The synthesis of Nα-protected amino hydroxamic acids from Nα-protected amino acids employing versatile chlorinating agent CPI-Cl

Article abstract of DOI:10.24820/ark.5550190.p010.727

Racemization free synthesis of N^α-protected amino hydroxamic acids from N^α-protected amino acids employing the versatile chlorinating reagent CPI-Cl has been described in one-pot. The present protocol has shown compability towards urethane protecting groups like Boc, Cbz and Fmoc, and side chain protections of amino acids showed complete tolerance.

Full text of DOI:10.24820/ark.5550190.p010.727

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The synthesis of Nα-protected amino hydroxamic acids from Nα-protected amino
acids employing versatile chlorinating agent CPI-Cl
U. Vathsala, C. Srinivasulu, L. Santhosh, and Vommina V. Sureshbabu*
#109, Peptide Research Laboratory, Department of Chemistry, Central College Campus,
Bangalore University, Bangalore-560001, India
Email: hariccb@hotmail.com; hariccb@gmail.com
Received 08-20-2018
Accepted 11-27-2018
Published on line 12-19-2018
Abstract
Racemization free synthesis of Nα-protected amino hydroxamic acids from Nα-protected amino acids
employing the versatile chlorinating reagent CPI-Cl has been described in one-pot. The present protocol has
shown compability towards urethane protecting groups like Boc, Cbz and Fmoc, and side chain protections of
amino acids showed complete tolerance.
Keywords: Nα-Protected amino hydroxamic acid, 3,3-dichloro-1,2-diphenylcyclopropene, hydroxyl amine
hydrochloride, racemization-free
DOI: https://doi.org/10.24820/ark.5550190.p010.727
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Introduction
Hydroxamic acids are an important class of compounds, which display a broad spectrum of biological
activities. Owing to their strong chelating ability towards Zn (II) ions, hydroxamic acids are excellent inhibitors
of matrix metalloproteinases.1 Compounds containing this functional moiety serve as anti-fungal (Figure 1a),2
anti-cancer (Figure 1b),3 anti-inflammatory (Figure 1c),4 anti-bacterial (Figure 1d),5 and other therapeutics.
They are also known as plant growth regulators.6 Furthermore, hydroxamic acids are important precursors to
obtain ureas,7 carbamates8 and thiocarbamates9 via Lossen rearrangement.
Figure 1. Selective display of biologically active hydroxamic acids.
Generally, hydroxamic acids are prepared by coupling activated carboxylic acid with O/N-protected
hydroxylamine or deprotonated hydroxylamine hydrochloride. For this purpose, activation of carboxylic acids
via converting them to acid halides were achieved using thionyl chloride (SOCl2),10 and
tetramethylfluoroformamidinium hexafluorophosphate (TFFH) and benzyl triphenylphosphonium dihydrogen
trifluoride (PTF).11 Hydroxamic acids have been prepared by coupling mixed anhydride, obtained by the
addition of carboxylic acid and ethyl chloroformate, with hydroxylamine.7, 12
14
Other coupling reagents like cyanuric chloride,13 N, N’-carbonyldiimidazole (CDI),
1-[(1-(cyano-2-
hexafluoro-
ethoxy-2-oxoethylidenaminooxy)-dimethylamino-morpholinomethylene)]
methaneaminium
phosphate (COMU),15 propanephosphonic acid anhydride (T3P),16,17 and ethyl-2-cyano-2-(4-
nitrophenylsulfonyloxyimino)acetate (NBsOXY),18 were also employed to obtain hydroxamic acids from their
corresponding carboxylic acids.
Literature has recorded a plethora of reports on synthesis of hydroxamic acids from carboxylic acid
esters.19-23 Especially, esters were found to be almost indispensable precursors to obtain hydroxymic acids in
solid phase strategies.24-26
However, use of toxic and irritative ethyl chloroformate adds inconvenience, though the synthesis of
hydroxamic acid from the corresponding carboxylic acid uses minimum reaction duration. Most of the
reactions employed require long reaction durations, especially with the use of ester as precursor. Particularly,
solid phase synthesis of hydroxamic acids have major disadvantages like they require profuse use of reagents,
longer reaction times, and an additional step to release hydroxamic acid from resins makes solid phase
strategy less preferred way to obtain hydroxamic acids. There are reports where hydroxamic acids were
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prepared from aldehydes;27-29 however, use of catalysts and prolonged reaction durations makes the protocol
less attractive. In addition, in amino acid chemistry, carboxylic acids are readily available to obtain hydroxamic
acids compared to their aldehyde derivatives.
As aforementioned not many halogenating reagents have been reported to obtain hydroxamic acids
from their corresponding carboxylic acids, especially in the synthesis of amino acid derived hydroxamic acids,
as it involves acid sensitive Boc group. The halogenating reagents like SOCl2, oxalyl chloride and POCl3 are not
suitable for halogenation for Boc-protected amino acids. Therefore, activating carboxylic acids with
halogenating reagent having compatibility towards acid sensitive groups to obtain hydroxamic acids can be
considered a useful strategy.
3,3-Dichloro-1,2-diphenylcyclopropene (CPI-Cl) has found its use not only as stoichiometric reagent but
also as catalyst. As a result, it has been employed in many synthetic processes. CPI-Cl has been converted to
cyclopropenylidene carbene complexe catalysts and employed for C-C bond formation reactions.30,31 It was
also used to chlorinate carboxylic acids 32 and alcohols. 33 It was found to catalyze Beckmann rearrangement to
obtain amides/lactams from corresponding oximes.34,35 Recently, Sureshbabu et al. reported Nα-protected
amino acid azides employing stoichiometric CPI-Cl from Nα-protected amino acid without affecting acid
sensitive groups. 36
In this regard, we herein demonstrate a useful application of CPI-Cl in the preparation of Nα-protected
amino hydroxamic acids from Nα-protected amino acids.
Results and Discussion
In the initial trial reaction, Cbz-Gly (1a, 1.0 mmol) was taken as model substrate in CH2Cl2, and DIPEA (1.1
mmol) and CPI-Cl (generated by the treatment of 2, 3-diphenylcyclopropenone with oxalyl chloride in CH2Cl2,
1.0 mmol) were added at room temperature and stirred at the same temperature for ten minutes.
o
The reaction mixture was then cooled to -10 C and treated with deprotonated hydroxylamine
hydrochloride (obtained by treating hydroxylamine hydrochloride with methanolic potassium hydroxide) and
stirred till the completion of the reaction. After simple aqueous workup and column chromatography, the
compound 2a was isolated in good yield. In the next reaction, Boc-Phg was taken and the protocol was found
to be compatible towards acid sensitive Boc-group, too. Similarly, Fmoc-amino acid hydroxamic acids were
obtained in good yields. During the present study, water miscible solvents like acetonitrile, acetone and N, N’-
dimethylformamide were also tested and found to give poor yields of products.
Scheme 1. Synthesis of Nα-protected amino hydroxamic acids using CPI-Cl
Nα-protected serine, threonine and cystiene when employed in the present protocol, resulted in side
reactions and found not suitable to use without their side chain protections. Fortunately, on employing their
side chain protected analogues resulted in hydroxamic acids without affecting their side chain protections.
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Side chains of amino acids protected by benzyl, tertiary butyl and trityl groups were found to be stable
towards present approach.
Table 1. Library of Nα-protected amino hydroxamic acids
#β-amino acid
Interestingly, 2, 3-diphenylcyclopropenone generated at the end of the reaction can be isolated by
column chromatography and reused to prepare CPI-Cl.
Fmoc-D-Phe-NHOH was prepared and compared with Fmoc-L-Phe-NHOH by RP-HPLC studies using
chiral column. Retention time (tR) of Fmoc-L-Phe-NHOH was found to be 8.14 min and of Fmoc-D-Phe-NHOH
was 5.51 min (Figures 2 and 3). Further, Fmoc-L-Phg-NHOH and Fmoc-D-Phg-NHOH showed retention times
9.16 min and 7.08 min, respectively, and thier equimolar mixture gave two distinct peaks at 9.77 and 7.56 min.
These studies confirmed the present method is racemization free.
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Figure 2. RP-HPLC profile of Fmoc-L-Phe-NHOH
Figure 3. RP-HPLC profile of Fmoc-D-Phe-NHOH
Conclusions
We have developed a mild protocol to synthesize Nα-protected amino hydroxamic acids from Nα-protected
amino acids employing CPI-Cl. The reaction found to tolerate commonly employed N-protecting groups such
as Boc, Cbz and Fmoc and retain optical purity of amino acids.
Experimental Section
General. All the chemicals were purchased from Sigma Aldrich Company, USA. All the solvents were freshly
distilled and dried whenever required. TLC analysis was carried out using Merck aluminium TLC sheets (Silica
gel 60 F254), the chromatograms were visualized by UV light and also by exposing in an iodine chamber.
Column chromatography using mixtures of ethyl acetate and hexane as eluents through silica gel (100-200
1
13
mesh). HRMS spectra were recorded in a XEVO-G2-XS-Q-TOF mass spectrometer. H and
C NMR were
determined in Brucker AV NMR (400 MHz, 100 MHz) spectrometer. Melting points were determined in an
open capillary and are uncorrected. Optical rotations were recorded at 25 oC. The RP-HPLC analysis of epimers
was carried out using an Agilent instrument (method: gradient 0.1% TFA water-acetonitrile (0–100%) in 20
min; VWD at λ 254 nm; flow rate: 1.0 mL/min; column: Agilent Eclipse, XDB-C18, pore size 5 μm, diameter ×
length = 4.6 × 150 nm).
General procedure for the synthesis of Nα-protected amino hydroxamic acid. To a solution of 3,3-dichloro-
o
1,2-diphenylcyclopropene at -10 C [1.1 equiv. prepared by adding diphenylcyclopropenone (1.0 equiv.) in
DCM and oxalyl chloride (1.0 equiv.) at rt and stirred until the gas evolution has ceased] was added a solution
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of Nα-protected amino acid (1.0 equiv.) and diisopropylethylamine (1.2 equiv.) in dichloromethane and stirred.
After 10 min hydroxylamine hydrochloride (NH2OH.HCl, 1.5 equiv.) in methanolic potassium hydroxide was
added to the reaction mixture and stirred of another 45 min. The solvent was removed under reduced
pressure; the residue obtained was diluted with EtOAc, washed with 10% citric acid solution, water and brine
solution. The organic phase was dried over anhydrous Na2SO4 and removed under reduced pressure. The
crude residue was purified by column chromatography using n-hexane and ethyl acetate as eluents.
Benzyl (2-(hydroxyamino)-2-oxoethyl)carbamate (2a). White solid; Yield 93%; [α]25 ˗1.6 (c 1.0, MeOH); mp
D
115-117 °C; 1H NMR (400 MHz, DMSO-d6): δ 10.52 (s, 1H), 8.79 (s, 1H), 7.46-7.31 (m, 5H), 5.04 (s, 2H), 3.54 (d,
J 8 Hz, 2H);13C NMR (100 MHz, DMSO-d6): δ166, 156.3, 137, 128.3, 127.7, 127.6, 65.4, 41.4; HRMS (ESI): calcd.
for C10H12N2O4 [M+ Na]+: 247.0695, found: 247.0696.
Benzyl 2-(hydroxycarbamoyl)pyrrolidine-1-carboxylate (2b). White solid; Yield 92%; [α]25 ˗ 49.6 (c 1.0,
D
MeOH); mp 81-83 °C; 1H NMR (400 MHz, DMSO-d6): δ 10.6 (s, 1H), 7.39-7.31 (m, 5H), 5.08 (dd, J 12 Hz, 12 Hz,
2H), 4.15-4.05 (m, 1H), 3.51-3.39 (m, 2H), 1.95-1.76 (m, 4H);13C NMR (100 MHz, DMSO-d6): δ 168.7, 153.9,
136.9, 128.3, 128.2, 127.7, 127.4, 126.9, 65.8, 57.9, 47, 30, 23.8; HRMS (ESI): calcd. for C13H16N2O4 [M+ Na]+:
287.1008, found: 287.1007.
(S)-tert-Butyl (1-(hydroxyamino)-1-oxopropan-2-yl)carbamate (2c). White solid; Yield 89%; [α]25D ˗20.1 (c 1.0,
MeOH); mp 114-116 °C; 1H NMR (400 MHz, CDCl3): δ 9.51 (s, 1H), 5.05 (s, 1H), 4.19-4.09 (m, 1H), 1.43 (s, 9H),
1.38 (d, J 4Hz, 3H);13C NMR (100 MHz, CDCl3): δ170.3, 155.8, 72.4, 47.5, 28.3, 17.6; HRMS (ESI): calcd. for
C8H16N2O4 [M+ Na]+: 227.1008, found: 227.1008.
tert-Butyl (1-(hydroxyamino)-3-methyl-1-oxobutan-2-yl)carbamate (2d). White solid; Yield 90%; [α]25 ˗10.5
D
1
(c 1.0, DMSO); mp 137-139 °C; H NMR (400 MHz, DMSO-d6): δ 10.53 (s, 1H), 8.81 (s, 1H), 3.58 (t, J 8 Hz, 1H),
1.90-1.81 (m, 1H), 1.39 (s, 1H), 0.85 (d, 6H);13C NMR (100 MHz, DMSO-d6): δ168, 155.2, 77.8, 57.6, 30.1, 28.1,
19, 18.6; HRMS (ESI): calcd. for C10H20N2O4 [M+ Na]+: 255.1321, found: 255.1321.
tert-Butyl (3-(hydroxyamino)-3-oxopropyl)carbamate (2e). White solid; Yield 87%; mp 73-75 °C; 1H NMR (400
MHz, DMSO-d6): δ 10.39 (s, 1H), 8.73 (s, 1H), 3.12 (dd, J 12 Hz, 4 Hz, 2H), 2.12 (t, J 8 Hz, 2H), 1.38 (s, 9H);13C
NMR (100 MHz, DMSO-d6): δ167.2, 155.4, 77.5, 36.6, 32.7, 28.1; HRMS (ESI): calcd. for C8H16N2O4[M+ Na]+:
227.1008, found: 227.1008.
(S)-tert-butyl (2-(hydroxyamino)-2-oxo-1-phenylethyl)carbamate) (2f). The spectral data of the compound
was similar to that of earlier report.13
(9H-Fluoren-9-yl)methyl (1-(hydroxyamino)-1-oxo-3-phenylpropan-2-yl)carbamate (2g). White solid; Yield
81%; [α]25D ˗45.5 (c 1.0, DMSO); mp 151-153 °C; 1H NMR (400 MHz, DMSO-d6): δ 10.72 (s, 1H), 8.88 (s, 1H),
7.90-7.18 (m, 13H), 4.22-4.02 (m, 4H), 2.96-2.82 (m, 2H);13C NMR (100 MHz, DMSO-d6): δ 168.1, 155.6, 143.7,
140.6, 137.9, 129.1, 128, 127.5, 127, 126.2, 125.2, 120, 65.6, 53.9, 46.5, 37.6; HRMS (ESI): calcd. for C24H22N2O4
[M+ Na]+: 425.1477, found: 425.1478.
(9H-Fluoren-9-yl)methyltert-butyl (6-(hydroxyamino)-6-oxohexane-1,5-diyl)dicarbamate (2h). White solid;
1
Yield 81%; [α]25 ˗12.7 (c 1.0, MeOH); mp 120-122 °C; H NMR (400 MHz, DMSO-d6): δ 10.61 (s, 1H), 8.82 (s,
D
1H), 7.91-7.33 (m, 8H), 6.76 (s, 1H), 4.27-4.14 (m, 3H), 3.85 (dd, J 12 Hz, 8 Hz, 1H), 2.91-2.84 (m, 2H), 1.84-1.71
(m, 2H), 1.60-1.54 (m, 2H), 1.38 (s, 9H), 1.30-1.15 (m, 2H);13C NMR (100 MHz, DMSO-d6): δ168.6, 155.8, 155.5,
143.7, 140.6, 127.6, 127, 125.3, 120, 77.3, 65.6, 52.3, 46.6, 42.5, 31.6, 29.1, 28.2, 22.7; HRMS (ESI): calcd. for
C26H33N3O6 [M+ Na]+: 506.2267, found: 506.2266.
(S)-(9H-Fluoren-9-yl)methyl(3-(tert-butoxy)-1-(hydroxyamino)-1-oxopropan-2yl)carbamate (2i). White solid;
1
Yield 79%; [α]25D+2.1 (c 1.0, MeOH); mp 136-138 °C; H NMR (400 MHz, CDCl3): δ 9.49 (s, 1H), 7.75-7.26 (m,
8Hz), 5.80 (d, J 4 Hz, 1H), 4.40-4.17 (m, 4H), 3.72 (m, 1H), 3.43 (t, J 8 Hz, 1H), 1.17 (s, 9H);13C NMR (100 MHz,
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CDCl3): δ 168.5, 156.2, 143.6, 141.3, 127.7, 127.1, 125.1, 120, 79.81, 67.3, 61.3, 53.1, 47.1, 27.3; HRMS (ESI):
calcd. for C22H26N2O5 [M+ Na]+: 421.1739, found: 421.1737.
tert-Butyl ((2S, 3S)-3-(benzyloxy)-1-(hydroxyamino)-1-oxobutan-2-yl)carbamate (2j). White solid; Yield 88%;
[α]25D+7.8 (c 1.0, MeOH); mp 98-100 °C; 1H NMR (400 MHz, CDCl3): δ 8.76 (s, 1H), 7.34-7.26 (m, 5H), 5.48 (d, J
8Hz, 1H), 4.53 (dd, J 24Hz, 8Hz, 2H), 4.32-4.24 (m, 1H), 4.19-4.13 (m, 1H), 1.43 (s, 9H), 1.16 (d, J 8Hz, 3H);13C
NMR (100 MHz, CDCl3): δ 168.3, 159.4, 137.7, 128.5, 127.9, 127.8, 80.6, 74.2, 71.6, 56.4, 28.2, 15.7; HRMS
(ESI): calcd. for C16H24N2O5 [M+ Na]+: 347.1583, found: 347.1582.
(S)-(9H-fluoren-9-yl)methyl (2-(hydroxyamino)-2-oxo-1-phenylethyl)carbamate (2k). White solid; Yield 91%;
1H NMR (400 MHz, DMSO-d6): δ 10.93 (s, 1H), 8.96 (s, 1H), 7.87 (d, J 8 Hz, 2H), 7.76 (d, J 8 Hz, 2H), 7.45-7.27
(m, 9H), 5.12 (d, J 12 Hz, 1H), 4.24-4.17 (m, 3H); 13C NMR (100 MHz, DMSO-d6): δ 166.6, 155.6, 143.6, 140.6,
138.5, 128.1, 127.5 (4C), 127, 125.4 (3C), 120, 65.9, 55.8, 46.5.
(R)-(9H-fluoren-9-yl)methyl (2-(hydroxyamino)-2-oxo-1-phenylethyl)carbamate (2k*). White solid; Yield 90%;
1H NMR (400 MHz, DMSO-d6): δ 10.93 (s, 1H), 8.96 (s, 1H), 7.87 (d, J 8 Hz, 2H), 7.76 (d, J 8 Hz, 2H), 7.45-7.27
(m, 9H), 5.13 (d, J 8 Hz, 1H), 4.24-4.17 (m, 3H); 13C NMR (100 MHz, DMSO-d6): δ 166.6, 155.6, 143.6, 140.6,
138.5, 128.1, 127.5, 127 (4C), 125.4, 125.3, 120, 65.9, 55.8, 46.5.
(R)-(9H-fluoren-9-yl)methyl (1-(hydroxyamino)-1-oxo-3-(tritylthio)propan-2-yl)carbamate (2l). White solid;
1
Yield 85%; H NMR (400 MHz, DMSO-d6): δ 10.69 (s, 1H), 8.88 (s, 1H), 7.87 (d, J 8 Hz, 2H), 7.74 (d, J 8 Hz, 2H),
7.41-7.19 (m, 19H), 4.26-4.16 (m, 3H), 4.02 (d, J 16Hz, 1H), 2.39-2.28 (m, 2H); 13C NMR (100 MHz, DMSO-d6): δ
166.4, 155.4, 144.2, 143.6, 140.6, 129, 128, 127.5, 126.9, 126.7, 125.3, 120, 65.8, 59.6, 51.6, 46.5, 33.9.
Acknowledgements
We thankfully acknowledge SERB (EMR/2016/002432) for funding this work. Indian Institute of Science (IISc)
for providing spectral studies is also thankfully acknowledged.
Supplementary Material
1H NMR, 13C NMR, Mass and RP-HPLC data for the title compounds.
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