Table 2 Hydroxamic acid synthesis using PS-HOBt and unprotected hydroxylamine
Hydroxamic acid
Cyclea
Yieldb (%)
Purityc (%)
[MHϩ] Calcd/Found
Benzohydroxamic acid
Benzohydroxamic acid
Benzohydroxamic acid
Benzohydroxamic acid
2,4-Dimethoxybenzohydroxamic acid
Phenylacetohydroxamic acid
Phenylacetohydroxamic acid
2-Naphthohydroxamic acid
N-Fmoc-L-Phe-NHOH
1
2
3
4
1
1
2
1
1
84d
91d
87e
78f
84e
55d
34d
76e
79e
84g
87g
83h
85h
96h
91g
90g
96h
98h
138.14/138.13
138.14/138.13
138.14/138.22
138.14/138.22
198.19/198.33
152.16/152.23
152.16/152.23
188.18/188.23
403.45/403.11
a Number of use of the same batch of PS-HOBt. b Calculated from the weight of hydroxamic acid isolated and based on the amount of hydroxyl-
amine used relative to the loading of the PS-HOBt resin (see footnotes d, e and f ). c Determined by reversed-phase HPLC analysis. d 0.5 Equiv.
hydroxylamine. e 0.6 Equiv. hydroxylamine. f 0.8 Equiv. hydroxylamine. g Detection at 235 and 280 nm (see Table 1 footnote c). h Detection at 220 and
235 nm (see Table 1 footnote c).
washing with common solvents, is completely automated and
complete in 4 hours. On the other hand, for amino acid
derivatives, the same batch of PS-HOBt resin can be used to
protect the amino group,16 and introduce, after resin recycling,
the hydroxamic acid functionality.
carboxylic acid in DMF (853 µl, 0.38 M) were then added to the
resin and the mixture was agitated for 2 min. A solution of DIC
in DCM (565 µl of a 1.65 M) was then added to the resin and
the resulting mixture was agitated for 3 h. The resin was
collected by filtration, washed successively with DMF, DCM,
DMF and THF (3 × 5 ml aliquots of each) and dried under
nitrogen.
Conclusion
To the resin-bound esters was added a solution of O-tert-
butyldimethylsilylhydroxylamine in THF (3 ml, 0.5–0.8
equivalents relative to the loading of the PS-HOBt resin) or a
solution of anhydrous hydroxylamine (3 ml, prepared from
hydroxylamine hydrochloride and sodium methoxide9c) in
2.3 : 1 THF–MeOH, and the mixture was agitated for 5 h. The
solution was filtered, the resin washed with THF (3 ml) and the
combined organic solutions evaporated under reduced pressure.
The cleavage of the tert-butyldimethylsilyl protecting group
was carried out by dissolution in 95 : 5 TFA–H2O (3 ml) and
stirring for 15 h. The solution was then concentrated with
nitrogen and the resulting solid was dried in vacuo.
This method describes a convenient synthesis of hydroxamic
acids combining the advantages inherent to both solid phase
synthesis and polymer assisted solution phase synthesis.17
Designed for the synthesis of low molecular weight hydroxamic
acids, it is also suitable for the preparation of hydroxamates of
higher molecular weight, by multi-step synthesis when the
sequence of reactions does not involve an N-nucleophile, or by
generating the hydroxamic acid functionality in the last step,18
by taking advantage of the mild reaction conditions.
Experimental
Materials and methods
Recycling of the PS-HOBt
The 1-hydroxybenzotriazole-6-sulfonamidomethyl polystyrene
resin (PS-HOBt, High Loading) was purchased from Argonaut
Technologies. All other reagents and solvents were purchased
from Aldrich and used without further purification.
The recycling of the solid phase to synthesise the same
hydroxamic acid was achieved by washing the spent resin with
THF (3 × 5 ml), DCM (3 × 5 ml), DMF (3 × 5 ml), DCM (3 × 5
ml), DMF (3 × 5 ml) and THF (3 × 5 ml).
The experiments were carried out on a Quest 210 ASW
Argonaut Technologies Organic Synthesiser. All reactions were
performed under an atmosphere of nitrogen. All percentage
yields in Tables 1 and 2 were estimated from the isolated weight
of each hydroxamic acid and are based on the amount
of O-tert-butyldimethylsilylhydroxylamine or hydroxylamine
used. Chromatographic analysis was performed on a PerSeptive
Biosystems BioCAD SPRINT Perfusion Chromatography
Workstation using POROS 20R2 Reversed Phase Perfusion
Chromatography packing (column: 4.6 mm d/100 mm L, self-
packed, A mobile phase: 0.1% TFA in water; B mobile phase:
0.1% TFA in acetonitrile – gradient: 2 to 60% B in 18 column
volumes at 7 ml minϪ1 flow rate). Purities were determined from
the area percent of the hydroxamic acid relative to the total area
of all UV absorbing components. While this is routine practice
in combinatorial chemistry, the reported purities are approxi-
mate since the relative responses of the various components are
neglected (although the carboxylic acids, the main impurities in
the present work, are expected to have similar responses to the
corresponding hydroxamic acids). Mass spectrometry was per-
formed on a PE Sciex API 3000 LC/MS/MS in electrospray
mode.
Recycling of the solid phase to produce
a different
hydroxamic acid was carried out as follow: the resin was washed
with DMF, then reacted with isopropylamine (5 equivalents
relative to the loading of the PS-HOBt resin) in DCM, and
washed with DCM (3 × 5 ml), DMF (3 × 5 ml), DCM (3 × 5
ml), NMP (3 × 5 ml), DCM (3 × 5 ml), DMF (3 × 5 ml), THF
(3 × 5 ml) and dried under nitrogen.
Acknowledgements
We thank the Irish Government under its ‘Programme for
Research in Third Level Institutions’, the Research Committee
of The Royal College of Surgeons in Ireland and the Health
Research Board for financial support; Mr Brendan Harhen,
Department of Clinical Pharmacology, RCSI, for MS analysis.
References
1 M. J. Miller, Chem. Rev., 1989, 89, 1563–1579.
2 (a) S. S. C. Tam, D. H. S. Lee, E. Y. Wang, D. G. Munroe and
C. Y. Lau, J. Biol. Chem., 1995, 270, 13948–13955; (b) B. De,
M. G. Natchus, M. Cheng, S. Pikul, N. G. Almstead, Y. O. Taiwo,
C. E. Snider, L. Chin, B. Barnett, F. Gu and M. Dowty, Ann. N. Y.
Acad. Sci., 1999, 878, 40–60; (c) A. J. Stemmler, J. W. Kampf,
M. L. Kirk and V. L. Pecoraro, J. Am. Chem. Soc., 1995, 117,
6368–6369.
Synthesis of hydroxamic acids
In a typical experiment, the PS-HOBt (150 mg, 1.41 mmol gϪ1
loading, 0.2 mmol) was pre-swollen with DMF. A solution of
DMAP in 1 : 1 DMF–DCM (2.42 ml, 0.05 M) and a solution of
3 R. Zamora, A. Grzesiok, H. Weber and M. Feelisch, Biochem. J.,
1995, 312, 333–339.
O r g . B i o m o l . C h e m . , 2 0 0 3 , 1, 8 5 0 – 8 5 3
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