Synthesis and antibacterial activity of C-2(S)-substituted pleuromutilin derivatives

Article abstract of DOI:10.1016/j.cclet.2010.01.001

In order to probe the effect of C-2(S)-substituted groups in the antibacterial activity, a series of novel C-2(S)-substituted pleuromutilin analogues of SB-225586 were synthesized and evaluated for their in vitro antibacterial activity. The results of ant

Full text of DOI:10.1016/j.cclet.2010.01.001

Available online at www.sciencedirect.com
Chinese Chemical Letters 21 (2010) 507–510
www.elsevier.com/locate/cclet
Synthesis and antibacterial activity of C-2(S)-substituted
pleuromutilin derivatives
Li Qiang Fu a,1, Xin Sheng Guo b,1, Xin Liu a, Hui Li He a,
Yu Lin Wang b, Yu She Yang a,
*
a Shanghai Institute of Materia Medica, Chinese Academy of Sciences, Shanghai 201203, China
b School of Pharmaceutical Engineering, Shenyang Pharmaceutical University, Shenyang 110015, China
Received 12 November 2009
Abstract
In order to probe the effect of C-2(S)-substituted groups in the antibacterial activity, a series of novel C-2(S)-substituted
pleuromutilin analogues of SB-225586 were synthesized and evaluated for their in vitro antibacterial activity. The results of
antibacterial activities indicated that C-2(S)-substituted pleuromutilin derivatives retained appreciable antibacterial activity, and the
2-fluorination compounds 6a and 6b are more potent than the corresponding 2-hydroxylation analogues 7a and 7b.
# 2010 Yu She Yang. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.
Keywords: Synthesis; Pleuromutilin; C-2(S)-substituted; Antimicrobial activity
Pleuromutilin (Fig. 1) was first isolated in 1951 from basidiomycetes Pleurotus and P. passeckerianus and display
modest activity against Gram-positive organisms in vitro [1,2]. Further studies have shown that this class of antibiotics
interfere with bacterial protein synthesis via a specific interaction with the 23S rRNA of the 50S bacterial ribosome
subunit [3]. These compounds offer a distinct profile and show no cross-resistance with any other class of antibiotics.
Tiamulin and valnemulin as semisynthetic derivatives are currently employed as veterinary medicine to treat serious
infections in swine and poultry [4,5]. GlaxoSmithKline’s novel pleuromutilin analogue retapamulin [6], with excellent
activity in vitro, was first approved for human use as a topical antimicrobial agent to treat skin infections in 2007.
Although semisynthetic pleuromutilin analogues generally exhibit potent antibacterial activity, they displayed poor
oral absorption and a short half-life due to rapid metabolism and subsequent excretion. Further studies have shown that
hepatic metabolism mediated by cytochrome P450 represents the major obstacle to developing a semisynthetic
pleuromutilin for oral treatment of bacterial infections in the community [7]. Our previous research [8] has revealed
that a water-soluble phosphate prodrug of pleuromutilin analogue SB-225586 exhibited potent protective effects
against MSSA and MRSA strains in vivo, but a short half-life was also found (less than 1 h). However, reports have
also shown that metabolic stability can be increased by involvement of fluorination or hydroxylation at C-2 [9]. In
order to probe the effect of C-2(S)-substituted groups in the antibacterial activity, pleuromutilin analogues SB-225586
* Corresponding author.
E-mail address: ysyang@mail.shcnc.ac.cn (Y.S. Yang).
1
Both the authors contributed equally to this work.
1001-8417/$ – see front matter # 2010 Yu She Yang. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.
doi:10.1016/j.cclet.2010.01.001

508
L.Q. Fu et al. / Chinese Chemical Letters 21 (2010) 507–510
Fig. 1. Structures of pleuromutilin derivatives.
were selected as C-2 unsubstituted parent. In this letter, we report the synthesis and antibacterial activity of C-2(S)-
substituted derivatives of SB-225586.
Compounds 6a and 6b were prepared as shown in Scheme 1. The 2-hydroxymethylene-11-formate 1 was
synthesized form commercially available pleuromutlilin in two steps according to reported method [10]. A solution of
1 in dichloromethane was treated with tosylazide to generate a-diazo carbonyl intermediate 2 [10]. C-2(S) fluoro-
substituted compound 4 was obtained by the reaction of 2 with hydrogen fluoride-pyridine in moderate yield [10]. The
nucleophilic substitution was taken place from less hinder side and the main side product of this step was the enones 3
in 20–30% yield. Intermediate 5 was prepared in satisfying yield by treatment 4-acetoxybenzoyl chloride, silver
cyanate with compound 4 in dichloromethane [11]. Then 5 was treated with 0.5 N potassium hydroxide in ethanol
solution at ambient temperature to give the deprotected compound 6a [12]. The reduced compounds 6b was obtained
by hydrogenation of the double bond in high yield (95.1%) without further purification.
Compounds 7a and 7b were prepared as shown in Scheme 2. A solution of 2 in dichloromethane was treated
dropwise with dichloracetic acid. The mixture was stirred continued until the solution was colourless, compound 8 was
obtained by silica gel chromatography [13], and the enones 3 was also observed as the main side product. Thus, 8
reacted with 4-acetoxybenzoyl chloride and silver cyanate successfully to produce compound 9, which was subjected
to deprotection and smoothly to afford the target compound 7a. The reduced compounds 7b was obtained by
hydrogenation as compound 6b [12].
The in vitro antibacterial activity of the compounds against a spectrum of resistant and susceptible Gram-positive
bacteria was tested with SB-225586 and Linezolid as positive controls. Minimum inhibitory concentration (MIC)
values were determined using agar dilution method according to NCCLS. The result is summarized in Table 1. It
Scheme 1. Regents and conditions: (a) TEA, TsN3, CH2Cl2, ꢀ10 8C to r.t., 44.1%; (b) HF-Py, ether, ꢀ0 8C to r.t., 46.9%; (c) 4-acetoxybenzoyl
chloride, CH2Cl2, AgOCN, 50 8C to r.t., 85.8%; (d) KOH, EtOH, r.t., 1 h, 88.3%; (e) H2, 10% Pd-C, THF, 95.1%.

L.Q. Fu et al. / Chinese Chemical Letters 21 (2010) 507–510
509
Scheme 2. Regents and conditions: (a) Cl2CHCOOH, CH2Cl2, 0 8C to r.t., 58.1%; (b) 4-acetoxybenzoyl chloride, CH2Cl2, AgOCN, 50 8C to r.t.,
72.9%; (c) KOH, EtOH, r.t., 1 h, 70.2%; (d) H2, 10% Pd-C, THF, r.t., 96.0%.
Table 1
In vitro antibacterial activity of synthetic compounds.
Compounds
MIC(mg/mL)
MSSAa n = 5
MRSAb n = 6
MSSEc n = 5
MRSEd n = 5
S.p.e n = 3
SB-225586
0.125
0.0625
0.0625
1
0.125
0.0625
0.0625
1
0.0625
0.125
0.0625
1
0.125
0.125
0.0625
1
0.25
0.125
0.125
0.5
6a
6b
7a
7b
0.5
0.5
0.5
0.5
0.25
1
Linezolid
0.5
1
0.5
1
a
MSSA = Methicillin-susceptible Staphylococcus aureus.
MRSA = Methicillin-resistant Staphylococcus aureus.
MSSE = Methicillin-susceptible Staphylococcus epidermidis.
MRSE = Methicillin-resistant Staphylococcus epidermidis.
S.p. = Streptococcus pneumoniae.
b
c
d
e
clearly showed that all of the compounds displayed excellent antibacterial activities and most of the compounds had
more potent activities compared with Linezolid. C-2(S) fluorination compounds 6a and 6b are were twofold more
potent than 2-unsubstitued SB-225586, however, C-2(S) hydroxylation compounds 7a and 7b were less potent than
references. Moreover, the double bond reduction compounds 6b and 7b are more potent than the corresponding vinyl
analogues 6a and 7b.
In conclusion, a series of C-2(S)-substituted derivatives of pleuromutilin analogues SB-225586 were synthesized
and evaluated. The results of antibacterial activities indicated that C-2(S)-substituted pleuromutilin derivatives
retained appreciable antibacterial activity, especially the C-2(S)-fluoro derivatives 6a and 6b which antimicrobial
activity has been obviously improved.
References
[1] F. Kavanagh, A. Hervey, W.J. Robbins, Proc. Natl. Acad. Sci. U.S.A. 37 (1951) 570.
[2] M. Anchel, J. Biol. Chem. 199 (1952) 133.
[3] C. Davidovich, A. Bashan, R. Yaggie, et al. Proc. Natl. Acad. Sci. U.S.A. 104 (2007) 4291.
[4] K.M. Islam, U. Klein, G.S. Burch, Poult. Sci. 88 (2009) 2353.

510
L.Q. Fu et al. / Chinese Chemical Letters 21 (2010) 507–510
[5] I.A. Aitken, J.H. Morgan, R. Dalziel, et al. Vet. Rec. 144 (1999) 128.
[6] S. Rittenhouse, S. Biswas, J. Broskey, et al. Antimicrob. Agents Chemother. 50 (2006) 3882.
[7] I. Schuster, C. Fleschurz, J. Hildebrandt, 13th Int. Cong. Chemother., Aug 28–Sept 2 Vienna, 1983, Abst PS 4.6/2-11.
[8] L.Q. Fu, Z.T. Jiang, Y.S. Yang, et al. Bioorg. Med. Chem. Lett. 19 (2009) 5407.
[9] E. Hunt, Drug Future 25 (2000) 1163.
[10] G. Brooks, E. Hunt, WO 00/07974 [P].
[11] G. Brooks, W. Burgess, D. Colthurst, et al. Bioorg. Med. Chem. 9 (2001) 1221.
20
[12] The data of 6a, 6b; 7a, 7b; 6a: White solid, yield 88.3%, mp: 127–129 8C; ½aꢁD ꢀ4.7 (c 0.75, MeOH); 1H NMR (300 MHz, CDCl3): d 0.88 (d,
3H, J = 7.0 Hz), 0.94 (d, 3H, J = 6.4 Hz), 1.07–1.38 (m, 4H), 1.08 (s, 3H), 1.39 (s, 3H), 1.80–1.95 (m, 4H), 2.05–2.10 (m, 2H), 2.36 (s, 1H),
3.36 (d, 1H), 4.79 (t, 1H, J = 8.1 Hz), 5.25 (d, 1H, J = 17.9 Hz), 5.36 (d, 1H, J = 11.2 Hz), 5.76 (d, 1H, J = 8.1 Hz), 6.41 (dd, 1H, J = 18.0 Hz,
11.2 Hz), 7.23 (d, 2H, J = 8.6 Hz), 7.80 (d, 2H, J = 8.5 Hz), 9.40 (s, 1H). MS (ESI) m/z: 500.2 ([MꢀH]ꢀ); Anal. Calcd. for C28H36FNO6:C,
20
67.23; H, 7.14; N, 2.95. Found: C, 67.05; H, 7.23; N, 2.79; 6b: White solid, yield 95.1%, mp: 141–143 8C; ½aꢁD +5.6 (c 1.0, MeOH); 1H NMR
(300 MHz, CDCl3): d 0.87 (d, 3H, J = 6.8 Hz), 0.91 (d, 3H, J = 6.7 Hz), 0.96 (t, 3H), 1.09–1.35 (m, 6H), 1.13 (s, 3H), 1.41 (s, 3H), 1.77–1.94
(m, 4H), 2.03–2.10 (m, 2H), 2.35 (s, 1H), 3.15 (d, 1H), 4.75 (t, 1H, J = 8.0 Hz), 5.74 (d, 1H, J = 8.2 Hz), 7.20 (d, 2H, J = 8.3 Hz), 7.79 (d, 2H,
J = 8.4 Hz), 9.65 (s, 1H). MS (ESI) m/z: 502.2 ([MꢀH]ꢀ); Anal. Calcd. for C28H38FNO6:C, 66.61; H, 7.33; N, 2.76. Found: C, 66.78; H, 7.61;
20
N, 2.78; 7a: White solid, yield 70.2%, mp: 161–163 8C; ½aꢁD ꢀ1.6 (c 0.5, MeOH); 1H NMR (300 MHz, CDCl3): d 0.87 (d, 3H, J = 7.1 Hz),
0.92 (d, 3H, J = 7.4 Hz), 1.05–1.39 (m, 4H), 1.10 (s, 3H), 1.36 (s, 3H), 1.60–1.86 (m, 4H), 2.06–2.10 (m, 2H), 2.39 (s, 1H), 3.32 (d, 1H), 4.04 (t,
1H, J = 8.1 Hz), 5.21 (d, 1H, J = 17.7 Hz), 5.39 (d, 1H, J = 11.0 Hz), 5.58 (d, 1H, J = 8.4 Hz), 6.20 (dd, 1H, J = 17.9 Hz, 11.4 Hz), 6.80 (d, 2H,
J = 8.8 Hz), 7.66 (d, 2H, J = 8.4 Hz), 10.44 (s, 1H). MS (ESI) m/z: 498.2 ([MꢀH]ꢀ); Anal. Calcd. for C28H38NO7:C, 67.06; H, 7.27; N, 2.91.
20
Found: C, 67.31; H, 7.43; N, 2.80; 7b: White solid, yield 96.0%, mp: 192–194 8C; ½aꢁD +5.2 (c 0.5, MeOH); 1H NMR (300 MHz, CDCl3): d
0.81 (d, 3H, J = 7.2 Hz), 0.90 (d, 3H, J = 6.9 Hz), 0.96 (t, 3H), 1.08–1.40 (m, 6H), 1.14 (s, 3H), 1.34 (s, 3H), 1.65–1.91 (m, 4H), 2.09–2.13 (m,
2H), 2.43 (s, 1H), 3.19 (d, 1H), 4.09 (t, 1H, J = 7.9 Hz), 5.65 (d, 1H, J = 8.1 Hz), 6.87 (d, 2H, J = 8.3 Hz), 7.73 (d, 2H, J = 8.1 Hz), 10.87 (s,
1H). MS (ESI) m/z: 500.2 ([MꢀH]ꢀ); Anal. Calcd. for C28H39NO7:C, 66.87; H, 7.67; N, 2.92. Found: C, 67.04; H, 7.84; N, 2.79.
[13] G. Brooks, E. Hunt, WO 01/74788 [P].