Synthesis of protected dithiol amino acids for potential use in peptide chemistry

Article abstract of DOI:10.1080/00397910802136631

Two N-Fmoc and S,S′-protected α-amino acids were synthesized by introducing the derivatives of dithioether into a Tyrosine side chain under the Mitsunobu protocol using the diisopropylazodicarboxylate/triphenylphosphine (DIAD/TPP) system. These amino acids provide a potential use in peptide chemistry. Copyright Taylor & Francis Group, LLC.

Full text of DOI:10.1080/00397910802136631

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Synthesis of Protected Dithiol
Amino Acids for Potential Use
in Peptide Chemistry
Yong Sun a , Xiang-Qin Li a , Yun-Ping Meng b & Chao
Wang a
a Department of Medicinal Chemistry , School
of Pharmaceutical Sciences, Peking University ,
Beijing, China
b Biochemical Engineering College of Beijing Union
University , Beijing, China
Published online: 12 Sep 2008.
To cite this article: Yong Sun , Xiang-Qin Li , Yun-Ping Meng & Chao Wang (2008)
Synthesis of Protected Dithiol Amino Acids for Potential Use in Peptide Chemistry,
Synthetic Communications: An International Journal for Rapid Communication of
Synthetic Organic Chemistry, 38:19, 3303-3310, DOI: 10.1080/00397910802136631
To link to this article: http://dx.doi.org/10.1080/00397910802136631
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Synthetic Communications1, 38: 3303–3310, 2008
Copyright # Taylor & Francis Group, LLC
ISSN: 0039-7911 print/1532-2432 online
DOI: 10.1080/00397910802136631
Synthesis of Protected Dithiol Amino Acids for
Potential Use in Peptide Chemistry
Yong Sun,1 Xiang-Qin Li,1 Yun-Ping Meng,2 and Chao Wang1
1Department of Medicinal Chemistry, School of Pharmaceutical Sciences,
Peking University, Beijing, China
2Biochemical Engineering College of Beijing Union University,
Beijing, China
Abstract: Two N-Fmoc and S,S0-protected a-amino acids were synthesized by
introducing the derivatives of dithioether into a Tyrosine side chain under the Mitsu-
nobu protocol using the diisopropylazodicarboxylate=triphenylphosphine (DIAD=
TPP) system. These amino acids provide a potential use in peptide chemistry.
Keywords: Amino acid, 1,3-dithiols, Mitsunobu reaction
Amino acids are the constituents of both peptides and proteins. Design
and synthesis of specific functional amino acids is an important task
for peptide chemists. The compounds containing 1,3-dithiols, such as
dihydrolipoic acid, have particular activity as antioxidants and free-
radical scavengers and efficient metal-ion coordinating properties.[1–4]
In the present article, the structures of protected dithiols were introduced
into the side chain of Tyrosine to generate a kind of novel amino acid
that can be applied in peptide synthesis. The final aim of our research
is to synthesize a series of bioactive peptides that contain the key struc-
tural feature of multithiols in one amino acid residue, and that residue
can be incorporated into a different position of a peptide sequence by
different synthetic programmers to coordinate with heavy metals, such
as [99mTc] to achieve the bioactive peptide labeling.[5]
For the purpose mentioned, 3,30-(diacetamidomethyl-dithioether)-
isobutanol (5) and 2,2-dimethyl-1,3-dithiane-5-methanol (6) as key
Received December 22, 2007.
Address correspondence to Chao Wang, Department of Medicinal Chemistry,
School of Pharmaceutical Sciences, Peking University, Beijing 100083, China.
E-mail: chaowang@bjmu.edu.cn
3303

3304
Y. Sun et al.
intermediates were synthesized first using diethyl malonate as the starting
material describing in Scheme 1.
Diethyl bis(hydroxymethyl) malonate (1),[6] 3,30-dibromo-isobutyric
acid (2),[7] and 3,30-dibromo-isobutanol (3)[8] in Scheme 1 are known sub-
stances that have been published in the literature. 3,30-Dimercapto-
isobutanol (4), 5, and 6 were prepared by modifying the procedures in
the literature.[9–11] Compound 4 was obtained as an oil and directly used
for the next reaction without further purification. In the purifications of 5
or 6, silica-gel column chromatography (CHCl3:CH3OH:AcOH
20:1:0.4) was used.
It must be emphasized that the analogs of compound 6 have been
synthesized by Bertini et al.[12] from 1,3-dithian-5-one through a Wittg
reaction followed by hydrolysis and reduction with NaBH4, and the
intermediate 1,3-dithian-5-one had to be prepared using mercaptoacetic
acid as starting material to undergo a five-step reaction.[13] As compared
with the reacted process mentioned previously, the synthetic route in our
experiment should be cheaper and more convenient than that published
in the literature.
The desired compounds, N-Fmoc-O-((S,S0-diacetamidomethyl)-3,30-
dithioether)-isobutyl-L-Tyrosine (8) and N-Fmoc-O-((S,S0-isopropylidene)
-3,30-dithioether)-isobutyl-L-Tyrosine (11), were synthesized from protected
Tyrosine derivatives with compound 5 or 6 as described in Scheme 2.
The alkylation of phenols with alcohols effected by the Mitsunobu
reaction using the diisopropylazodicarboxylate=triphenylphosphine
Scheme 1. Reagents and conditions: (a) HCHO, Et3N, 78%; (b) HBr=H2O, D,
50%; (c) BH3ꢀTHF, 98%; (d) NaHS=CH3OH, 98%; (e) HOCH2NCOCH3,
50%; (f) CH3COCH3, BF3, 60%. Acm ¼ acetamidomethyl.

Protected Dithiol Amino Acids
3305
Scheme 2. Reagents and conditions: (a) DIAD=TPP, 50%; (b) TFA, 0ꢁC, 50%; (c)
DIAD=TPP, 71%; (d) NaOH=H2O, 52%; (e) HCl=AcOEt, 90%; (f) Fmoc-Osu,
Na2CO3, 76%. Fmoc ¼ 9-fluorenylmethoxycarbonyl; Boc ¼ tert-butoxycarbonyl.
(DIAD=TPP) system is well documented in the literature.[14] To increase
the convertibility of the alcohol 5 or 6 in our experiment, we treated 5 or
6 with two- to three-fold excess of the phenol component, TPP, and
DIAD in tetrohydrofuran (THF), and meanwhile prolonged the reaction
time to 36 h at ambient temperature, affording 9 in 71% yield and 7 in
only 50%. This result can be explained because the effect of steric hin-
drance in the substitution reaction for open-chain compound 5 is larger
than that of cyclic compound 6. Other reactions mentioned in Scheme
2 are common protection and deprotection in peptide chemistry related
to either the amino group or carboxyl group, as described in references.

3306
Y. Sun et al.
Two Fmoc-amino acids with the structures of protected dithiol
moieties in side chains were synthesized as described here. These a-amino
acids can be used in solution peptide synthesis or solid-phase peptide syn-
thesis, and the blocks of the sulfhydryl group can be removed by deprotec-
tion procedures to be commonly used in peptide chemistry via disulfide
bond formation, which can be easily reduced to regenerate the 1,3-dithiol
compounds. The details of the use for these amino acids in peptide
synthesis in Fmoc strategy will be reported later in other articles. In con-
clusion, the incorporation of these novel residues into bioactive peptides
should represent a new approach and application in peptide chemistry.
EXPERIMENTAL
Melting points were measured using a microscope hot-stage apparatus
and are uncorrected. Spectroscopic data were obtained on the instru-
1
ments listed: H NMR (VXR-300); MS (ZQ-2000). Elemental analyses
were performed on a PE-2400 elemental analyzer. All reactions were
monitored by thin-layer chromatography (TLC) using precoated plates
of silica gel 60 F 254 (Merck).
3,30-(Diacetamidomethyl-dithioether)-isobutanol (5)
It was prepared using the modified procedures in the literature[10] to yield
the title compound (50%) as a colorless oil. ESI-MS: m=z 281 (M þ H)þ,
1
303 (M þ Na)þ, 319 (M þ K)þ; H NMR (500 MHz, DMSO-d6): d 1.693
(m, 1H), 1.846 (s, 6H), 2.612 (m, 4H), 3.414 (br, s, 1H), 3.463 (d, 2H),
4.918 (m, 4H), 8.595 (t, 2H).
2,2-Dimethyl-1,3-dithiane-5-methanol (6)
It was prepared using the modified procedures in the literature[11] to yield
the title compound (60%) as a white crystal, mp 68–69 ꢁC. ESI-MS: m=z
1
179 (M þ H)þ, 201 (M þ Na)þ; H NMR (300 MHz, CDCl3): d 1.400 (t,
1H, J ¼ 11.1 Hz), 1.648 (s, 3H), 1.715 (s, 3H), 1.975 (m, 1H), 2.775 (dd,
2H, J1 ¼ 8.7 Hz, J2 ¼ 14.4 Hz), 2.894 (dd, 2H, J1 ¼ 3.0 Hz, J2 ¼ 14.4 Hz),
3.720 (t, 2H, J ¼ 12.0 Hz).
N-Fmoc-O-((S,S0-diacetamidomethyl)-3,30-dithioether)-isobutyl-L-
Tyrosine tert-Butyl Ester (7)
A solution of compound 5 (1.40 g, 5.0 mmol), Fmoc-Tyr-OtBu (4.60 g,
10.0 mmol), and PPh3 (3.93 g, 1.5 mmol) in 10 ml of THF was stirred

Protected Dithiol Amino Acids
3307
under argon at 0 ꢁC. DIAD (3.53 g, 17.5 mmol) was added dropwise to
the reaction mixture. The cooling bath was removed, and the mixture
was stirred at room temperature for 36 h. The solution was evaporated
in vacuo, and the residue was purified by silica-gel column chromato-
graphy (CHCl3–CH3OH–AcOH 20:1:0.4) to yield 1.80 g (50%) of the
title compound as an oil. ESI-MS: m=z 721 Mþ, 722 (M þ H)þ, 744
(M þ Na)þ, 760 (M þ K)þ, 501 (M-Fmoc)þ.
N-Fmoc-O-((S,S0-diacetamidomethyl)-3,30-dithioether)
-isobutyl-L-Tyrosine (8)
Compound 7 (1.07 g, 1.5 mmol) was dissolved in 30 ml of TFA=
DCM=DMS (1:1:0.2) at 0 ꢁC, and the solution was stirred under argon
at 0 ꢁC for 2 h. The solution was evaporated in vacuo to dryness, and the
residue was purified by silica-gel column chromatography (CHCl3–
CH3OH–AcOH 20:1:0.4), and the obtained colorless powder was tritu-
rated with petroleum ether to give 0.50 g (50%) of the title compound,
mp 74–75 ꢁC; [a]D þ 26 ꢁ (c 1.0, CHCl3); ESI-MS: m=z 666 (M þ H)þ,
688 (M þ Na)þ, 704 (M þ K)þ, 664 (M ꢂ H)ꢂ; 1H NMR (300 MHz,
CDCl3): d 1.985 (s, 6H), 2.240 (m, 1H), 2.683 (d, 4H, J ¼ 6.0 Hz), 3.086
(m, 2H), 4.039 (m, 2H), 4.168 (d, 4H, J ¼ 6.6 Hz), 4.330 (t, 1H, J ¼ 6.9 Hz),
4.415–4.513 (m, 2H), 4.627 (m, 1H), 5.482 (d, 1H, J ¼ 7.5 Hz), 6.276 (br s,
1H), 6.780 (d, 2H, J ¼ 7.8 Hz), 6.912 (t, 2H, J ¼ 6.6 Hz), 7.030 (d, 2H,
J ¼ 7.8 Hz), 7.292 (t, 2H, J ¼ 7.2 Hz), 7.384 (t, 2H, J ¼ 7.2 Hz), 7.549 (m,
2H), 7.738 (d, 2H). Anal. calcd. for C34H39N3O7S2ꢀ0.5H2O: C, 60.51; H,
5.97; N, 6.23. Found: C, 60.27; H, 6.18; N, 6.01.
N-Boc-O-((S,S0-isopropylidene)-3,30-dithioether)-isobutyl-L-Tyrosine
Methyl Ester (9)
A solution of compound 6 (3.10 g, 17.4 mmol), Boc-Tyr-OMe (7.20 g,
24.4 mmol), and PPh3 (8.31 g, 31.7 mmol) in 15 ml of THF was stirred
under argon at 0 ꢁC. DIAD (8.33 g, 41.2 mmol) was added dropwise to
the reaction mixture. The cooling bath was removed, and the mixture
was further stirred at room temperature for 36 h. The solution was eva-
porated in vacuo, and the residue was purified by silica-gel column chro-
matography (AcOEt–petroleum ether 1:2) to yield 5.60 g (71%) of the
title compound as an oil. ESI-MS: m=z 456 (M þ H)þ, 478 (M þ Na)þ,
494 (M þ K)þ, 356 (M-Boc)þ; 1H NMR (500 MHz, CDCl3) d 1.450
(s, 9H), 1.725 (s, 3H), 1.763 (s, 3H), 2.338 (m, 1H), 2.926 (dd, 2H,
J1 ¼ 8.0 Hz, J2 ¼ 14.5 Hz), 3.038 (dd, 2H, J1 ¼ 3.5 Hz, J2 ¼ 14.5 Hz),
3.065 (m, 1H), 3.115 (m, 1H), 3.741 (s, 3H), 4.007 (d, 2H, J ¼ 6.5 Hz),

3308
Y. Sun et al.
4.566 (m, 1H), 4.976 (d, 1H, J ¼ 7.5 Hz), 6.864 (d, 2H, J ¼ 8.0 Hz), 7.056
(d, 2H, J ¼ 8.0 Hz).
N-Boc-O-((S,S0-isopropylidene)-3,30-dithioether)
-isobutyl-L-Tyrosine (10)
A solution of compound 9 (5.00 g, 1.1 mmol) in methanol (40 ml) was
surrounded by a ice-water bath, and 2 N aqueous NaOH (1.1 ml) was
added with stirring. The mixture was stirred at room temperature for
3 h. The methanol was removed in vacuo, and a dilute solution of KHSO4
was added to change the pH to 1–2. The aqueous phase was extracted
with ethyl acetate (3 ꢃ 50 ml). The combined organic layers were washed
twice with water and dried over anhydrous Na2SO4. After elimination of
the solvent at reduced pressure, the residue was purified by silica-gel col-
umn chromatography (AcOEt–petroleum ether–AcOH 1:3:0.12), and the
obtained colorless powder was triturated with petroleum ether to give
2.50 g (52%) of the title compound, mp 56–57 ꢁC; [a]D þ 43 ꢁ (c 1.0,
CHCl3); ESI-MS: m=z 442 (M þ H)þ, 464 (M þ Na)þ, 480 (M þ K)þ,
342 (M-Boc)þ; 1H NMR (300 MHz, CDCl3): d 1.443 (s, 9H), 1.715
(s, 3H), 1.751 (s, 3H), 2.328 (m, 1H), 2.991 (dd, 2H, J1 ¼ 8.1 Hz,
J2 ¼ 14.4 Hz), 3.030 (dd, 2H, J1 ¼ 3.0 Hz, J2 ¼ 14.4 Hz), 3.070 (m, 1H),
3.120 (m, 1H), 4.070 (d, 2H, J ¼ 6.3 Hz), 4.573 (m, 1H), 4.951 (d, 1H,
J ¼ 6.9 Hz), 6.870 (d, 2H, J ¼ 8.7 Hz), 7.115 (d, 2H, J ¼ 8.7 Hz).
N-Fmoc-O-((S,S0-isopropylidene)-3,30-dithioether)
-isobutyl-L-Tyrosine (11)
Compound 10 (3.20 g, 7.2 mmol) was dissoved in 40 ml of HCl=AcOEt
(4 N), and the solution was stirred at room temperature for 2 h. After
elimination of the solvent at reduced pressure, the residue was triturated
with ethyl acetate to obtain a white powder. The solid product was
dissolved in a solution of Na2CO3 (0.83 g, 7.8 mmol) in water (30 ml).
Dioxane (25 ml) was added, and the mixture was stirred in an ice-water
bath. Fmoc-OSu (2.43 g, 7.2 mmol) was added in portions and stirring
was continued at 0 ꢁC for 3 h and then at room temperature for 12 h. After
elimination of the solvent at reduced pressure, the residue was redissolved
in a 10% solution of Na2CO3 in water, extracted with ethyl ether
(2 ꢃ 40 ml). The aqueous phase was acidified at 0 ꢁC with a dilute solution
of KHSO4 to pH 1–2 and extracted with ethyl acetate (3 ꢃ 50 ml), and the
combined organic layers were dried over anhydrous Na2SO4. After elimin-
ation of the solvent at reduced pressure, the residue was purified by silica-
gel column chromatography (AcOEt–petroleum ether–AcOH 1:2:0.08),

Protected Dithiol Amino Acids
3309
and the obtained colorless powder was triturated with petroleum ether to
give 2.78 g (69%) of the title compound, mp 80–81 ꢁC; [a]D þ 29 ꢁ (c 1.0,
CHCl3); ESI-MS: m=z 564 (M þ H)þ, 586 (M þ Na)þ, 602 (M þ K)þ,
1
342 (M-Fmoc)þ; H NMR (500 MHz, CDCl3): d 1.727 (s, 3H), 1.758
(s, 3H), 2.316 (m, 1H), 2.895 (dd, 2H, J1 ¼ 8.0 Hz, J2 ¼ 14.0 Hz), 3.009
(dd, 2H, J1 ¼ 3.5 Hz, J2 ¼ 14.0 Hz), 3.095 (dd, 1H, J1 ¼ 6.0 Hz,
J2 ¼ 13.5 Hz), 3.192 (dd, 1H, J1 ¼ 5.0 Hz, J2 ¼ 13.5 Hz), 4.034 (d, 2H,
J ¼ 6.0 Hz), 4.232 (t, 1H, J ¼ 7.0 Hz), 4.380 (dd, 1H, J ¼ 10.5 Hz,
J ¼ 7.0 Hz), 4.484 (dd, 1H, J ¼ 10 Hz, J ¼ 7.0 Hz), 4.705 (m, 1H), 5.239
(d, 1H, J ¼ 8.0 Hz), 6.855 (d, 2H, J ¼ 8.0 Hz), 7.086 (d, 2H, J ¼ 8.0 Hz),
7.336 (t, 2H, J ¼ 7.5 Hz), 7.432 (t, 2H, J ¼ 7.5 Hz), 7.580 (m, 2H), 7.796
(d, 2H, J ¼ 7.5 Hz). Anal. calcd. for C34H39N3O7S2ꢀ0.5H2O: C, 65.01; H,
5.98; N, 2.45. Found: C, 64.98; H, 6.29; N, 2.19.
REFERENCES
1. Packer, L.; Witt, E. H.; Tritschler, H. J. Alpha-lipoic acid as a biological anti-
oxidant. Free Radical Biol. Med. 1995, 19, 227–250.
2. Lodge, J. K.; Traber, M. G.; Packer, L. Thiol chelation of Cu2 by dihydro-
lipoic acid prevents human low density lipoprotein peroxidation. Free Rad-
ical Biol. Med. 1998, 25, 287–297.
3. Hacnen, G. R. M. M.; Bast, A. Scavenging of hypochlorous acid by lipoic
acid. Biochem. Pharmacol. 1991, 42, 2244–2246.
4. Ho, Y. S.; Lai, C. S.; Liu, H. I.; Ho, S. Y.; Tai, C.; Pan, M. H.; Wang, Y. J.
Dihydrolipoic acid inhibits skin tumor promotion through anti-inflammation
and anti-oxidation. Biochem. Pharmacol. 2007, 73, 1786–1795.
5. Liu, S.; Edwards, D. S. 99mTc-labeled small peptides as diagnostic radiophar-
maceuticals. Chem. Rev. 1999, 99, 2235–2268.
6. Andrei, G.; Harri, L. bis(Hydroxymethylation) of the active methylene group
of 1,3-dicarbonyl and related compounds. Synthesis 1997, 11, 1281–1284.
7. Arthur, F. F. The action of mineral acid on diethyl bis-(hydroxymethyl)ma-
lonate. J. Org. Chem. 1955, 20, 780–786.
8. Akbar, A. A.; Torbjorn, F.; Goran, M. 3-Bromo-2-bromomethylpropyl
glycosides in the preparation of double-chain bis-sulfide neo-glycolipids.
Carbohyd. Res. 1987, 161, 225–233.
9. Adams, E. P.; Doyle, F. P.; Hatt, D. L.; Holland, D. O.; Hunter, W. H.;
Mansford, K. R. L.; Nayler, J. H. C.; Queen, A. Antituberculous sulphur
compound part I: New mercapto-derivatives of alkanols, sulphides, and
hydroxy-sulphides. J. Chem. Soc. 1960, 2649–2659.
10. Albericio, F.; Grandas, A.; Porta, A.; Pedroso, E.; Giralt, E. One-pot
synthesis of S-acetamidomethyl-L-cysteine (Fmoc-Cys(Acm)-OH). Synthesis
1987, 3, 271–272.
11. Curtis, N. J.; Brown, R. S. Nitrogen- and sulfur-containing models for
metallo-enzymes, part I: Synthesis and physical studies of 2-(2-pyridyl)-1,

3310
Y. Sun et al.
3-dithioalkyl-2-propanols, 2-(2-pyridyl)- and 2-(2-imidazolyl)-1,3-dimer-
capto-2-propanols. Can. J. Chem. 1981, 59, 65–75.
12. Bertini, V.; Lucchesini, F.; Pocci, M.; Alfei, S. Soluble and insoluble
polymeric 1,3-dithiane reagents for the synthesis of aldehydes from alkyl
halides. Tetrahedron 2005, 61, 9519–9526.
13. Howard, E. G.; Lindsey, R. V. Jr. The chemistry of some 5-m-dithianones
and dithiacycloalkanones. J. Am. Chem. Soc. 1960, 82, 158–164.
14. Hughes, D. L.; Reamer, R. A.; Bergan, J. J.; Grabowski, E. J. J. A mechan-
istic study of the Mitsunobu esterification reaction. J. Am. Chem. Soc. 1988,
110, 6487–6491.