Syntheses and biological evaluation of novel pseudomycin side-chain analogues. Part 2

Article abstract of DOI:10.1016/S0960-894X(00)00424-8

A series of aliphatic side-chain analogues of pseudomycin was synthesized and evaluated during the course of our side-chain SAR effort. We found that several of the pseudomycin side-chain analogues (e.g., 10) exhibited good in vitro activity against all t

Full text of DOI:10.1016/S0960-894X(00)00424-8

Bioorganic & Medicinal Chemistry Letters 10 (2000) 2107±2110
Syntheses and Biological Evaluation of Novel Pseudomycin
Side-Chain Analogues. Part 2
Shu-Hui Chen,* Xicheng Sun, Robert Boyer, Jonathan Paschal, Doug Zeckner,
William Current, Mark Zweifel and Michael Rodriguez
Lilly Research Laboratories, A Division of Eli Lilly and Company, Lilly Corporate Center, Indianapolis, IN 46285, USA
Received 1 June 2000; accepted 12 July 2000
AbstractÐA series of aliphatic side-chain analogues of pseudomycin was synthesized and evaluated during the course of our side-
chain SAR e€ort. We found that several of the pseudomycin side-chain analogues (e.g., 10) exhibited good in vitro activity against
all three major fungi responsible for systemic fungal infections. # 2000 Elsevier Science Ltd. All rights reserved.
Introduction
when both PSB and PSC0 were evaluated against
Aspergillus, they exhibited only modest activity, with
MIC values in the range of 10±20 mg/mL. In order to
further extend the therapeutic utility of natural occur-
ring pseudomycins, it would be desirable to synthesize
analogues that possess balanced activity against all three
major fungi that are responsible for systemic fungal
infections. Bearing these considerations in mind, we
synthesized several series of ridged side-chain bearing
pseudomycin analogues.6 A recent report by Rodriguez
et al.7 demonstrated that PSC0 1 was more potent than
its 30-epimer 2 and the 30-racemate 3. Furthermore, all
three pseudomycin analogues (1±3) displayed better
activity than the 30-deoxy analogue 4. Taking all this
information into account, it became evident that the
presence of the 30a hydroxyl group is required for opti-
mal anti-fungal activity for PSC0.7 Encouraged by this
important ®nding, we decided to extend the side-chain
SAR by preparing additional side-chain analogues as
shown in Figure 1. Compounds 8 and 9 contain C-14
side-chain. Three C-18 and three C-20 aliphatic side-
chain analogues (10±15) were also synthesized. In this
paper, we wish to report the synthesis and preliminary
evaluation of these novel pseudomycin side-chain ana-
logues.
Systemic fungal infections (SFI) are those infections
that involve deep viscera such as liver and lung. Clini-
cally, it has been shown that such SFI can cause serious
life-threatening diseases in normal healthy humans.
Candida albican, Cryptococcus neoformans, and Asper-
gillus fumigatus are the major opportunistic pathogens
responsible for systemic infections.1,2 In recent years,
there has been a signi®cant increase in the incidence of
SFI. This is mainly due to the growing population of
immunocompromised individuals including those suf-
fering from HIV, patients receiving chemotherapy, and
patients undergoing heart surgery and organ transplan-
tation, etc. Although amphotericin B and several azole
based drugs (e.g., ¯uconazole) are used to treat systemic
fungal infections, the usefulness of these drugs are lim-
ited either by severe toxicity (e.g., amphotericin B)3 or
by the lacking of broad spectrum of activity against all
three major fungi (e.g., ¯uconazole).4 Therefore, there is
an urgent need for the development of new antifungal
agents that can be used safely for the treatment of sys-
temic fungal infections caused by all of the three major
fungi mentioned above.
Recently, we and others reported the structure and pre-
liminary biological pro®les of three novel antifungal
agents: pseudomycin A,5 pseudomycin B (PSB, 7),6 and
pseudomycin C0(PSC0, 1).7 When tested against Candida
and Cryptococcus, PSB and PSC0 displayed better
activity than that of amphoterin B (AMB). However,
Chemical Syntheses
The synthetic scheme employed for the preparation of
new side-chain analogues is brie¯y outlined in Scheme 1.
Three requisite 30-racemic acids (19a±19c) were pre-
pared from their corresponding aldehydes (16a±16c) via
a sequence consisting of allylation, osmylation, vicinal
*Corresponding author. Tel.: +1-317-276-2076; fax: +1-317-276-5431;
e-mail: chen_shu-hui@lilly.com
0960-894X/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved.
PII: S0960-894X(00)00424-8

2108
S.-H. Chen et al. / Bioorg. Med. Chem. Lett. 10 (2000) 2107±2110
Figure 1. Structures of pseudomycin side-chain analogues.
Scheme 1. Synthesis of pseudomycin side-chain analogues 10±15.
diol cleavage and NaClO2 mediated aldehyde oxidation
as shown in Scheme 1. Most of the chemistry described
in this sequence was done without puri®cation of the
intermediates. Thus, the crude acids (19a±19c) were
coupled with ZPSN 6 in the presence of HOBt and
EDCI in DMF to a€ord three pairs of the 30-diaster-
eomers (20a±20c) in low yields. Direct hydrogenolysis
on (20a±20c) provided three 30-diastereomers 9 (10%),
12 (18%), and 15 (7%), respectively. Satisfactory mass
spectra for all ®nal products were obtained.
chemical shift di€erence between 7 and 8 was noted for
the 1a proton (see Figure 1) with 4.59 ppm for PSB 7
and 4.63 ppm for 30-epi PSB 8. To further con®rm the
structural assignments, additional spiking studies were
performed on both samples. The results from these
investigations indicated that proton NMR spectrum of
synthetic PSB 7 (TFA salt) was identical to that obtained
from standard PSB-TFA adduct. Whereas compound 8
was indeed the 30-epimer of PSB.
Following the same procedure described for 7 and 8,
four pure 30-diastereomers, 10 and 11 (C-18 side chain)
along with 13 and 14 (C-20 side chain), were obtained
from either 20b or 20c in yields ranging from 20±38%.
The proton NMR spectra of 10, 11, 13, and 14 were
assigned according to the method described for PSB 7
and its 30-epimer 8. The detailed peak assignments for
these new analogues are listed in Table 1. It should also
be mentioned that all of the pseudomycin side-chain
analogues with same side chain length (e.g., C-14 side-
chain analogues: 7±9), regardless of the stereochemistry
at C-30 position, displayed identical retention time on
our HPLC system. All of the new pseudomycin side-
Alternatively, reverse-phase HPLC separation of the 30-
diastereomers 20a a€orded two pure 30-distereomers
20a(a) and 20a(b). In accordance with our previous
experience obtained with PSC0 analogues, the 30a-OH
bearing isomer 20a(a) had a relatively shorter retention
time than that observed with the 30b-OH bearing coun-
terpart 20a(b).7 Both 20a(a) and 20a(b) thus obtained
were converted to the desired products 7 and 8, via
hydrogenolysis, in good yield. The proton NMR spectra
of 7 and 8 showed only one small apparent di€erence in
the vicinity of 4.5±4.7 ppm. A COSY experiment was
used to assign the resonances of that region. The biggest

S.-H. Chen et al. / Bioorg. Med. Chem. Lett. 10 (2000) 2107±2110
2109
analogues discussed herein (7, 8, 10, 11, 13, and 14)
exhibited satisfactory mass spectra.
side-chain analogues.8 Following the new route shown
in Scheme 2, we resynthesized the C-18 side-chain ana-
logue 10. Thus, the homoallylic ether 23 was prepared
from the chiral acetal 22 with good yield and stereo-
selectivity (>6:1 versus the undesired 30-epimer). Com-
pound 23 was then converted to the homoallylic alcohol
17b(ꢀ) via a one-pot procedure. From this point on, the
C-18 side-chain precursor 17b(ꢀ) was processed as that
shown in Scheme 1 to provide the ®nal product 10
(ꢀ17% overall yield). Presumably, the same procedure
shown in Scheme 2 can be used for the preparation of
other related side-chain bearing analogues.
Although the chemistry shown in Scheme 1 permitted
the synthesis of each 30-diastereomers of interest, the
HPLC separation of the intermediates (20a±c) was very
tedious and time consuming. Furthermore, this proce-
dure proved to be more troublesome for C-18 or C-20
side-chain analogues. In view of this diculty, we
devised a chiral acetal based stereoselective route for
Table 1. Proton NMR data of pseudomycin side-chain analogues 7±
11 and 13±14
Biological Evaluation
Positions
7
8
10
11
13
14
All eight newly synthesized pseudomycin analogues (8±
15) were evaluated in vitro against Candida albican,
Cryptococcus neoformans and Aspergillus fumigatus.
Pseudomycin B 7 was used as positive control. As
shown in Table 2, when tested against Candida and
Cryptococcus, 30-epi PSB 8 was found to be less potent
than 30-racemate 9, with the latter being 2-fold less
potent than PSB 7. None of these C-14 side-chain ana-
logues showed signi®cant activity against Aspergillus.
These results are in good agreement with that found
with PSC0 series analogues reported by Rodriguez et al.7
Residue 1
a
b1
b2
4.59
4.38
4.53
4.63
4.38
4.53
4.59
4.39
4.53
4.62
4.38
4.52
4.59
4.38
4.53
4.63
4.38
4.52
Residue 2
a
b1
b2
g1
g2
4.13
2.01
2.07
2.90
2.97
4.14
2.01
2.07
2.90
2.97
4.13
2.01
2.07
2.91
2.98
4.13
2.00
2.07
2.90
2.96
4.13
2.01
2.08
2.91
2.97
4.13
2.00
2.06
2.91
2.96
Residue 3
a
b1
b2
4.55
2.82
2.87
4.55
2.82
2.87
4.54
2.82
2.86
4.55
2.82
2.86
4.54
2.83
2.86
4.55
2.82
2.86
When three C-18 side-chain bearing analogues (10±12)
were evaluated, it was found that in vitro antifungal
potencies (against all three fungi) decrease according to
the following order: 10>12>11. From the data shown
in Table 2, it is again evident that 30a-hydroxyl group is
required for optimal antifungal activity. It is encoura-
ging to note that, when compared with PSB 7, the C-18
side chain analogue 10 showed improved potency
against Cryptococcus (4-fold) and Aspergillus (8-fold).
Furthermore, better activity (vs 7) towards Aspergillus
were also observed with compounds 11 and 12.
Residue 4
a
4.13
1.75
1.78
1.26
1.32
1.53
1.56
2.84
4.13
1.75
1.78
1.26
4.12
1.74
1.78
1.26
1.33
1.55
1.55
2.84
4.15
1.73
1.77
1.28
1.34
1.55
1.55
2.84
4.12
1.73
1.78
1.26
1.32
1.55
1.55
2.84
4.15
1.74
1.78
1.25
1.32
1.55
1.55
2.85
b1
b2
g1
g2
d1
d2
e
1.32
ꢀ1.55
1.55
2.84
Residue 5
a
b1
b2
g1
g2
4.29
1.99
2.14
2.89
2.91
4.29
1.99
2.14
2.90
2.90
4.29
1.99
2.14
2.90
2.90
4.30
1.99
2.15
2.90
2.90
4.29
2.00
2.14
2.90
2.90
4.30
2.00
2.14
2.90
2.90
Further side-chain elongation by 2-carbons led to three
C-20 analogues 13 through 15, all displayed reduced
activity against Candida and Cryptococcus in compar-
ison to PSB 7. However, compounds 13 and 15 showed
improved activity towards Aspergillus, (ranging from
2.5±5.0 mg/mL), which was ꢀ5-fold more potent than
Residue 6
a
b
g
4.27
3.92
1.18
4.28
3.92
1.18
4.27
3.91
1.18
4.28
3.91
1.18
4.27
3.91
1.18
4.28
3.91
1.18
Residue 7
b
g
6.53
1.70
6.51
1.70
6.53
1.70
6.51
1.70
6.53
1.70
6.51
1.70
Table 2. In vitro antifungal activity of pseudomycin derivatives 7±
15
Residue 8
Compounds
30
Con®guration
MIC (mg/mL)a
a
b
4.96
4.75
4.95
4.75
4.96
4.75
4.95
4.75
4.96
4.75
4.95
4.75
C. albicans C. neoformans A. fumigatus
Residue 9
a
b
g1
g2
4.87
4.31
3.44
3.50
4.89
4.32
3.44
3.51
4.87
4.31
3.44
3.51
4.88
4.30
3.44
3.51
4.87
4.31
3.44
3.51
4.88
4.30
3.44
3.51
7 (PSB)
8
9
10
11
12
13
14
15
a
0.312±1.25 0.039±0.312
5.0±10 1.25±2.5
0.625±1.25 0.078±0.312
>20
>20
>20
b
Racemates
a
0.625
2.5
1.25
5.0
0.01
2.5
10
5.0
5.0
>20
2.5
b
Racemates
0.312
0.02±0.078
1.25
5.0
0.625
Side chain
20a
2.25
2.36
3.86
1.38
2.25
2.35
3.90
1.38
2.25
2.36
3.86
1.38
2.25
2.35
3.89
1.38
2.25
2.36
3.86
1.38
2.25
2.36
3.90
1.38
20b
a
b
30
5.0
40
Racemates 10
5±13,17,19
14/18/20
ꢀ1.22
ꢀ1.22
ꢀ1.22
ꢀ1.22
ꢀ1.22
ꢀ1.22
aMIC, lowest drug concentration required to inhibit 90±100% of visi-
ble growth compared to controls.
0.83
0.83
0.82
0.83
0.82
0.82

2110
S.-H. Chen et al. / Bioorg. Med. Chem. Lett. 10 (2000) 2107±2110
Scheme 2. Stereoselective synthesis of 10 via coupling of ZPSN 6 with 30a-hydroxyacid 19b(a).
References and Notes
PSB 7. It is also interesting to note that the activity
against Candida for 13 and 14 is independent of the 30-
stereochemistry on the side chain.
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of these side-chain analogues (e.g., 10, 12, 13, and 15)
showed improved activity against Aspergillus. In parti-
cular, the C-18 side-chain analogue 10 demonstrated
balanced activity against all three major fungi respon-
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encouraging result, further in vivo evaluation of 10 is
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Acknowledgements
We are indebted to Drs. J. Munroe and B. Laguzza for
their support.