Incorporation of the α-mercapto acid unit into peptides

Article abstract of DOI:10.1055/s-2004-822341

Incorporation of α-mercapto acid units into the backbone of potential drug candidates improves the metal ion complexing properties. So far, only the methodology for the N-terminal incorporation of mercapto acid units into peptides has been described. Now, we report on a modified strategy for the incorporation of the α-mercapto acid unit into any position of a peptide backbone starting from hexafluoroacetone-protected thiomalic acid. Concomitantly with the incorporation of the α-mercapto unit the direction of the peptide chain is inverted.

Full text of DOI:10.1055/s-2004-822341

1088
PAPER
Incorporation of the a-Mercapto Acid Unit into Peptides
I
ncorporationof th
s
e
a
-Merca
e
ptoAcidU
n
nit intoPept
i
ides a Pumpor,a Jan Spengler,a,b Fernando Albericio,*b Alois Haas,c Klaus Burger*a
a
Department of Organic Chemistry, University of Leipzig, Johannisallee 29, 04103 Leipzig, Germany
Fax +49(341)9736599; E-mail: burger@chemie.uni-leipzig.de
b
c
Institut de Recerca Biomèdica de Barcelona, Parc Cientific Barcelona, Josef Samitier 1-5, 08028 Barcelona, Spain
Ruhr-Universität Bochum, 44780 Bochum; new address: Medonstraße 17, 14532 Kleinmachnow/Berlin, Germany
Received 19 January 2004; revised 12 March 2004
Dedicated to Prof. Dr. G.-V. Röschenthaler on the occasion of his 60th birthday
Abstract: Incorporation of a-mercapto acid units into the backbone
of potential drug candidates improves the metal ion complexing
properties. So far, only the methodology for the N-terminal incorpo-
ration of mercapto acid units into peptides has been described. Now,
we report on a modified strategy for the incorporation of the a-mer-
capto acid unit into any position of a peptide backbone starting from
hexafluoroacetone-protected thiomalic acid. Concomitantly with
the incorporation of the a-mercapto unit the direction of the peptide
chain is inverted.
Key words: hexafluoroacetone, thiomalic acid, dielectrophiles, thi-
ols, acylations, peptides
A growing number of reports focus on peptidomimetics
built from two or more different types of monomers.1
Scheme 1
Consequently, the development of improved methodolo-
gy for synthesis and incorporation of new types of mono-
mers for peptide modification is of current interest.
Surprisingly, mercapto acid derivatives have been virtual-
ly neglected as building blocks for peptide modifica-
tion,2,3 although some applications in medicinal chemistry
are described.
achieved in one step, involves a heterocyclic intermedi-
ate.2,3 (Scheme 2). The disadvantage of this strategy is,
that the mercapto acid unit can be incorporated only into
N-terminal position of a peptide. However, this drawback
can be overcome by modifying the above discussed ‘1,3-
oxathiolan-5-one route’, by using multifunctional mer-
capto acids like thiomalic acid for the construction of a tri-
functional, orthogonal protected monomer.
A b-mercapto acid is a subunit of Captopril, a reliable
drug for treating hypertension.4 a-Mercapto subunits are
present in Omapatrilat and Gemopatrilat; both are va-
sopeptidase inhibitors which are currently under clinical
evaluation (Scheme 1).5 a-Mercapto acid amides are use-
ful subunits in metal ion complexing agents6 and often ex-
hibit strong inhibitory effects on metal-containing
enzymes (metallozymes). Inhibition of matrix metallo-
proteases has been postulated as a possible therapeutic ap-
proach to a number of disease states.7 Some of the zinc-
containing angiotensine converting enzymes (ACE) pos-
sess a mercaptoacyl substructure.8
A recently described protection/activation concept for
site-selective derivatization of multifunctional a-amino10
and a-hydroxy acids11 was successfully adapted to a-mer-
capto acids. Hexafluoroacetone reacts with thiomalic acid
1, which was used as a racemate, in DMSO at room tem-
perature to give exclusively 2,2-bis(trifluoromethyl)-1,3-
oxathiolan-5-one 2 in 80% yield.3 In only one step, pro-
tection of the a-mercapto and the adjacent carboxy group
are achieved. Concomitantly, the a-carboxy group is acti-
vated and can be selectively derivatized. Furthermore, the
trifluoromethyl groups function as 19F NMR label and al-
The synthetic repertoire for incorporation of a-mercapto
acid derivatives into peptides is limited. Conventional
multistep syntheses via acylation of amino acid deriva-
tives by carboxy-activated S-protected mercapto acid de-
rivatives have been described.9
A more concise approach in which mercapto group pro-
tection and carboxy group activation as well as amide
bond formation and deblocking of the mercapto group are
SYNTHESIS 2004, No. 7, pp 1088–1092
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x
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Advanced online publication: 14.04.2004
DOI: 10.1055/s-2004-822341; Art ID: T00504SS
© Georg Thieme Verlag Stuttgart · New York
Scheme 2

PAPER
Incorporation of the a-Mercapto Acid Unit into Peptides
1089
low monitoring of transformations of compound 3
(Scheme 3).
Scheme 3
A carboxy group present in the side-chain remains unaf-
fected and can be transformed separately after activation.
On heating 2 in the presence of an excess of thionyl chlo-
ride the corresponding b-acid chloride 3 is formed in high
yield (86%). Compound 3 is a member of a preparative
useful new class of dielectrophiles,12 with two centers of
different reactivity. Under carefully controlled conditions
two consecutive acylation processes can be achieved in a
site-selective way. In the first step the acid chloride 3 re-
acts with equimolar amounts of N-nucleophiles, like pri-
mary, secondary amines and amino acid derivatives in the
presence of a tertiary base. Surprisingly, lactams and dike-
topiperazines can be N-acylated or N,N¢-diacylated with
equimolar amounts of 3 in excellent yields (87–100%) in
boiling toluene without the need of a tertiary base
(Scheme 4) while the lactone moiety acts as a protective
group. Compounds 4–11 are stable at room temperature
when stored on exclusion of water.
Scheme 5
The above-described synthetic sequences are well suited
for the generation of libraries of peptidomimetics
equipped with mercapto groups in any position of the
backbone. Remarkably, with the incorporation of the a-
mercapto unit the direction of the peptide chain is invert-
ed. On further applications of the mercapto modified pep-
tidomimetics like S-glycosylation, introduction of
lipophilic anchors and disulfide formation13 we will report
elsewhere. The present reaction may therefore offer con-
siderable potential for the construction of low molecular
weight libraries of metal complexing peptidomimetics.
Solvents were purified and dried prior to use. Reagents were used
as purchased. Mps were determined on a Boetius heating table. MS
were recorded on a VG-250 (Masslab) EI spectrometer (70 eV) or
by a VG ZAB-HSQ FAB spectrometer. 1H (200 MHz or 300 MHz),
13C (50 MHz or 75 MHz) and 19F NMR (188 MHz or 282 MHz)
spectra were recorded on a Varian Gemini 2000 or a Varian Gemini
300 spectrometer. TMS was used as reference standard for 1H and
13C NMR spectra (internal), and TFA for 19F NMR spectra (exter-
nal). Flash chromatography was performed by using silica gel (32–
63 m, ICN Biomedicals) with solvents systems given in the text.
[2,2-Bis(trifluoromethyl)-5-oxo-1,3-oxathiolan-4-yl]-acetyl
Chloride (3)
Compound 2 (25.0 g, 83.8 mmol) was heated with freshly distilled
thionyl chloride (20 mL) under reflux for 6 h. After removal of the
excess of thionyl chloride, the remaining liquid was distilled off in
vacuo.
Yield: 88% (24.4 g); slightly yellow liquid; mp ca. 15 °C; bp 80 °C/
4.5 torr.
Scheme 4
IR (film): 1800 (br), 1385 cm–1.
In a consecutive step the lactone moiety reacts as an acti-
vated ester. By this means, amino acid derivatives 11–14
were obtained (Scheme 5).
1H NMR (CDCl3): d = 3.47 (dd, J = 19.0 Hz, J = 10.0 Hz, 1 H), 3.86
(dd, J = 19.0 Hz, J = 4.0 Hz, 1 H), 4.55 (dd, J = 10.0 Hz, J = 4.0 Hz,
1 H).
13C NMR (CDCl3): d = 41.6, 49.8, 83.9 (sept, J = 35 Hz), 120.9 (q,
J = 284 Hz), 121.4 (q, J = 285 Hz), 169.1, 171.5.
Synthesis 2004, No. 7, 1088–1092 © Thieme Stuttgart · New York

1090
K. Pumpor et al.
PAPER
19F NMR (CDCl3): d = 1.60 (q, J = 9.0 Hz, 3 F), 2.40 (q, J = 9.0 Hz,
1H NMR (CDCl3): d = 1.85–1.91 (m, 4 H), 2.58–2.62 (m, 2 H), 3.50
(dd, J = 19.0 Hz, J = 11.0 Hz, 1 H), 3.74–3.78 (m, 2 H), 4.00 (dd, J
= 19.0 Hz, J = 3.0 Hz, 1 H), 4.50 (dd, J = 11.0 Hz, J = 3.0 Hz, 1 H).
13C NMR (CDCl3): d = 19.9, 22.1, 34.6, 42.0, 44.5, 44.8, 83.4 (sept,
J = 35 Hz), 120.8 (q, J = 283 Hz), 121.3 (q, J = 284 Hz), 170.4,
172.5, 173.2.
3 F).
For further data see lit.3
[2,2-Bis(trifluoromethyl)-5-oxo-1,3-oxathiolan-4-yl]-acetamide
(4)
A solution of 3 (3.17 g, 10.0 mmol) in anhyd Et2O (50 mL) was
cooled to –78 °C, then NH3 (gas) (0.34 g, 20.0 mmol) was added
with stirring. After warming to r.t., the precipitate was filtered off
and the organic layer was evaporated to dryness. The residue was
dissolved in CH2Cl2 (100 mL), washed with water (3 × 100 mL) and
dried (MgSO4). The solvent was removed in vacuo and the residue
was recrystallized from CHCl3.
19F NMR (CDCl3): d = 0.78 (q, J = 9.0 Hz, 3 F), 1.54 (q, J = 9.0 Hz,
3 F).
MS (EI): m/z (%) = 379 [M]+, 360 [M – F]+, 100 [C5H10NO]+.
Anal. Calcd for C12H11F6NO4S (379.27): C, 38.00; H, 2.92; N, 3.69.
Found: C, 38.14; H, 3.00; N, 3.72.
N,N¢-Di{[2,2-bis(trifluoromethyl)-5-oxo-1,3-oxathiolan-4-yl]-
acetyl}glycine Anhydride (7)
A solution of 3 (3.1 g, 10.0 mmol) and glycine anhydride (0.57 g,
5.0 mmol) in toluene (50 mL) was refluxed for 3 d. The solvent was
removed under reduced pressure to give product 7.
Yield: 52% (1.55 g); mp 78 °C.
IR (KBr): 3700–2600, 1800, 1690, 1620 cm–1.
1H NMR (CDCl3): d = 2.81 (dd, J = 17.0 Hz, J = 11.0 Hz, 1 H), 3.28
(dd, J = 17.0 Hz, J = 3.0 Hz, 1 H), 4.53 (dd, J = 11.0 Hz, J = 3.0 Hz,
1 H), 5.60 (br s, 1 H), 5.79 (br s, 1 H).
13C NMR (DMSO-d6): d = 37.7, 41.4, 90.2 (sept, J = 32 Hz), 120.8
(q, J = 283 Hz), 121.4 (q, J = 283 Hz), 170.0, 171.0.
Yield: 100% (3.37 g); mp 208 °C; mixture of diastereomers in ca.
1:1 ratio.
IR (KBr): 1810, 1720 cm–1.
1H NMR (DMSO-d6): d = 3.45, 3.46 (dd, J = 19.0 Hz, J = 9.5 Hz, 2
H), 3.97, 3.98 (dd, J = 19.0 Hz, J = 2.7 Hz, 2 H), 4.61–4.63 (m, 4
H), 5.18–5.21 (m, 2 H).
13C NMR (DMSO-d6): d = 41.1, 41.1, 41.7, 41.7, 47.2, 47.2, 82.3,
82.3 (sept, J = 35 Hz), 120.8, 120.8 (q, J = 283 Hz), 121.3, 121.3 (q,
J = 284 Hz), 166.5, 166.5, 169.9, 169.9, 170.6, 170.6.
19F NMR (CDCl3): d = 0.96 (q, J = 9.0 Hz, 3 F), 1.66 (q, J = 9.0 Hz,
3 F).
MS (EI): m/z (%) = 297 [M]+, 280 [M – NH3]+, 131 [M – HFA]+, 69
[CF3]+, 59 [H3CCONH2]+.
Anal. Calcd for C7H5F6NO3S (297.16): C, 28.29; H, 1.70; N, 4.71.
Found: C, 28.38; H, 1.78; N, 4.73.
19F NMR (DMSO-d6): d = 1.28, 1.28 (q, J = 9.3 Hz, 6 F), 2.36, 2.36
(q, J = 9.3 Hz, 6 F).
[2,2-Bis(trifluoromethyl)-5-oxo-1,3-oxathiolan-4-yl]-N,N-di-
benzylacetamide (5)
MS (EI): m/z (%) = 674 [M]+, 655 [M – F]+, 627 [655 – CO]+, 115
To a solution of 3 (3.17 g, 10.0 mmol) in anhyd Et2O (25 mL) was
added a solution of dibenzylamine (1.97 g, 10.0 mmol) and N-eth-
yldiisopropylamine (1.29 g, 10.0 mmol) in anhyd Et2O (25 mL) at
–78 °C. After 30 min, the solvent was removed in vacuo. The resi-
due was purified by column chromatography (CH2Cl2).
[C4H7N2O2]+, 69 [CF3]+.
Anal. Calcd for C18H10F12N2O8S2 (674.39): C, 32.06; H, 1.49; N,
4.05. Found: C, 32.10; H, 1.50; N, 4.24.
8a–c, 9 and 10; General Procedure
Yield: 67% (3.20 g); oil.
To a solution of 3 (3.17 g, 10.0 mmol) in anhyd CH2Cl2 (20 mL) at
–30 °C the solutions of amino acid ester hydrochloride (10.0 mmol)
and N-ethyldiisopropylamine (2.58 g, 20.0 mmol) in anhyd CH2Cl2
(20 mL) were simultaneously dropped. The reaction mixture was
stirred for 1 h keeping the temperature below –20 °C. After the re-
action was complete, the solvent was removed in vacuo, EtOAc and
sat. NaCl were added, and the layers were separated. The organic
phase was washed with citric acid (10% solution, 2 × 10 mL), sat.
NaHCO3 solution (2 × 10 mL) and water (5 × 10 mL). The organic
layer was dried (MgSO4) and filtered. The filtrate was concentrated
to dryness to give a crude product, which was purified by flash chro-
matography (EtOAc–CH2Cl2, 1:1).
IR (film): 1810, 1640 cm–1.
1H NMR (CDCl3): d = 2.93 (dd, J = 16.8 Hz, J = 11.5 Hz, 1 H), 3.49
(dd, J = 16.8 Hz, J = 3.2 Hz, 1 H), 4.43 (s, 2 H), 4.50 (d, J = 14.7
Hz, 1 H), 4.61 (dd, J = 11.5 Hz, J = 3.2 Hz, 1 H), 4.72 (d, J = 14.7
Hz, 1 H), 7.11 (s, 1 H), 7.15 (s, 1 H), 7.20 (s, 1 H), 7.23 (s, 1 H),
7.32 (s, 3 H), 7.36 (s, 2 H), 7.40 (s, 1 H).
13C NMR (CDCl3): d = 39.1, 43.0, 49.0, 50.0, 83.9 (sept, J = 35 Hz),
121.0 (q, J = 284 Hz), 121.5 (q, J = 284 Hz), 126.3, 128.0, 128.2,
128.5, 128.9, 129.4, 135.1, 136.4, 169.1, 171.0.
19F NMR (CDCl3): d = –0.39 (q, J = 9.0 Hz, 3 F), 0.19 (q, J = 9.0
Hz, 3 F).
MS (EI): m/z (%) = 477 [M]+, 386 [M – C7H7]+, 106 [C7H7NH]+, 91
N-[2,2-Bis(trifluoromethyl)-5-oxo-1,3-oxathiolan-4-yl]acetyl-L-
alanine Methyl Ester (8a)
Yield: 74% (2.84 g); oil; mixture of diastereomers, in ca. 1:1 ratio.
IR (film): 3375, 1820, 1745, 1645 cm–1.
[C7H7]+.
Anal. Calcd for C21H17F6NO3S (477.43): C, 52.83; H, 3.59; N, 2.93.
Found: C, 52.50; H, 3.62; N, 3.10.
1H NMR (CDCl3): d = 1.43, 1.43 (d, J = 7.5 Hz, 3 H), 2.79 (dd, J =
16.5 Hz, J = 1.0 Hz, 0.5 H), 2.81 (dd, J = 16.5 Hz, J = 11.0 Hz, 0.5
H), 3.24 (dd, J = 16.5 Hz, J = 3.5 Hz, 0.5 H), 3.27 (dd, J = 16.5 Hz,
J = 3.5 Hz, 0.5 H), 3.77, 3.77 (s, 3 H), 4.50 (dd, J = 11.0 Hz, J = 3.5
Hz, 0.5 H), 4.51 (dd, J = 11.0 Hz, J = 3.5 Hz, 0.5 H), 4.57 (dq, J =
7.0 Hz, J = 2.0 Hz, 0.5 H), 4.59 (dq, J = 7.0 Hz, J = 2.0 Hz, 0.5 H),
6.28–6.30, 6.28–6.30 (m, 1 H).
1-{[2,2-Bis(trifluoromethyl)-5-oxo-1,3-oxathiolan-4-yl]-acetyl}-
piperidin-2-one (6)
A solution of 3 (1.58 g, 5.0 mmol) and valerolactame (0.5 g, 5.0
mmol) was heated in toluene (20 mL) under reflux. The reaction
mixture was concentrated in vacuo until the product starts to crys-
tallize.
13C NMR (CDCl3): d = 18.5,18.5, 39.8, 39.8, 42.1, 42.1, 48.6, 48.6,
52.8, 52.8, 83.8, 83.8 (sept, J = 35 Hz), 120.9, 120.9 (q, J = 285 Hz),
121.5, 121.5 (q, J = 284 Hz), 167.7, 167.7, 170.8, 170.8, 173.2,
173.2.
Yield: 87% (1.65 g); mp 95 °C.
IR (KBr): 1825, 1715, 1670 cm–1.
Synthesis 2004, No. 7, 1088–1092 © Thieme Stuttgart · New York

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Incorporation of the a-Mercapto Acid Unit into Peptides
1091
19F NMR (CDCl3): d = 0.87, 0.87 (q, J = 9.0 Hz, 3 F), 1.65, 1.65 (q,
J = 9.5 Hz, 3 F)
MS (EI): m/z (%) = 383 [M]+, 364 [M – F]+, 351 [M – CH3OH]+,
324 [M – CH3OCO]+, 281 [M – CH3O2CCH(CH3)NH]+, 252 [M –
Ala-OMe, – CO]+, 102 [CH3O2CCH(CH3)NH]+, 87 [CH3O2C-
CHCH3]+, 69 [CF3]+, 59 [CH3O2C]+.
1H NMR (CDCl3): d = 1.46 (s, 4.5 H), 1.47 (s, 4.5 H), 1.90–2.23 (m,
3 H), 2.19–2.23 (m, 0.5 H), 2.16–2.27 (m, 0.5 H), 2.69 (dd, J = 16.5
Hz, J = 11.5 Hz, 0.5 H), 2.81 (dd, J = 17.0 Hz, J = 11.5 Hz, 0.5 H),
3.28 (dd, J = 16.5 Hz, J = 3 Hz, 0.5 H), 3.42 (dd, J = 17.0 Hz, J = 3
Hz, 0.5 H), 3.43–3.48 (m, 1 H), 3.58–3.61 (m, 1 H), 4.23 (dd, J =
8.0 Hz, J = 3.0 Hz, 0.5 H), 4.40 (dd, J = 8.5 Hz, J = 3.0 Hz, 0.5 H),
4.53 (dd, J = 11.5 Hz, J = 3.0 Hz, 0.5 H), 4.55 (dd, J = 11.5 Hz, J =
3.0 Hz, 0.5 H).
13C NMR (CDCl3): d = 22.5, 24.5, 27.8, 27.8, 29.3, 31.4, 39.5, 39.8,
42.2, 42.4, 46.9, 47.1, 59.8, 60.3, 83.1, 81.9, 83.8 (sept, J = 35 Hz),
121.0 (q, J = 285 Hz), 121.4 (q, J = 284 Hz), 167.0, 167.4, 170.5,
170.9, 171.1, 171.2.
Anal. Calcd for C11H11F6NO5S (383.27): C, 34.47; H, 2.89; N, 3.65.
Found: C, 34.30; H, 2.58; N, 3.58.
N-[2,2-Bis(trifluoromethyl)-5-oxo-1,3-oxathiolan-4-yl]-acetyl-
L-valine tert-Butyl Ester (8b)
Yield: 76% (3.45 g); mp 55 °C; mixture of diastereomers in ca. 1:1
ratio.
19F NMR (CDCl3): d = 0.84 (q, J = 9.0 Hz, 1.5 F), 0.88 (q, J = 9.0
Hz, 1.5 F), 1.36 (q, J = 9.0 Hz, 1.5 F), 1.45 (q, J = 9.0 Hz, 1.5 F).
IR (KBr): 3355, 1820, 1715, 1665 cm–1.
MS (EI): m/z (%) = 350 [M – CO2C(CH3)3]+, 70 [C4H8N]+, 57
1H NMR (CDCl3): d = 0.94, 0.94 (d, J = 7.0 Hz, 3 H), 0.97, 0.97 (d,
J = 7.0 Hz, 3 H), 1.52, 1.52 (s, 9 H), 2.17–2.22 (m, 1 H), 2.84 (dd,
J = 16.0 Hz, J = 11.5 Hz, 0.5 H), 2.88 (dd, J = 16.0 Hz, J = 11.0 Hz,
0.5 H), 3.31 (dd, J = 16.5 Hz, J = 3.5 Hz, 0.5 H), 3.34 (dd, J = 16.5
Hz, J = 3.5 Hz, 0.5 H), 4.47–4.51 (m, 1 H), 4.54–4.58 (m, 1 H), 6.33
(br s, 1 H).
13C NMR (CDCl3): d = 17.7, 17.7, 19.0, 19.0, 28.1, 28.1, 31.6, 31.7,
40.0, 40.0, 42.2, 42.2, 57.9, 58.0, 82.9, 82.9, 83.8, 83.8 (sept, J = 35
Hz), 121.0, 121.0 (q, J = 283 Hz), 121.5, 121.5 (q, J = 284 Hz),
167.9, 168.0, 170.7, 170.9, 171.0, 171.0.
19F NMR (CDCl3): d = 0.87, 0.87 (q, J = 9.0 Hz, 3 F), 1.65, 1.65 (q,
J = 9.0 Hz, 3 F).
MS (EI): m/z (%) = 453 [M]+, 352 [M – CO2C(CH3)3]+, 281
[C7H3F6O3S]+, 252 [M – Val-t-OBu, –CO]+, 183 [HSC(CF3)2]+, 113
[SCCF3]+, 57 [C(CH3)3]+.
[C(CH3)3]+.
Anal. Calcd for C16H19F6NO5S (451.39): C, 42.57; H, 4.24; N, 3.10.
Found: C, 42.51; H, 4.24; N, 3.18.
N-[2,2-Bis(trifluormethyl)-5-oxo-1,3-oxathiolan-4-yl]-acetyl-
sarcosine tert-Butyl Ester (10)
Yield: 69% (2.93 g); oil; mixture of conformers in ca. ratio 2:1.
IR (film): 1815, 1735, 1650 cm–1.
1H NMR (CDCl3): d = 1.40, 1.43 (s, 9 H), 2.81 (dd, J = 17.0 Hz, J
= 11.5 Hz, 0.66 H), 2.70 (dd, J = 16.5 Hz, J = 11.5 Hz, 0.33 H), 3.00,
2.93 (s, 3 H), 3.42 (dd, J = 17.0 Hz, J = 3.0 Hz, 0.66 H), 3.26 (dd, J
= 16.5 Hz, J = 3.0 Hz, 0.33 H), 3.36 (d, J = 17.0 Hz, 0.66 H), 3.38
(s, 0.66 H), 4.07 (d, J = 17.0 Hz, 0.66 H), 4.45, 4.18 (dd, J = 11.5
Hz, J = 3.0 Hz, 1 H).
13C NMR (CDCl3): d = 28.2, 28.1, 36.5, 35.4, 39.1, 38.9, 42.7, 42.9,
50.6, 52.4, 82.5, 83.6, 84.6 (sept, J = 35 Hz), 121.1 (q, J = 285 Hz),
122.4 (q, J = 285 Hz), 167.7, 167.2, 169.2, 169.2, 171.1, 171.1.
Anal. Calcd for C16H21F6NO5S (453.40): C, 42.39; H, 4.67; N, 3.09.
Found: C, 42.50; H, 4.13; N, 3.01.
N-[2,2-Bis(trifluoromethyl)-5-oxo-1,3-oxathiolan-4-yl]-acetyl-
L-phenylalanine tert-Butyl Ester (8c)
19F NMR (CDCl3): d = 0.90, 0.93 (q, J = 9.0 Hz, 3 F), 1.49, 1.43 (q,
J = 9.0 Hz, 3 F).
Yield: 67% (3.36 g); mp 89 °C; mixture of diastereomers, ratio ca.
1:1.
IR (KBr): 3340–3310, 2980, 1820, 1730, 1650 cm–1.
MS (EI): m/z (%) = 425 [M]+, 352 [M – OC(CH3)3]+, 324 [352 –
CO]+, 280 [M – Sar-t-BuO]+, 252 [280 – CO]+, 131 [C4H3O3S]+, 87
[C3H3OS]+, 57 [C(CH3)3]+.
1H NMR (CDCl3): d = 1.43 (s, 4.5 H), 1.43 (s, 4.5 H), 2.68 (dd, J =
16.0 Hz, J = 11.5 Hz, 0.5 H), 2.76 (dd, J = 16.0 Hz, J = 11.0 Hz, 0.5
H), 3.09–3.11 (m, 1 H), 3.09–3.11 (m, 1 H), 3.15 (dd, J = 16.0 Hz,
J = 3.5 Hz, 0.5 H), 3.22 (dd, J = 16.5 Hz, J = 3.5 Hz, 0.5 H), 4.47
(dd, J = 11.5 Hz, J = 3.5 Hz, 0.5 H), 4.50 (dd, J = 11.0 Hz, J = 3.5
Hz, 0.5 H), 4.70–4.76 (m, 0.5 H), 4.70–4.76 (m, 0.5 H), 6.35–6.39
(m, 1 H), 7.15–7.17 (m, 2 H), 7.30–7.34 (m, 3 H).
13C NMR (CDCl3): d = 28.0, 28.0, 37.9, 38.1, 39.6, 39.8, 42.1, 42.1,
54.0, 54.1, 83.1, 83.1, 83.8, 83.8 (sept, J = 35 Hz), 121.0, 121.0 (q,
J = 287 Hz), 121.4, 121.4 (q, J = 282 Hz), 127.3, 127.3, 128.6,
128.6, 129.5, 129.5, 135.7, 135.9, 167.6, 167.6, 170.6, 170.6, 170.7,
170.8.
Anal. Calcd for C14H17F6NO5S (425.34): C, 39.53; H, 4.03; N, 3.29.
Found: C, 38.10; H, 3.86; N, 2.78.
2-Mercaptosuccinic Acid 1-(l-Phenylalanine Amide) 4-(N,N-
Dibenzylamide) (11)
Under a nitrogen atmosphere, a solution of 5 (0.95 g, 2.0 mmol) in
MeCN (15 mL) was reacted at r.t. with S-phenylalanine amide (0.39
g, 2.4 mmol). After a few hours the product began to crystallize. Af-
ter completion of the reaction (19F NMR analysis) the precipitate
was filtered off, washed with Et2O (10 mL) and dried in vacuo.
Yield: 94% (0.89 g); mp 198–200 °C, mixture of diastereomers in
ca. 1:1 ratio.
19F NMR (CDCl3): d = 0.90 (q, J = 9.0 Hz, 1.5 F), 0.90 (q, J = 9.0
Hz, 1.5 F), 1.59 (q, J = 9.0 Hz, 1.5 F), 1.66 (q, J = 9.0 Hz, 1.5 F).
IR (KBr): 3565–3235, 1675, 1655, 1625 cm–1.
1H NMR (acetone-d6): d = 2.24, 2.24 (d, J = 8.0 Hz, 1 H), 2.35–2.44,
2.35–2.44 (m, 2 H), 2.60, 2.60 (dd, J = 16.5 Hz, J = 9.5 Hz, 1 H),
2.78, 2.78 (dd, J = 14.0 Hz, J = 4.0 Hz, 1 H), 3.42–3.45, 3.42–3.45
(m, 1 H), 3.91, 3.91 (d, J = 15.0 Hz, 1 H), 3.95–4.00, 3.95–4.00 (m,
1 H), 4.02, 4.02 (d, J = 17.0 Hz, 1 H), 4.15, 4.15 (d, J = 15.0 Hz, 1
H), 4.18, 4.18 (d, J = 17.0 Hz, 1 H), 6.74–6.99, 6.74–6.99 (m, 17 H),
8.07, 8.07 (d, J = 8.0 Hz, 1 H).
13C NMR (acetone-d6): d = 36.8, 36.9, 38.7, 48.0, 49.7, 53.9, 126.2,
126.6, 127.1, 127.4, 127.7, 128.1, 128.4, 128.7, 129.2, 136.7, 137.2,
138.2, 170.9, 172.0, 173.0.
MS (EI): m/z (%) = 501 [M]+, 400 [M – t-BuO2C]+, 252 [M – Phe-
t-BuO – CO]+, 183 [252 – CF3]+, 69 [CF3]+, 57 [(CH3)3C]+.
Anal. Calcd for C20H21F6NO5S (501.45): C, 47.91; H, 4.22; N, 2.79.
Found: C, 47.70; H, 4.20; N, 2.74.
(S)-N-[2,2-Bis(trifluormethyl)-5-oxo-1,3-oxathiolan-4-yl]-
acetyl-L-proline tert-Butyl Ester (9)
Yield: 40% (1.80 g); mp 136 °C; mixture of diastereomers in ca. 1:1
ratio.
IR (KBr): 3000, 1810, 1735, 1645 cm–1.
MS (EI): m/z (%) = 475 [M]+, 458 [M – NH3]+, 442 [M – SH]+, 425
[442 – NH3]+, 384 [M – C7H7]+, 367 [384 – NH3]+, 312 [M –
Synthesis 2004, No. 7, 1088–1092 © Thieme Stuttgart · New York

1092
K. Pumpor et al.
PAPER
NHCH(CH2Ph)CONH2]+, 278 [M
[(PhCH2)2N]+, 91 [C7H7]+, 55 [CH2CHCO]+.
–
(PhCH2)2NH]+, 196
2-Mercaptosuccinic Acid 1-(L-Proline Benzyl Ester) 4-(L-Valine
tert-Butyl Ester) (14)
Yield: 22% (0.54 g); oil; mixture of diastereomers.
IR (film): 3430–3130, 2970, 1730, 1635 cm–1.
Anal. Calcd for C27H29N3O3S (475.61): C, 68.19; H, 6.15; N, 8.84.
Found: C, 67.90; H, 6.40; N, 8.57.
1H NMR (CDCl3): d = 0.87, 0.87 (d, J = 6.5 Hz, 6 H), 1.46, 1.46 (s,
9 H), 2.02–2.23 (m, 6 H), 2.68, 2.68 (dd, J = 14.5 Hz, J = 4.5 Hz, 1
H), 3.04, 3.04 (dd, J = 14.5 Hz, J = 10.0 Hz, 1 H), 3.60–3.65 (m, 1
H), 3.76–3.94 (m, 2 H), 4.34, 4.34 (dd, J = 9.0 Hz, J = 5.0 Hz, 1 H),
4.49–4.54 (m, 1 H), 5.09, 5.09 (d, J = 12.0 Hz, 1 H), 5.23, 5.23 (d,
J = 12.0 Hz, 1 H), 6.19, 6.19 (d, J = 9.0 Hz, 1 H), 7.34, 7.34 (s, 5 H).
13C NMR (CDCl3): d = 17.8, 19.0, 24.9, 28.3, 29.3, 31.6, 35.8, 43.1,
47.3, 57.8, 59.4, 67.3, 82.4, 128.6, 128.8, 129.0, 135.8, 170.3,
170.8, 171.2, 171.4.
2-Mercaptosuccinic Acid 1-(L-Phenylalanine Methyl Ester)
4-(2-Oxopiperidine) (12)
Under a nitrogen atmosphere, to a solution of 9 (0.76 g, 2.0 mmol)
in anhyd Et2O (10 mL), was added dropwise a solution of L-phenyl-
alanine methyl ester (0.54 g, 3.0 mmol) in anhyd Et2O (5 mL). The
reaction was stirred for 4 h at r.t., and then the precipitate was fil-
tered off, dissolved in EtOAc, and the solvent was evaporated in
vacuo.
Yield: 52% (0.41 g); mp 120 °C; mixture of diastereomers.
IR (KBr): 3350, 2970, 1740, 1700, 1650, 1545 cm–1.
MS (ESI): m/z (%) = 1007 [2 × M + Na]+, 986 [2 × M + H]+, 493
[M + H]+.
1H NMR (DMSO-d6): d = 1.70–1.73 (m, 2 H), 1.70–1.73 (m, 2 H),
2.47–2.51 (m, 1 H), 2.47–2.51 (m, 1 H), 2.90–3.06 (m, 2 H), 3.14,
3.14 (dd, J = 18.0 Hz, J = 8.5 Hz, 2 H), 3.35–3.37 (m, 0.5 H), 3.35–
3.37 (m, 0.5 H), 3.55–3.58 (m, 1 H), 3.55–3.58 (m, 1 H), 3.58 (s, 1.5
H), 3.60 (s, 1.5 H), 3.81–3.83 (m, 0.5 H), 3.81–3.83 (m, 0.5 H),
4.45–4.48 (m, 0.5 H), 4.45–4.48 (m, 0.5 H), 7.22–7.26 (m, 5 H),
8.55 (d, J = 8.0 Hz, 0.5 H), 8.57 (d, J = 8.0 Hz, 0.5 H).
13C NMR (DMSO-d6): d = 19.4, 19.4, 21.6, 21.6, 34.2, 34.2, 36.3,
36.3, 36.7, 36.8, 43.3, 43.4, 44.8, 44.9, 51.7, 51.7, 53.6, 53.6, 126.5,
126.5, 128.1, 128.1, 129.0, 129.1, 136.9, 136.9, 171.5, 171.5, 171.6,
171.6, 173.1, 173.1, 173.2, 173.2.
Anal. Calcd for C25H36N2O6S (492.63): C, 60.95; H, 7.37; N, 5.69.
Found: C, 61.17; H, 7.43; N, 5.48.
Acknowledgment
We are grateful to Stiftung Volkswagenwerk, Hannover, for finan-
cial support.
References
(1) (a) Soth, M. J.; Nowick, J. S. J. Org. Chem. 1999, 64, 276.
(b) Recent Advances in Peptidomimetics, Tetrahedron
Symposium in Print No 83, Vol. 56; Aubé, J., Ed.; Elsevier:
Amsterdam, 2000, 9725; and references cited therein.
(2) Kraus, G. A.; Bae, J.; Choudhury, P. K. Synthesis 2003, 19.
(3) Pumpor, K.; Windeisen, E.; Burger, K. J. Heterocycl. Chem.
2003, 40, 435.
(4) Kleemann, A.; Engel, J. Pharmazeutische Chemie,
Synthesen-Patente-Anwendungen; Georg Thieme: Stuttgart,
1987, 1104.
(5) (a) Robl, J. A. Bristol-Meyers-Squibb, US Patent, 5508272,
1996; Chem. Abstr. 1996, 125, 33695h. (b) Karanewsky, D.
S.; Robl, J. A. Bristol-Meyers-Squibb, US Patent, 5552397,
1996; Chem. Abstr. 1996, 125, 328738a. (c) Zhu, J.; You,
L.; Zhao, S. X.; White, B.; Chen, J. G.; Skonezny, P. M.
Tetrahedron Lett. 2002, 43, 7585.
MS (EI): m/z (%) = 392 [M]+, 293 [M – C5H9NO]+, 99 [C5H9NO]+,
91 [C7H7]+.
Anal. Calcd for C19H24N2O5S (392.47): C, 58.15; H, 6.16; N, 7.14.
Found: C, 57.96; H, 6.03; N, 7.10.
13 and 14; General Procedure
Under a nitrogen atmosphere, a solution of the hexafluoroacetone-
protected isopeptide (5.0 mmol), the amino acid ester hydrochloride
(6.0 mmol) and N-ethyldiisopropylamine (6.0 mmol) in MeCN (15
mL) was stirred at r.t. After the reaction was complete, the solvent
was evaporated in vacuo, and the residue was purified by flash chro-
matography (EtOAC–CH2Cl2, 1:5).
2-Mercaptosuccinic Acid 1-(L-Alanine tert-Butyl Ester) 4-(L-
Valine tert-Butyl Ester) (13)
Yield: 51% (1.10 g); mp 49–51 °C; mixture of diastereomers, in ca.
3:1 ratio.
IR (KBr): 3355, 3290, 1735, 1675, 1645, 1530 cm–1.
(6) (a) Baidoo, K. E.; Lever, S. Z. Bioconjugate Chem. 1990, 1,
132. (b) Bryson, N.; Dewan, J. C.; Lister-James, J.; Jones, A.
G.; Davison, A. Inorg. Chem. 1988, 27, 2154.
(7) Fray, M. J.; Ellis, D. Tetrahedron 1998, 54, 13825.
(8) (a) Robl, J. A.; Sun, C.-Q.; Stevenson, J.; Ryono, D. E.;
Simpkins, L. M. J. Med. Chem. 1997, 40, 1570. (b) Ashton,
W. T.; Cantone, C. L.; Tolman, R. L.; Greenlee, W. J.;
Lynch, R. J.; Schorn, T. W.; Stroese, J. F.; Siegl, P. K. S. J.
Med. Chem. 1992, 35, 2772.
(9) Bell, R. A.; Dickson, K. C.; Valliant, J. F. Can. J. Chem.
1999, 77, 146.
(10) Pires, R.; Spengler, J.; Schedel, H.; Burger, K. Amino Acids
1997, 13, 73.
1H NMR (CDCl3): d = 0.90 (d, J = 7.0 Hz, 1.5 H), 0.89 (d, J = 7.0
Hz, 1.5 H), 0.93 (d, J = 7.0 Hz, 1.5 H), 0.91 (d, J = 7.0 Hz, 1.5 H),
1.39 (d, J = 7.0 Hz; 1.5), 1.36 (d, J = 7.0 Hz, 1.5 H), 1.46 (s, 4.5 H),
1.46 (s, 4.5 H), 1.47 (s, 4.5 H), 1.47 (s, 4.5 H), 2.10–2.17 (m, 0.5 H),
2.10–2.17 (m, 0.5 H), 2.43 (d, J = 10.0 Hz, 0.5 H), 2.38 (d, J = 10.0
Hz, 0.5 H), 2.76 (dd, J = 15.0 Hz, J = 6.5 Hz, 0.75 H), 2.78 (dd, J =
15.0 Hz, J = 5.5 Hz, 0.25 H), 2.97 (dd, J = 15.0 Hz, J = 6.0 Hz, 0.75
H), 2.95 (dd, J = 15.0 Hz, J = 7.5 Hz, 0.25 H), 3.67–3.78 (m, 0.5 H),
3.67–3.78 (m, 0.5 H), 4.36–4.45 (m, 1 H), 4.36–4.45 (m, 1 H), 6.37
(d, J = 8.5 Hz, 0.75 H)/6.33 (d, J = 9.0 Hz, 0.25 H), 7.15 (d, J = 6.5
Hz, 0.75 H), 7.05 (d, J = 7.0 Hz, 0.25 H).
(11) (a) Coppola, G. M.; Schuster, H. F. a-Hydroxy Acids in
Enantioselective Synthesis; VCH: Weinheim, 1997, ; and
references cited therein. (b) Burger, K.; Windeisen, E.;
Heistracher, E.; Lange, T.; Abdel Aleem, A. A. H. Monatsh.
Chem. 2002, 133, 41.
13C NMR (CDCl3): d = 17.8, 17.9, 18.3, 18.5, 19.1, 19.1, 28.2, 28.2,
28.2, 28.2, 31.5, 31.5, 38.9, 39.4, 42.4, 42.5, 49.5, 49.5, 57.8, 57.9,
82.3, 82.3, 169.8, 169.8, 169.9, 169.9, 171.0, 171.0, 171.2, 171.2.
(12) Böttcher, C.; Spengler, J.; Essawy, S. A.; Burger, K.
Monatsh. Chem. 2004, in print.
(13) Radics, G.; Böttcher, C.; Spengler, J.; Burger, K.,
unpublished results.
MS (FAB): m/z (%) = 455 [M + Na]+, 433 [M + H]+.
Anal. Calcd for C20H36N2O6S (432.58): C, 55.53; H, 8.39; N, 6.48.
Found: C, 55.10; H, 8.15; N, 6.09.
Synthesis 2004, No. 7, 1088–1092 © Thieme Stuttgart · New York