Synthesis of p-amino-N,N′-dihydroxybenzamidine using a TBDMS protecting group protocol

Article abstract of DOI:10.1016/j.tetlet.2014.04.048

A synthetic route to p-amino-N,N′-dihydroxybenzamidine is established using a TBDMS protecting group strategy starting with p-nitrobenzhydroxamic acid chloride, which is transformed to O,O′-bis(tert-butyldimethylsilyl)- N,N′-dihydroxybenzamidine. Reduction with sodium dithionite occurs without degradation of the dihydroxyamidine functional group. Deprotection with ammonium fluoride is fast and efficient. This is important because no other possibility to synthesize this derivative has been found up to now. Furthermore, TBDMS protecting group strategy is proved to be adaptable to other substituted N,N′-dihydroxybenzamidines.

Full text of DOI:10.1016/j.tetlet.2014.04.048

Tetrahedron Letters 55 (2014) 3322–3324
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Synthesis of p-amino-N,N0-dihydroxybenzamidine using a TBDMS
protecting group protocol
⇑
Laura Schwarz, Ulrich Girreser, Bernd Clement
Pharmaceutical Institute, Department of Pharmaceutical and Medicinal Chemistry, Christian-Albrechts-University of Kiel, Gutenbergstraße 76, 24118 Kiel, Germany
a r t i c l e i n f o
a b s t r a c t
A synthetic route to p-amino-N,N0-dihydroxybenzamidine is established using a TBDMS protecting group
strategy starting with p-nitrobenzhydroxamic acid chloride, which is transformed to O,O0-bis(tert-butyl-
dimethylsilyl)-N,N0-dihydroxybenzamidine. Reduction with sodium dithionite occurs without degrada-
tion of the dihydroxyamidine functional group. Deprotection with ammonium fluoride is fast and
efficient. This is important because no other possibility to synthesize this derivative has been found up
to now. Furthermore, TBDMS protecting group strategy is proved to be adaptable to other substituted
N,N0-dihydroxybenzamidines.
Article history:
Received 25 March 2014
Revised 11 April 2014
Accepted 14 April 2014
Available online 23 April 2014
Keywords:
Silyl ether
Prodrugs
Ó 2014 Elsevier Ltd. All rights reserved.
Protective groups
Amidines
Synthesis
Introduction
tection have to be compatible with these requirements. Therefore,
there is a need for a synthetic strategy leading to para-amino-N,N0-
N,N0-Dihydroxybenzamidines are efficient prodrugs of benzami-
dines since they raise their bioavailability to a great extent.1,2 They
exceed characteristics of amidoximes. In turn, in vivo reduction to
the active agent amidine proceeds fast and complete.2,3 The syn-
thesis of various para-substituted N,N0-dihydroxybenzamidines
has been realized successfully. The general synthetic strategy starts
from substituted benzaldoximes4 which are chlorinated with N-
chlorosuccinimide (NCS) to benzhydroxamic acid chlorides5 using
published procedures. By reacting benzhydroxamic acid chlorides
with hydroxylamine a number of N,N0-dihydroxybenzamidines
were synthesized.6 Using this protocol for the synthesis of para-
amino-N,N0-dihydroxybenzamidine turned out not to be appropri-
ate for this special and hitherto unknown derivative because chlo-
rination of the respective oxime with NCS or tert-butyl-N-
chlorocyanamide (CBCA)7 does not lead to the intermediate p-
aminobenzhydroxamic acid chloride. The para-amino derivative 6
is of special interest because it represents the most simple phenyl-
ogous dihydroxyguanidine. Moreover, some anticoagulants con-
tain a p-aminobenzamidine group being essential for the desired
effect. N,N0-Dihydroxyamidines are known to be unstable against
acids8 and bases9,10, reductive conditions as well as high tempera-
ture, preferably leading to the respective benzamidoximes. Thus,
protecting group strategies are limited since protection and depro-
dihydroxybenzamidine 6 as a model compound to be applicable
also in more complex prodrugs.
Results and discussion
Previous results showed that already the approach of p-amin-
obenzhydroxamic acid chloride and its N-methyl, N-ethyl and
N,N-dimethyl derivatives resulted in
a mixture of undesired
byproducts especially caused by ring chlorination, so a synthetic
strategy using protecting groups was necessary. We considered
the tert-butyldimethylsilyl group as a sterically demanding pro-
tecting group being a suitable choice.11 There is continuing interest
in evaluating the formation and also special reagents and condi-
tions of cleavage for this group.12 The reaction started with
para-nitrobenzhydroxamic acid chloride 1 (Table 1). Addition of
O-tetrahydro-2H-pyranylhydroxylamine (NH2OTHP)13 or tert-
butyldimethylsilylhydroxylamine (NH2OTBDMS)14 which were
synthesized according to literature procedures led to O,O-dipro-
tected p-nitro-N,N0-dihydroxybenzamidines 2 and 4. Both deriva-
tives were purified by flash chromatography which was very
comfortable in the case of 4 because of its high lipophilic character
allowing an elution with pure cyclohexane. The obtained com-
pounds in turn had to be reduced to the appropriate p-amino
derivatives 3 and 5. Hydrogenation with catalytic amounts of pal-
ladium on charcoal (Pd/C) under mild conditions with a pressure of
2 atm for 10 h led to p-aminobenzamidine. Also reducing the
⇑
Corresponding author. Tel.: +49 431 8801126; fax: +49 431 8801352.
E-mail address: bclement@pharmazie.uni-kiel.de (B. Clement).
http://dx.doi.org/10.1016/j.tetlet.2014.04.048
0040-4039/Ó 2014 Elsevier Ltd. All rights reserved.

L. Schwarz et al. / Tetrahedron Letters 55 (2014) 3322–3324
3323
Table 1
Reaction conditions for the different steps of the synthesis of p-amino-N,N’-dihydroxybenzamidine 6
Entry Reaction step Reactant Reagent
Product Temperature Solvent
Reaction-time Mobile phase (flash chromatography) Yield (%)
1
2
3
4
5
6
a
b
c
d
e
f
1
2
3
1
4
5
2 equiv NH2OTBDMS
10 equiv Na2S2O4
11 equiv NH4F
4 equiv NH2OTHP
10 equiv Na2S2O4
2 equiv PTSA
2
3
6
4
5
6
rt
rt
rt
rt
rt
0 °C
DCM
Water/THF
MeOH
DCM
Water/THF
MeOH
10 d
3 h
15 min
6 d
3 h
1 h
Cyclohexane
Cyclohexane/ethyl acetate 8:2
DCM/MeOH 9:1
Cyclohexane/ethyl acetate 9:1
Cyclohexane/ethyl acetate 5:5
DCM/MeOH 9:1
75
23
41
53
17
11
reactant 2 or 4 could be detected by TLC analysis. FeCl3 reacted
with O,O0-di-TBDMS derivatives 2 and 3 to give a typical blue col-
our which was also observed for the O,O0-THP derivatives 4 and 5
upon heating, allowing simple colorimetric detection of the prod-
ucts on TLC.
Yields were moderate and suffered from amidine and amidox-
ime byproducts which further demonstrated the sensitivity of
dihydroxyamidines towards reductive conditions even in a pro-
tected form. Flash chromatography (cyclohexane/ethyl acetate) of
both product mixtures afforded the desired products 3 and 5.
Deprotection of p-amino-O,O0-ditetrahydro-2H-pyranyldihydroxy-
benzamidine 5 was performed with MgBr2 in ethyl ether16 and
with acetyl chloride in methanol,17 described as mild and efficient
methods but did not succeed with 5. With p-toluenesulfonic acid
(PTSA) in methanol18 p-amino-N,N0-dihydroxybenzamidine 6 could
be detected but yields were very low. p-Amino-O,O0-di-tert-butyl-
dimethylsilyl-N,N0-dihydroxy-benzamidine 3 was cleaved very fast
with a great excess of ammonium fluoride in methanol19 leading to
better yields.20 Less ammonium fluoride and longer reaction time
led to partially uncleaved products. Flash chromatography with
dichloromethane/methanol was applied. In contrast to all other
dihydroxyamidines prepared previously6 6 is insoluble in ethyl
acetate. Its properties as the simplest phenylogous dihydroxygua-
nidine have to be investigated. p-Amino-N,N0-dihydroxybenzami-
dine 6 is of special interest as a model compound for prodrugs of
p-aminobenzamidines.
Figure 1. General synthesis of para-substituted O,O0-di-tert-butyldimethylsilyl-
N,N0-dihydroxybenzamidines 2a–c.
pressure and reaction time to 1 atm and 1 h resulted in an unde-
sired reduction of the functional dihydroxyamidine group to p-
aminobenzamidine and -amidoxime. Microwave irradiation for
15 min and 80 °C with cyclohexene and Pd/C as a smooth reduction
method led to the same products. p-Nitro-O,O0-diprotected N,N0-
dihydroxybenzamidines 2 and 4 were still detected when micro-
wave irradiation time and temperature were decreased, but no p-
amino-O,O0-diprotected derivative 3 or 5 (or even p-amino-N,N0-
dihydroxybenzamidine 6) was found. Successful reduction was
eventually achieved by using an access of the smooth reagent
sodium dithionite dissolved in water.15 This solution was dropped
slowly to 2 or 4 dissolved in THF with stirring at room tempera-
ture. After three hours the reaction was completed since no more
Table 2
29
15N and Si NMR values of the synthesized para-substituted O,O0-bis(tert-butyldimethylsilyl)-N,N0-dihydroxybenzamidines 2a–c (Fig. 1) in comparison with the reagent
NH2OTBDMS
Entry
R
Hammett’s
r
Reactant
Product
Reaction time
Yield (%)
15Nc
A
15Nc
B
1JNH in Hzd
29Sie
A
29Sie
B
1a
2
3
—
—
––
1a
1b
1c
—
NH2OTBDMS
—
122.0
317.1
292.1
310.0
306.6
—
26.07
29.09
27.43
27.39
26.71
NO2
OH
H
0.81
ꢀ0.38
0
ꢀ0.57
2a
2b
2c
3
10 d
6 d
8 d
—
75
55
58
—
141.3
138.2
142.6
142.7
80
80
81
80
30.60
30.10
29.59
29.31
4
5b
NH2
a
b
c
Reagent was synthesized according to literature procedure.14
For additional information see Table 1.
15N NMR spectra were recorded in CDCl3. Values are given in ppm, relative to external CH3NO2 in CDCl3 (d = 381.6 ppm).
Measured with two-dimensional NMR experiments (1H, 15N HMBC).
d
e
29Si NMR spectra were recorded in CDCl3. Values are given in ppm, relative to external TMS in CDCl3 (d = 0.00 ppm).

3324
L. Schwarz et al. / Tetrahedron Letters 55 (2014) 3322–3324
shifts, mass spectra, IR spectra, melting points, elementary analy-
ses)) associated with this article can be found, in the online ver-
sion, at http://dx.doi.org/10.1016/j.tetlet.2014.04.048.
References and notes
1. Clement, B.; Reeh, C. WO 2008009264A1 2007.
2. Reeh, C.; Wundt, J.; Clement, B. J. Med. Chem. 2007, 50, 6730–6734.
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4. Ismail, T.; Shafi, S.; Singh, P. P.; Qazi, N. A.; Sawant, S. D.; Ali, I.; Khan, I. A.;
Kumar, H. M. S.; Qazi, G. N.; Alam, M. S. Indian J.Chem., Sect. B: Org. Chem. Incl.
Med. Chem. 2008, 47B, 740–747.
5. Liu, K.-C.; Shelton, B. R.; Howe, R. K. J. Org. Chem. 1980, 45, 3916–3918.
6. Schwarz, L.; Girreser, U.; Clement, B. Eur. J. Org. Chem. 2014, 1961–1975.
7. Kumar, V.; Kaushik, M. Synlett 2007, 2937–2951.
8. Armand, J.; Minvielle, R. M. Compt. Rend. 1965, 260, 2512–2515.
9. Wieland, H.; Bauer, H. Ber. Dtsch. Chem. Ges. 1906, 39, 1480–1488.
10. Valentini, F.; Gouzerh, P. C.R. Acad. Sci., Ser. IIc: Chim. 1972, 275, 123–126.
11. (a) Kocienski, P. J. Protecting Groups, 3rd ed.; Thieme: Stuttgart, 2005; (b) Green,
T. W.; Wuts, P. G. M. Protecting Groups in Organic Synthesis, 3rd. ed.; John Wiley
& Sons: New York, 1999.
29
Figure 2. Correlation of Si NMR chemical shifts of the functional group of O,O0-
bis(tert-butyldimethylsilylated) para-substituted N,N0-dihydroxybenzamidines 2a–
c and 3 with Hammett
(B)) = 0.74
+ 29.93, r2 = 0.63, see Figure 1.
r r
values. j d (29Si (A)) = 1.59 + 27.71, r2 = 0.93; Ç d (29Si
r
12. (a) Das, B.; Ravinder Reddy, R.; Thirupathi, P. Tetrahedron Lett. 2006, 47, 5855–
5857; (b) González-Calderón, D.; Benítez-Puebla, L. J.; González-González, C.
A.; Assa-Hermández, S.; Fuentes-Benítez, A.; Cuevas-Yañez, E.; Corona-Becerril,
D.; González-Romero, C. Tetrahedron Lett. 2013, 54, 5130–5132.
13. Haslanger M. F.; Karanewsky D. S. US 4604407 1985.
14. Bottaro, J. C.; Bedford, C. D.; Dodge, A. Synth. Commun. 1985, 15, 1333–1335.
15. Louis-Andre, O.; Gelbard, G. Bull. Soc. Chim. Fr. 1986, 565–577.
16. Kim, S.; Park, J. H. Tetrahedron Lett. 1987, 28, 439–440.
17. Chang-Eun, Y.; Yong, J. S.; Moon, K. B. Bull. Korean Chem. Soc. 2007, 28, 103–107.
18. Corey, E. J.; Niwa, H.; Knolle, J. J. Am. Chem. Soc. 1978, 100, 1942–1943.
19. Zhang, W.; Robins, M. J. Tetrahedron Lett. 1992, 33, 1177–1180.
20. To a solution of 3.16 mmol (1.25 g) p-amino-O,O0-bis(tert-butyldimethylsilyl)-
N,N0-dihydroxybenzamidine 3 in 10 mL methanol 11 equiv of ammoniuim
fluoride (34.81 mmol, 1.313 g) in 10 mL methanol are added. Reaction mixture
The sensitivity of O,O0-unsubstituted dihydroxyamidines espe-
cially towards bases and reductive conditions affords convenient
protecting groups for the synthesis of more complex compounds.
The option to synthesize di-tert-butyldimethylsilyl derivatives of
different para-substituted benzhydroxamic acid chlorides 1 dem-
onstrated the adaptability of the synthesis strategy to a wide range
of hydroxamic acid chlorides (Fig. 1).
Reaction conditions and yields of every derivative 2a–c were
similar and deprotection with ammonium fluoride afforded all p-
substituted N,N0-dihydroxybenzamidines as the desired prod-
ucts.21 Thus, a variety of TBDMS-protected dihydroxyamidines
can be synthesized (Table 2). The 29Si NMR shifts show a correla-
tion with the Hammett constant which is rather crude due to the
small number of data points (Fig. 2). Comparable correlations have
been reported by Schraml for silylated benzhydroxamic acids.22
is stirred for further 5 min and disappearance of
3 is followed by TLC
(cyclohexane/ethylacetate 1:1). The product 6 occurs near the baseline of the
TLC plate indicated as blue spots with 2% aqueous FeCl3 solution. Methanol is
evaporated and the residue is dissolved in ethyl acetate. Insoluble byproduct is
filtered off. The filtrate is applied on silica for flash chromatography with
dichloromethane/methanol gradient as the mobile phase and silica as the solid
phase. The isolated crude product is triturated with ethyl acetate/cyclohexane
2:1 to give 1.288 mmol (215 mg) 6 as a light yellow solid in 41% yield.
21. To a solution of 16 mmol (3.208 g) of p-nitrobenzhydroxamic acid chloride in
50 mL of DCM is added 4.704 g (32 mmol) of TBDMS-hydroxylamine and the
solution is stirred for 10 d at room temperature. Precipitated byproduct is
filtered off. The solvent is evaporated and the remainder is subjected to flash
chromatography on silica gel (Merck, 60) with elution with cyclohexane.
Evaporation of the solvent affords 12 mmol (5.101 g) of p-nitro-O,O0-di-tert-
butyldimethylsilyl-N,N0-dihydroxybenzamidine 2a in 75% yield.
The correlations with Hammett’s
r
for all nuclei (1H, 13C, 15N,
29Si) are given in the Supplementary material section. Unequivocal
assignment of the chemical shifts especially for the two different
silyl residues was performed with two-dimensional NMR measure-
ments, that is 1H, 13C HSQC, 1H, 29Si HMBC and 1H, 1H NOESY
experiments.
ˇ
22. (a) Schraml, J.; Kvicalová, M.; Soukupová, L.; Blechta, V. Magn. Reson. Chem.
1999, 37, 427–429; (b) Schraml, J. Appl. Organomet. Chem. 2000, 14, 604–610.
Supplementary data
Supplementary data (full synthetic protocols and analytical and
spectroscopic data of the compounds (1H and 13C NMR chemical