Synthesis of (S)-5,6-dibromo-tryptophan derivatives as building blocks for peptide chemistry

Article abstract of DOI:10.1016/j.tetlet.2011.03.041

5,6-Dibromo-tryptophan is an interesting amino acid whose derivatives and analogues are found in a variety of highly bioactive natural compounds. Notwithstanding its relevance no data concerning this compound are found in the literature. Here an efficient pathway for the synthesis of 5,6-dibromo- tryptophan derivatives is reported. The reaction is performed by using 6-Br-isatin as starting material. Selective bromination at position 5 was followed by BH₃ reduction of the intermediate α-keto-amide and alkylation with Ser-OH in Ac₂O/AcOH. Optical resolution was effected by enzymatic de-acetylation of the obtained racemic mixture. Finally, in situ N^α-Boc protection of the optically pure S form yielded the desired N^α-Boc-(S)-5,6-dibromo-tryptophan.

Full text of DOI:10.1016/j.tetlet.2011.03.041

Tetrahedron Letters 52 (2011) 2583–2585
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Synthesis of (S)-5,6-dibromo-tryptophan derivatives as building blocks
for peptide chemistry
Adriano Mollica a, , Azzurra Stefanucci a, Federica Feliciani a, Gino Lucente b, Francesco Pinnen a
⇑
a Dipartimento di Scienze del Farmaco, Università di Chieti-Pescara ‘G. d’Annunzio’, Via dei Vestini 31, 66100 Chieti, Italy
b Istituto di Chimica Biomolecolare (CNR) c/o Dipartimento di Chimica e Tecnologie del Farmaco, ‘Sapienza’, Università di Roma, P.le A. Moro, 00185 Roma, Italy
a r t i c l e i n f o
a b s t r a c t
Article history:
5,6-Dibromo-tryptophan is an interesting amino acid whose derivatives and analogues are found in a
variety of highly bioactive natural compounds. Notwithstanding its relevance no data concerning this
compound are found in the literature. Here an efficient pathway for the synthesis of 5,6-dibromo-
tryptophan derivatives is reported. The reaction is performed by using 6-Br-isatin as starting material.
Received 21 December 2010
Revised 7 March 2011
Accepted 8 March 2011
Available online 16 March 2011
Selective bromination at position 5 was followed by BH3 reduction of the intermediate
a-keto-amide
and alkylation with Ser-OH in Ac2O/AcOH. Optical resolution was effected by enzymatic de-acetylation
Keywords:
Brominated amino acids
Indole
a
of the obtained racemic mixture. Finally, in situ N -Boc protection of the optically pure S form yielded
a
the desired N -Boc-(S)-5,6-dibromo-tryptophan.
Ó 2011 Elsevier Ltd. All rights reserved.
Isatin
Racemic resolution
Tryptophan
1. Introduction
of the aromatic bromine atoms. Methods for efficient synthesis of
tryptophan analogues with monobromo- or dibromo-substitution
In a large number of biochemical processes brought about by
functional proteins the presence of the essential amino acid trypto-
phan plays a crucial role. The indolic side chain of this residue is in
fact unique for structural and chemical properties. Its planar and
heteroaromatic moiety is the largest among the naturally occurring
amino acids and possesses electrostatic and amphiphilic properties
capable of highly influencing the protein function. The usual
H-bonding activity is accompanied by more specific interactions
connected with the presence of the electron-rich indole nucleus
and this property is responsible for the interaromatic ring-stacking
on the indole ring are therefore highly desirable.
Representative examples of natural compounds related to
5,6-dibromo-tryptophan as the bioprecursor and possessing inter-
esting biological activity, often associated with unusual structural
features, are reported in Figure 1.
A wide range of indoles13 and tryptophan derivatives14 have
been previously synthesized by using enzymatic and chemical
methods. When enzymes are involved14a the accessibility of the
enzyme active site may be sensibly restricted by substituents on
the tryptophan indole ring. Substitution at the 4- or 7-positions
leads to poor substrates while substituents at the 5- or 6-positions
are generally accepted with the large iodo or nitro groups usually
poorly tolerated.14c Concerning the series of halogenated trypto-
phans, an examination of the literature shows that, whereas some
monohalo-tryptophans have recently been described,14a–g synthe-
sis and property of dihalo-tryptophans are not known.
effectsandcation–pinteractionswhichoftencontrolproteinconfor-
mation and enzymatic activity.1–3
In accordance with the above reported multifunctional charac-
ter tryptophan represents a target substrate for both synthetic and
biosynthetic transformations in search of lead compounds and
building blocks for pharmaceutical applications.4a,b Literature
examination shows that halogenated derivatives and in particular
brominated indole derivatives have attracted the attention of both
chemists and biologists.5–7 The presence of halogen atoms in the
indole nucleus may in fact significantly influence bioactivity and
bioavailability5 of bioactive compounds offering, at the same time,
suitable models for SAR studies and selective functionalization
strategies through coupling reactions mediated by the presence
Taking into account the low availability of marine organisms
which are the main source of dihalo-tryptophan containing mole-
cules as well as the potential of dihalo-tryptophans as building
blocks for the chemistry of peptides, we report here synthesis and
the properties of 5,6-dibromo-tryptophan (di-BrTrp) derivatives.
2. Chemistry
The adopted synthetic protocol is reported in Scheme 1. Selec-
tive bromination of 6-bromo-isatin was performed by following
⇑
Corresponding author. Tel.: +39 08713554476; fax: +39 08713554477.
E-mail address: a.mollica@unich.it (A. Mollica).
0040-4039/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2011.03.041

2584
A. Mollica et al. / Tetrahedron Letters 52 (2011) 2583–2585
HO
O
NH2
N
Br
Br
Br
N
H
CH3
Br
N
N
H
Br
N
H
Br
H
5,6-Dibromoabrine
5,6-Dibromotryptamine
5,6-Dibromo-N,N-dimethyltryptamine
HN
HN
N
N
H2
H
N
N
O
O
H
N
NH2
N
H
Br
Br
Br
Br
Br
O
S
S
N
N
H
N
H
Br
OMe
Kottamide E
Aplicyanin E
Meridianin F
Figure 1. Literature examples of 5,6-dibromoindole containing natural products: 5,6-dibromo-abrine (see Ref. 8); 5,6-dibromo-tryptamine (see Refs. 8,9); 5,6-dibromo-N,N-
dimethyltryptamine (see Ref. 9); aplicyanin E (isolated from the ascidians Aplidium cyaneum and characterized by cytotoxic activity on the human tumour cells and
antimitotic activity; see Ref. 10a,c); meridianin F (isolated from the tunicate Aplidium meridianum; see Refs. 10b,11); kottamide E (from ascidian Pycnoclavella kottae; see Refs.
12a,b).
O
O
Br
Br
Br
Br
b
c
a
O
O
N
N
N
Br
H
H
H
2
1
HO
HO
HO
O
O
O
N
Ac
N
Ac
N
R
H
H
Br
Br
Br
Br
H
Br
Br
d
+
N
H
N
H
N
H
3
4
R = H : 5
e
R = Boc : 6
a
a
Scheme 1. Synthesis of N -Ac-5,6-(R)-di-BrTrp-OH 4 and N -Boc-(S)-5,6-di-BrTrp-OH 6. Reagents and conditions: (a) Br2, THF, AcOH, reflux, 48 h, 57%; (b) 1 M BH3ꢀTHF, THF,
rt, 3 h, 68%; (c) (S)-Ser-OH, Ac2O, AcOH, under N2 atm, reflux, 3.5 h, 50%; (d) amano acylase, CoCl2ꢀ6 H2O, phosphate buffer at pH 8, 37 °C, 6 h; (e) Boc2O, 10% NaOH, H2O/
dioxane 1:3, rt, 5 h, 23% from 3.
the procedure described by Vine et al.15 5,6-Dibromo-isatin (1)16
was carefully purified by multiple crystallization from AcOH and
then reduced by a solution of BH3 in THF to give 5,6-dibromo-in-
dole (2) in good yield (68%).17 As previously reported, attempts
to obtain 2 by using LiAlH4 or NaBH4 were unsuccessful while
reduction with LiBH4 in THF gave only very low yields.18 Synthesis
of the 5,6-dibromo-tryptophan derivative (3) was performed by
exploiting the nucleophilic reactivity of the indole ring at the C-3
carbon atom. Thus, 5,6-dibromo-indole (2) was reacted with
(S)-serine in a mixture of acetic acid/acetic anhydride to give in
good yields the desired N-acetyl-(R,S)-5,6-dibromo-tryptophan
an enzyme commonly used for optical resolutions of racemic N-
acetyl amino acids was used to resolve the racemic N-acyl deriva-
tive 3. As previously reported14c the two large substituents at the
5- and 6-positions of the indole ring sensibly slow down the enzy-
matic hydrolysis rate as compared with the N-acetyl-(R,S)-trypto-
phan. Thus, in order to obtain in one step the (S)-form as
derivative suitable for peptide synthesis, the usually reported pro-
cedures14c,e have been modified. The aqueous buffered solution
was acidified (pH 3) and extracted with EtOAc to give N-acetyl-
(R)-5,6-dibromo-tryptophan (4) ca. 90% ee. Subsequent evapora-
tion of the aqueous layer and treatment of the residue with
tert-butyl dicarbonate (Boc2O) gave the desired enantiopure
N-Boc-(S)-5,6-dibromo-tryptophan (6), ca. 82% ee.20 The enantio-
meric excess of products (4) and (6) was tested by the HPLC
(3).19 The reaction proceeds through N-acetyl-
a,b-unsaturated spe-
cies as electrophilic intermediates14c,d and this leads, as expected,
to complete loss of the (S)-serine chiral centre. ‘Amano’ acylase,

A. Mollica et al. / Tetrahedron Letters 52 (2011) 2583–2585
2585
method reported by Jin et al.21 Chromatography was performed at
room temperature on a polysaccaride-derived chiral stationary
phase (CSP) covalently bonded on silica matrix. The packing com-
position of the used column (Chiralpak IA) is amylose tris(3,5-dim-
15. Vine, K. L.; Locke, J. M.; Ranson, M.; Pyne, S. G.; Bremner, J. B. Bioorg. Med. Chem.
2007, 15, 931–938.
16. Procedure for preparation of 1: To a mixture of 6-bromoisatin (0.88 mmol) in
AcOH (9 ml), Br2 (0.88 mmol) was added and the mixture was stirred for 48 h
at reflux. After this time the reaction was cooled at 0 °C, and the solid residue
was paper filtered off, washed with AcOH and dried in the oven. The crude
product was then recrystallized from AcOH to give the pure orange coloured
product 1, 57%; mp >270 °C with decomposition. 1H NMR, 300 MHz, DMSO-d6
d = 7.23 (1H, s, H7); 7.82 (1H, s, C4), 11.22 (1H, brs, NH). 13C NMR, 75 MHz,
DMSO-d6 d = 117.40, 117.64, 119.61, 129.33, 133.81, 150.67, 159.83, 183.12.
17. Procedure for preparation of 2: To a solution of 1 (0.327) in THF (10 ml) a
solution of 1 M BH3ꢀTHF (2.5 equiv) was added dropwise at 0 °C, then the
mixture was stirred at rt for 3 h. After this time, the reaction was quenched by
slowly adding 2 ml of demineralized water, then 2N HCl was added to pH 3.
THF was removed under reduced pressure and the aqueous residue extracted
with two portions of EtOAc. The organic layers were washed with two portions
of brine, dried over Na2SO4 and evaporated under reduced pressure to give a
yellow oil. The crude product was purified by silica gel chromatography
ethylphenylcarbamate) immobilized on 5
l
m silica-gel,22 a chiral
selector system which shows high enantioselectivity for the reso-
lution of N-Boc-
a
-amino acids and their esters.21
3. Conclusion
In conclusion, an efficient synthesis of the N-Boc 5,6-dibromo-
(S)-tryptophan and N-acetyl 5,6-dibromo-(R)-tryptophan, useful
and still unknown building blocks for both peptide chemistry
and synthesis of natural products, has been developed by using a
combined chemical and enzymatic approach.
(petroleum benzene/Et2O 9:1) to give the pure product
2 as a greyish
crystalline solid, mp = 154–155 °C, Rf = 0.2 (Et2O/petroleum ether 1:5), 68%.
1H NMR, 300 MHz, DMSO-d6 d = 6.42 (1H, m, H3), 7.42 (1H, t, H2), 7.75 (1H, s,
H7), 7.93 (1H, s, H4), 11.35 (1H, brs, NH). 13C NMR, 75 MHz, DMSO-d6
d = 101.53, 113.62, 115.45, 116.71, 124.96, 128.76, 129.44, 136.35.
Supplementary data
18. (a) Tsintsadze, T. G.; Khoshtariya, T. E.; Kurkovskaya, L. N.; Mirziashvili, N. T.;
Sikharulidze, M. I. Chem. Heterocycl. Compd. 2002, 38, 472–476; (b)
Khoshtariya, T. E.; Matnadze, M. M.; Mirziashvili, N. T.; Kurkovskaya, L. N.;
Sikharulidze, M. I.; Dzhashi, T. O. Chem. Heterocycl. Compd. 2004, 40, 1454–
1459.
Supplementary data (1H NMR spectra, 13C NMR spectra, chiral
HPLC runs are reported in Supplementary data) associated with
this article can be found, in the online version, at doi:10.1016/
j.tetlet.2011.03.041.
19. Procedure for preparation of 3: To a mixture of 2 (0.363 mmol) in Ac2O (0.31 ml)
and AcOH (0.7 ml) was added (S)-Ser-OH (0.72 mmol) under reflux. The
reaction mixture was stirred for 3 h at reflux under N2 atmosphere. Then the
reaction was allowed to rt and the pH was adjusted at 11 by adding a few drops
of 30% NaOH solution. The solvent was then removed under reduced pressure
and the residue taken up in demineralized water. The aqueous layer was
extracted with two portions of Et2O and then the pH was adjusted to 3 by
adding 5% HCl. The aqueous layer was extracted with EtOAc and the organic
phase was dried on Na2SO4, evaporated under reduced pressure to give a crude
3. This was purified by silica gel column chromatography (EtOAc/AcOH 99:1 to
EtOAc/AcOH 97:3) to give product 3 as an oil, Rf = 0.4 (EtOAc/AcOH 9:1), 50%.
1H NMR data, and MS are consistent with the enantiopure product 4.
20. Typical procedure: 3 (0.254 mmol) was dissolved in a 3.1 ml of phosphate buffer
pH 8 (Sigma Aldrich) containing CoCl2 (3.9 ꢁ 10ꢂ4 mmol) and the mixture was
added to a mixture of Amano Acylase (103 mg) in phosphate buffer (10.3 ml) at
37 °C for 24 h. Then the mixture was concentrated under reduced pressure and
the pH adjusted to 3 by adding 2N HCl. The suspension was extracted by two
portions of EtOAc and the organic layer dried on Na2SO4, filtered and
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Rt = 16.6 min to give the pure
6 as a colourless oil, overall yield 23%.
Compound 4: 1H NMR, 300 MHz, DMSO-d6 d = 1.75 (3H, s, CH3CO), 3.07 (2H,
m, bCH2), 4.29 (1H, m, a
CH), 7.18 (1H, s, H2), 7.69(1H, s, H7), 7.88 (1H, s, H4),
7.96 (1H, d, NHCO), 11.13 (1H, s, indolic NH). 13C NMR, 75 MHz, DMSO-d6
d = 23.06, 27.46, 53.62, 110.81, 113.32, 115.49, 116.74, 123.48, 125.10, 129.07,
136.49, 169.87, 173.95; MS (ESꢂ): 403.0 [MꢂH]ꢂ a 2D0
½ ꢃ : ꢂ13.7, c = 1 (DMF).
,
Compound 6: 1H NMR, 300 MHz, DMSO-d6 d = 1.37 (9H, s, Boc), 3.15 (2H, m,
bCH2), 4.38 (1H, s, a
CH); 5.55 (1H, d, Boc-NH); 7.04 (1H, s, H2), 7.61 (1H, s, H7),
7.78 (1H, s, H4), 10.62 (1H, s, indolic NH). 13C NMR, 75 MHz, 10% DMSO-d6 in
CDCl3 d = 27.45, 28.32, 54.13, 79.13, 109.54, 113.43, 115.64, 116.09, 123.00,
125.84, 128.77, 136.09, 155.13, 173.64; MS (ES+): 485.1 [M+Na]+; ½a D20
ꢃ : ꢂ2.5,
c = 1 (DMF).
21. Jin, J. Y.; Bae, S. K.; Lee, W. Chirality 2009, 21, 871–877.
22. Chromatographic conditions. Column: Chiralpak IA 250 mm L ꢁ 4.6 mm I.D.
Conditions: detection UV at 220 nm; flow rate: 0.9 ml/min; injection of 20
lL
of a solution containing 1 mg/mL of analyte; isocratic elution with a mixture of
hexane/2-propanol/TFA (85:15:0.1) in 30 min, for chiral HPLC runs of product
3, 4 and 6 (further details in Supplementary data).