Rapid, effective deprotection of tert-butoxycarbonyl (Boc) amino acids and peptides at high temperatures using a thermally stable ionic liquid

Article abstract of DOI:10.1039/c5ra21279k

A method for high temperature Boc deprotection of amino acids and peptides in a phosphonium ionic liquid is described. The ionic liquid had low viscosity, high thermal stability and demonstrated a beneficial effect. The study extended the possibility for extraction of water soluble polar organic molecules using ionic liquids. Trace water significantly improved product purity and yield, while only 2 equiv. TFA led to deprotection within 10 min. The trityl group was also deprotected.

Full text of DOI:10.1039/c5ra21279k

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Rapid, effective deprotection of tert-
butoxycarbonyl (Boc) amino acids and peptides at
high temperatures using a thermally stable ionic
liquid†
Cite this: RSC Adv., 2015, 5, 95854
Received 13th October 2015
Accepted 29th October 2015
*
Sumit S. Bhawal, Rahul A. Patil and Daniel W. Armstrong
DOI: 10.1039/c5ra21279k
www.rsc.org/advances
A method for high temperature Boc deprotection of amino acids and reported in subcritical water (150 ꢁC < T < 370 ꢁC, 0.4 < p <
peptides in a phosphonium ionic liquid is described. The ionic liquid 22 MPa) to take advantage of the higher dissociation of water.9
had low viscosity, high thermal stability and demonstrated a beneficial Extraction of water soluble organic molecules using ionic
effect. The study extended the possibility for extraction of water liquids have been explored with the advent of [BMIM]PF6 ionic
soluble polar organic molecules using ionic liquids. Trace water liquid.10 Recently, Majumdar et al. reported deprotection of
significantly improved product purity and yield, while only 2 equiv. TFA several N-Boc protected aromatic and heteroaromatic
led to deprotection within 10 min. The trityl group was also compounds using imidazolium based protic ionic liquid and
deprotected.
also discussed the selectivity of their approach.11 Interestingly,
use of phosphonium based ionic liquids in organic synthesis
has been very limited. This is surprising, given some of their
interesting properties like lack of an acidic proton (which may
lead to carbene formation), generally lower density than water
(helpful in liquid–liquid extraction), tolerance towards air,
moisture and high thermal stability.12
To the best of our knowledge, this is the rst report on
a generalized Boc-deprotection of natural amino acids and
peptides in ionic liquid media. There are challenges in puri-
cation of water soluble compounds involving many ILs tradi-
tionally used in organic synthesis (due to their own aqueous
solubility). This problem is circumvented by the current
approach.
The development of simple, effective and environmentally
compliant organic transformations are increasingly sought
aer as a means to drive sustainable processes.1 Ionic liquids
(ILs) have appeared in the literature as a new class of designer
solvents and have been found to be attractive for organic
synthesis.2 They offer several advantages over traditional
solvents including negligible vapor pressure (safety), recycla-
bility (lowering cost) and modulation of properties (scope).3
Sequential protection and deprotection of amine functional
groups play a signicant role in organic synthesis.4 The tert-
butoxycarbonyl (Boc) group is one of the more widely used
amine protecting group.5 The most common method for its
deprotection uses TFA and generally requires large excesses
(TFA : CH2Cl2 (1 : 1)), reaction times ranging from 2–16 h
depending on the substrate and a tedious purication process.6
Another well-established method for faster (ꢀ10–30 min) and
selective deprotection (in most cases) of Boc group utilizes (4 M)
HCl/dioxane.7 In a classic publication, Wang et al. reported
deprotection of N-Boc aliphatic, aromatic and heterocyclic
amine substrates using boiling water as a reaction medium,
indicating the role of water as a dual acid/base catalyst at
elevated temperature.8 However, reaction times with water
insoluble substrates could be signicantly longer, when water is
the only solvent. A few Boc deprotections have also been
The current work, utilizes trihexyltetradecylphosphonium
bis(triuoromethane)sulfonimide (TTP–NTf2) ionic liquid as
a water insoluble solvent having a relatively low kinematic
viscosity of 218.6 cSt @ 30 ꢁC,12 density of 1.065 g cmÀ3 12 high
,
thermal stability (ꢀ380 ꢁC),12 broad substrate solubility and
thermal range (À22 to 350 ꢁC).12
Three different methods were employed to liberate the built
in gaseous entity in the Boc group (Scheme 1 and Table 1).
Department of Chemistry and Biochemistry, The University of Texas at Arlington, 700
Planetarium Place, Arlington, Texas 76019-0065, USA. E-mail: sec4dwa@uta.edu
† Electronic supplementary information (ESI) available: For spectra of
compounds refer to ESI. See DOI: 10.1039/c5ra21279k
Scheme 1 Boc deprotection of amino acids using phosphonium ionic
liquids.
95854 | RSC Adv., 2015, 5, 95854–95856
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Table 1 Deprotection of N-Boc amino acid and peptide substrates
Method Cc TFA +
D, yield (%), time
(h)
Method Aa neat +
D, yield (%), time (h)
Method A
product
Method Bb H2O +
D, yield (%), time (h)
Method B
product
Method C
product
#
Substrate
Amino acidsa,b,c,d
1
2
3
4
5
6
7
8
N-Boc-L-Gly-OH
71% (5 h)
67% (5 h)
29% (6 h)
Dd (6 h)
H-L-Gly-OH
H-L-Ala-OH
H-L-Val-OH
NA
NA
H-L-Phe-OH
NA
NA
H-L-Thr-OH
NA
NA
H-L-Asp-OH
H-L-Glu-OH
96% (3 h)
91% (3 h)
92% (6 h)
89% (6 h)
94% (6 h)
93%(5 h)
95% (5 h)
92% (5 h)
90% (5 h)
92% (6 h)
91% (6 h)
97% (2.5 h)
93% (4 h)
H-L-Gly-OH
H-L-Ala-OH
H-L-Val-OH
H-L-Leu-OH
H-L-Ile-OH
H-L-Phe-OH
H-L-Pro-OH
H-L-Met-OH
H-L-Thr-OH
H-L-His-OH
H-L-Lys-OH
H-L-Asp-OH
H-L-Glu-OH
93% (7 min)
98% (7 min)
93% (10 min)
98% (10 min)
98% (10 min)
95% (10 min)
96% (10 min)
97% (10 min)
92% (10 min)
92% (10 min)
93% (11 min)
99% (10 min)
96% (10 min)
H-L-Gly-OH
N-Boc-L-Ala-OH
N-Boc-L-Val-OH
N-Boc-L-Leu-OH
N-Boc-L-Ile-OH
N-Boc-L-Phe-OH
N-Boc-L-Pro-OH
N-Boc-L-Met-OH
N-Boc-L-Thr-OH
N-Boc-L-His-OH
Boc-L-Lys(Boc)-OH
N-Boc-L-Asp-OH
N-Boc-L-Glu-OH
H-L-Ala-OH
H-L-Val-OH
H-L-Leu-OH
H-L-Ile-OH
H-L-Phe-OH
H-L-Pro-OH
H-L-Met-OH
H-L-Thr-OH
H-L-His-OH
H-v-Lys-OH
H-L-Asp-OH
H-L-Glu-OH
Dd (6 h)
20% (6 h)
Dd (6 h)
Dd (6 h)
9
Dd (6 h)
10
11
12
13
Dd (6 h)
Dd (6 h)
87% (5 h)
27% (5 h)
Peptidese,f,g
14
15
16
N-Boc-Ala-Ala-OH
13%
ICe
—
—
NA
92% (2.5 h)f
94% (2.5 h)f
92% (2.5 h)f
H-Ala-Ala-OH
H-Gly-Val-OH
H-Gly-His-Lys-OH
94% 10 ming
95% 10 ming
95% 7–8 ming
H-Ala-Ala-OH
H-Gly-Val-OH
H-Gly-His-Lys-
OH
N-Boc-Gly-Val-OH
Boc-Gly-His(Trt)-
Lys(Boc)-OH
ICe
a
TTP–NTf2, 150 ꢁC, 5–6 h (incomplete reaction with unreacted starting material for entry 1, 2, 3, 6, 12 and 13. For entry 14 A, trace condensation by
product observed along with unreacted starting material). b TTP–NTf2, 12–14% water as additive (based on TPP–NTf2), 150 ꢁC, 2.5–6 h. c TTP–NTf2
2 equiv. TFA as additive (based on substrate), 130 ꢁC, ꢀ10 min. d D (decomposed), TTP–NTf2 150 ꢁC, 6 h. e IC (trace product with unreacted starting
f
material and some decomposition observed), TTP–NTf2, 120 ꢁC, 3 h. TTP–NTf2, 110 ꢁC, 12–14% water as additive (based on TPP-NTf2), 2.5 h.
g
TTP–NTf2, 2 equiv. TFA as additive based on substrate, 110 ꢁC, 7–10 min. Yields reported are isolated.
Simple thermal treatment of Boc protected amino acids at conventional methods that requires 2–5 h and uses high
around 150 ꢁC gave the desired product for selected substrates, concentrations of TFA (generally 1 : 1 TFA : CH2Cl2).6 The
however the yields were not satisfactory with incomplete method was extended for the deprotection of Boc groups of
conversions and some product decomposition was observed dipeptides and two Boc and a trityl group for a tripeptide.
(Table 1, 1A–16A).
However, some condensation products were observed for entry
Addition of small amount of water led to signicant 14B which was avoided by performing the reaction at 110 ꢁC for
improvement in yield and product purity as evident in (Table 1, 2.5 h.
1B–16B). Deprotection of Boc-L-aspartic and glutamic acid
In summary, the current work provides a simple and
(having two free acid groups) conversion took the least time. It convenient procedure for Boc-deprotection reaction in ionic
is well known that quaternary ammonium and phosphonium liquid media. The lower viscosity, broad substrate solubility and
cations act as phase transfer agents.13 Therefore, reactions were thermal stability of the ionic liquid provides scope for carrying
also performed in tetrabutylammonium NTf2 (mp 84–85 ꢁC) out high temperature organic reactions and opens up the
with N-Boc-L-Ala-OH as substrate under reaction conditions possibility of extraction and purication for water soluble
identical to method B, which gave alanine in 97% yield. A organic compounds using ionic liquids. The rapid deprotection;
similar reaction with butylmethylimidazolium NTf2 resulted in using only two equivalent of TFA (little excess of which is mostly
incomplete conversion with 67% of L-alanine as product. These removed during the water distillation under reduced pressure)
observations may indicate the possibility of phosphonium allowed the whole process to be very efficient. The various
cation acting as a phase transfer catalyst, while acidic pH in the amino acids were extracted without any chromatographic
aqueous phase would drive deprotection of Boc group at high purication (as it can be challenging to separate ionic liquid
temperature. Lewis acidic type interaction of carbonyl group and water miscible organic compounds in the normal phase
with imidazolium based ionic liquids have been indicated in mode using silica gel chromatography). Also the said ionic
earlier reports.11,14 For phosphonium ionic liquids, especially in liquid from all batches were pooled together and recycled for
the absence of water as a separate phase, similar interactions further use by column purication using silica gel chromatog-
have been suggested.15–17 This however does not appear to be raphy (ethylacetate/hexane 70 : 30 to ethyl acetate 100%).
operative in the present case.
Reactions performed with the recycled ionic liquid, (92%
In the third method variation, (Table 1, 1C–16C) addition of recovery and characterized by NMR spectroscopy) gave similar
only 2 equiv. TFA resulted in accelerated deprotection within efficiencies. Study of efficient and waste free high temperature
10 min in almost all the cases studied compared to organic transformations in phosphonium based ionic liquid
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and their role in solvent assisted catalysis are underway in our acid by adding acetone. All the products were characterized by
laboratory. We would like to thank Dr Zachary S. Breitbach and 1H NMR, 13C NMR. ESI-MS was done in some cases.
Mohsen Talebi for technical support. We also acknowledge
funding for this work from Robert A. Welch Foundation
(Y0026).
Notes and references
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4 T. W. Greene and P. G. M. Wuts, in Protective Groups in
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A mixture of (0.25 g, 0.32–1.43 mmol) N-Boc protected amino
acid/peptides and TTP–NTf2 (7 g, 9.16 mmol) was stirred
(ꢀ150 rpm) for 5–6 h at 150 ꢁC in a round bottom ask (50 mL)
equipped with a reux condenser. Completion of reaction was
monitored by TLC using ethyl acetate/methanol (95 : 5) and
ninhydrin as staining agent. On cooling, the reaction mixture
was transferred into separating funnel with 3 Â 10 mL (CH2Cl2/
water 1 : 1). In addition, CH2Cl2 (100 mL) and water (50–80 mL,
depending on the substrate) was added to the separating fun-
nel. The layers were allowed to separate aer shaking and the
organic layer (containing the ionic liquid) was removed. To the
aqueous layer (containing the amino acids) fresh CH2Cl2 (2 Â
75 mL) was added to ensure removal of any trace of ionic liquid
remaining. The aqueous layer was evaporated to dryness to
obtain the product. In most of the cases, this gave us the puri-
ed amino acid, but otherwise a nal washing with ꢀ5 mL
isopropanol (using centrifuge). All the products were charac-
1
terized by H NMR, 13C NMR. ESI-MS was done in some cases.
Method B (TTP–NTf2 and water)
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To a mixture of (0.25 g, 0.32–1.43 mmol) N-Boc protected amino
acid/peptides and TTP–NTf2 (7 g, 9.16 mmol) was added
deionized water 12–14% (based on TTP–NTf2). The resulting
mixture was stirred (ꢀ200 rpm) for 2.5–6 h at 150 ꢁC (for amino
acids) and 2.5 h at 110 ꢁC (for peptides). The remaining work-up
is similar to method A. All the products were characterized by
1H NMR, 13C NMR. ESI-MS was done in some cases.
L-Histidine and L-lysine tends to form emulsions especially
for method B. They were centrifuged for 5 min at around
5000 rpm (the Drucker Co., Model 614 B) to obtain the puried
compound. For peptides, the water was generally removed at
ꢁ
ꢀ40–45 C under reduced pressure.
Method C (TTP–NTf2 and TFA)
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added TFA (2 equiv., based on substrate). The resulting mixture 15 J. McNulty, J. Dyck, V. Larichev, A. Capretta and
was stirred (ꢀ150 rpm) for 7–10 min at 100 ꢁC (for peptides) and
A. J. Robertson, Lett. Org. Chem., 2004, 1, 137–139.
130 ꢁC (for amino acids). The remaining procedure is similar to 16 S. R. Sarda, W. N. Jadhav, A. S. Shete, K. B. Dhopte,
method A.
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air drying at higher ow in a crystallizing dish using an inverted 17 P. Ludley and N. Karodia, Tetrahedron Lett., 2001, 42, 2011–
funnel at room temperature or direct precipitation of amino
2014.
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