Clarithromycin-adenine and related conjugates

Article abstract of DOI:10.1016/j.tetlet.2006.01.079

Macrolide-nucleoside and macrolide-nucleobase conjugates (chimeras) have been synthesised by linking erythromycin A oxime derivatives and clarithromycin C11,C12-oxazolidinone derivatives with 3′-amino-3′-deoxythymidine or adenine through different spacers; clarithromycin-adenine conjugate 16a is the most active species against Micrococcus luteus.

Full text of DOI:10.1016/j.tetlet.2006.01.079

Tetrahedron Letters 47 (2006) 1919–1922
Clarithromycin–adenine and related conjugates
Jorge Esteban,a Anna M. Costa,a,* M. Carmen Cruzado,b Montserrat Faja,a
b
a,
*
´
Pilar Garcıa and Jaume Vilarrasa
a
`
´
Departament de Quı´mica Organica, Facultat de Quımica, Universitat de Barcelona, 08028 Barcelona, Catalonia, Spain, EU
bFYSE-ERCROS, Paseo del Deleite s/n, 28300 Aranjuez, CA de Madrid, Spain, EU
Received 27 December 2005; accepted 17 January 2006
Available online 3 February 2006
Abstract—Macrolide–nucleoside and macrolide–nucleobase conjugates (chimeras) have been synthesised by linking erythromycin A
oxime derivatives and clarithromycin C11,C12-oxazolidinone derivatives with 30-amino-30-deoxythymidine or adenine through dif-
ferent spacers; clarithromycin–adenine conjugate 16a is the most active species against Micrococcus luteus.
Ó 2006 Elsevier Ltd. All rights reserved.
X
Erythromycin-related macrolides are among the safest
N
and most effective antibiotics for the treatment of respi-
ratory infections. Although erythromycin A (EA, 1a) is
9
OR
HO
OH
6
H
unstable in the acidic media of the stomach (owing to
the well-known formation of hemiketals involving C9)
and causes gastrointestinal side effects, semi-synthetic
derivatives such as clarithromycin (1b) and roxithro-
mycin (1c), a derivative of EA oxime (1d), overcome
most of these problems.1 However, the development of
resistance to these antibiotics is on the rise. Although
certain modifications of the macrolide backbone confer
activity against macrolide-resistant pathogens,2 better
antibiotics are still needed.3 Preparation of conjugates
and hybrid compounds (or chimeras) is one of the new-
est strategies that are currently being investigated.4 Since
macrolide antibiotics inhibit bacterial protein synthesis
by selectively binding to ribosomal RNA,1b,5 we felt that
conjugation of a macrolide with a nucleoside or nucleo-
tide could enhance the affinity for the ribosome by
hydrogen bonding and/or electrostatic interactions.
Third-generation macrolide antibiotics such as telithro-
mycin6 (Fig. 1) or ABT-7737 already use a similar stra-
tegy, incorporating heteroaromatic rings capable of
further interactions with the ribosome; these rings
appear to be crucial for activity against macrolide–linco-
samide–streptogramin B (MSLB) resistant strains.
N
NMe
2
N
O
O
O
O
O
O
OMe
OH
O
O
N
OMe
O
O
H
EA
NMe
2
1a, R = H, X = O,
1b, R = Me, X = O,
O
O
clarithromycin
roxithromycin
EA oxime
O
O
1c, R = H, X = NOMEM,
1d, R = H, X = NOH,
O
O
telithromycin
O
NH
N
NHR
O
HO
H
(
)
O
O
n
O
N
OMe
O
O
N
NH
H
(
)
NMe
2
n
N
O
O
O
O
O
O
OH
O
OMe
OH
NH
HO
OH
H
NMe
2
O
O
O
O
O
O
NH
2
O
O
OMe
N
N
N
HO
O
O
OH
3, R =
N
O
4, R =
N
2
CH CH –
NHCO–
2
2
Figure 1.
We report here the synthesis of compounds 2–4. In the
series 2a–c, the erythromycin backbone has been linked
through an oxime ether to a nucleoside;8 as a first
approach, a thymidine was chosen. In clarithromycin
derivatives, the link between the antibiotic and
Keywords: Macrolide antibiotics; Erythromycin–thymidine chimeras;
Clarithromycin–adenine chimeras; Oxime-linked conjugates; Oxazo-
lidinone-linked conjugates.
*
Corresponding authors. Tel.: +34 934039113; fax: +34 933397878;
e-mail addresses: amcosta@ub.edu; jvilarrasa@ub.edu
0040-4039/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2006.01.079

1920
J. Esteban et al. / Tetrahedron Letters 47 (2006) 1919–1922
nucleoside (see 3a–c) and between the antibiotic and
adenine (see 4a,b) has been established through a cyclic
carbamate, or oxazolidinone, at positions C11,C12.9,10
the E compounds, in relation to the Z ones (Scheme
1). Furthermore, the 13C NMR chemical shift for 10-
Me is characteristic: it appears at ca. 15 ppm in the
(E)-oxime ethers, and at ca. 11 ppm for the (Z)-oxime
ethers. This is most probably due to a shielding effect
caused by the steric interaction between the alcoxyimino
group and 10-Me.12,15
For the preparation of 2 (Scheme 1), we envisaged the
coupling of the thymidine derivative 5 prepared11 from
AZT with a suitably alkylated erythromycin oxime.
Mesylates 6a–c with different chain lengths were chosen
as alkylating agents. Alkylation of the oxime 1d was at-
tempted under a variety of conditions (e.g., with MeO-
Na in DMF and with Na2CO3 in EtOH) but no oxime
ether could be isolated. However, when the reaction
was performed with K2CO3 as the base and acetone as
the solvent, the desired oximes (E)-7a–c were obtained
in good yields. The corresponding (Z)-oximes, arising
from the partial isomerisation of EA (E)-oxime in the
basic reaction medium,12 were also isolated.
Removal of the benzyloxycarbonyl group of the major
compounds, (E)-7a–c, by catalytic hydrogenation affor-
ded the free amines 8a–c in quantitative yield. These
amines were coupled with thymidine derivative 5 in
CH2Cl2–DMF to yield protected chimeras 9a–c.
Removal of the TBS group was accomplished by treat-
ment with HF–pyridine in THF as the solvent to give
the desired 2a–c.
Chimeras 3a–c were prepared by coupling amines 12a–c
with thymidine 5, as shown in Scheme 2. The clarithro-
mycin 20,400-diacetate, 10, was converted to 12-O-imidaz-
olylcarbonyl macrolide 11 by treatment with an excess
of 1,10-carbonyldiimidazole and NaH in DMF–THF.
This transformation proceeds through a 11,12-cyclic
carbonate intermediate.9a The 11,12-carbamate group
(oxazolidinone ring) was introduced by treating crude
11 with the corresponding diamines followed by depro-
tection of the 20-acetate. Only the natural 10R epimer
was formed. For 12a–c, the stereochemistry at C10
was determined by NMR spectroscopy. Previous studies
The E or Z configuration of 7a–c was determined by
1
careful comparison of their H and 13C NMR spectra
with those of (E)- and (Z)-oximes of 1d12,13 and 1b.14
The proton H11 appears at a lower field for the Z-oxime
ethers due to the deshielding effect of the alcoxyimino
group. The same effect causes H8 to be deshielded in
NHR
NHR
( )
O
( )
O
n
n
δ
C 14.5 0.1
δC 11.2 0.2
δ
H 2.88 0.05
δ
H 3.72 0.03
N
N
13
showed that the C signal for C10 is diagnostic:9a it
δ
H 3.80 0.03
HO
δ
H 3.70 0.02
9
10
11
8
OH
OH
HO
OH
(a)
OH
+
1d
O
ODes
OCla
N
O
ODes
O
O
1
O
O
OCla
N
OMe
O
OMe
HO
(E)-7a–c, R = COOCH Ph
(Z)-7a–c, R = COOCH Ph
OH
O
2
2
(b)
AcO
O
2'
NMe
(a)
(b)
NMe
2
(E)-8a–c, R = H
AcO
O
2
O
1b
O
O
O
O
O
(c)
O
O
OMe
OMe
OAc
O
O
NH
N
NH
OAc
10
O
11
(c)
4"
O
N
HO
TBSO
O
O
O
O
O
H
N
H
N
NH
O
NH
NH
O
( )
( )
N
N
n
n
δ
C 38.9 0.1
N
R O
1
O
O
δC 45.6 0.1
O
O
(d)
NH
2
O
OH
OH
( )
n
HO
HO
OH
O
OH
O
NH
NH
( )
N
OMe
O
O
ODes
O
O
ODes
H
(d)
NMe
O
O
2
n
N
O
O
OCla
9a–c
O
OCla
2a–c
O
O
OMe
O
O
OMe
OAc
O
H
NMe
O
O
2
O
O
O
O
12a, n = 2
12b, n = 3
12c, n = 5
O
OMe
OR
O
NH
( )
N
TBSO
O
O
2
O
n
O
MsO
NHCOOCH Ph
2
13a–c, R = TBS, R = Ac
14a–c, R = H, R = Ac
1
2
6a, n = 2
6b, n = 3
6c, n = 6
(e)
NH
1
2
(f)
3a–c , R = R = H
1
2
N
5
N
Scheme 2. Reagents and conditions: (a) Ac2O, Et3N, DMAP, CH2Cl2,
rt, 17 h, 99%; (b) Im2CO, NaH, DMF, THF, rt, 4 h; (c)
H2N(CH2)nNH2 (n = 2, 3, 5), CH3CN, rt, 18 h, then MeOH, reflux,
4 h, 73–90% (overall, for the three last steps); (d) 5, DMF, CH2Cl2, rt,
4 h, 80–86%; (e) HF–pyridine, THF, 0–4 °C, 18 h, 92–97%; (f)
NH4OH, MeOH, rt, 4 d, 77–91%.
Scheme 1. Reagents and conditions: (a) 6a–c, K2CO3, acetone, reflux,
24 h, 50–65% of (E)-oxime plus 15–25% of (Z)-oxime; (b) H2, Pd/C,
MeOH, rt, 1.5–4 h, 94–99%; (c) 5, CH2Cl2, DMF, rt, 2–3 h, 72–80%;
(d) HF–pyridine, THF, 0–4 °C, 14–24 h, 76–84%.

J. Esteban et al. / Tetrahedron Letters 47 (2006) 1919–1922
1921
NH
appears at ca. 37 ppm in the natural isomer, whereas it is
2
N
N
shifted to ca. 52 ppm for the unnatural 10S epimer.
N
N
Coupling of amines 12a–c with 5 took place smoothly in
CH2Cl2–DMF. Removal of the TBS group (to convert
13a–c into 14a–c, respectively), followed by treatment
with aqueous ammonia to cleave the 400-OAc groups,
furnished the desired compounds 3a–c.
O
NH
( )n
N
O
OMe
(a)
O
12a,b
H
NMe
2
O
O
O
O
NH
2
Compounds 2a–c, 12a–c, 13a–c, 14a–c and 3a–c were
tested against Micrococcus luteus ATCC 9341 by the
standard agar dilution method.16 All compounds
showed weak antibacterial activity (Table 1). The most
active ones were amines 12a–c, although with potencies
below 40% of that of the reference antibacterial agent
(EA, 1a). Under identical conditions the value that we
have determined for clarithromycin, 1b, is 122% and
that for azithromycin is 110%. Long-chain spacers
(n = 6 for the case of 2c and its precursors, n = 5 for
the case of 3c and its precursors) did not afford any
advantage. All these side chains seem to be too bulky.
Perhaps they also lack basic centres that could be
required for the interaction with the nucleotides in
the peptidyl-transferase tunnel.5
O
OMe
N
O
N
N
OR
N
O
4"
OHC
16a,b R = Ac
4a,b R = H
15
(b)
Scheme 3. Reagents and conditions: (a) NaBH3CN, CH3CN–H2O,
pH 4–5, rt, 24 h, 42–50%; (b) NH4OH, MeOH, rt, 3 d, 84–90%.
noted. Samples of 16a,b, 4a and 4b have also been sub-
mitted to evaluation;18 results will be reported in due
course.
In summary, syntheses of erythromycin–thymidine chi-
meras 2a–c and of clarithromycin–thymidine chimeras
3a–c are disclosed. Clarithromycin–adenine conjugates
4a and 4b are also reported and characterised for the
first time. The biological studies are preliminary but
indicate some detrimental factors to be avoided if the
discovery of new leads is pursued. Further biological
tests as well as virtual screenings (by computational
docking into the known ribosome structure of series of
alternative conjugates) are underway.
Thus, we set out to synthesise macrolide–adenine chime-
ras 4a,b, in which the link between the two components
is through an amine moiety. They were prepared by
reductive amination of amines 12a,b with the hydrate
of aldehyde 15,17 followed by hydrolysis of the OAc
group (Scheme 3).
Compounds 4a,b and their precursors 16a,b were tested
against M. luteus ATCC 9341, as above indicated.16 The
values for their biological activity are also shown in
Table 1. Among all the samples tested so far, 400-O-acetyl
derivative 16a showed the highest potency (Table 1,
entry 10), although it is still lower than desired. A spacer
with an additional CH2 (from n = 2–3) is detrimental;
a HB donor at 400 (400-OH) is also detrimental (entry
11) with regard to its 400-OAc derivative (entry 10).
Acknowledgements
Antimycobacterial data were provided by the TAACF
program (Alabama, USA). J.E. thanks the Universitat
of Barcelona, for a doctoral fellowship. Financial sup-
port from the Spanish MCyT (currently Ministerio de
´
Educacion y Ciencia) and the Generalitat de Catalunya
is acknowledged.
The antimycobacterial activities of compounds 2a–c and
3a–c were checked against Mycobacterium tuberculosis
H37Rv.18 Inhibition percentages around 73–83% were
Supplementary data
Table 1. Percentage of potency against Micrococcus luteus, in relation
to EA, 1a
Spectral data for compounds 2a–c, 3a–c and 4a,b can be
found in the Supplementary data associated with this
article in the online version, at doi:doi:10.1016/j.tetlet.
2006.01.079.
Entry
Compound
Percentage of activity of each
series (%)
a
b
c
a
a
a
a
a
a
1
2
3
4
5
6
7
8
(E)-7
(Z)-7
(E)-8
(E)-9
2
30.6
31.4
24.8
7.4
7.8
37.5
4.1
1.6
1.3
49.8
33.0
—
—
—
—
—
—
References and notes
1.3
8.4
36.2
6.4
1.7
1.7
3.4
5.5
17.2
2.2
3.7
2.5
—
1. (a) Sunazuka, T.; Omura, S.; Iwasaki, S.; Omura, S. In
Macrolide Antibiotics: Chemistry, Biology and Practice,
2nd ed.; Omura, S., Ed.; Academic Press, Elsevier:
Orlando, 2002; pp 99–180; (b) Katz, L.; Ashley, G. W.
Chem. Rev. 2005, 105, 499–527.
2. Ma, Z.; Clark, R. F.; Brazzale, R.; Wang, S.; Rupp, M. J.;
Li, L.; Griesgaber, G.; Zhang, S.; Yong, H.; Tam Pham,
L.; Nemoto, P. A.; Chu, D. T. W.; Plattner, J. J.; Zhang,
X.; Zhong, P.; Cao, Z.; Nilius, A. M.; Shortridge, V. D.;
12
13
14
9
10
11
3
16
29.5
7.3
4
—
a Not determined.

1922
J. Esteban et al. / Tetrahedron Letters 47 (2006) 1919–1922
Flamm, R.; Mitten, M.; Meulbroek, J.; Ewing, P.; Alder,
J.; Or, Y. S. J. Med. Chem. 2001, 44, 4137–4156.
3. (a) Chu, D. T. W.; Plattner, J. J.; Katz, L. J. Med. Chem.
1996, 39, 3853–3874; (b) Walsh, C.; Wright, G. Chem. Rev.
2005, 105, 391–393.
4. Reviews: (a) Tietze, L. F.; Bell, H. P.; Chandrasekhar, S.
Angew. Chem., Int. Ed. 2003, 42, 3996–4028; (b) Mehta,
G.; Singh, V. Chem. Soc. Rev. 2002, 31, 324–334.
14. Spectral data of clarithromycin (E)-oxime: 1H NMR
(CDCl3, 400 MHz): d 0.84 (t, J = 7.4, 3H, Me15), 0.99
(d, J = 6.8, 3H, 8-Me), 1.08 (d, J = 8.0, 3H, 4-Me), 1.13 (s,
3H, 12-Me), 1.14 (d, J = 8.0, 3H, 10-Me), 1.20 (d, J = 6.8,
3H, 2-Me), 1.23 (d, J = 6.0, 3H, 50-Me), 1.25 (s, 3H, 300-
Me), 1.31 (d, J = 6.4, 3H, 500-Me), 1.48 (s, 3H, 6-Me),
1.48–1.61 (m, 4H, H7a+H7b+H14a+H200a), 1.67 (m, 1H,
H40b), 1.89–1.98 (m, 2H, H4+H14b), 2.30 (s, 6H, NMe2),
2.37 (d, J = 15.2, 1H, H200b), 2.44 (m, 1H, H30), 2.58 (q,
J = 7.2, 1H, H10), 2.89 (dq, J = 9.2, J = 7.2, 1H, H2), 3.03
(m, 1H, H400), 3.11 (s, 3H, 300-OMe), 3.21 (dd, J = 10.0,
7.2, 1H, H20), 3.33 (s, 3H, 6-OMe), 3.48 (m, 1H, H50), 3.66
(d, J = 7.2, 1H, H5), 3.74–3.80 (m, 2H, H3+H8), 3.75 (s,
1H, H11), 4.03 (dq, J = 9.2, 6.4, 1H, H500), 4.44 (d,
J = 7.2, 1H, H10), 4.94 (d, J = 4.4, 1H, H100), 5.11 (dd,
J = 11.2, 2.0, 1H, H13); 13C NMR (CDCl3, 100.6 MHz): d
9.1 (4-Me), 10.6 (C15), 14.9 (10-Me), 16.0 (2-Me), 16.1
(12-Me), 18.6 (500-Me), 18.7 (8-Me), 20.0 (6-Me), 21.2,
21.5, 21.5 (C14, 50 -Me, 300-Me), 25.4 (C8), 28.8 (C40), 32.9
(C10), 34.9 (C200), 37.4 (C7), 39.1 (C4), 40.3 (NMe2), 45.1
(C2), 49.5 (300-OMe), 51.2 (6-OMe), 65.5 (C30), 65.7 (C500),
68.6 (C50), 70.2 (C11), 71.1 (C20), 72.7 (C300), 74.1 (C12),
76.9 (C13), 78.0 (C400), 78.5 (C6), 78.8 (C3), 80.5 (C5), 96.1
(C100), 102.8 (C10), 170.9 (C9), 175.7 (C1). The assignments
indicated throughout have been established or corrobo-
rated by 2D NMR experiments (HSQC and COSY).
(Z)-Oxime: 1H NMR (CDCl3, 500 MHz): d 0.80 (t,
J = 7.3, 3H, Me15), 1.0–1.4 (m, 28H, H40a+2-Me+4-
Me+6-Me+8-Me+10-Me+12-Me+50-Me+300-Me+500-Me),
1.40–1.50 (m, 2H, H7a+H14a), 1.51–1.57 (m, 2H,
H7b+H200a), 1.64 (m, 1H, H40b), 1.86–1.95 (m, 2H,
H4+H14b), 2.27 (s, 6H, NMe2), 2.33 (d, J = 15.0, 1H,
H200b), 2.43 (ddd, J = 12.5, 10.5, 3.3, 1H, H30), 2.55 (m,
1H, H10), 2.73 (m, 1H, H8), 2.84 (dq, J = 9.3, 7.3, 1H,
H2), 2.98 (d, J = 9.0, 1H, H400), 3.06 (s, 3H, 300-OMe), 3.18
(dd, J = 10.3, 7.3, 1H, H20), 3.33 (s, 3H, 6-OMe), 3.44 (m,
1H, H50), 3.58 (d, J = 7.5, 1H, H5), 3.73 (d, J = 9.5, 1H,
H3), 3.93 (br s, 1H, H11), 3.98 (dq, J = 9.3, 6.3, 1H, H500),
4.39 (d, J = 7.0, 1H, H10), 4.89 (d, J = 4.5, 1H, H100), 5.04
(dd, J = 11.0, 2.0, 1H, H13); 13C NMR (CDCl3,
75.4 MHz): d 9.1 (4-Me), 10.6 (C15), 11.6 (10-Me), 15.9,
16.6 (2-Me, 12-Me), 18.6 (500-Me), 19.8, 19.9 (8-Me, 6-Me),
21.3, 21.5, 21.5 (C14, 6-Me, 300-Me), 28.9 (C40), 34.2
(C10), 34.9 (C200), 36.1 (C8), 37.4 (C7), 39.1 (C4), 40.2
(NMe2), 45.2 (C2), 49.4 (300-OMe), 50.2 (6-OMe), 65.4
(C30), 65.6 (C500), 68.6 (C50), 70.5 (C11), 71.1 (C20), 72.7
(C300), 74.8 (C12), 76.6 (C13), 78.0 (C400), 78.6 (C6), 78.9
(C3), 80.5 (C5), 96.1 (C100), 102.8 (C10), 167.1 (C9), 175.9
(C1).
5. Schlunzen, F.; Zarivach, R.; Harms, J.; Bashan, A.; Tocilj,
¨
A.; Albrecht, R.; Yonath, A.; Franceschi, F. Nature 2001,
413, 814–821.
6. (a) Agouridas, C.; Denis, A.; Auger, J.; Benedetti, Y.;
Bonnefoy, A.; Bretin, F.; Chantot, J. F.; Dussarat, A.;
Fromentin, C.; D’Ambrieres, S. G.; Lachaud, S.; Laurin,
P.; Le Martret, O.; Loyau, V.; Tessot, N. J. Med. Chem.
1998, 41, 4080–4100; (b) Denis, A.; Agouridas, C.; Auger,
J.-M.; Benedetti, Y.; Bonnefoy, A.; Bretin, F.; Chantot, J.
F.; Dussarat, A.; Fromentin, C.; D’Ambrieres, S. G.;
Lachaud, S.; Laurin, P.; Martret, O. L.; Loyau, V.; Tessot,
N.; Pejac, J.-M.; Perron, S. Bioorg. Med. Chem. Lett.
1999, 9, 3075–3080; (c) Berisio, R.; Harms, J.; Schluenzen,
F.; Zarivach, R.; Hansen, H. A. S.; Fucini, P.; Yonath, A.
J. Bacteriol. 2003, 185, 4276–4279.
7. Or, Y. S.; Clark, R. F.; Wang, S.; Chu, D. T. W.; Nilius,
A. M.; Flamm, R. K.; Mitten, M.; Ewing, P.; Alder, J.;
Ma, Z. J. Med. Chem. 2000, 43, 1045–1049.
8. For reports of 9-oxime derivatives with good antibacterial
activity, see: (a) Chantot, J. F.; Bryskier, A.; Gasc, J. C. J.
Antibiot. 1986, 39, 660–668; (b) Denis, A.; Pejac, J.-M.;
Bretin, F.; Bonnefoy, A. Bioorg. Med. Chem. 2003, 11,
2389–2394; (c) Kawashima, Y.; Yamada, Y.; Asaka, T.;
Misawa, Y.; Kashimura, M.; Morimoto, S.; Ono, T.;
Nagate, T.; Hatayama, K. Chem. Pharm. Bull. 1994, 42,
1088–1096; (d) Akemi, N.; Narita, K.; Ohmoto, S.;
Takahashi, Y.; Yoshizumi, S.; Yoshida, T.; Kada, N.;
Okezaki, E.; Kato, H. Chem. Pharm. Bull. 2001, 49, 1120–
1127.
9. This ring is known to enhance antibacterial activity
against resistant pathogens, by increasing the conforma-
tional rigidity of the macrolactone. See: (a) Baker, W. R.;
Clark, J. D.; Stephens, R. L.; Kim, K. H. J. Org. Chem.
1988, 53, 2340–2345; (b) Fernandes, P. B.; Baker, W. R.;
Freiberg, L. A.; Hardy, D. J.; MacDonald, E. J. Anti-
microb. Agents Chemother. 1989, 33, 78–81.
10. For ketolides with heteroaromatic appendages (including
two examples with adenine) linked via a thioether, see:
Hunziker, D.; Wyss, P.-C.; Angehrn, P.; Mueller, A.;
Marty, H.-P.; Halm, R.; Kellenberger, L.; Bitsch, V.;
Biringer, G.; Arnold, W.; Sta¨mpfli, A.; Schmitt-Hoffmann,
A.; Cousot, D. Bioorg. Med. Chem. 2004, 12, 3503–3519.
11. Costa, A. M.; Vilarrasa, J. Tetrahedron Lett. 2000, 41,
3371–3375.
15. McGill, J. M.; Johnson, R. Magn. Res. Chem. 1993, 31,
273–277.
16. The assays were performed in accordance with the Code of
Federal Regulations guidelines: CFR Title 21, Part
436105.
12. Wilkening, R. R.; Ratcliffe, R. W.; Doss, G. A.; Bartizal,
K. F.; Graham, A. C.; Herbert, C. M. Bioorg. Med. Chem.
Lett. 1993, 3, 1287–1292.
17. Xu, Z.-Q.; Qiu, Y.-L.; Chokekijchai, S.; Mitsuya, H.;
Zemlicka, J. J. Med. Chem. 1995, 38, 875–882.
´
13. (a) Esteban, J.; Costa, A. M.; Urpı, F.; Vilarrasa, J.
Tetrahedron Lett. 2004, 45, 5563–5567; (b) J. Esteban,
Master Thesis, Universitat de Barcelona, 2002; (c) Wil-
kening, R. R. EP 0,503,932 A1, 1992.
18. Tuberculosis Antimicrobial Acquisition and Coordinating
Facility (TAACF, Alabama, USA), cf. http://www.taacf.
org.