Efficient Conversions of Carboxylic Acids into O-Alkyl, N-Alkyl and O,N-Dialkylhydroxamic Acids

Article abstract of DOI:10.1055/s-2003-42488

Carboxylic acids were conveniently converted into unsubstituted, N-alkyl-, O-alkyl-, and O,N-dialkylhydroxamic acids via acylbenzotriazole intermediates. The ready availability of the reagents, mild conditions, and easy handling of the intermediates are advantageous.

Full text of DOI:10.1055/s-2003-42488

PAPER
2777
Efficient Conversions of Carboxylic Acids into O-Alkyl, N-Alkyl and
O,N-Dialkylhydroxamic Acids
C
onversions of Ca
l
r
boxyli
a
c
A
cids int
n
o
O
-Alkyl,
N
-Alky
R
l,
O
,
N
-Dialkylhy
.
doxamic
Aci
K
ds atritzky,* Nataliya Kirichenko, Boris V. Rogovoy
Center for Heterocyclic Compounds, Department of Chemistry, University of Florida, Gainesville, FL 32611-7200, USA
Fax +1(352)3929199; E-mail: Katritzky@chem.ufl.edu
Received 8 May 2003; revised 21 July 2003
azine and N-methylmorpholine;22 e) tetrafluoroborate or
hexafluorophosphate thiouronium salts derived from 2-
mercaptopyridone N-oxide and tetramethylurea or N,N’-
dimethylpropyleneurea;23 and f) after conversion to mixed
anhydrides24 or esters;11 (ii) lactones or esters in the pres-
ence of dimethylaluminum chloride;25 (iii) esters in the
presence of isopropyl magnesium chloride,26a and (iv)
acylbenzotriazoles.26b
Abstract: Carboxylic acids were conveniently converted into un-
substituted, N-alkyl-, O-alkyl-, and O,N-dialkylhydroxamic acids
via acylbenzotriazole intermediates. The ready availability of the
reagents, mild conditions, and easy handling of the intermediates
are advantageous.
Key words: hydroxamic acid, Weinreb amide, carboxylic acids, re-
gioselectivity, nucleophilic
The conversion of carboxylic acids into amides, esters and
derivatives of hydroxamic acid in the presence of carbon-
yl diimidazole (CDI) described by Staab et al.27 is a gen-
eral method which has found wide application. The use of
CDI recently has been reported for the synthesis of hy-
droxamic acids28 and Weinreb amides.29
Acylbenzotriazoles are neutral acylating agents,30 suc-
cessfully used for the preparation of primary, secondary
and tertiary amides30a and cinnamoyl hydrazides,31 forma-
mides,32 trifluoroacetylamides,33 and oxamides.34 Recent-
ly, various acylbenzotriazoles were used for the
preparation of Weinreb amides.26b We now report further
investigations on the use of acylbenzotriazoles for the
preparation of diverse O-alkyl, N-alkyl and O,N-dialkyl
hydroxamic acids.
Hydroxamic acids are important biologically active com-
pounds with a broad spectrum of applications. They pos-
sess matrix metalloproteinase inhibition properties1 and
antibiotic activity.2 The therapeutic potential of hydrox-
amic acids and the synthesis of hydroxamic acid based si-
derophores have been summarized.3 Preparative methods
for hydroxamic acids and their applications have been in-
vestigated for 150 years.4
As starting materials, hydroxamic acids provide access to
many compounds of biological and synthetic interest,
such as , -difluoro- -hydroxy ketones,5 1,2-benzisox-
azolin-3-ones,6 protected amino aldehydes,7 -amino and
-hydroxy acids,8 -amino acids,9 -fluoroalkyl ke-
tones,10 and 1-benzyloxyphenyl-3-phenylacetones.11
Recent reports on the synthesis of hydroxamic acids in-
clude preparations from (i) esters with O-benzylhydroxy-
lamine in the presence of trimethylaluminum in refluxing
toluene12 or when prone to enolization, with N-(tert-bu-
toxycarbonyl)-O-(2-pyridylsulfonyl)hydroxylamine;13
(ii) hydroxamoyl chlorides with olefins in the presence of
The acylbenzotriazoles 2a–d were conveniently prepared
from carboxylic acids by reaction with 1-methanesulfo-
nyl-1H-benzotriazole as reported previously.30 Reaction
of 1-benzoyl-1H-benzotriazole (2a) with hydroxylamine
hydrochloride in the presence of potassium tert-butoxide
in THF at 20 °C gave the desired hydroxamic acid 3a in
only 17% yield with benzotriazole and benzoic acid as by-
products. Similar reaction with O-ethylhydroxylamine
gave the corresponding O-ethylhydroxamic acid 3b (iso-
lated yield 24%) along with benzotriazole, O-unsubstitut-
ed benzoylhydroxamic acid and benzoic acid. However,
changing the base to triethylamine improved the yield of
O-ethylbenzoylhydroxamic acid up to 61%, with benzo-
triazole as the only by-product, which was easily removed
by washing with aqueous sodium carbonate to give O-eth-
yl benzoylhydroxamic acid (3b) as pure compound. The
yield of benzoylhydroxamic acid (3a) was 91% with the
use of triethylamine; the purification required column
chromatography. O-Benzylhydroxylamine gave O-ben-
zylbenzoylhydroxamic acid (3c) in 87% yield after wash-
ing the reaction mixture with aqueous sodium carbonate.
We extended the preparation of hydroxamic acid deriva-
tives using hydroxylamine, O-ethyl- and O-benzylhy-
droxylamine, N-methylhydroxylamine and N,O-
base;14
(iii)
N-acyloxazolidinones15a
and
2-
acyloxypyridines15b with N- and/or O-substituted hydrox-
ylamines; and (iv) from mixed anhydrides in neutral con-
ditions.16
Weinreb amides are hydroxamic acid derivatives which
play a special role as building blocks for the synthesis of
ketones and aldehydes17 including -amino derivatives.18
Synthetic approaches to Weinreb amides include reac-
tions of O,N-dimethylhydroxylamine with (i) carboxylic
acids in the presence of a) [bis(2-methoxyethyl)ami-
no]sulfur trifluoride (Deoxo-Fluor reagent);19 b) tribu-
tylphosphine, triethylamine and (2-pyridine N-
oxide)disulfide;20 c) pivaloyl chloride via an intermediate
mixed anhydride;21 d) 2-chloro-4,6-dimethoxy-1,3,5-tri-
SYNTHESIS 2003, No. 18, pp 2777–2780
x
x.
x
x
.
2
0
0
3
Advanced online publication: 17.11.2003
DOI: 10.1055/s-2003-42488; Art ID: M02003SS
© Georg Thieme Verlag Stuttgart · New York

2778
A. R. Katritzky et al.
PAPER
dimethylhydroxylamine hydrochlorides with a variety of We have developed a practical and convenient method for
acylbenzotriazoles (Scheme 1). The results are given in the preparation of O-alkyl, N-alkyl and O,N-dialkyl hy-
Table 1.
droxamic acids. The simplicity of the reagents, mild con-
ditions, the easy handling of the starting materials and
intermediates are advantageous. Acylbenzotriazoles were
shown to be efficient reagents for the preparation of a full
variety of hydroxamic acids in neutral conditions whereas
conversion of esters are limited to substituted hydroxyl-
amines or required reaction media with pH > 1016 or
strong base for activation.4,26a Other published procedures
to hydroxamic acids from esters provide high yields, but
utilize environmentally challenged pyridine as a solvent.4
N-Acyloxazolidinones in the presence of Sm(OTf)3 give
moderate to good yields of final compounds and require
removal of oxazolidinone as by-product by chromatogra-
phy.15a Our method avoids the use of dangerous com-
pounds and large quantities of solvents; environmentally
safe benzotriazole, the only by-product, can be easily re-
moved. The use of trimethylaluminum was limited only to
N-benzylhydroxylamine;12 use of hydroxamoyl chlorides
with olefins affords only N-vinylhydroxamates14 lacking
an O-substituent.
Independently of the substituent in both N-acylbenzotria-
zole and hydroxylamine, the desired O-alkyl, N-alkyl and
O,N-dialkyl hydroxamic acids were obtained as sole prod-
ucts as a result of nucleophilic displacement of the benzo-
triazolyl moiety by the hydroxylamine nitrogen.
Exceptionally, in the reaction of 2-furoylbenzotriazole
(2b) with N-methylhydroxylamine hydrochloride, the de-
sired N-hydroxy-N-methyl-2-furamide (3g) (Table 1, En-
try 7) was obtained along with 28% of the isomeric
product
N-[(2-furylcarbonyl)oxy]methanamine (4),
which is a result of nucleophilic attack by oxygen. Similar
reaction of N-methylhydroxylamine with 1H-benzotria-
zol-1-yl(2-pyridinyl)methanone (2c) gave only the de-
sired N-hydroxy-N-methyl-2-pyridinecarboxamide (3k).
No product of type 4 was detected in the reaction mixture.
R2ONHR3.HCl
Et3N,THF, r.t.
Bt
O
O
O
BtSO2CH3
O
THF,
rt, 18 h
R1
N
R2
R1
OH
1a-d
R1
2a-d
R3
a: R1 = Ph;
3a-oa
b: R1 = 2-furyl
c: R1 = 2-pyridyl
d: R1 = n-butyl
Melting points were determined using a capillary melting point ap-
paratus equipped with a digital thermometer and are uncorrected.
1H and 13C NMR spectra were recorded on a 300 MHz NMR spec-
trometer (300 and 75 MHz, respectively) using CDCl3 or acetone-
d6 as stated, with tetramethylsilane as an internal standard. Column
chromatography was conducted with silica gel 230–400 mesh. All
other reagents were of reagent grade and were used without purifi-
cation. Acylbenzotriazoles 2a–d were prepared as described.30
O
a For 3g, a by-product 4
was isolated in 28% yield
(R1 = 2-furyl, R2 = H, R3 = Me).
O
R1 HN R3
4
Scheme 1
Table 1 Preparation of O,N-Alkylated Hydroxamic Acids 3a–o
O-Alkyl, N-Alkyl and O,N-Dialkylhydroxamic Acids 3a–o;
General Procedure
Entry
Product
R1
R2
R3
Yield
(%)
Acylbenzotriazole 2a–d (1.5 mmol) was dissolved in anhyd THF
(10 mL) followed by the addition of the hydroxylamine hydrochlo-
ride of choice (1.65 mmol) and Et3N (0.23 mL, 1.65 mmol). The
mixture was stirred vigorously for 16 h at r.t. and filtered through
Celite to remove the hydrochloric salt of Et3N. For 3b,i,m, after ro-
tary evaporation of THF, the crude product was dissolved in CH2Cl2
and washed with aq Na2CO3; no additional purification was re-
quired. Compounds 3c,f,j were purified by recrystallization. Com-
1
2
3a
3b
3c
3d
3e
3f
Ph
H
H
91
61
87
84
78
80
64
89
64
91
92
80
79
94
65
Ph
Et
Bn
H
H
3
Ph
H
4
2-furyl
2-furyl
2-furyl
2-furyl
2-pyridyl
2-pyridyl
2-pyridyl
2-pyridyl
n-Bu
H
pounds
3a,d,e,g,h,k,l,n,o
were
purified
by
column
5
Et
Bn
H
H
chromatography.
6
H
N-Hydroxybenzamide (3a)
7
3g
3h
3i
Me
H
Off-white microcrystals after gradient column chromatography
with EtOAc–hexanes (from 1:3 to 1:1); mp 126–128 °C (Lit.35 mp
125–128 °C).
1H NMR (acetone-d6): = 7.43–7.57 (m, 3 H), 7.84–7.87 (m, 2 H),
8.92 (br s, 1 H), 10.93 (br s, 1 H).
13C NMR (acetone-d6): = 127.9, 129.4, 132.4, 133.1, 166.3.
8
H
9
Et
Bn
H
H
10
11
12
13
14
15
3j
H
3k
3l
Me
H
N-Ethoxybenzamide (3b)
H
White microcrystals were obtained after the removal of benzotria-
zole by washing with aq Na2CO3; mp 60–61 °C (Lit.36 mp 58–
59 °C).
1H NMR (CDCl3): = 1.26 (t, J = 7.0 Hz, 3 H), 4.04 (q, J = 7.0 Hz,
2 H), 7.34–7.40 (m, 2 H), 7.45–7.50 (m, 1 H), 7.75–7.79 (m, 2 H),
9.88 (br s, 1 H).
3m
3n
3o
n-Bu
Et
Bn
Me
H
n-Bu
H
n-Bu
Me
13C NMR (CDCl3): = 13.3, 71.8, 127.1, 128.3, 131.6, 131.8, 166.1.
Synthesis 2003, No. 18, 2777–2780 © Thieme Stuttgart · New York

PAPER
Conversions of Carboxylic Acids into O-Alkyl, N-Alkyl and O,N-Dialkylhydroxamic Acids
2779
N-(Benzyloxy)benzamide (3c)
N-Ethoxy-2-pyridinecarboxamide (3i)
Recrystallized from benzene to give white flakes; mp 103–104 °C
Light brown oil was obtained after the removal of benzotriazole by
washing with aq Na2CO3.
(Lit.37 mp 100–102 °C).
1H NMR (CDCl3): = 5.02 (s, 2 H), 7.35–7.45 (m, 7 H), 7.46–7.52
(m, 1 H), 7.66 (d, J = 7.3 Hz, 2 H), 8.81 (br s, 1 H).
13C NMR (CDCl3): = 78.3, 127.0, 128.6, 128.6, 128.8, 129.3,
131.9, 132.0, 135.2, 166.4.
1H NMR (CDCl3): = 1.36 (t, J = 7.0 Hz, 3 H), 4.13 (q, J = 7.0 Hz,
2 H), 7.43–7.48 (m, 1 H), 7.84–7.90 (m, 1 H), 8.19 (d, J = 7.7 Hz, 1
H), 8.51–8.54 (m, 1 H), 10.34 (br s, 1 H).
13C NMR (CDCl3): = 13.5, 72.4, 122.3, 126.6, 137.4, 148.1, 149.0,
161.9.
N-Hydroxy-2-furamide (3d)
Anal. Calcd for C8H10N2O2: C, 57.82; H, 6.07; N, 16.86. Found: C,
57.57; H, 6.19; N, 16.74.
White microcrystals were obtained after column chromatography
with EtOAc–hexanes (1:1); mp 124–125 °C (Lit.38 mp 121–
122 °C).
1H NMR (acetone-d6): = 6.59 (dd, J = 3.4, 1.8 Hz, 1 H), 7.09 (d,
J = 3.4 Hz, 1 H), 7.68 (dd, J = 1.8, 0.8 Hz, 1 H), 8.71 (br s, 1 H),
10.65 (br s, 1 H).
N-(Benzyloxy)-2-pyridinecarboxamide (3j)
Recrystallized from CHCl3–hexanes to give white needles; mp 95–
96 °C.
1H NMR (CDCl3): = 5.06 (s, 2 H), 7.37–7.49 (m, 6 H), 7.85 (td,
J = 7.7, 1.6 Hz, 1 H), 8.18 (d, J = 7.7 Hz, 1 H), 8.45–8.47 (m, 1 H),
10.23 (br s, 1 H).
13C NMR (acetone-d6): = 112.5, 114.7, 145.7, 147.4, 157.9.
N-Ethoxy-2-furamide (3e)
White microcrystals were obtained after gradient column chroma-
tography with EtOAc–hexanes (1:2 1:1); mp 81–82 °C.
1H NMR (CDCl3): = 1.31 (t, J = 7.0 Hz, 3 H), 4.08 (q, J = 7.0 Hz,
2 H), 6.50 (dd, J = 3.5, 1.8 Hz, 1 H), 7.20 (d, J = 3.5 Hz, 1 H), 7.44–
7.45 (m, 1 H), 9.61 (br s, 1 H).
13C NMR (CDCl3): = 78.5, 122.3, 126.7, 128.6, 128.7, 129.2,
135.2, 137.4, 148.2, 149.0, 161.9.
Anal. Calcd for C13H12N2O2: C, 68.41; H, 5.30; N, 12.27. Found: C,
68.68; H, 5.40; N, 12.22.
N-Hydroxy-N-methyl-2-pyridinecarboxamide (3k)
Off-white microcrystals were obtained after gradient column chro-
matography with EtOAc–hexanes (1:2 1:1), mp 80–81 °C.
1H NMR (CDCl3): = 3.50 (s, 3 H), 7.50–7.55 (m, 1 H), 7.99 (td,
13C NMR (CDCl3): = 13.3, 72.5, 111.8, 115.2, 144.4, 145.7, 157.2.
Anal. Calcd for C7H9NO3: C, 54.19; H, 5.85; N, 9.03. Found: C,
53.94; H, 5.92; N, 8.82.
J = 7.7, 1.6 Hz, 1 H), 8.30 (d, J = 7.7 Hz, 1 H), 8.51–8.53 (m, 1 H).
N-(Benzyloxy)-2-furamide (3f)
Recrystallized from CHCl3–hexanes to give white needles; mp
100–101 °C.
1H NMR (CDCl3): = 5.02 (s, 2 H), 6.49 (dd, J = 3.4, 1.8 Hz, 1 H),
7.19 (d, J = 3.4 Hz, 1 H), 7.36–7.46 (m, 6 H), 8.89 (br s, 1 H).
13C NMR (CDCl3): = 35.9, 125.3, 126.1, 138.6, 145.4, 151.8,
157.4.
Anal. Calcd for C7H8N2O2: C, 55.26; H, 5.30; N, 18.41. Found: C,
55.43; H, 5.29; N, 18.40.
13C NMR (CDCl3): = 78.8, 112.0, 115.7, 128.6, 128.8, 129.2,
135.0, 144.4, 145.6, 157.0.
N-Hydroxypentanamide (3l)
White microcrystals were obtained after column chromatography
with EtOAc–hexanes (1:1); mp 53– 54 °C (Lit.41 mp 50–51 °C).
Anal. Calcd for C12H11NO3: C, 66.35; H, 5.10; N, 6.45. Found: C,
66.52; H, 5.33; N, 6.76.
1H NMR (CDCl3): = 0.90 (t, J = 7.3 Hz, 3 H), 1.27–1.39 (m, 2 H),
1.55–1.65 (m, 2 H), 2.16 (t, 7.3 Hz, 2 H), 9.31 (br s, 2 H).
N-Hydroxy-N-methyl-2-furamide (3g)
13C NMR (CDCl3): = 13.7, 22.2, 27.5, 32.7, 172.1.
White microcrystals were obtained after gradient column chroma-
tography with EtOAc–hexanes (1:6 1:1); mp 103–104 °C (Lit.39
mp 103–105 °C).
1H NMR (CDCl3): = 3.34 (s, 3 H), 6.57 (dd, J = 3.4, 1.8 Hz, 1 H),
7.22 (dd, J = 3.4, 0.7 Hz, 1 H), 7.71 (dd, J = 1.8, 0.7 Hz, 1 H), 9.35
(br s, 1 H).
N-Ethoxypentanamide (3m)
Colorless oil was obtained after the removal of benzotriazole by
washing with aq Na2CO3.
1H NMR (CDCl3): = 0.91 (t, J = 7.3 Hz, 3 H), 1.25 (t, J = 7.0 Hz,
3 H), 1.31–1.39 (m, 2 H), 1.57–1.68 (m, 2 H), 2.14 (t, J = 7.0 Hz, 2
H), 3.95 (q, J = 7.0 Hz, 2 H), 9.92 (br s, 1 H).
13C NMR (CDCl3): = 37.2, 112.2, 117.9, 145.9, 147.2, 159.7.
13C NMR (CDCl3): = 13.2, 13.6, 22.2, 27.5, 32.8, 71.7, 171.2.
N-[(2-Furylcarbonyl)oxy]methanamine (4)
Colorless oil was separated from compound 3g.
1H NMR (CDCl3): = 2.95 (s, 3 H), 6.54 (dd, J = 3.4, 1.8 Hz, 1 H),
7.24 (d, J = 3.4 Hz, 1 H), 7.61–7.63 (m, 1 H).
Anal. Calcd for C7H15NO2: C, 57.90; H, 10.41; N, 9.65. Found: C,
58.30; H, 10.81; N, 9.77.
N-(Benzyloxy)pentanamide (3n)
Colorless oil was obtained after gradient column chromatography
with elution from CHCl3 CHCl3–MeOH (20:1).
1H NMR (CDCl3): = 0.89 (t, J = 7.3 Hz, 3 H), 1.25–1.38 (m, 2 H),
1.53–1.64 (m, 2 H), 2.00–2.10 (m, 2 H), 4.80–4.90 (m, 2 H), 7.36–
7.39 (m, 5 H), 8.56 (br s, 1 H).
13C NMR (CDCl3): = 39.9, 11.9, 118.5, 142.8, 146.7, 159.0.
Anal. Calcd for C6H7NO3: C, 51.06; H, 5.00; N, 9.92. Found: C,
51.41; H, 5.11; N, 9.71.
N-Hydroxy-2-pyridinecarboxamide (3h)
White microcrystals were obtained after column chromatography
13C NMR (CDCl3): = 13.6, 22.2, 27.4, 33.0, 78.0, 128.5, 128.6,
129.2, 135.4, 171.1.
with EtOAc–hexanes (1:1); mp 119–120 °C (Lit.40 mp 120 °C).
1H NMR (acetone-d6): = 7.54–7.59 (m, 1 H), 7.99 (td, J = 7.7, 1.6
Hz, 1 H), 8.08 (d, J = 7.7 Hz, 1 H), 8.58 (d, J = 4.7 Hz, 1 H), 8.85
(br s, 1 H), 11.08 (br s, 1 H).
Anal. Calcd for C12H17NO2: C, 69.54; H, 8.27; N, 6.76. Found: C,
69.28; H, 8.52; N, 7.06.
13C NMR (acetone-d6): = 122.8, 127.4, 138.5, 149.5, 150.5, 162.6.
Synthesis 2003, No. 18, 2777–2780 © Thieme Stuttgart · New York

2780
A. R. Katritzky et al.
PAPER
N-Methoxy-N-methylpentanamide (3o)
Yellow oil was obtained after gradient column chromatography
with EtOAc–hexanes (1:6 1:1).
1H NMR (CDCl3): = 0.96 (t, J = 7.3 Hz, 3 H), 1.30–1.44 (m, 2 H),
1.56–1.67 (m, 2 H), 2.42 (t, J = 7.3 Hz, 2 H), 3.18 (s, 3 H), 3.69 (s,
3 H).
(22) De Luca, L.; Giacomelli, G.; Taddei, M. J. Org. Chem. 2001,
66, 2534.
(23) Bailen, M. A.; Chinchilla, R.; Dodsworth, D. J.; Najera, C.
Tetrahedron Lett. 2001, 42, 5013.
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38, 2685.
(26) (a) Williams, J. M.; Jobson, R. B.; Yasuda, N.; Marchesini,
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1995, 36, 5461. (b) Katritzky, A. R.; Yang, H.; Zhang, S.;
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13C NMR (CDCl3): = 13.7, 22.4, 26.7, 31.5, 32.1, 61.1, 174.7.
Anal. Calcd for C7H15NO2: C, 57.90; H, 10.41; N, 9.65. Found: C,
58.08; H, 10.87; N, 9.56.
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Synthesis 2003, No. 18, 2777–2780 © Thieme Stuttgart · New York