Salicylanilide esterification: unexpected formation of novel seven-membered rings

Article abstract of DOI:10.1016/j.tetlet.2006.05.110

Novel benzoxazepines were prepared upon esterification of biologically active salicylanilides with some N-protected amino acids. While the desired conjugates of the salicylanilides with the amino acids were obtained when sterically more demanding amino acids were used, benzoxazepines were formed as a result of a seven-exo-trig cyclization in the case of N-protected glycine and alanine. The structures of the products were confirmed by 2D NMR methods, and further transformations of the acyclic conjugates provided additional support for the proposed mechanism of cyclization.

Full text of DOI:10.1016/j.tetlet.2006.05.110

Tetrahedron Letters 47 (2006) 5007–5011
Salicylanilide esterification: unexpected formation of novel
seven-membered rings
*
´
´
´
Alesˇ Imramovsky´, Jarmila Vinsˇova, Juana Monreal Ferriz, Jirˇı Kunesˇ,
Milan Pour and Martin Dolezˇal
Faculty of Pharmacy Charles University, 500 05 Hradec Kra´love´, Czech Republic
Received 5 April 2006; revised 9 May 2006; accepted 18 May 2006
Abstract—Novel benzoxazepines were prepared upon esterification of biologically active salicylanilides with some N-protected
amino acids. While the desired conjugates of the salicylanilides with the amino acids were obtained when sterically more demanding
amino acids were used, benzoxazepines were formed as a result of a seven-exo-trig cyclization in the case of N-protected glycine and
alanine. The structures of the products were confirmed by 2D NMR methods, and further transformations of the acyclic conjugates
provided additional support for the proposed mechanism of cyclization.
ꢀ 2006 Elsevier Ltd. All rights reserved.
1. Introduction
ties could be advantageous. The goal of this work
was the preparation of esters of amino acids with
highly active salicylanilides as a new group of prodrugs
with high activity, improved solubility and low
toxicity.
Salicylanilides display potent antifungal and antibacte-
rial activity.1 They have shown activity against gram-
positive pathogens including methicillin-resistant
Staphylococcus aureus and vancomycin-resistant
Enterococcus faecium, strains representing a significant
problem in clinical practice.2,3 Recently, antimycobac-
terial activity of salicylanilides has been reported.4–6
They have shown activity not only against classical
Mycobacterium tuberculosis H37Rv, but also against
atypical strains of mycobacteria, mainly Mycobacte-
rium avium, fortuitum and kansassii where standard
antituberculotics fail. In 1998, a new mechanism for
their action based on the inhibition of two-component
regulatory systems in bacteria was proposed.7 Electron-
accepting substituents on the salicylic moiety and
hydrophobic groups on the anilide ring are essential
for their activity.8 Hydrophobicity is another factor
that may influence biological activity: while the pres-
ence of phenolic hydroxyls seems necessary for the
activity, and is probably responsible for oxidative
phosphorylation cleavage,9 they also add irritative
properties to the compounds. Therefore, temporary
masking of phenolic hydroxyls with hydrophilic moie-
The starting salicylanilides 1 were routinely prepared by
the reaction of substituted salicylic acids with the appro-
priate anilines in chlorobenzene with PCl3.4 Using a
microwave reactor, we shortened the reaction times
from several hours (5–8) to 10–15 min with yields of
about 90%. As direct esterification of a non-protected
amino acid with a salicylanilide or amino acyl derivative
was not successful, we employed DCI-mediated conden-
sation of salicylanilides 1 with N-protected amino acids
2 in the hope of obtaining the desired compounds 3
(Scheme 1).
As expected, the reactions proceeded uneventfully in
most cases, affording esters 3a–d in high yields. How-
ever, when Z-Gly and Z-L-Ala were used, the isolated
products lacked the N-benzyloxycarbonyl group, while
possessing the ester arrangement (m CO 1764 cmꢀ1).
The structures of the compounds were elucidated based
on compounds 6a and 6b by spectral methods. The mass
spectrum for 6b showed a molecular ion at m/z 364, con-
firming the composition C16H10Cl2N2O4. Hence, a com-
plex cyclization process involving the loss of benzyl
alcohol was strongly suspected. Two possible modes of
cyclization could be envisaged: (1) attack of the amide
Keywords: Salicylanilide; Amino acid ester; Seven-membered ring;
Benzoxazepines; 2D NMR.
Corresponding author. Tel.: +420 495 067 343; fax: +420 495 514
330; e-mail: vinsova@faf.cuni.cz
*
0040-4039/$ - see front matter ꢀ 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2006.05.110

5008
A. Imramovsky´ et al. / Tetrahedron Letters 47 (2006) 5007–5011
R3
O
cess depicted as path b in Scheme 2. Since cyclization
was observed in the cases where R3 = H or CH3, steric
demand around the carbamic nitrogen appears to be
the major factor influencing the formation of ben-
zoxazepines 6 (Scheme 2).
O
R2
N
H
HOOC
N
H
O
+
R1
OH
2
1
1a R1=5-Cl, R2=4-Cl
1b R1=5-Cl, R2=3-Cl
2a R3= (
S) CH3
2b R3= H
It is also interesting to note that esters 3a–d showed
melting points in the range of 130–150 ꢁC, while those
of benzoxazepines 6 were substantially higher, in the
range of 180–240 ꢁC. Both types of compounds differed
in solubility, esters 3 were soluble in ethyl acetate and
chloroform, while heterocycles 6 were only slightly
soluble in ethyl acetate and nearly insoluble in
chloroform.
1
2
3
a
1c
2c
2d
R =5-Cl, R =4-Br
R = (
R = (
R) CH(CH3)2
) CH2C6H5
3
S
O
O
R2
N
H
R1
H
N
O
O
The proposed mechanism of cyclization is in accordance
with the course of subsequent conversions of esters 3.
While N-deprotection of esters 3 (R3 = R/S CH2Ph,
R/S CH(CH3)2) by hydrogenolysis on Pd/C was unsuc-
cessful, acidolysis (33% HBr in anhydrous acetic acid)
gave hydrobromide amino salts 7 (Scheme 3). Subse-
quent amino group liberation by triethylamine under
anhydrous conditions yielded product 10. This com-
pound possessed neither ester nor free amino groups,
but the presence of a phenolic hydroxyl was clearly
apparent. The structure of 10 was unequivocally corro-
borated by 2D NMR. All HMBC correlations are shown
in Figure 1. The salicylic amide carbonyl at 166.3 ppm
displayed cross-peaks to both the H6 salicylic and the
R3
O
3
Yield (%)
) CH(CH3)2 36
3a R1=4-Cl, R2=4-Cl, R3= (
R
S
S
1
2
3
3b
3c
3d
R =4-Cl, R =4-Cl, R = (
) CH2C6H5
) CH2C6H5
52
62
57
1
2
3
R =4-Cl, R =3-Cl, R = (
1
2
3
R =4-Cl, R =4-Br, R = (S) CH2C6H5
Scheme 1. Synthesis of Z-amino acid esters with substituted salicy-
lanilides. Reagents and conditions: (a) DCI, DMF, ꢀ15 ꢁC.
nitrogen on the carbonyl of the carbamic moiety result-
ing in nine-membered ring closure (path a), and (2) at-
tack of the amide carbonyl by the carbamic NH group
(path b). The latter process would give rise to a seven-
membered cyclic intermediate 4, which would further
react with the substituted aniline released as a leaving
group in the previous step, to furnish the final product
6. MS fragmentation of 6b led to intense signals at
m/z 238 (loss of ClC6H4NH), and 210 (loss of
ClC6H4NHCO). Strong correlation between the NH
proton and carbons in the ortho-position on the p-chloro-
phenyl ring was observed in the gHMBC spectrum of
6a. Thus, the initially formed esters 3 were converted
to benzoxazepines 6 via a seven-exo-trig cyclization pro-
1
NH proton observable as a doublet in the H spectrum.
Hence, this NH group must be adjacent to a CH moiety.
The CH group, which was directly coupled to the benz-
ylic protons, showed correlation to the other carbonyl at
170.1 ppm.
Consequently, the remaining 3-chloroaniline residue
must have been a component of this amide fragment.
Obviously, the deprotected amino group immediately
attacked the amide carbonyl thus releasing substituted
aniline, which opened cyclic ester 9 to furnish diamide
10.
R2
O
R2
O
HN
H2N
O
O
N
H
b
R1
R2
O
O
R3
+
a
N
N
-PhCH2OH
O
R3
R1
R1
H
N
O
O
O
O
O
O
R3
O
R2
3
4
6
Yield (%)
22
35
1
2
3
6a
R =7-Cl, R =4-Cl, R = (
S
) CH3
O
6b R1=7-Cl, R2=4-Cl, R3= H
O
N
O
R1
NH
O
R3
5
Scheme 2. Possible modes of cyclization.

A. Imramovsky´ et al. / Tetrahedron Letters 47 (2006) 5007–5011
5009
O
O
O
R2
R2
a
N
H
N
R1
R1
H
O
H
N
-
NH3+Br
O
O
O
O
3
7
1
2
1
2
b
7a
7b
7c
R =4-Cl, R =4-Cl
3b
3c
3d
R =4-Cl, R =4-Cl
1
2
1
2
R =4-Cl, R =3-Cl
R =4-Cl, R =3-Cl
1
2
1
2
R =4-Cl, R =4-Br
R =4-Cl, R =4-Br
O
O
O
R2
NH
O
N
H
R1
R1
H2N
+
R2
O
NH2
O
8
9
O
O
O
R2
O
N
H
R1
O
N
H
+
R1
HN
NH
OH
R2
O- K+
O
11
10
Yield (%)
R =5-Cl, R =3-Cl 60
R =5-Cl, R =4-Br 44
1
2
10a
10b
1
2
Scheme 3. Reagents: (a) 33% HBr/CH3COOH, (b) triethylamine.
vacuo. The crude product (3 or 6) was purified by crys-
tallization from ethyl acetate–hexane.
H
H
O
H
Cl
O
N
H
H
H
H
N
H
Cl
3. Example of ester 3
OH
3.1. 4-Chloro-2-{[(4-chlorophenyl)amino]carbonyl}phenyl
(2S)-2-{[(benzyloxy)carbonyl]amino}-3-phenylpropanoate
(3b)
Figure 1. HMBC correlations present in 10.
23
Diamide 10 was also obtained by the reaction of the
Leuch anhydride of L-phenylalanine10 11 with the
appropriate salicylanilide salt (Scheme 3).
White solid; yield 62%; mp 151–153 ꢁC; ½aꢁD ꢀ13.2 (c 1;
CHCl3). IR (KBr): 3420, 1767, 1706, 1659, 1597, 1531,
1493, 1402, 1315, 1199, 1102. 1H NMR (300 MHz,
CDCl3) d 8.22 (1H, br s, NH), 7.78 (1H, d, J = 2.2 Hz,
H6), 7.59–7.46 (2H, m, H20, H60), 7.40 (1H, dd,
J = 8.8 Hz, J = 2.5 Hz, H4), 7.36–7.21 (10H, m, H30,
H50, Ar-phenyl), 7.19–7.11 (2H, m, Ar-phenyl), 6.92
(1H, d, J = 8.5 Hz, H3), 5.29 (1H, d, J = 7.1 Hz, NH),
5.04 (1H, d, J = 12.1 Hz, OCH2), 4.96 (1H, d,
J = 12.1 Hz, OCH2), 4.75 (1H, q, J = 7.4 Hz, NCH),
3.23 (1H, dd, J = 13.9 Hz, J = 6.3 Hz, CH2), 3.10 (1H,
dd, J = 13.9 Hz, J = 7.4 Hz, CH2). 13C NMR
(75 MHz, CDCl3) d 170.1, 162.1, 155.9, 145.7, 136.0,
135.7, 134.9, 132.2, 132.0, 129.9, 129.5, 129.1, 129.0,
128.9, 128.5, 128.3, 128.0, 127.5, 124.3, 121.9, 67.3,
55.3, 37.3. Anal Calcd for C30H24Cl2N2O5 (563.43) C,
63.95; H, 4.29; N 4.97. Found: C, 63.83; H, 4.29; N, 4.69.
2. General procedure for the synthesis of ester 3 or
benzoxazepine 6
N-Benzyloxycarbonyl protected a-amino acid
2
(10 mmol) and substituted salicylanilide 1 (10 mmol)
were dissolved in dry N,N0-dimethylformamide (DMF,
45 ml). The solution was cooled to ꢀ10 ꢁC and N,N0-
dicyclohexylcarbodiimide (DCI, 11 mmol) was added
in three portions over 1 h. The mixture was then stirred
for 3 h at the same temperature and stored at 4 ꢁC for
20 h. The precipitate of N,N0-dicyclohexylurea was re-
moved by filtration and the solvent was evaporated in

5010
A. Imramovsky´ et al. / Tetrahedron Letters 47 (2006) 5007–5011
4. Ilustrative example of cycle 6
tronꢂ Harrison Research Model 7924T (toluene/ethyl
acetate 4:1) or flash chromatography (toluene/ethyl
acetate 9:1).
4.1. (3S)-7-chloro-N-(4-chlorophenyl)-3-methyl-2,5-dio-
xo-2,3- dihydro-1,4-benzoxazepine-4(5H)-carboxamide
(6a)
6.1. (S)-5-chloro-N-(1-(3-chlorophenylamino)-1-oxo-3-
phenylpropan-2-yl)-2-hydroxybenzamide (10a)
24
White solid; yield 22%; mp 188–191 ꢁC; ½aꢁD ꢀ67.8 (c
25
2.2; DMSO). IR (KBr): 3455, 1763, 1708, 1655, 1533,
1493, 1437, 1343, 1271, 1092, 827. 1H NMR
(300 MHz, (CD3)2CO) d 9.30 (1H, br s, NH), 7.97
(1H, d, J = 2.6 Hz, H6), 7.84 (1H, dd, J = 8.8 Hz,
J = 2.6 Hz, H4), 7.58–7.52 (2H, m, AA0, BB0, H20,
H60), 7.44 (1H, d, J = 8.8 Hz, H3), 7.30–7.24 (2H, m,
AA0, BB0, H30, H50), 5.57 (1H, q, J = 6.9 Hz, CH),
1.66 (3H, d, J = 6.9 Hz, CH3). 13C NMR (75 MHz,
(CD3)2CO) d 167.8, 160.4, 152.3, 138.6, 136.9, 131.0,
129.3, 128.8, 127.6, 122.4, 122.3, 119.3, 117.1, 53.4,
13.8. Anal Calcd for C17H12Cl2N2O4 (379.19): C,
53.85; H, 3.19; N 7.39. Found: C, 53.98; H, 3.48; N,
7.50.
White solid; yield 60%; mp 181–183 ꢁC; ½aꢁD 38.25 (c 1;
ethyl acetate). IR (KBr pellet): 3297, 1672, 1633, 1594,
1536, 1483, 1416, 1288, 1236, 1180, 865, 823, 746, 693,
1
675, 535. H NMR (300 MHz, DMSO) d 12.03 (1H,
br s, NH), 10.45 (1H, br s, OH), 9.09 (1H, d,
J = 7.4 Hz, NH) 7.97 (1H, d, J = 2.5 Hz, H6), 7.79
(1H, t, J = 1.9 Hz, H20), 7.49–7.09 (9H, m, H4, H40,
H50, H60, H200, H300, H400, H500, H600), 6.94 (1H, d,
J = 8.8 Hz, H3), 4.95–4.81 (1H, m, CH), 3.24–3.03
(2H, m, CH2). 13C NMR (75 MHz, DMSO) d 170.1,
166.4, 157.6, 140.3, 137.4, 133.5, 133.3, 130.7, 129.4,
128.6, 128.5, 126.8, 123.5, 122.9, 119.4, 119.1, 118.0,
117.7, 55.6, 37.5. Anal Calcd for C22H18Cl2N2O3
(429.30): C, 61.55; H, 4.23; N, 6.53. Found: C, 61.48;
H, 4.41; N 6.57. MS (ESI) m/z 428.9 [M+].
5. Preparation of hydrobromide salt 7
In conclusion, we have shown that depending on the
structure of the amino acids, their conjugates with bio-
active salicylanilides may undergo an interesting cycliza-
tion reaction leading to novel benzoxazepines. The
scope and limitations of this reaction as well as its syn-
thetic potential and biological activity of the products
are now being intensively studied and the results will
be reported in due course.
A solution of hydrogen bromide in acetic acid (33%) was
slowly added to N-benzyloxycarbonyl-protected esters 3
with stirring. The suspension was stirred at room tem-
perature for 30 min. Over this time, the suspension
turned into a clear brown solution, and evolution of car-
bon dioxide was observed. When the gas evolution
ceased, dry diethyl ether (DEE) was added. The precip-
itate was collected by filtration, washed with DEE
(3 · 15 ml) and dried. The isolated crystals were sus-
pended in dry chloroform at room temperature, filtered
and dried in vacuo at room temperature. The yield of
hydrobromide salt 7 was about 90%.
Acknowledgements
This study was supported by IGA of the Ministry of
Health of the Czech Republic No. 1A/8238–3 and by
the Ministry of Education of the Czech Republic
(MSM 0021620822).
5.1. 4-Chloro-2-{[(4-chlorophenyl)amino]carbonyl}phenyl
(2S)-2-amino-3-phenylpropanoate hydrobromide (7a)
Yield 92%; mp 214–216 ꢁC. IR (KBr): 3421, 1763, 1658,
References and notes
1
1595, 1493, 1400, 1204, 1102, 825, 701, 507. H NMR
(300 MHz, DMSO) d 10.72 (1H, br s, NH), 8.61 (3H,
br s, NH2ÆHBr) 7.87 (1H, d, J = 2.7 Hz, H6), 7.78–
7.73 (2H, m, AA0, BB0 overlapped, H20, H60), 7.73
(1H, dd overlapped, J = 8.5 Hz, J = 2.7 Hz, H4), 7.45–
7.39 (2H, m, AA0, BB0, H30, H50), 7.33–7.21 (6H, m
H3, H200, H300, H400, H500, H600), 3.20 (1H, dd,
J = 14.4 Hz, J = 6.7 Hz, CH2), 3.13 (1H, dd,
J = 14.4 Hz, J = 6.7 Hz, CH2). 13C NMR (75 MHz,
DMSO) d 167.5, 162.6, 146.1, 137.9, 134.8, 132.0,
131.0, 130.8, 129.7, 129.4, 128.9, 128.8, 127.9, 127.5,
125.2, 121.7, 53.5, 35.6. MS (ESI) m/z 511.2 [M+H]+.
´
´
1. Vinsˇova, J.; Imramovsky, A. Ces. Slov. Farm. 2004, 53,
294–299.
2. Hiramatsu, K.; Hanaki, H.; Ino, T.; Yabuta, K.; Oguri,
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Jiraskova, L.; Pour, M.; Klimesova, V.; Kubicova, L.;
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5. Kunes, J.; Balsanek, V.; Pour, M.; Waisser, K.; Kaustova,
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6. Waisser, K.; Hladuvkova, J.; Kunes, J.; Kubicova, L.;
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Wells, M. A.; Barret, J. F. Bioorg. Med. Chem. Lett. 1998,
8, 1923–1928.
6. General procedure for the synthesis of diamide 10
Triethylamine (0.95 mmol) was added to a stirred
suspension of hydrobromide salt 7 (1 mmol) in dry
chloroform (10 ml) at room temperature. After
30 min of stirring, insoluble material was filtered off
and the filtrate was purified by using a Chromato-
8. Macielag, M. J.; Demers, J. P.; Fraga-Spano, S. A.;
Hlasta, D. J.; Johnson, S. C.; Kanojia, R. M.; Eussell,

A. Imramovsky´ et al. / Tetrahedron Letters 47 (2006) 5007–5011
5011
23:3
R. K.; Sui, Z.; Widner-Wells, M. A.; Werblood, H.;
Folen, B. D.; Goldschmidt, R. M.; Loeloff, M. J.; Webb,
G. C.; Barrett, J. F. J. Med. Chem. 1998, 41, 2939–
2945.
91–93 ꢁC.11 ½aꢁD ꢀ23.8 (c 8; DMF); IR 1847 cmꢀ1 and
1784 cmꢀ1
.
N-(1S)-1-Benzyl-2-[(3-chlorophenyl)amino]-2-oxoethyl-5-
chloro-2-hydroxybenzamide To a solution of 5-chloro-N-
(3-chlorophenyl)-2-hydroxybenzamide potassium salt (4.2
mmol) in dry THF (20 ml), cetyltrimethylammonium
bromide (0.4 mmol) and a solution of L-phenylalanine-
N-carboxyanhydride (4.2 mmol) in THF (20 ml) were
added. The reaction was performed under direct sonica-
tion at 40 ꢁC for 1 h. The solvent was then evaporated
under reduced pressure, the residue suspended in dry ethyl
acetate and heated at reflux for 1 h. Insoluble material was
filtered off, and the filtrate was evaporated and purified by
flash chromatography (toluene/ethyl acetate 4:1).
9. Terada, H.; Goto, S.; Yamamoto, K.; Takeuchi, I.;
Hamada, Y.; Miyake, K. Biochim. Biophys. Acta 1988,
936, 504–512.
10. L-Phenylalanine-N-carboxyanhydride (11) A solution of
0.89 g (3 mmol) of bis(trichloromethyl)carbonate in THF
(25 ml) was added to a suspension of 0.83 g (5 mmol) of
L-phenylalanine in tetrahydrofuran (10 ml). The reaction
mixture was stirred for 3 h at 50 ꢁC under an argon
atmosphere (the suspension became a transparent solu-
tion), the solvent was evaporated under reduced pressure,
and the oil recrystallized from a mixture of THF/petro-
leum ether and dried at room temperature. Yield 78%, mp
11. Wilder, R.; Mobashery, S. J. Org. Chem. 1992, 57, 2755–
2756, mp 91–93 ꢁC.