Tetrabutylammonium Fluoride as a Mild and Versatile Reagent for Cleaving Boroxazolidones to Their Corresponding Free α-Amino Acids

Article abstract of DOI:10.1002/ejoc.201700063

Protection of α-amino acids with 9-borabicyclo[3.3.1]nonane (9-BBN) to give their corresponding boroxazolidones is highly attractive, as it concurrently masks both the amino and the carboxylic acid functionalities. However, the harsh methods required for deprotection of these boroxazolidones have limited their use. Herein, we report that tetrabutylammonium fluoride serves as a mild and versatile reagent that can be used to cleave boroxazolidones to their corresponding free α-amino acids. The reaction conditions were explored, including the use of various nucleophilic fluoride sources, solvents, and reaction temperatures. Nucleophilic fluoride sources comprising an ammonium cation proved superior to other countercations. The scope of the reaction was extended to the cleavage of B,B-diphenyl- and B,B-diethyl boroxazolidone complexes. Furthermore, a wide range of α-amino acid side-chain functionalities were shown to be compatible, including acids, esters, amides, thiols, thioethers, alkynes, phenols, basic heterocycles, and important biorelevant molecules such as glutathione, (S)-adenosyl-l-homocysteine, and l-biocytin.

Full text of DOI:10.1002/ejoc.201700063

A Journal of
Accepted Article
Title: Tetrabutylammonium Fluoride (TBAF) as a Mild and Versatile
Reagent for Cleaving Boroxazolidones to their Corresponding
Free α-Amino Acids
Authors: Christian B. M. Poulie and Lennart Bunch
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To be cited as: Eur. J. Org. Chem. 10.1002/ejoc.201700063
Link to VoR: http://dx.doi.org/10.1002/ejoc.201700063
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10.1002/ejoc.201700063
European Journal of Organic Chemistry
Tetrabutylammonium Fluoride (TBAF) as a Mild and Versatile Reagent
for Cleaving Boroxazolidones to their Corresponding Free α-Amino
Acids
Christian B. M. Poulie and Lennart Bunch*
Department of Drug Design and Pharmacology, Faculty of Health and Medical Sciences, University of
Copenhagen, Denmark
*Corresponding author, LB: lebu@sund.ku.dk, homepage: http://drug.ku.dk/research/medicinal-
chemistry-research/chemistry-and-medicinal-chemistry/bunch_research_group/
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10.1002/ejoc.201700063
European Journal of Organic Chemistry
Abstract
Protection of α-amino acids with 9-borabicyclo[3.3.1]nonane (9-BBN) to give their corresponding
boroxazolidones is highly attractive as it concurrently masks both the amino and the carboxylic acid
functionalities. However, the required harsh methods for their deprotection have limited its use. Here
we report that tetrabutylammonium fluoride (TBAF) is a mild and versatile reagent for cleaving
boroxazolidones to their corresponding free α-amino acids. Reaction conditions were explored
including use of various nucleophilic fluoride sources, solvents and reaction temperatures. Nucleophilic
fluoride sources which comprise an ammonium cation proved superior to other counter cations. The
scope of the reaction was extended to cleavage of B,B-diphenyl- and B,B-diethyl boroxazolidone
complexes. Furthermore, a wide range of α-amino acid side chain functionalities were shown to be
compatible, including acids, esters, amides, thiols, thioethers, alkynes, phenols, basic heterocycles and
important bio-relevant molecules, such as glutathione (GSH), (S)-adenosyl-L-homocysteine (SAH) and
L-biocytin.
Keywords
Boroxazolidone
Fluorides
Amino acids
Cleavage reactions
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10.1002/ejoc.201700063
European Journal of Organic Chemistry
Introduction
Functional group transformation of α-amino acids side chains often requires prior protection of the
amino and acid group functionalities.[1] Aside from the standard two-step ester-carbamate protection of
the free amino acid,[1] protection with 9-borabicyclo[3.3.1]nonane (9-BBN) provides the corresponding
boroxazolidone as a fully protected amino acid, in only one step.[2–6] Other boron substituted
boroxazolidones have also been reported which include the B,B-diphenyl,[7] the B,B-diethyl[7] and the
B,B-diisopinocampheylborane (B,B-(Ipc)2) analogs.[8]
Scheme 1. Previously reported method a-e for cleaving boroxazolidones to their corresponding free α-amino acids and this
work.
A number of methods for cleaving the boroxazolidone to its corresponding free α-amino acid have been
reported (Scheme 1). a) Hydrolysis of the boroxazolidone with a strong acid.[2,5,6,9–12] Most frequently
used method is stirring the boroxazolidone in a 1:1 mixture of conc. HCl and MeOH, which provides
the corresponding α-amino methyl ester, in good yield (65 – 95%).[2,5,6] Besides the harsh acidic
conditions, limitations are the need for an additional hydrolysis step to obtain the corresponding free α-
amino acid. b) The ethanolamine/ethylenediamine methods are based on the principle of exchanging
the boroxazolidones complex with the more stable boroxazolidine/diazaborolidine complexes,
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10.1002/ejoc.201700063
European Journal of Organic Chemistry
respectively.[13] Yields reported are inconsistent, ranging from 30 – 90%.[2,5,6,14–20] c) Stirring the
boroxazolidones in chloroform, with a trace amount of methanol leads to slow decomposition of the
boroxazolidone complex to give the α-amino acid in 60 – 95%.[6,20–24] The residual acidity of the
chloroform is believed to be the key driver for this conversion. d) Oxidative cleavage of the carbon-
boron bonds by peracids proceeds with high conversion (>90%).[25] Trifluoroperacetic acid (TFPAA)
was shown to be superior to meta-chloroperbenzoic acid (mCPBA).[25] However, the scope of this
method is obviously limited by incompatibility with oxidizable functional groups.
Discussions
We set out the search for a versatile, non-oxidative and mild method for cleaving boroxazolidones. On
the note that the boron-oxygen and the boron-nitrogen bond are roughly the same in length, 1.623 Å
and 1.534 Å, respectively,[26] it was previously theorized that the boroxazolidone complex exists in an
equilibrium between the coordinative N→B bond and the coordinative C=O→B bond.[26] We then
hypothesized that these states could exist in equilibrium with their corresponding dissociated states, in
which the boron is sp2 hybridized. The vacant p orbital on boron would then be available for
nucleophilic attack. Concurrently, boron is considered fluorophilic,[27] and others have pointed to its
[28]
weak lewis acidity as the reason for it susceptibility towards attack by hard anions such as fluoride.
Summarily, we believed that addition of a nucleophilic fluoride to a boroxazolidone would yield the
corresponding free α-amino acid.
We set out to explore the nucleophilic fluorides KHF2, SbF3, CsF, pyridinium poly(hydrogen fluoride)
(PPHF), ammonium fluoride and tetrabutylammonium fluoride trihydrate (TBAF3H2O), for cleaving
the 9-BBN boroxazolidone complex of phenylalanine (2a) (Table 1). All reactions were initially
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10.1002/ejoc.201700063
European Journal of Organic Chemistry
performed with a large excess (5 equiv.) of fluoride. The reagent KHF2 in MeOH gave only 9%
conversion after 24 h (Table 1, entry 1). This was surprising as this reagent is used for converting
boronic acids to the corresponding potassium organotrifluoroborates.[29] SbF3 has been reported in the
successful conversion of the 9-BBN hydride-bridged dimer into the monofluoride analogue in
heptane.[30] Due to insolubility of 2a in heptane the reaction was instead performed in MeCN. After 24
hours, a 31% conversion to the free α-amino acid was observed (Table 1, entry 2). CsF was
investigated both in the presence of and without acetic acid. Acetic acid was added to increase the
solubility of the CsF.[31] In our hands, the addition of acetic acid indeed enhanced the conversion, from
79 to 90%, respectively (Table 1, entry 3 and 4). A trans-fluorination with HBF4·Et2O has been
reported on an aminoboryl compound.[32] HBF4·Et2O cleaved the boroxazolidone quantitatively within
1 hour (Table 1, entry 5). However, the acidity of HBF4·Et2O might be responsible rather than action of
fluoride. Finally, three fluoride sources were investigated which comprised an organic cation. Firstly,
PPHF was explored, which resulted in a 46% conversion (Table 1, entry 6). Ammonium fluoride was
dissolved in a 3:1 mixture of THF:H2O and stirred vigorously. After 24 hours, a conversion of 69%
was observed, and after a total of 48 hours, this had increased to 83% (Table 1, entry 7). But most
satisfyingly, a 1M THF solution of TBAF3H2O, gave quantitative conversion within only 1 hour
(Table 1, entry 8).
These results prompted us to optimise the reaction conditions for the NH4F and TBAF reagents (Table
2). One equivalent of NH4F at rt only led to 22% conversion after 24 hours (Table 2, entry 1), whereas
reflux gave 86% conversion (Table 2, entry 2). Addition of an additional equivalent of NH4F under
reflux did, however, not drive the reaction to completion even after 48 hours (Table 2, entry 3). One
equivalent of a TBAF3H2O-THF solution resulted in 68% cleavage of the boroxazolidone after 1 hour.
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Extending the reaction time to 2 hours did not increase the conversion (Table 2, entry 4) (see
supporting information for investigation on conversion). However, two equivalents gave near complete
conversion, after 1 hour (Table 2, entry 5) (see supporting information for investigation of conversion).
Table 1. Investigation of the influence on reactivity by the nucleophilic fluoride sources[a]
Entry
Fluoride source (5 equiv)
Solvent
MeOH
MeCN
MeOH
MeOH
Reaction time (h)
Conversion (%)[b]
1
2
3
4
5
6
7
8
KHF2
SbF3
CsF
24
24
24
24
1
24
24
1
9
31
79
90
CsF + AcOH (30 equiv.)
HBF4·Et2O
THF
THF
THF:H2O (3:1)
THF
100
64
PPHF
NH4F
69[c]
100
TBAF3H2O (1M in THF) [d]
[a] The reactions (0.10 M) were carried out with 5.0 equiv. of fluoride source at rt. [b] Conversion was determined by
HPLC, after 24 hours, with isochroman as internal standard. [c] 83% conversion after 48 h. [d] Contains 5 – 7 wt% H2O.
Table 2. Optimization of ammonium based fluoride sources[a]
Temp
(°C)
rt
Reflux
Reflux
React. Time
Conversion
Entry
Reagent
Equiv.
Solvent
(h)
24
24
24
1
1
1
24
24
2
4
1
(%)[b]
22
1
2
3
4
5
6
7
8
9
NH4F
NH4F
NH4F
1.0
1.0
2.0
1.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
THF:H2O (3:1)
THF:H2O (3:1)
THF:H2O (3:1)
THF
THF
THF
THF:H2O (1:1)
MeOH
86
85
68
99
96
16
61
99
TBAF3H2O (1M in THF)[c]
TBAF3H2O (1M in THF)[c]
TBAF3H2O (solid)
rt
rt
rt
rt
rt
rt
rt
rt
TBAF3H2O (1M in THF)[c]
TBAF3H2O (1M in THF)[c]
TBAF3H2O (1M in THF)[c]
TBAF3H2O (1M in THF)[c]
TBAF3H2O (1M in THF)[c]
MeCN
toluene
DMF
10
11
95
98
[a] The reactions were performed at a 0.10 M concentration. [b] Determined by HPLC, with isochroman as internal
standard. [c] 5 – 7 wt % H2O.
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10.1002/ejoc.201700063
European Journal of Organic Chemistry
The commercial TBAF3H2O-THF solution used has a water content of 5–7 wt%. We were interested
to examine what effect water has on the reaction and therefore repeated the reaction with solid
TBAF3H2O (Table 2, entry 6). Interestingly, we did not observe any difference in reactivity between
the two reagents, but when the experiment was run in THF:H2O (1:1), conversion was reduced to only
16% (Table 2, entry 7). Similarly, the conversion was also lowered when the reaction was performed in
MeOH (Table 2, entry 8). The solvent compatibility of the deprotection reaction was investigated in
DMF, MeCN and toluene, all wherein the reaction smoothly advanced to completion (Table 2, entry 9-
11).
Noteworthy was, that upon isolation of 1a, we observed that to fully liberate the α-amino acid, the
reaction mixture needed to be stirred for a minimum of five minutes in a protic solvent, i.e. H2O or
MeOH, before submitting the mixture directly to ion-exchange chromatography (DOWEX® 50WX2
400 or DOWEX® 1x8 400). In the case of stirring with MeOH, no esterification of the α-carboxylate
group was observed.
Possible epimerization of the α-carbon in 1a was addressed by submitting the isolated material to chiral
HPLC (Chirobiotic T column). The ee% was determined to be >99.9% (see supporting information),
which is in agreement with the enatiopurity of the commercial starting material.
With the optimized procedure in hand, we set out to investigate the scope and limitation of the reaction.
Firstly, the two boroxazolidone complexes of phenylalanine were prepared, comprising different
substituents on boron, i.e. the B,B-diphenyl- (2b) and B,B-diethyl phenylalanine boroxazolidone (2c)
analogs. Secondly, side chain functional group compatibility was investigated by preparing a series of
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10.1002/ejoc.201700063
European Journal of Organic Chemistry
9-BBN boroxazolidone complexes 2d-o from their corresponding zwitter ionic α-amino acids (1d-o)
(Table 3).[8,33,34] Side chains comprising silyl ethers were not included in the study due to their high
fluorophilic nature, which most likely would result in desilylation under TBAF conditions.[24]
Table 3. Formation of the boroxazolidiones 2a-o, and cleavage to give corresponding free amino acids 1a-o
React. Time
Yield
React. Time
Yield
Entry
α-Amino Acid
RBoron
(formation)
(formation)
(cleaving)
(cleaving)
(h)[a]
(%)[b]
98
(%)[b]
93
(h)[c]
1
1
2
3
4
5
6
7
8
L-Phe (1a)
L-Phe (1b)
L-Phe (1c)
Cyclooctane-1,5-diyl (2a)
Diphenyl (2b)
1
5
4
5
1
1
1
1
99
66
89
78
98
71
99
24
1
7
5
5
80
89
88
81
79
82
96
Diethyl (2c)
L-Glu (1d)
Cyclooctane-1,5-diyl (2d)
Cyclooctane-1,5-diyl (2e)
Cyclooctane-1,5-diyl (2f)
Cyclooctane-1,5-diyl (2g)
Cyclooctane-1,5-diyl (2h)
L-Glu-5-OMe (1e)
L-Gln (1f)
L-Pro (1g)
L-Met (1h)
7
1
L-Propargylglycine
(1i)
9
Cyclooctane-1,5-diyl (2i)
1
82
3
83
10
11
12
L-DOPA (1j)
L-Trp (1k)
L-Pyridylalanine (1l)
L-Glutathione (GSH)
(1m)
Cyclooctane-1,5-diyl (2j)
Cyclooctane-1,5-diyl (2k)
Cyclooctane-1,5-diyl (2l)
2
1
1
81
76
93
2
2
3
86
76
77
13
Cyclooctane-1,5-diyl (2m)
1
96
7
76
(S)-Adenosyl-L-
14
15
homocysteine (SAH) Cyclooctane-1,5-diyl (2n)[d]
(1n)
4[e]
4
75
58
3[f]
2
64
73
L-Biocytin (1o)
Cyclooctane-1,5-diyl (2o)
[a] The reactions (0.10 M) were carried out in MeOH with 1.2 equiv. of borinic methyl ester at reflux. [b] Isolated yield [c]
The reactions (0.10 M) were carried out in THF with 2.0 equiv. of TBAF3H2O at rt. [d] Double protected. [e] 2.5 equiv of
9-BBN-B-OMe. [f] 4 equiv of TBAF3H2O.
The synthesized boroxazolidones 2a–o were submitted to the optimized conditions and results are
summarized in Table 3. The two boron analogs, B,B-diphenyl complex 2b and the B,B-diethyl complex
2c gave 80% and 89%, respectively, which is in agreement with yield obtained for 2a to 1a. All side
chain functionalities comprised in the analogs 2d-o were compatible with reaction conditions and gave
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10.1002/ejoc.201700063
European Journal of Organic Chemistry
the corresponding free amino acid 1d-o in high yield (64–96%). In details, acidic functionalities
include carboxylic acids 2d and 2m and catechol 2j, which gave 1d, 1m and 1j in 88%, 76% and 86%,
respectively. The pH-neutral functionalities were well accepted and exemplified by phenyl 2a, diol 2n,
ether 2n, ester 2e, primary amide 2f, secondary amide 2m and 2o, urea 2o, and terminal alkyne 2i. The
heterocycles indole 2k, pyridine 2l and adenosine 2n, were also smoothly converted to their
corresponding α-amino acid in 76%, 77% and 64%, respectively. Finally, the sulfur containing analogs;
thioethers 2h, 2n and 2o and thiol 2m, were all compatible and did not undergo undesired oxidation.
Conclusions
In summary, we have shown that boroxazolidone complexes can be cleaved to the corresponding free
α-amino acids using different nucleophilic fluorides sources. Our study shows that ammonium based
fluorides sources were significantly more reactive then other fluoride sources investigated. Application
of 2 equiv. of the TBAF3H2O (1M THF solution or solid) was superior, leading to conversions of up
to 96%. A wide range of side chain functionalities were shown to be compatible with these reaction
conditions including carboxylic acids, esters, amides, ureas, ethers, diols, thiols, thioethesr, alkynes,
phenols (catechols), basic heterocycles. Furthermore, B,B-diphenyl and B,B-diethyl boroxazolidone
complexes are also cleaved with this procedure
Acknowledgements
The authors would like to thank the University of Copenhagen for financial support. We would like to
thank Eloi Astier for the synthesis of 2g.
Supporting Information
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European Journal of Organic Chemistry
See the supporting information for the experimental procedures, chiral analytical HPLC
13
chromatograms of isolated 1a together with reference compounds and hard copy of 1H- and C-NMR
spectra of 2a-o.
Conflicts of Interest
The authors declare no conflict of interest.
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TOC graphics
Tetrabutylammonium fluoride (TBAF) is a mild and versatile reagent for cleaving boroxazolidones to
their corresponding free α-amino acids. The reagent is compatible with a large number of functional
groups including acids, esters, amides, thiols, thioethers, alkynes, phenols, basic heterocycles and bio-
relevant molecules, such as glutathione (GSH), (S)-adenosyl-L-homocysteine (SAH) and L-biocytin.
This article is protected by copyright. All rights reserved.