O-Protected N-(2-Nitrophenylsulfonyl)hydroxylamines: Novel Reagents for the Synthesis of Hydroxamates

Article abstract of DOI:10.1055/s-2001-14567

Preparative methods for novel O-protected N-(2-nitrophenylsulfonyl)hydroxylamines (8a-e) are described. Their versatility as intermediates en route to polyhydroxamates is examplified by the synthesis of a non-amide DFO analog 22.

Full text of DOI:10.1055/s-2001-14567

1086
PAPER
O-Protected N-(2-Nitrophenylsulfonyl)hydroxylamines: Novel Reagents for
the Synthesis of Hydroxamates
N
ovel O-Prote
o
cted
N
-(2-N
i
r
tropheny
e
lsulfonyl)h
d
ydroxylamin
d
e
s
y Amruta Reddy,a Otto F. Schall,a James R. Wheatley,a Leonard O. Rosik,a Joseph P. McClurg,a Garland R.
Marshall,a,b Urszula Slomczynska*a
a
MetaPhore Pharmaceuticals, Inc., 1910 Innerbelt Business Service Center Drive, Saint Louis, MO 63114, USA
Fax +1(314)4267491; E-mail: uslomczynska@metaphore.com
b
Department of Biochemistry and Molecular Biophysics, Washington University School of Medicine, 660 South Euclid Avenue, Saint
Louis, MO 63110, USA
Received 12 January 2001; revised 14 March 2001
PG
PG
O
N
Abstract: Preparative methods for novel O-protected N-(2-nitro-
phenylsulfonyl)hydroxylamines (8a–e) are described. Their versa-
tility as intermediates en route to polyhydroxamates is exemplified
by the synthesis of a non-amide DFO analog 22.
O
N
PG
H
O
N
RSH, Base
NO2
Alkylation
R1X
H
SO2
R1
SO2
R1
NO2
3
Key words: hydroxamate, nosyl protecting group, solid-phase syn-
thesis, alkylation, Mitsunobu reaction
1
2
PG
O
H
O
O
N
O
N
Deprotection
The naturally occurring siderophore Desferrioxamine B
(Desferal, DFO),1 which contains three hydroxamate
groups that form a hexadentate-complex with ferric ion,
has been used for the treatment of iron overload for many
years, and still remains the drug of choice.2 The short-
comings of this chelating therapy, mainly poor compli-
ance of patients associated with subcutaneous infusion of
the drug, necessitates the development of a new iron che-
lator that is orally active and safe. The solution-phase syn-
theses of several important siderophores, including
DFO3,4 and analogs,5,6 have been accomplished. The solu-
tion synthetic methods used, however, are not suitable for
generating large numbers of structurally diverse hydrox-
amate analogs.
Acylation
R2COY
R2
R2
R1
R1
5
4
R = alkyl or aryl
R1 = alkyl, arylalkyl, or appropriately derivatized polystyrene-based resin
R2 = alkyl, aryl, or arylalkyl
Scheme 1
In this context, we wish to report the synthesis and char-
acterization of novel O-protected N-(2-nosyl)hydroxy-
lamines, 8a–e (Scheme 2). The choice of 2-nosyl group
for N-protection is based on the reported utility of o-and
p-nitrophenylsulfonamides in selective N-alkylation of
primary amines7,8 and esters of -amino-acids,9,10 as well
as the generation of N-alkyl peptides on the solid sup-
port.11–13 In addition, the thiolate-labile nosyl group is or-
thogonal not only to the O-protecting groups utilized here,
but also to most linkers to the solid support. The choice of
O-protecting groups,14 particularly, acid-labile t-Bu, THP,
and 2,4-dimethoxybenzyl15 makes the subsequent auto-
mated high-throughput solid-phase synthesis a more ro-
bust process, as the protecting group can also be removed
In order to facilitate structure–activity studies on naturally
occurring hydroxamates, such as DFO, and pursue chem-
ical diversity with novel structures capable of binding
iron, we have developed a flexible synthetic strategy in-
volving N,O-bisprotected hydroxylamine 1 as the key in-
termediate (Scheme 1). The presence of the temporary
N-protecting 2-nitrophenylsulfonyl (nosyl) group acti-
vates the nitrogen for alkylation with alkyl halides by con-
ventional methods or alcohols under Mitsunobu reaction
conditions to give 2. Selective removal of the nosyl group
with thiolate anion then leads to 3 that facilitates further
elaboration of the molecule by N-acylation to afford the
intermediate 4. Removal of the permanent O-protecting
group (PG) leads to the desired hydroxamate 5. This ap-
proach is very versatile and is suitable for both solid- and
solution-phase methods.
R
O
SO2Cl
N
R
H
H
SO2
NO2
O
N
Pyridine or Et3N
NO2
+
CHCl3 or CH2Cl2
H
−5 °C to r.t.
7
6a: R = Bn
8a: R = Bn (70%)
6b: R = allyl
6c: R = t-Bu
6d: R = THP
8b: R = allyl (51%)
8c: R = t-Bu (80%)
8d: R = THP (79%)
Synthesis 2001, No. 7, 01 06 2001. Article Identifier:
1437-210X,E;2001,0,07,1086,1092,ftx,en;M00101SS.pdf.
© Georg Thieme Verlag Stuttgart · New York
ISSN 0039-7881
6e: R = 2,4-(MeO)2Bn
8e: R = 2,4-(MeO)2Bn (54%)
Scheme 2

PAPER
Novel O-Protected N-(2-Nitrophenylsulfonyl)hydroxylamines
1087
during the final cleavage from the solid support. Some of during denosylation of some 4-nitrophenylsulfonamides
the known N,O-bisprotected hydroxylamine derivatives (displacement of nitro group by thiolate rather than sul-
used for the synthesis of hydroxamates, including BOC- fonamide cleavage)23 prompted us to discontinue the use
NH-OTHP,16 BOC-NH-OTBDMS,16 BOC-NH-OBn,3,6 of 4-substituted analogs.
and O-(2,4)-dimethoxybenzyl-N-(2,4,6)-trimethoxyben-
During the above synthesis of 8e, the hydroxylamine de-
rivative 6e was prepared in 76% yield by reacting 2,4-
dimethoxybenzylalcohol (9) and N-hydroxyphthalimide
(10) under Mitsunobu conditions [Ph3P, diethyl azodicar-
boxylate (DEAD), CHCl3, room temperature, 17 hours],
zylhydroxylamine,15 and certain previously reported N-
arylsulfonyl-O-substituted
hydroxylamine
deriva-
tives,17,18 suffer mainly from a lack of orthogonality of the
N- and O-protecting groups during deprotection.
To this end, commercially available O-benzylhydroxy- followed by hydrazinolysis with hydrazine in EtOH (re-
lamine (6a) hydrochloride was reacted with 2-nosyl chlo- flux, 30 minutes) according to a reported method.15 How-
ride (7, 0.8 or 1.5 equivalents) in CH2Cl2 in the presence ever, this involved extensive chromatographic
of N,N-diisopropylethylamine (DIPEA) (2.6 or 4.0 equiv- purification of the intermediate phthalimide derivative 11
alents) for 2 hours at room temperature. This approach from byproducts, particularly, copious amounts of Ph3PO.
failed to give a reasonable yield of the desired product 8a. Additionally, after the hydrazinolysis step, compound 6e
The acidic NH group in the O-protected hydroxamate could not be obtained entirely free of the corresponding
(pKa ~10) is susceptible to further sulfonylation in the byproduct phthalhydrazide. In an attempt to avoid this
presence of strongly basic DIPEA (pKa ~11), and in fact, chromatographic step, we first carried out the Mitsunobu
the major product isolated in the reaction was the corre- reaction in CH2Cl2; it was then feasible to wash the excess
sponding disubstituted derivative.19
10 with alkali solution. Subsequent hydrazinolysis was ef-
fected with methylhydrazine in toluene at 80 °C for 2
hours (Scheme 3), as described in the synthesis of 6d.20
The insoluble byproduct N-methylpthalhydrazide was
separated by filtration and the neutral Ph3PO carried over
from the Mitsunobu reaction was separated by chemical
purification (acid/base work up). The crude O-protected
hydroxylamine 6e (57% yield), thus obtained, was reacted
with 2-nosyl chloride (7) in CH2Cl2 in the presence of py-
ridine as described above to give the product 8e in 49%
yield (overall 28% versus 41% in the previous method).
Consequently, the reaction was carried out in pyridine
(pKa = 5.23) with stoichiometric amounts of the reagents
for 30 minutes at –5 °C and then for 2 hours at room tem-
perature, and the desired product 8a was obtained in good
yield (70 %). A similar reaction of 6b with 7 (1.5 equiva-
lents) in pyridine gave 8b in 51% yield. Similarly, com-
mercially available O-tert-butyllhydroxylamine (6c)
hydrochloride was treated with 7 (1 equivalent) in the
presence of Et3N (2.1 equivalents) as the base in CHCl3
for 2 hours at –5 °C, and then overnight at room tempera-
ture to afford 8c in 80% yield. No dinosyl-substituted
byproduct was isolated, probably due to the steric bulk of
the adjacent tert-butyl group. The reaction was slightly
modified for the preparation of THP derivative 8d, where-
in pyridine and CH2Cl2 were substituted for Et3N and
CHCl3, respectively. Thus, the reaction of stoichiometric
amounts of 6d20 and 7 in the presence of pyridine (1.5
equivalents) in CH2Cl2 for 2 hours at –5 °C, and then over-
night at room temperature, gave 8d in 79% yield. A simi-
lar reaction of 6e15 with 7 (1.1 equivalents) in the presence
of pyridine (2.0 equivalents) in CH2Cl2 gave 8e in 54%
yield. It should be mentioned that 4-nitro-analogs of 8a
and 8c were also prepared by reacting 6a and 6c with
stoichiometric amounts of 4-nosyl chloride in pyridine
(–5 °C, 2 hours and room temperature, 2 hours; 87%) and
in the presence of Et3N (2.1 equivalents) in CHCl3 as the
solvent (–5 °C, 2 hours and room temperature, 3 hours;
59%), respectively.21
O
OH
O
N
O
MeO
MeO
a
b, c
6e (57%)
OMe
OMe
9
11
Scheme 3 Reagents and conditions: (a) N-Hydroxyphthalimide
(10), Ph3P, DEAD, CH2Cl2, r.t., 18 h; (b) MeNHNH2, PhMe, 80°C, 2
h; (c) chemical purification
An example highlighting the utility of nosylhydroxy-
lamine derivatives of type 8 in the synthesis of hydroxam-
ates is shown in Scheme 4. A non-amide analog of DFO,
22, was synthesized by solution methods using tert-butyl
as the O-protecting group for nosylhydroxylamine. Thus,
6-bromohexanoic acid (12) was esterified with benzyl al-
cohol, and the resulting known benzyl ester 1324 was em-
ployed in the N-alkylation of O-tert-butyl-N-
nosylhydroxylamine (8c) in the presence of Cs2CO3, lead-
ing to 14 in quantitative yield. Deprotection of the nosyl
group in 14 with dilithio-mercaptoacetic acid furnished
one of the desired intermediates 15. Treatment of 15 with
acetic anhydride/pyridine followed by saponification of
the benzyl ester supplied the other key intermediate 17.
Based on our initial observation that removal of the
4-nosyl group demanded longer reaction times than that
required for the corresponding 2-nosyl group, we have not
explored the use of these compounds in solid-phase syn-
theses. Reports of minor byproduct formation during the
alkylation of 4-nitrophenylsulfonamides of amino esters
with less reactive alkylating reagents (intramolecular
transfer of the nitrophenyl group to the -carbon of the
amino ester),9,22 and the poor regioselectivity observed
Synthesis 2001, No. 7, 1086–1092 ISSN 0039-7881 © Thieme Stuttgart · New York

1088
P. Amruta Reddy et al.
PAPER
NO2
O
S
OCH2Ph
OH
OCH2Ph
a
b
c
N
Br
Br
O
90%
100%
80%
Ot-Bu
O
N
O
O
14
12
13
O
O
H
N
OCH2Ph
OH
O
OCH2Ph
d
e
N
98%
100%
Ot-Bu
O
Ot-Bu
Ot-Bu
O
17
15
16
O
O
OCH2Ph
OH
f
e
f
N
N
94%
96%
97%
Ot-Bu
O
Ot-Bu
O
2
2
18
19
O
O
N
O
OCH2Ph
OH
OH
e
g
N
N
98%
76%
Ot-Bu
O
Ot-Bu
O
OH
O
3
3
3
20
21
22
Scheme 4 Reagents and conditions: (a) PhCH2OH, DIC, DMAP, THF, r.t., 4 h; (b) t-BuONHNs (8c), Cs2CO3, CH3CN, reflux, 6 h; (c)
HSCH2CO2H, LiOH, DMF, r.t., 2.5 h; (d) Ac2O, pyridine, r.t., 3 h; (e) 1 N NaOH, EtOH, r.t.; (f) 15, HATU, DIPEA, DMF, r.t., 2 h; (g) TFA,
r.t., 10 h
The coupling of the N,O-bisprotected carboxylic acid 17
N-(2–Nitrophenylsulfonyl)-O-(phenylmethyl)hydroxylamine
to the hydroxylamine moiety of the intermediate 15 was
achieved with N-[(dimethylamino)-1H-1,2,3-triazo-
lo[4,5-b]pyridin-1-ylmethylene]-N-methylmethanamini-
um hexafluorophosphate N-oxide (HATU) in DMF
leading to the dimeric ester 18 in high yield. Subsequent
hydrolysis of the ester followed by coupling once again
with 15 furnished trimeric ester 20. After saponification,
the tert-butyl groups of the trimeric acid 21 were depro-
tected with TFA under anhydrous conditions, to give the
target trihydroxamate 22 in good overall yield (46%). Al-
though the example shown here is based on solution
chemistry, our primary focus is solid-phase synthesis. In
fact, we have generated several polyhydroxamate libraries
(manuscripts in preparation) by solid-phase syntheses.25
(8a)
A 250-mL round-bottom flask fitted with an addition funnel was
charged with O–benzylhydroxylamine (6a) hydrochloride (5.00 g,
31.3 mmol) and the solid was partially dissolved in anhyd pyridine
(60 mL) by stirring under a flow of N2. The flask was immersed in
an ice-salt water bath and cooled to about –5 ºC. A greenish brown
solution of 2-nitrobenzenesulfonyl chloride (7, 7.1 g, 32 mmol) in
anhyd pyridine (20 mL) was added dropwise at a rate of ~1 drop per
second, while the temperature was maintained at –5 °C throughout
the addition. Following the addition, the brownish orange solution
was stirred for 30 min more at this temperature, then allowed to
warm to r.t. and stirred for a total of 2 h. H2O (15 mL) was added to
terminate the reaction and afford a clear solution, and the solvents
were removed. The resulting dark amber syrup was taken up in
EtOAc–H2O (1:1, 400 mL) and partitioned in a separatory funnel.
The organic layer was washed successively with 5% HCl, H2O, and
sat. NaHCO3 (200 mL each). Solvent removal gave a brownish or-
ange solid, which was dissolved in boiling EtOH–H2O (9:1, 200
mL), treated with charcoal (2 g), and filtered. The filtrate was re-
heated and allowed to cool to r.t., depositing small-to medium-sized
light yellow rhombic crystals of the product 8a (6.7 g, 70%). Homo-
geneous by TLC. Rf = 0.52 (EtOAc–hexanes, 3:7).
In summary, we have developed methods for the prepara-
tion of O-substituted N-(2-nitrophenylsulfonyl)hydroxy-
lamines 8a–e that are useful intermediates in the synthesis
of polyhydroxamates.
Unless otherwise noted, all reagents were used as supplied. Organic
extracts were dried either with Na2SO4 or MgSO4 and filtered. Sol-
vents were removed on a rotary evaporator under reduced pressure.
Analytical TLC was performed on Analtech silica gel GF plates
(250 m). Flash chromatography was carried out using Fisher Sci-
entific silica gel 60A (38–90 m). All mps were determined on a
Thomas-Hoover (Model 67T108) capillary melting point apparatus
and are uncorrected. NMR spectra were recorded on a Varian Gem-
ini-300 spectrometer. Chemical shifts ( ) are expressed in ppm rel-
ative to TMS as an internal standard. Coupling constants (J) are
reported in Hz. Electrospray mass spectra were obtained on a Ther-
moQuest AQA spectrometer. Elemental analyses were carried out
by Atlantic Microlab, Inc., Norcross, GA.
Mp 149–150 °C.
1H NMR (CDCl3): = 8.27–8.23 (m, 1H, NsH-3), 8.12 (s, 1H, NH),
7.90–7.76 (m, 3H, NsH-4, 5, 6), 7.37 (s, 5H, BnH), 5.07 (s, 2H,
OCH2Ph).
13C NMR (DMSO-d6): = 148.91, 135.58, 135.29, 132.68, 130.22,
129.62, 129.25, 128.63, 128.54, 124.62, 78.69.
ESI MS: m/z = 309 (M + H)+, 326 (M + NH4)+.
Anal. Calcd for C13H12N2O5S: C, 50.65; H, 3.92; N, 9.09; S, 10.40.
Found: C, 50.51; H, 3.91; N, 8.99; S, 10.53.
N-(2–Nitrophenylsulfonyl)-O-(2-propenyl)hydroxylamine (8b)
The reaction of O–allylhydroxylamine (6b) hydrochloride (3.00 g,
27.4 mmol) with 2-nitrobenzenesulfonyl chloride (7, 9.1 g, 41
mmol) in anhyd pyridine (50 mL) was carried out as described
Synthesis 2001, No. 7, 1086–1092 ISSN 0039-7881 © Thieme Stuttgart · New York

PAPER
Novel O-Protected N-(2-Nitrophenylsulfonyl)hydroxylamines
1089
above in the preparation of 8a. After the usual work up, the crude
product was isolated as a dark brown solid. It was dissolved in boil-
ing EtOH–H2O (9:1, 100 mL), treated with charcoal (1 g), and fil-
tered. The filtrate was reheated and allowed to cool in a refrigerator
overnight, whereby light orange flakes of the product 8b (3.5 g,
51%) were separated. Homogeneous by TLC. Rf = 0.35 (EtOAc–
hexanes, 3:7).
nearly colorless powder.26 Homogeneous by TLC. Rf = 0.49
(EtOAc–hexanes, 1:1).
Mp 108–109 °C.
1H NMR (CDCl3): = 8.23 8.17 (m, 1H, NsH-3), 7.96 7.78 (m,
3H, NsH-4, 5, 6), 5.22 (t, 1H, J = 3.2, OCH), 3.91 3.83 (m, 1H,
OCH2), 3.70 3.60 (m, 1H, OCH2), 1.82 1.50 [m, 6H, (CH2)3].
13C NMR (CDCl3): = 148.80, 135.10, 133.73, 133.09, 130.62,
125.82, 103.88, 63.01, 28.36, 24.89, 18.86.
Mp 110–111 °C.
1H NMR (CDCl3): = 8.26–8.20 (m, 1H, NsH-3), 8.12 (s, 1H, NH),
7.94–7.78 (m, 3H, NsH-4, 5, 6), 6.00–5.86 (m, 1H, CH=CH2),
5.38–5.29 (m, 2H, CH=CH2), 4.56 (d, 2H, J = 6.3, OCH2).
13C NMR (CDCl3): = 148.74, 135.07, 133.96, 133.11, 131.91,
130.45, 125.76, 120.82, 78.66.
ESI MS: m/z = 325 (M + Na)+, 301 (M H) .
Anal. Calcd for C11H14N2O6S: C, 43.70; H, 4.67; N, 9.27; S, 10.61.
Found: C, 43.76; H, 4.68; N, 9.22; S, 10.73.
O-[(2,4-Dimethoxyphenyl)methyl]-N-(2-nitrophenylsulfonyl)-
hydroxylamine (8e) Method A:
ESI MS: m/z = 259 (M + H)+, 276 (M + NH4)+.
Anal. Calcd for C9H10N2O5S: C, 41.86; H, 3.90; N, 10.85; S, 12.41.
Found: C, 41.86; N, 3.97; N, 10.68; S, 12.43.
To a solution of O-2,4-dimethoxybenzylhydroxylamine (6e, 12.1 g,
66.0 mmol)15 and pyridine (8.0 mL, 99 mmol) in anhyd CH2Cl2
(180 mL), was added slowly (approx. 1 h) a solution of 2-
nitrobenzenesulfonyl chloride (7, 16.1 g, 72.6 mmol) in anhyd
CH2Cl2 (85 mL) containing pyridine (2.66 mL, 33.0 mmol) with
stirring at –5 °C under N2. The resulting orange solution was slowly
warmed to r.t. (approx. 4 h) and stirred overnight (approx. 14 h).
The dark colored reaction mixture was successively washed with
H2O, ice-cold 1 N HCl, 5% NaHCO3, H2O, and brine (150 mL
each). Removal of the solvent gave 24.3 g of an orange viscous res-
idue, which was dissolved in boiling CH2Cl2 (50 mL) with stirring,
and then boiling EtOH (200 mL) was added slowly. Upon cooling
to r.t., the solution was seeded with a crystal of pure product and left
overnight, whereupon product 8e (8.21 g) was separated as a pale
yellow crystalline solid. The filtrate was concentrated subjected to
flash chromatography over silica gel (120 g) using CH2Cl2 hexanes
(3:2) to obtain an additional 4.91 g of the product 8e as a pale yellow
foam, bringing the overall yield to 13.12 g (54%). Homogeneous by
TLC. Rf = 0.58 (CH2Cl2 hexanes, 4:1).
O-(1,1-Dimethylethyl)-N-(2-nitrophenysulfonyl)hydroxylamine
(8c)
A 1-L round bottom flask fitted with an addition funnel was charged
with O-tert-butylhydroxylamine (6c) hydrochloride (25.3 g, 201
mmol) and the solid was dissolved in anhyd CHCl3 (400 mL) by
stirring under N2. The flask was immersed in an ice-salt water bath,
cooled to about –5 °C and Et3N (58.8 mL, 422 mmol) added drop-
wise (approx. 15 min). Then a solution of 2-nitrobenzenesulfonyl
chloride (7, 44.64g, 201.0 mmol) in anhyd CHCl3 (250 mL) was
added slowly (approx. 1.25 h), and the resulting brownish orange
solution was stirred for 2 h at –5 °C, allowed to warm to r.t., and
stirred overnight. The contents were transferred to a 2-L separatory
funnel and washed successively with H2O, 1 N HCl ( 2), H2O, 5%
NaHCO3 ( 2), H2O, and brine (200 mL each). Solvent removal
gave 50.98 g of the crude product, which was recrystallized from
EtOH (550 mL) to afford the product 8c (44.26 g, 80%) as a color-
less crystalline solid. Homogeneous by TLC. Rf = 0.40 (CH2Cl2–
hexanes, 7:3).
Mp 124–125 °C.
1H NMR (CDCl3): = 8.27 8.22 (m, 1H, NsH-3), 8.02 (br s, 1H,
NH), 7.87 7.73 (m, 3H, NsH-4, 5, 6), 7.28 (d, 1H, J = 8.2, BnH-6),
6.49 6.44 (m, 2H, BnH-3 & 5), 5.07 (s, 2H, OCH2Ph), 3.82 (s, 3H,
OMe), 3.81 (s, 3H, OMe).
Mp 149–150 °C.
1H NMR (CDCl3): = 8.19–8.15 (m, 1H, NsH-3), 7.90–7.75 (m,
3H, NsH-4, 5, 6), 7.61 (s, 1H, NH) 1.28 (s, 9H, CMe3).
13C NMR (CDCl3): = 148.76, 134.76, 133.96, 132.88, 130.91,
125.54, 83.00, 26.74.
ESI MS: m/z = 275 (M + H)+, 292 (M + NH4)+.
13C NMR (CDCl3): = 162.17, 159.73, 148.69, 134.81, 134.02,
133.44, 132.96, 130.71, 125.60, 115.79, 104.29, 98.70, 74.73,
55.57, 55.50.
ESI MS: m/z = 369 (M + H)+, 386 (M + NH4)+, 391 (M + Na)+.
Anal. Calcd for C10H14N2O5S: C, 43.79; H, 5.14; N, 10.21; S, 11.69.
Found: C, 43.96; H, 5.11; N, 10.10; S, 11.79.
Anal. Calcd for C15H16N2O7S: C, 48.91; H, 4.38; N, 7.60; S, 8.70.
Found: C, 48.98; H, 4.39; N, 7.55; S, 8.82.
N-(2-Nitrophenylsulfonyl)-O-(tetrahydro-2H-pyran-2-
yl)hydroxylamine (8d)
Method B:
A 1-L round bottom flask equipped with an addition funnel was
charged with 2,4-dimethoxybenzyl alcohol (9, 10.09 g, 60.00
mmol), N-hydroxyphthalimide (10, 12.72 g, 78.00 mmol), Ph3P
(20.46 g, 78.00 mmol), and anhyd CH2Cl2 (200 mL), and the sus-
pension was stirred vigorously for 15 min at r.t. under N2. Then an
orange solution of DEAD (12.28 mL, 78.00 mmol) in anhydrous
CH2Cl2 (40 mL) was added slowly (approx. 35 min) and stirred
overnight (approx. 20 h) at r.t. The clear yellow reaction mixture
was washed with 1 N NaOH ( 2), H2O ( 2), and brine (100 mL
each). Removal of the solvent gave crude N-[(2,4-dimethoxyphe-
nyl)methyl]phthalimide (11, 43.91 g) as a thick yellow semi-solid,
which was dried under high vacuum overnight.
To a solution of O-(tetrahydro-2H-pyran-2-yl)hydroxylamine (6d,
101.0 mmol, 11.82 g),20 and pyridine (151.5 mmol, 12.2 mL) in an-
hyd CH2Cl2 (275 mL), was added slowly (approx. 1 h) a solution of
2-nitrobenzenesulfonyl chloride (7, 101.0 mmol, 22.42 g) in anhyd
CH2Cl2 (125 mL) containing pyridine (50.5 mmol, 4.1 mL) with
stirring at –5 °C under N2. The cooling bath was removed after 2 h,
and the yellow solution was stirred overnight (approx. 14 h) at r.t.
The reaction mixture was diluted with CH2Cl2 (200 mL), washed
with 5% NaHCO3 (3 200 mL), and the solvent was removed to
give 35.87 g of a dark viscous residue. Quick filtration through a
bed of silica gel (100 g) packed in a fritted glass funnel using
CH2Cl2–hexanes (4:1), followed by trituration of the concentrate
with Et2O–hexanes (1:3), afforded 26.01 g of slightly impure prod-
uct as an off-white powder. Further purification was accomplished
by flash chromatography over silica gel (300 g) using EtOAc–hex-
anes (2:3) containing Et3N (0.5%) to give 8d (24.14 g, 79%) as
The above crude phthalimide 11 (60.0 mmol) was co-evaporated
with toluene (100 mL) and then additional toluene (220 mL) was
added. The suspension was heated to 80 °C, and after dissolution of
the solid, methylhydrazine (3.51 mL, 66.0 mmol) was added and the
reaction mixture was stirred at that temperature for 2 h. After cool-
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P. Amruta Reddy et al.
PAPER
6-(N-tert-Butyloxy)aminohexanoic Acid Benzyl Ester (15)
ing in an ice bath, the contents were filtered through a fritted glass
funnel. The filtrate was chilled to near 0 °C in an ice bath and
washed with ice-cold 2 N HCl (4 7 5 mL). The combined aqueous
layers were washed with CH2Cl2 (2 100 mL), chilled to near 0 °C,
and made alkaline by the slow addition of 3 N NaOH (solution be-
comes foggy-white upon becoming alkaline). The product was ex-
tracted with CH2Cl2 (4 125 mL),27 and the combined CH2Cl2
extract washed successively with H2O and brine (100 mL each).
Solvent removal gave a clear amber-colored oil, which was dried
under high vacuum to give O-[(2,4-dimethoxyphenyl)methyl]hy-
droxylamine (6e, 6.23 g, 57% for two steps) [ESI MS: m/z = 184 (M
+ H)+, 167 (M – NH3)+].
To a suspension of LiOH H2O (1.93 g, 46.0 mmol) in DMF (30
mL), mercaptoacetic acid (1.92 mL, 27.6 mmol) was added with
stirring at r.t. under N2. After the reaction mixture became homoge-
neous (approx. 15 min), a solution of 14 (5.50 g, 11.5 mmol) in
DMF (10 mL) was added dropwise (approx. 10 min), and the result-
ing yellow suspension stirred at r.t. for 2.5 h. The reaction mixture
was poured into ice-water (500 mL), saturated by the addition of
solid NaCl, and then extracted with Et2O (4 125 mL). The com-
bined Et2O extracts were successively washed with H2O ( 2), sat.
NaHCO3, H2O, and brine (125 mL each). Removal of the solvent
gave 3.67 g of a pale yellow oil, which upon flash chromatography
over silica gel (100 g) (CH2Cl2), furnished pure 15 (2.69 g, 80%) as
a colorless oil. Further elution gave 0.75 g (14%) of substrate 14.
1H NMR (CDCl3): = 7.40–7.33 (m, 5H, BnH), 5.12 (s, 2H,
OCH2Ph), 2.83 (t, 2H, J = 7.0, NCH2), 2.37 (t, 2H, J = 7.5, CH2CO),
1.69–1.35 [m, 6H, (CH2)3], 1.16 (s, 9H, CMe3).
13C NMR (CDCl3): = 173.80, 136.37, 128.78, 128.42, 76.75,
66.21, 52.89, 34.30, 27.07, 26.92, 26.86, 24.93.
The above crude hydroxylamine derivative 6e (6.23 g, 34.0 mmol)
was co-evaporated with anhyd CH2Cl2 (50 mL) and the residue re-
acted with 2-nitrobenzenesulfonyl chloride (7, 9.04 g, 40.8 mmol)
in anhyd CH2Cl2 (135 mL) in the presence of pyridine (6.60 mL,
81.6 mmol) as base as described above in Method A. After the usual
work up, the crude product (13.25 g) was treated with charcoal in a
minimum amount of boiling CH2Cl2–EtOH (1:4). The filtrate was
concentrated and passed through a short-path silica gel (15 g) col-
umn using CH2Cl2 (250 mL). Solvent removal followed by recrys-
tallization from CH2Cl2–EtOH (1:4) afforded 8e (5.88 g) as yellow
crystals. The filtrate was concentrated and subjected to flash chro-
matography over silica gel (60 g) using CH2Cl2 to give 0.31 g of ad-
ditional pure product 8e, bringing the total yield to 6.19 g (49%,
overall 28% from 9).
ESI MS: m/z = 294 (M + H)+.
Anal. Calcd for C17H27NO3: C, 69.59; H, 9.28; N, 4.77. Found: C,
69.73; H, 9.38; N, 4.92.
6-(N-Acetyl-N-tert-butyloxy)aminohexanoic Acid Benzyl Ester
(16)
To a solution of hydroxylamine derivative 15 (1.03 g, 3.50 mmol)
and pyridine (0.85 mL, 10.5 mmol) in CH2Cl2 (20 mL), protected
from moisture by a CaSO4 guard tube, was added acetic anhydride
(0.66 mL, 7.0 mmol) at ice-bath temperature and the resulting mix-
ture was stirred at r.t. for 3 h. The reaction mixture was poured into
50 g of crushed ice and allowed to stand for 1 h. The layers were
separated and the aqueous layer was extracted with CH2Cl2 (3 25
mL). The combined CH2Cl2 extracts were washed successively with
1 N HCl, H2O, sat. NaHCO3, H2O, and brine (30 mL each). Solvent
removal followed by flash chromatography over silica gel (CH2Cl2–
EtOAc, 9:1) afforded 16 (1.15 g, 98%) as a colorless viscous oil.
1H NMR (CDCl3): = 7.37–7.29 (m, 5H, BnH), 5.10 (s, 2H,
OCH2Ph), 3.59 (br m, 2H, NCH2), 2.35 (t, 2H, J = 7.4, CH2CO),
2.10 (s, 3H, CH3CO), 1.71–1.61 (m, 4H, CH2CH2CH2), 1.33–1.22
[m, 11H, CMe3 (singlet at 1.29) and CH2CH2CH2].
13C NMR (CDCl3): = 175.68, 173.37, 136.10, 128.54, 128.17,
82.55, 66.08, 49.81, 34.15, 27.73, 26.33, 25.97, 24.63, 21.54.
6-Bromohexanoic Acid Benzyl Ester (13)
A slightly turbid solution of 6-bromohexanoic acid (12, 4.33 g, 22.2
mmol) and DMAP (0.32 g, 2.60 mmol) in THF (40 mL) was stirred
and cooled to 0 °C, and treated with benzyl alcohol (1.90 mL, 18.5
mmol) followed by DIC (3.50 mL, 22.2 mmol) in THF (10 mL) un-
der N2. After 30 min at 0 °C, the cooling bath was removed and the
reaction mixture was allowed to warm to r.t (approx. 15 min) and
then stirred for 4 h. The reaction was quenched by the addition of
H2O (10 mL) and the solvent removed. The remaining light oil and
white crystals were taken up in EtOAc–H2O (100 mL each) and ex-
tracted with sat. NaHCO3. Removal of solvent followed by flash
chromatography over silica gel (EtOAc–hexanes, 1:9) afforded 13
(4.75 g, 90%) as a colorless oil.
1H NMR (CDCl3): Consistent with the reported data.24
ESI MS: m/z = 285 (M + H)+, 302 (M + NH4)+, 307 (M + Na)+.
6-[N-tert-Butyloxy-N-(2-nitrophenylsulfonyl)]aminohexanoic
Acid Benzyl Ester (14)
ESI MS: m/z = 336 (M + H)+, 353 (M + NH4)+, 358 (M + Na)+.
Anal. Calcd for C19H29NO4: C, 68.03; H, 8.71; N, 4.18. Found: C,
67.92; H, 8.79; N, 4.31.
An orange solution of hydroxylamine derivative 8c (2.74 g, 10.0
mmol) and ester 13 (3.08 g, 10.8 mmol) in anhyd CH3CN (50 mL),
containing Cs2CO3 (6.52 g, 20.0 mmol) in suspension, was stirred
and refluxed for 6 h under N2. After cooling to r.t., Cs2CO3 was fil-
tered off and the filtrate, which included acetone washings (25 mL),
was concentrated to give 5.13 g of dark brown viscous residue.
Flash chromatography over silica gel (30 g) (CH2Cl2) yielded 14
(4.79 g, quantitative) as a highly viscous pale yellow residue.
1H NMR (CDCl3): = 8.07 (dd, 1H, J = 7.8, 1.7, NsH-3), 7.78–7.66
(m, 2H, NsH-4, 5), 7.53 (dd, 1H, J = 7.8, 1.7, NsH-6), 7.37–7.34 (m,
5H, BnH), 5.11 (s, 2H, OCH2Ph), 3.39 (br m, 1H, NCH2), 3.05 (br
m, 1H, NCH2) (unresolved ‘AB’ pair), 2.34 (t, 2H, J = 7.4 Hz,
CH2CO), 1.80–1.30 [m, 6H, (CH2)3], 1.31 (s, 9H, CMe3).
6-(N-Acetyl-N-tert-butyloxy)aminohexanoic Acid (17)
A pale yellow solution of N-acetyl ester 16 (1.14 g, 3.40 mmol,) in
EtOH–1 N NaOH (40 mL; 9:1) was stirred at r.t. for 3.5 h. The bulk
of EtOH was removed, the residue was dissolved in H2O (40 mL)
and extracted with Et2O (2 30 mL). The aqueous layer was cooled
in an ice-bath and the solution made acidic (pH ~3.0–4.0) by the
dropwise addition of 3 N HCl (1.5 mL) while stirring. The aqueous
layer was saturated by the addition of solid NaCl (10 g) and extract-
ed with Et2O (4 30 mL). The solvent was removed and the residue
dried under high vacuum to give 17 (0.83 g, quantitative) as a very
pale yellow viscous material.
1H NMR (CDCl3): = 3.60 (br m, 2H, NCH2), 2.34 (t, 2H, J = 7.6,
CH2CO), 2.12 (s, 3H, CH3CO), 1.70–1.60 (m, 4H, CH2CH2CH2),
1.37–1.21 [m, 11H, CMe3 (singlet at 1.30) and CH2CH2CH2].
13C NMR (CDCl3): = 178.62, 176.03, 82.85, 49.75, 33.95, 27.76,
26.26, 25.95, 24.42, 21.39.
13C NMR (CDCl3): = 173.63, 149.73, 136.28, 134.75, 133.14,
131.03, 128.80, 128.43, 127.51, 123.62, 84.01, 66.27, 55.97, 34.10,
27.79, 26.94, 26.47, 24.54.
ESI MS: m/z = 496 (M + NH4)+.
Anal. Calcd for C23H30N2O7S: C, 57.73; H, 6.32; N, 5.85; S, 6.70.
Found: C, 57.84; H, 6.51; N, 5.84; S, 6.83.
ESI MS: m/z = 246 (M + H)+, 263 (M + NH4)+, 268 (M + Na)+.
Synthesis 2001, No. 7, 1086–1092 ISSN 0039-7881 © Thieme Stuttgart · New York

PAPER
Novel O-Protected N-(2-Nitrophenylsulfonyl)hydroxylamines
1091
Anal. Calcd for C12H23NO4: C, 58.75; H, 9.45; N, 5.71. Found: C,
58.49; H, 9.46; N, 5.69.
CH3CO), 1.67–1.50 (m, 12H, 3 CH2CH2CH2), 1.37–1.24 [m, 33
H, 3 CMe3 (singlets at 1.31, 1.30, and 1.29) and 3 CH2CH2CH2].
13C NMR (CDCl3): = 177.84 (br), 177.15, 175.80, 82.61, 82.49,
82.43, 49.84 (br), 33.82, 32.97, 32.91, 27.71, 27.66, 26.59, 26.20,
26.10, 25.94, 24.50, 24.40, 21.38.
7,14-Bis(tert-butyloxy)-8,15-dioxo-7,14-diazahexadecanoic
Acid Benzyl Ester (18)
A solution of acid 17 (0.83 g, 3.40 mmol), hydroxylamine deriva-
tive 15 (1.0 g, 3.4 mmol), and HATU (1.29 g, 3.40 mmol) in DMF
(6.8 mL) was cooled in an ice-bath and DIPEA (1.18 mL, 6.80
mmol) was added with stirring. The resulting yellow solution was
protected from moisture by a CaSO4 guard tube and stirred at r.t. for
2 h. Most of the DMF was removed and the residue was partitioned
between EtOAc and H2O (40 mL each). The layers were separated
and the aqueous layer was extracted with EtOAc (2 30 mL). The
combined organic extracts were successively washed with sat.
NaHCO3, H2O, 0.5 M KHSO4, H2O, and brine (30 mL each). The
solvent was removed, and the residue (2.71 g) was subjected to flash
chromatography over silica gel (CH2Cl2-EtOAc, 4:1) to furnish 18
(1.66 g, 94%) as a colorless viscous residue.
1H NMR (CDCl3): = 7.38–7.31 (m, 5H, BnH), 5.11 (s, 2H,
OCH2Ph), 3.60 (br m, 4H, 2 NCH2), 2.40 (br m, 2H, CH2CO),
2.35 (t, 2H, J = 7.4, CH2CO), 2.11 (s, 3H, CH3CO), 1.70–1.56 (m,
8H, 2 CH2CH2CH2), 1.32 1.19 [m, 22H, 2 CMe3 (singlets at
1.30 and 1.29) and 2 CH2CH2CH2].
ESI MS: m/z = 616 (M + H)+, 633 (M + NH4)+.
Anal. Calcd for C32H61N3O8: C, 62.41; H, 9.98; N, 6.82. Found: C,
62.32; H, 10.12; N, 6.83.
7,14,21-Trihydroxy-8,15,22-trioxo-7,14,21-triazatriicosanoic
Acid (22)
A pale brown solution of the trimeric acid 21 (1.02 g, 1.65 mmol)
in TFA (from a freshly opened bottle, 33 mL) was stirred at r.t. for
10 h while protected from moisture by a CaSO4 guard tube. Most of
the TFA was removed under high vacuum; the pale brown viscous
residue was dissolved in CH3CN (20 mL), and evaporated to dry-
ness. The crude product was further dried under high vacuum to
give 0.79 g of pale brown solid. Preparative chromatography over
C18 silica gel (Bakerbond, 40 m) (H2O–CH3CN, 7:3), followed by
recrystallization from H2O–CH3CN (7:3) afforded 0.559 g (76%) of
the pure product 22 as a white powder (in fact, the product precipi-
tated from the column fractions and was collected by filtration and
dried under high vacuum).
ESI MS: m/z = 521 (M + H)+, 538 (M + NH4)+, 543 (M + Na)+.
Mp 111–112 °C.
1H NMR (DMSO-d6): = 11.96 (br s, 1H, D2O exchangeable,
COOH), 9.67 (s, 1H, D2O exchangeable, OH), 9.56 (s, 2H, D2O ex-
changeable, OH), 3.46 (t, 6H, J = 6.9, 3 NCH2), 2.32 (t, 4H,
J = 7.1, 2 CH2CO), 2.19 (t, 2H, J = 7.1, CH2CO), 1.97 (s, 3H,
CH3CO), 1.55–1.43 (m, 12H, 3 CH2CH2CH2), 1.28–1.18 (m, 6H,
7,14-Bis(tert-butyloxy)-8,15-dioxo-7,14-diazahexadecanoic
Acid (19)
A pale yellow colored solution of dimeric ester 18 (1.64 g, 3.15
mmol,) in EtOH–1 N NaOH (35 mL; 9:1) was stirred at r.t. for 3 h.
Analytical TLC (CH2Cl2–EtOAc, 3:1) revealed the presence of a
substantial amount of starting material. At this stage, another 1.2
mL of 1 N NaOH was added and the reaction mixture was stirred
overnight (approx. 12 h) at r.t. The usual work up (CH2Cl2 was used
for final extraction) followed by drying under high vacuum supplied
19 (1.30 g, 96%) as a colorless residue.
3
CH2CH2CH2).
13C NMR (DMSO-d6): = 174.41, 172.54, 170.08, 47.03, 46.93,
46.84, 33.58, 31.62, 26.22, 26.05, 25.96, 25.67, 24.18, 23.95, 20.33.
ESI MS: m/z = 448 (M + H)+, 470 (M + Na)+, 486 (M + K)+; 446 (M
H) , 560 (M + TFA) .
1H NMR (CDCl3): = 3.60 (br m, 4H, 2 NCH2), 2.42 (br m, 2H,
CH2CO), 2.35 (t, 2H, J = 7.1, CH2CO), 2.14 (s, 3H, CH3CO), 1.72–
1.50 (m, 8H, 2 CH2CH2CH2), 1.38–1.26 [m, 22H, 2 CMe3 (sin-
glets at 1.31 and 1.30) and 2 CH2CH2CH2].
Anal. Calcd for C20H37N3O8: C, 53.68; H, 8.33; N, 9.39. Found: C,
53.68; H, 8.42; N, 9.40.
ESI MS: m/z = 431 (M + H)+, 448 (M + NH4)+.
Acknowledgement
7,14,21-Tris(tert-butyloxy)-8,15,22-trioxo-7,14,21-triazatri-
icosanoic Acid Benzyl Ester (20)
We gratefully acknowledge the National Institutes of Health for
partial financial support (SBIR 1 R43 DK54157–01).
The reaction of dimeric acid 19 (1.29 g, 3.00 mmol) with hydroxy-
lamine derivative 15 (0.88 g, 3.00 mmol) in the presence of HATU
(1.14 g, 3.00 mmol) and DIPEA (1.04 mL, 6.00 mmol) in DMF (6.0
mL) was carried out for 2 h as described in the preparation of 18.
After a similar work up, the crude product (2.64 g) was subjected to
flash chromatography over silica gel (CH2Cl2–EtOAc, 7:3) to give
20 (2.05 g, 97%) as a colorless viscous residue.
1H NMR (CDCl3): = 7.37–7.32 (m, 5H, BnH), 5.11 (s, 2H,
OCH2Ph), 3.60 (br m, 6H, 3 NCH2), 2.40 (br m, 4H, 2 CH2CO),
2.35 (t, 2H, J = 7.4, CH2CO), 2.12 (s, 3H, CH3CO), 1.71–1.50 (m,
12H, 3 CH2CH2CH2), 1.35–1.27 [m, 33H, 3 CMe3 (singlets at
1.30 for 9H and 1.29 for 18H) and 3 CH2CH2CH2).
References
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7,14,21-Tris(tert-butyloxy)-8,15,22-trioxo-7,14,21-triazatri-
icosanoic Acid (21)
Hydrolysis of trimeric ester 20 (2.04 g, 2.89 mmol) with EtOH–1 N
NaOH (45 mL, 9:1) for 15 h as described above in the preparation
of 19, afforded 21 (1.75 g, 98%) as a colorless residue, after the usu-
al work up followed by drying under high vacuum.
1H NMR (CDCl3): = 3.65 (br m, 6H, 3 NCH2), 2.42 (unresolved
triplet, 4H, 2 CH2CO), 2.34 (t, 2H, J = 7.1, CH2CO), 2.12 (s, 3H,
Synthesis 2001, No. 7, 1086–1092 ISSN 0039-7881 © Thieme Stuttgart · New York

1092
P. Amruta Reddy et al.
PAPER
(11) Miller, S. C.; Scanlan, T. S. J. Am. Chem. Soc. 1997, 119,
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(21) N-Bis(4-nitrophenylsulfonyl)-O-(phenylmethyl)-
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8.47 (d, 2 H, J = 8.8 Hz), 8.15 (d, 2 H, J = 8.8 Hz), 7.45–7.30
(m, 5 H), 4.94 (s, 2 H)] from 6a and O-(1,1-Dimethylethyl)-
N-(4-nitrophenylsulfonyl)hydroxylamine [1H NMR
(DMSO-d6): = 10.21 (s, 1H), 8.46 (d, 2H, J = 8.8 Hz), 8.09
(d, 2H, J = 8.8 Hz), 1.17 (s, 9H)] from 6c.
(22) Wilson, M. W.; Ault-Justus, S. E.; Hodges, J. C.; Rubin, J.
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(19) N-Bis(2-nitrophenylsulfonyl)-O-(phenylmethyl)-
hydroxylamine: 1H NMR (DMSO-d6): = 8.17–8.07 (m, 6
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(27) It is critical that the first 125-mL portion not be shaken, but
rather gently mixed and then separated. This will ensure the
separation of layers even after shaking well during all
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Synthesis 2001, No. 7, 1086–1092 ISSN 0039-7881 © Thieme Stuttgart · New York