/hexane 2 :1). M.p. 116 – 1208. 1H-NMR (500 MHz, CDCl3): 1.45 (d, J ¼ 7.3, 3 H); 3.73 (s, 3 H);
3.78 (s, 3 H); 3.84 (s, 2 H); 4.42 (d, J ¼ 4.9, 2 H); 4.53 – 4.59 (m, 1 H); 7.20 (d, J ¼ 6.3, 1 H); 9.99 (s, 1 H).
13C-NMR (50 MHz, CDCl3): 18.4; 47.8; 48.9; 51.4; 53.1; 168.0; 169.2; 173.5; 196.7. HR-ESI-MS: 299.0689
([M þ Na]þ, C10H16N2NaO5Sþ; calc. 299.0678).
Methyl N-{3-[(2-Methoxy-2-oxoethyl)amino]-3-thioxopropanoyl}valinate (3cg). Purification by CC
1
(AcOEt/hexane 5 :4). Orange oil. H-NMR (200 MHz, CDCl3): 0.92 (d, J ¼ 3.9, 3 H); 0.95 (d, J ¼ 3.9,
3 H); 2.14 – 2.20 (m, 1 H); 3.75 (s, 3 H); 3.79 (s, 3 H); 3.86 (s, 2 H); 4.46 (d, J ¼ 4.9, 2 H); 4.49 – 4.56 (m,
1 H); 7.07 (d, J ¼ 8.4, 1 H); 7.28 (s, 1 H). 13C-NMR (50 MHz, CDCl3): 17.7; 18.8; 31.0; 47.2; 51.0; 52.1;
52.4; 57.4; 167.7; 168.5; 171.8; 196.2. HR-ESI-MS: 327.0999 ([M þ Na]þ, C12H20N2NaO5Sþ; calc. 327.0991).
Methyl N-{3-[(2-Methoxy-2-oxoethyl)amino]-3-thioxopropanoyl}leucinate (3ch). Purification by CC
(AcOEt/hexane 5 :4). Yellow oil. 1H-NMR (500 MHz, CDCl3): 0.92 – 0.94 (m, 6 H); 1.58 – 1.70 (m, 3 H);
3.74 (s, 3 H); 3.79 (s, 3 H); 3.84 (s, 2 H); 4.38 – 4.48 (m, 2 H); 4.57 – 4.61 (m, 1 H); 7.05 (d, J ¼ 7.8, 1 H);
9.97 (s, 1 H). 13C-NMR (125 MHz, CDCl3): 22.1; 23.0; 25.1; 41.3; 47.6; 51.2; 51.4; 52.7; 52.8; 168.0; 168.9;
173.3; 196.5. HR-ESI-MS: 341.1140 ([M þ Na]þ, C13H22N2NaO5Sþ; calc. 341.1147).
Methyl N-{3-[(2-Methoxy-2-oxoethyl)amino]-3-oxopropanethioyl}valinate (3de). Purification by
CC (AcOEt/hexane 5 :4). Yellow oil. 1H-NMR (500 MHz, CDCl3): 0.95 (d, J ¼ 6.8, 3 H); 1.00 (d, J ¼ 6.8,
3 H); 2.27 – 2.35 (m, 1 H); 3.72 (s, 3 H); 3.85 (d, J ¼ 6.3, 2 H); 4.03 (d, J ¼ 5.4, 2 H); 4.95 – 4.98 (m, 1 H);
7.38 (s, 1 H); 9.95 (d, J ¼ 7.32, 1 H). 13C-NMR (50 MHz, CDCl3): 18.9; 19.2; 31.2; 41.8; 51.1; 52.7; 52.9;
64.0; 169.1; 170.4; 171.2; 196.6. HR-ESI-MS: 327.1002 ([M þ Na]þ, C12H20N2NaO5Sþ; calc. 327.0991).
Methyl N-(3-{[2-(Acetylamino)ethyl]amino}-3-oxopropanethioyl)glycinate (3ci). Purification by CC
(AcOEt/hexane 5 :4). M.p. 130 – 1348. 1H-NMR (500 MHz, CDCl3): 2.04 (s, 3 H); 3.44 (s, 4 H); 3.81 (s,
3 H); 3.82 (s, 2 H); 4.46 (d, J ¼ 4.8, 2 H); 6.89 (s, 1 H); 7.48 (s, 1 H); 10.10 (s, 1 H). 13C-NMR (50 MHz,
CDCl3): 22.5; 39.0; 39.4; 47.1; 51.7; 52.5; 168.9; 169.3; 172.5; 197.9. HR-ESI-MS: 298.0861 ([M þ Na]þ,
C10H17N3NaO4Sþ; calc. 298.0836).
Methyl N-(3-{[2-(Acetylamino)ethyl]amino}-3-oxopropanethioyl)valinate (3di). Purification by CC
(AcOEt/hexane 5 :4). Oil. 1H-NMR (500 MHz, CDCl3): 1.03 (d, J ¼ 6.8, 3 H); 1.07 (d, J ¼ 6.8, 3 H); 2.00
(s, 3 H); 2.33 – 2.40 (m, 1 H); 3.33 – 3.49 (m, 4 H); 3.78 (s, 3 H); 3.80 (s, 2 H); 4.96 – 4.98 (m, 1 H); 6.71 (s,
1 H); 7.48 (s, 1 H); 10.07 (d, J ¼ 6.84, 1 H). 13C-NMR (125 MHz, CDCl3): 18.8; 19.2; 23.4; 30.8; 39.7; 40.1;
51.7; 52.6; 64.0; 169.3; 171.4; 171.9; 197.1. HR-ESI-MS: 340.1302 ([M þ Na]þ, C13H23N3NaO4Sþ; calc.
340.1307).
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Received September 1, 2011