Synthesis of photoswitchable hemithioindigo-based ω-amino acids and application in Boc-based peptide assembly

Article abstract of DOI:10.1055/s-2005-918461

Efficient procedures for the preparation of the N-Boc-protected aldehydes 5a and 5b are described which are valuable precursors for the synthesis of hemithioindigo-based ω-amino acids and peptides. Georg Thieme Verlag Stuttgart.

Full text of DOI:10.1055/s-2005-918461

PAPER
3297
Synthesis of Photoswitchable Hemithioindigo-Based w-Amino Acids and
Application in Boc-Based Peptide Assembly
HBStoeemcp-hiBtahanisoeHidnedPrrieegp,otW-iBdeansAceksdseewSm-tAbeilmnylien,oKaArocildasR: üAckp-pBlircaautino*n in
Institut für Chemie, Technische Universität Berlin, Str. d. 17. Juni 135, Sekr. C3, 10623 Berlin, Germany
Fax +49(30)31479651; E-mail: krueck@chem.tu-berlin.de
Received 16 September 2005
tection to the carbamates 4a and 4b, and removal of the
acetal protecting group provided the N-Boc-protected al-
dehydes 5a and 5b in excellent overall yields (Scheme 1).
For the synthesis of the meta-substituted w-amino acid 1b,
the aldehyde 5b and the literature-known thioindoxyl car-
boxylic acid chloride 6 were condensed using 1% sodium
hydroxide/tert-butyl alcohol (Scheme 2) as previously de-
scribed by us.4 These conditions provided the hemithioin-
digo 1b in 70% isolated yield and >95% purity after
purification by chromatography on Florisil.
Abstract: Efficient procedures for the preparation of the N-Boc-
protected aldehydes 5a and 5b are described which are valuable
precursors for the synthesis of hemithioindigo-based w-amino acids
and peptides.
Key words: hemithioindigos, amino acids, peptides, photoswitches
Driven by the development of new experimental tech-
niques, the investigation and modulation of fast processes
of peptide and protein folding gain increasing interest.1 In
addition, the reversible photocontrol of the conformation
and activity of biomolecules is intensively being ex-
plored.2,3 In an attempt to develop modular and tunable
photoswitches for these applications, we are exploring the
synthesis of novel hemithioindigo-derived compounds.4,5
Our design is based on significant light-induced end-to-
end distance changes and changes in the dipole moment.
Recently, we have demonstrated that a hemithioindigo-
based a-amino acid can be applied as a light switch for the
photomodulation of ionic current through modified gram-
icidin ion channels.6 Based on this promising approach,
the Boc-protected hemithioindigo-derived pseudo-w-ami-
no acids 1a and 1b (Figure 1) are developed in our labo-
ratories for their incorporation into cyclic peptides.
1. LAH, Et2O (3a: 92%; 3b: 80%)
2. Boc2O, Et3N, MeOH (4a: 80%; 4b: 85%)
O
O
H
O
3. PTSA (cat.), acetone/H2O (5a: 97%; 5b: 98%)
R
R
R'
R'
5a: R = H, R' = CH2NHBoc
5b: R = CH2NHBoc, R' = H
2a: R = H, R' = CN
2b: R = CN, R' = H
Scheme 1
In Boc-based solution-phase peptide synthesis, trifluoro-
acetic acid in combination with scavengers (e.g., H2O,
DMS, ethanedithiol, phenols, cresols, thiocresols, or thio-
anisols) is commonly used for deprotection strategies.
However, we found treatment with hydrogen chloride in
dioxane to be superior. Thus, Boc-deprotection of the
dipeptide 74 (Scheme 3), derived from 1a4 by EDC cou-
pling, was carried out with 4 M hydrogen chloride in di-
oxane (r.t.) and gave the hydrochloride 8 quantitatively
after evaporation of the solvent.
R'
OH
O
R
1a: R = H, R' = CH2NHBoc
S
1b: R = CH2NHBoc, R' = H
O
Figure 1
OH
O
BocHN
O
Cl
5b
So far, these pseudo-amino acids have been synthesized
and used as valuable building blocks in the synthesis of di-
and tripeptides via Boc-based peptide assembly in solu-
tion.
S
S
NaOH/t-BuOH,
∆, 3 h (70%)
1b
O
O
6
For the synthesis of the hemithioindigo-based w-amino
acids 1a and 1b, the Boc-protected aldehydes were pre-
pared starting from 3-cyano- or 4-cyanobenzaldehyde.
Protection of the aldehydes to give the dioxolanes 2a and
2b,7,8 reduction to the benzylamines 3a and 3b,7 Boc-pro-
Scheme 2
Coupling of 8 with (S)-Boc-Lys(Cbz)-OH proved to be a
challenge. By coupling with EDC/HOBt/DIPEA
(1.1:1.1:1) in N,N-dimethylformamide or HBTU/HOBt/
DIPEA (1:1:1) in N-methyl-2-pyrrolidinone, compound
formation varied and occasionally decomposition was ob-
served. A retro-aldol reaction could be ruled out based on
the analysis of the 1H NMR spectra of the crude material
SYNTHESIS 2005, No. 19, pp 3297–3300
x
x.
x
x
.
2
0
0
5
Advanced online publication: 14.11.2005
DOI: 10.1055/s-2005-918461; Art ID: C08205SS
© Georg Thieme Verlag Stuttgart · New York

3298
S. Herre et al.
PAPER
3-(1,3-Dioxolan-2-yl)benzonitrile (2b)8
Colorless solid.
isolated. Similar results were obtained for the coupling of
(S)-Boc-Lys(Cbz)-OSu with DIPEA or of (S)-Boc-
Lys(Aloc)-OH with EDC/HOBt and DIPEA in dimethyl- Yield: 12.0 g (90%).
formamide. Fortunately, tripeptide 9 was obtained using
(S)-Boc-Lys(Cbz)-OH and dipeptide salt 8 with DPPA
and DIPEA in N,N-dimethylformamide. Treatment of the
crude product with methanol followed by centrifugation
gave 9 in 76% isolated yield and >95% purity.
Mp 26 °C.
Rf = 0.67 (EtOAc–pentane, 1:4).
1H NMR (200 MHz, CDCl3): d = 7.75–7.73 (m, 1 H), 7.65 (dt, J =
7.8, 1.6 Hz, 1 H), 7.62 (dt, J = 7.8, 1.6 Hz, 1 H), 7.44 (t, J = 7.8 Hz,
1 H), 5.77 (s, 1 H), 4.12–3.96 (m, 4 H).
13C NMR (50.3 MHz, CDCl3): d = 139.8, 132.8, 131.1, 130.3,
R
129.3, 118.7, 112.6, 102.4, 65.5.
H
O
N
COOMe
4-(1,3-Dioxolan-2-yl)benzylamine (3a)7
Hygroscopic colorless solid.
S
Yield: 14.2 g (92%).
Mp 108 °C.
O
NHCbz
1H NMR (500 MHz, CDCl3): d = 7.39 (d, J = 8.2 Hz, 2 H), 7.26 (d,
J = 7.9 Hz, 2 H), 5.75 (s, 1 H), 4.07–3.95 (m, 4 H), 3.81 (s, 2 H),
1.41 (s, 2 H).
7 R = NHBoc
Boc-Lys(Cbz)-OH, DPPA,
DIPEA, DMF, r.t.,
15 h (76%)
4M HCl,
dioxane
(quant.)
8 R = NH2⋅HCl
13C NMR (125.7 MHz, CDCl3): d = 144.2, 136.4, 127.0, 126.6,
103.6, 65.2, 46.2.
O
BocHN
N
H
H
COOMe
N
3-(1,3-Dioxolan-2-yl)benzylamine (3b)
O
Hygroscopic colorless oil.
S
Yield: 6.4 g (80%).
1H NMR (500 MHz, CDCl3): d = 7.41 (s, 1 H), 7.33–7.29 (m, 3 H),
5.77 (s, 1 H), 4.12–3.99 (m, 4 H), 3.85 (s, 2 H), 1.46 (s, 2 H).
NHCbz
NHCbz
O
9
13C NMR (125.7 MHz, CDCl3): d = 143.7, 138.2, 128.7, 128.0,
125.1 (2C), 103.8, 65.4, 46.5.
Scheme 3
tert-Butyl [4-(1,3-Dioxolan-2-yl)benzyl]carbamate (4a); Typical
Procedure
In summary, for the Boc-protected w-amino acid building
blocks 1a and 1b syntheses and coupling strategies for the
solution-phase peptide synthesis of di- and tripeptides
were developed. The synthesis of 5a and 5b should have
general applicability to other N-Boc-protected aldehydes.
Benzylamine 3a (9.07 g, 0.05 mol) was dissolved in anhyd MeOH
(80 mL). While stirring, Et3N (7.02 mL, 0.05 mol) was added fol-
lowed by Boc2O (14.37 g, 0.07 mol, 1.3 equiv). After 2.5 h, Et2O
(250 mL) was added and the organic layer was separated and
washed with sat. aq NH4Cl (300 mL). The organic layer was dried
(MgSO4), filtered, and evaporated in vacuo. The residue was recrys-
tallized (Et2O–pentane, 20:1) to give 4a as a colorless solid.
1H NMR and 13C NMR spectra were recorded on Bruker spectrom-
eters AC 200 and DRX 500. Solvents are mentioned for the partic-
ular substances. The MS and HRMS spectra were recorded on a
Finnigan MAT 95 SQ or Varian MAT 711. The MS (EI) or HRMS
(EI) samples were ionized at an ionization potential of 70 eV. Com-
pound 6 was prepared following the literature procedure.4
Yield: 11.34 g (80%).
Mp 101 °C.
Rf = 0.76 (EtOAc–pentane, 2:1).
IR (film): 3355, 2881, 1679, 1527 cm–1.
4-(1,3-Dioxolan-2-yl)benzonitrile (2a); Typical Procedure7
4-Cyanobenzaldehyde (10.00 g, 0.08 mol) was dissolved in toluene
(150 mL) and treated with ethylene glycol (17.0 mL, 0.31 mol, 4
equiv) followed by PTSA (10 mg, 0.06 mmol). The mixture was re-
fluxed in a Dean Stark apparatus overnight. After cooling to r.t., the
solution was washed with 5% aq NaHCO3 (80 mL). The organic
layer was dried (MgSO4), filtered, and concentrated in vacuo. The
light yellow residue was recrystallized (Et2O–pentane, 3:1) to give
2a as a colorless solid.
1H NMR (500 MHz, DMSO-d6): d = 7.38 (d, J = 8.0 Hz, 2 H), 7.38
(s, 1 H), 7.26 (d, J = 7.9 Hz, 2 H), 5.70 (s, 1 H), 4.15 (d, J = 5.8 Hz,
2 H), 4.07–3.90 (m, 4 H), 1.41 (s, 9 H).
13C NMR (125.7 MHz, DMSO-d6): d = 155.8, 141.2, 136.5, 126.7,
126.5, 102.8, 77.8, 64.8, 43.2, 28.2.
MS (EI, 70 eV): m/z (%) = 279 (<5) [M+], 278 (<5) [M+ – H], 222
(100), 149 (33), 73 (25), 57 (59).
HRMS (EI): m/z calcd for C15H20NO4 [M+ – H]: 278.1392; found:
278.1399.
Yield: 12.1 g (90%).
Mp 39 °C.
Anal. Calcd for C15H21NO4: C, 64.50; H, 7.58; N, 5.01. Found: C,
64.59; H, 7.65; N, 4.78.
Rf = 0.41 (EtOAc–pentane, 1:4).
1H NMR (200 MHz, CDCl3): d = 7.67–7.63 (m, 2 H), 7.58–7.53 (m,
tert-Butyl [3-(1,3-Dioxolan-2-yl)benzyl]carbamate (4b)
Yield: 9.1 g (85%).
2 H), 5.82 (s, 1 H), 4.13–4.01 (m, 4 H).
13C NMR (125.7 MHz, CDCl3): d = 142.2, 131.3, 126.3, 117.7,
112.0, 101.5, 64.5.
Mp 68 °C.
Rf = 0.79 (EtOAc–pentane, 2:1).
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Hemithioindigo-Based w-Amino Acids: Application in Boc-Based Peptide Assembly
3299
IR (film): 3354, 2976, 1710, 1697 cm–1.
13C NMR (125.7 MHz, DMSO-d6): d = 187.6, 166.5, 155.9, 147.0,
143.2, 136.9, 133.5, 132.4, 131.3, 131.1, 131.0, 129.9, 129.3, 128.9,
127.8, 127.0, 126.1, 77.9, 43.3, 28.3.
1H NMR (500 MHz, DMSO-d6): d = 7.41 (br s, 1 H), 7.35–7.32 (m,
2 H), 7.29 (d, J = 7.3 Hz, 1 H), 7.26 (d, J = 7.2 Hz, 1 H), 5.71 (s, 1
H), 4.15 (d, J = 6.0 Hz, 2 H), 4.05–3.93 (m, 4 H), 1.41 (s, 9 H).
MS (EI, 70 eV): m/z (%) = 412 (10) [M+ + H], 295 (60), 57 (100).
13C NMR (125.7 MHz, DMSO-d6): d = 155.8 (C-10), 140.3 (C-7),
138.1 (C-3), 128.1 (C-8), 127.7 (C-5), 125.0 (C-4, C-6), 102.9 (C-
2), 77.9 (C-11), 64.8 (C-1), 43.3 (C-9), 28.2 (C-12). The assignment
was confirmed by HMQC measurements.
HRMS (EI): m/z calcd for C22H21NO5S: 411.1141; found:
411.1144.
Methyl N-{(2Z)-2-[4-(Aminomethyl)benzylidene]-3-oxo-2,3-di-
hydrobenzo[b]thiophen-7-yl}carbonyl-N6-[(benzyloxy)carbon-
yl]lysinate Hydrochloride (8)
Compound 7 (0.19 g, 0.28 mmol) was dissolved in 4 M HCl in di-
oxane (10 mL). The solution was stirred for 1 h at r.t. under a nitro-
gen atmosphere. Then, the solvent was removed in vacuo to give 8
as a light-yellow solid.
HRMS (EI): m/z calcd for C15H21NO4: 279.1471; found: 279.1472.
Anal. Calcd for C15H21NO4: C, 64.50; H, 7.58; N, 5.01. Found: C,
64.68; H, 7.65; N, 4.62.
tert-Butyl (4-Formylbenzyl)carbamate (5a); Typical Procedure
Carbamate 4a (11.34 g, 0.04 mol) was dissolved in acetone (190
mL). After addition of H2O (23 mL) and PTSA (10 mg, 0.06 mmol)
the solution was stirred at r.t. overnight. The mixture was diluted
with CH2Cl2 (100 mL), and the organic layer was separated and
washed with 5% aq NaHCO3 (50 mL), dried (MgSO4), filtered, and
concentrated in vacuo to give 5a as colorless crystals.
Yield: 170 mg (quant).
Mp 194 °C.
IR (film): 3323, 2952, 1684, 1534 cm–1.
1H NMR (DMSO-d6, 200 MHz): d = 9.16 (d, J = 7.2 Hz, 1 H), 8.46
(d, J = 7.4 Hz, 1 H), 8.40 (br s, 3 H), 8.08 (d, J = 7.2 Hz, 1 H), 7.92
(s, 1 H), 7.88 (d, J = 8.4 Hz, 2 H), 7.68 (d, J = 8.4 Hz, 2 H), 7.58 (t,
J = 7.6 Hz, 1 H), 7.33–7.23 (m, 6 H), 4.98 (s, 2 H), 4.47 (q, J = 5.4
Hz, 1 H), 4.11 (br d, J = 5.0 Hz, 2 H), 3.68 (s, 3 H), 3.01–2.99 (m,
2 H), 1.86–1.76 (m, 2 H), 1.43–1.35 (m, 4 H).
13C NMR (DMSO-d6, 50.3 MHz): d = 187.5, 172.7, 169.3, 165.1,
156.0, 146.1, 137.2, 137.1, 136.3, 133.9, 132.6, 132.2, 131.3, 131.0,
129.7, 129.2, 128.6, 128.2, 127.6, 127.1, 65.0, 51.8, 51.6, 41.8,
30.5, 28.9, 22.3 (one C atom overlapped by the adjacent DMSO sig-
nal).
Yield: 9.25 g (97%).
Mp 81 °C.
Rf = 0.91 (EtOAc–pentane, 2:1).
IR (film): 3351, 2978, 1693, 1168 cm–1.
1H NMR (500 MHz, DMSO-d6): d = 9.97 (s, 1 H), 7.86 (d, J = 7.7
Hz, 2 H), 7.51 (s, 1 H), 7.45 (d, J = 7.7 Hz, 2 H), 4.22 (d, J = 5.8 Hz,
2 H), 1.40 (s, 9 H).
13C NMR (125.7 MHz, DMSO-d6): d = 192.7, 155.8, 147.3, 135.0,
129.6, 127.4, 78.0, 43.3, 28.2.
FAB-MS: m/z (%) = 588 (100) [M+ – HCl].
Anal. Calcd for C13H17NO3: C, 66.36; H, 7.28; N, 5.95. Found: C,
66.09; H, 7.26; N, 5.72.
Tripeptide 9
To a solution of compound 8 (152 mg, 0.24 mmol) in DMF (5 mL)
were gradually added Boc-Lys(Cbz)-OH (92 mg, 0.24 mmol),
DPPA (74 mg, 0.26 mmol), and DIPEA (26 mg, 0.24 mmol). After
the solution was stirred under nitrogen for 15 h, a mixture of
EtOAc–benzene (4:1, 100 mL) was added. The organic layer was
separated and washed with 2% citric acid (2 × 50 mL) followed by
H2O (2 × 50 mL) and brine (2 × 50 mL). The organic layer was dried
(MgSO4) and the solvent was removed in vacuo. The residue was
treated with MeOH and isolated by centrifugation (3 ×) to give tri-
peptide 9 as a light-yellow solid.
tert-Butyl (3-Formylbenzyl)carbamate (5b)
Yield: 7.0 g (98%).
Rf = 0.89 (EtOAc–pentane, 2:1).
Mp 59 °C.
IR (film): 3350, 2978, 1605, 1163 cm–1.
1H NMR (500 MHz, DMSO-d6): d = 10.00 (s, 1 H), 7.79–7.77 (m,
2 H), 7.57–7.54 (m, 2 H), 7.50 (s, 1 H), 4.22 (d, J = 5.6 Hz, 2 H),
1.40 (s, 9 H).
Yield: 175 mg (76%); 95% purity.
Mp 128 °C.
13C NMR (125.7 MHz, DMSO-d6): d = 193.1, 155.8, 141.4, 136.3,
133.1, 129.1, 128.5, 127.2, 77.9, 43.0, 28.2.
Rf = 0.34 (CH2Cl2–MeOH, 1:1).
IR (film): 3324, 2935, 1700, 1532, 1251 cm–1.
HRMS (EI): m/z calcd for C13H16NO3 [M – H]+: 234.1130; found:
234.1120.
1H NMR (CDCl3, 500 MHz): d = 8.01 (d, J = 7.5 Hz, 1 H), 7.97 (d,
J = 7.0 Hz, 1 H), 7.84 (s, 1 H), 7.68 (d, J = 7.5 Hz, 2 H), 7.39 (t, J =
7.5 Hz, 1 H), 7.35–7.20 (m, 13 H), 7.19–7.14 (m, 1 H), 6.88 (br s, 1
H), 5.31–5.26 (m, 1 H), 5.01–4.98 (m, 5 H), 4.80–4.79 (m, 1 H),
4.44 (br s, 2 H), 4.12–4.01 (m, 1 H), 3.80 (s, 3 H), 3.18 (br s, 4 H),
2.00–1.98 (m, 1 H), 1.92–1.85 (m, 2 H), 1.69–1.66 (m, 1 H), 1.60–
1.48 (m, 5 H), 1.46–1.30 (m, 3 H), 1.41 (s, 9 H).
Anal. Calcd for C13H17NO3: C, 66.36; H, 7.28; N, 5.95. Found: C,
66.21; H, 7.29; N, 5.55.
(2Z)-2-(3-{[(tert-Butoxycarbonyl)amino]methyl}benzylidene)-
3-oxo-2,3-dihydrobenzo[b]thiophene-7-carboxylic Acid (1b)
Yield: 795 mg (70%).
Mp 223 °C.
13C NMR (CDCl3, 50.3 MHz): d = 188.4, 172.8, 172.3, 165.5,
156.8, 156.7, 156.0, 140.6, 136.5, 136.3, 134.0, 133.3, 132.5, 132.4,
131.6, 130.0, 129.7, 128.5, 128.1, 128.0, 127.9, 127.5, 126.1, 125.2,
120.7, 120.2, 120.1, 77.2, 66.7, 52.8, 52.7, 43.0, 40.3, 40.2, 31.7,
31.4, 29.6, 29.4, 28.3, 22.5, 22.4 (two C atoms are overlapped by
adjacent signals).
Rf = 0.66 (EtOAc–AcOH, 80:1).
IR (film): 3336, 1700, 1682, 1274 cm–1.
1H NMR (500 MHz, CD3OD): d = 8.28 (dd, J = 7.5, 1.1 Hz, 1 H),
7.95 (dd, J = 7.5, 1.3 Hz, 1 H), 7.86 (s, 1 H), 7.77 (d, J = 7.7 Hz, 1
H), 7.71 (s, 1 H), 7.49–7.47 (m, 1 H), 7.40–7.37 (m, 2 H), 4.31 (s, 2
H), 1.47 (s, 9 H).
FAB-MS: m/z (%) = 951 (26) [M+ + H], 950 (2) [M+], 850 (100).
Anal. Calcd for C51H59N5O11S: C, 64.47; H, 6.26; N, 7.37; S, 3.37.
Found: C, 64.38; H, 6.22; N, 7.27; S, 3.41.
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S. Herre et al.
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(4) Steinle, W.; Rück-Braun, K. Org. Lett. 2003, 5, 141.
(5) Priewisch, B.; Steinle, W.; Rück-Braun, K. In Peptides
2004, Proceedings of the Third International and Twenty-
Eighth European Peptide Symposium, Prague, September
5–10, 2004; Mayflower: Kingswinford UK, 2005, in press.
(6) Lougheed, T.; Borisenko, V.; Hennig, T.; Rück-Braun, K.;
Woolley, G. A. Org. Biomol. Chem. 2004, 2, 2798.
Acknowledgment
This work was supported by the Deutsche Forschungsgemeinschaft
(Sonderforschungsbereich 498).
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Synthesis 2005, No. 19, 3297–3300 © Thieme Stuttgart · New York