Function-oriented synthesis of liponucleoside antibiotics

Article abstract of DOI:10.1002/ejoc.201400140

Function-oriented synthesis of a class of liponucleoside antibiotics was investigated through rational simplification guided by previous structure-activity relationship studies of caprazamycins and muraymycins to address the issue associated with their molecular complexity. A lactam-fused isoxazolidine scaffold was designed, and a diverse set of lactam-fused isoxazolidines derivatives were constructed by intramolecular 1,3-dipolar cycloaddition of alkenyl nitrones. Several analogues exhibited moderate activity against a range of Gram-positive drug-resistant bacterial pathogens.

Full text of DOI:10.1002/ejoc.201400140

SHORT COMMUNICATION
DOI: 10.1002/ejoc.201400140
Function-Oriented Synthesis of Liponucleoside Antibiotics
Tetsuya Tanino,[a] Mayumi Yamaguchi,[a] Akira Matsuda,[a] and Satoshi Ichikawa*[a,b]
Keywords: Natural products / Antibiotics / Domino reactions / Cycloaddition
Function-oriented synthesis of a class of liponucleoside anti-
biotics was investigated through rational simplification
guided by previous structure–activity relationship studies of
caprazamycins and muraymycins to address the issue associ-
ated with their molecular complexity. A lactam-fused isox-
azolidine scaffold was designed, and a diverse set of lactam-
fused isoxazolidines derivatives were constructed by intra-
molecular 1,3-dipolar cycloaddition of alkenyl nitrones. Sev-
eral analogues exhibited moderate activity against a range
of Gram-positive drug-resistant bacterial pathogens.
philic side chain.[8–10] During the course of this study, we
also found that truncated MRY analogue 1, which has an
Introduction
Although natural products are a rich source for drug de- arginine residue as an accessory group, retained antibacte-
velopment,[1] many natural products possess large, complex, rial activity comparable to that of the parent MRYs (Fig-
or labile chemical structures that may restrict chemical ure 2).[10] As for the CPZs, the diazepanone ring could play
modifications in a structure–activity relationship (SAR) a role as a scaffold, on which each of the key functional
study. Function-oriented synthesis (FOS)[2] is a concept not groups hang in the right orientation.[8,9] A simpler scaffold
only to be considered in terms of step economy to synthe- would be utilized to install the key functional groups, and
size target molecules but also as a strategy to pursue for the we designed lactam-fused isoxazolidine scaffold 3 instead
design of less-complex structured targets with comparable of such a complicated ring system. The choice of the
activity. This concept is important for drug development lactam-fused isoxazolidine scaffold enabled us to modulate
based on natural products. Muraymycins (MRYs)[3] and ca- the three dimensional orientation of the key functional
prazamycins (CPZs)[4] (Figure 1) are a class of liponucleo- groups simply by changing the stereochemistry or the ring
side antibiotics that exhibit potent antibacterial activity size. Step economy is also an important issue for rapid and
in vitro without significant toxicity in mice, and they inhibit efficient access to the target molecules. A set of diverse
peptidoglycan biosynthesis through a novel mode of ac- lactam-fused isoxazolidines 3 was readily constructed by in-
tion.[5] Accordingly, this class of natural products is a prom- tramolecular 1,3-dipolar cycloaddition of alkenyl nitrone
ising lead as a novel antibacterial agent against drug-resist- 4.[11]
ant bacterial pathogens.[6] However, the 5Ј-O-aminoribosyl-
5Ј-C-glycyluridine moiety as well as the diazepanone moi-
ety, which is a seven-membered ring found in caprazamy-
cins, are not only prone to β-elimination and epimerization,
but they also require rather lengthy steps for their prepara-
tion because of their molecular complexity.[7] Herein we de-
scribe the FOS of this class of nucleoside antibiotics
through rational simplification to address the issue associ-
ated with their molecular complexity. Our efforts on the
SAR of MRYs and CPZs elucidated that the key functional
groups essential for antibacterial activity are the uridine
moiety, the amino group of the aminoribose, and a lipo-
[a] Faculty of Pharmaceutical Sciences, Hokkaido University,
Kita-12, Nishi-6, Kita-ku, Sapporo 060-0812, Japan
E-mail: ichikawa@pharm.hokudai.ac.jp
http://www.pharm.hokudai.ac.jp/org/yakka01.html
[b] Center for Research and Education on Drug Discovery,
Hokkaido University,
Kita-12, Nishi-6, Kita-ku, Sapporo 060-0812, Japan
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201400140.
Figure 1. Structures of caprazamycins (CPZs) and muraymycins
(MRYs).
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Function-Oriented Synthesis of Liponucleoside Antibiotics
Figure 2. Function-oriented synthesis of muraymycins and caprazamycins; TBS = tert-butyldimethylsilyl.
tion auxiliary[15] as well as a precursor to the accessory
Results and Discussion
functional group, as described later. The impact of the
newly introduced cyanoethyl group on the cycloaddition
proved to be immense, and the cycloaddition of 4d pro-
ceeded even at room temperature to afford bicyclo[4.3.0]-
The preparation of the key lactam-fused isoxazolidine
core structures is described in Scheme 1. Mild zinc-pro-
moted Horner–Wadsworth–Emmons reaction[12] of phos-
phonates 5a–i[13] with uridine 5Ј-aldehyde derivative 6,
which was obtained from uridine in three steps, gave trans-
α,β-unsaturated amides 7a–i. Then, the primary alcohol
functionality of 7a–i was oxidized by 2-iodoxybenzoic acid
(IBX) in MeCN/DMSO, and the resulting aldehydes were
subjected to a hydroxylamine in MeCN to form nitrones
4a–i. Then, the intramolecular 1,3-dipolar cycloaddition
was investigated, and the results are summarized in Table 1.
Lactam-fused bicyclo[4.3.0]-type isoxazolidines are known
structures.[11] Upon heating nitrone 4a at reflux in MeCN,
desired cis-fused bicyclo[4.3.0]-type isoxazolidines 8a and
9a were obtained in 77% yield (8a/9a = 1.4:1) over three
steps from 7a (Table 1, entry 1). In contrast, a decrease in
the yield was observed upon increasing the size of the
lactam ring, which has not yet been previously investigated.
Thus, bicyclo[5.3.0]-type isoxazolidines 8b and 9b were ob-
tained from 7b in 34% yield (Table 1, entry 2), and no reac-
tion occurred to give bicyclo[6.3.0]-type isoxazolidines 8c
and 9c from 7c (Table 1, entry 3). Generally, a carboxamide
exists predominantly as the trans conformer, and these con-
formational features suggest that nitrones 4a–c adopt ex-
tended conformations, in which the target olefin and the
nitrone are far apart. N-Protection of the carboxamide to
induce a conformational change is an established strategy
to improve the yields of certain intramolecular reactions,
especially those that provide highly strained macrocycles.[14]
A cyanoethyl group was introduced at the nitrogen atom of
Scheme 1. Reagents and conditions: (a) Zn(OTf)2 (1.2 equiv.),
TMEDA (1.2 equiv.), Et3N (4 equiv.), THF, r.t., 34–65%; (b) IBX
(1.5 equiv.),
MeCN/DMSO,
r.t.;
(c) BnNHOH
or
BocNHCH2CH2CH2NHOH (1.2 equiv.), 4 Å molecular sieves,
MeCN, r.t. or reflux, see Table 1. TMEDA = N,N,NЈ,NЈ-tetra-
the carboxamide moiety (i.e., 4d–9i) as a removable cycliza- methylethylenediamine.
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T. Tanino, M. Yamaguchi, A. Matsuda, S. Ichikawa
SHORT COMMUNICATION
Table 1. Intramolecular 1,3-dipolar cycloaddition of alkenyl nitrones.
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Function-Oriented Synthesis of Liponucleoside Antibiotics
type isoxazolidines 8d and 9d in 82% yield (Table 1, en- the cyanoethyl group of 8g was removed by tBuOK in THF
try 4). Bicyclo[5.3.0]-type 8e/9e and bicyclo[6.3.0]-type 8f/9f in 79% yield, and the azide of 10a was reduced to the corre-
were obtained in 78 and 82% yield, respectively, although sponding amine, which was acylated with 11 (Scheme 2).[13]
heating was necessary for the cyclization to proceed Finally, global deprotection afforded 12a in 70% yield over
(Table 1, entries 5 and 6). The structures of 8a and 9a were three steps. The cyanoethyl group can be used as an access-
determined with the materials obtained from 8d and 9d af- ory group. Namely, reduction of the azide group of 8g was
ter deprotection of the cyanoethyl group. The relative ste- followed by palmitoylation to give 13a in 55% yield over
reochemistry of cis-fused bicyclo[4.3.0]-type isoxazolidines two steps. The cyano group of 13a was reduced by hydro-
8d and 9d were in good accordance with those of related genation catalyzed by PtO2 in the presence of Me3N·HCl,
compounds previously reported.[11] The absolute stereo- and the liberated amine was converted into the protected
chemistry was predicted by NOE experiments in conjunc- guanidine with BocHNC(=NBoc)NHTf (Tf = trifluorome-
tion with molecular modeling, because of the difficulty in thylsulfonyl). Global deprotection provided 14a in 69%
obtaining the crystal structure of the cycloadducts (for de- yield over three steps. The other analogues were prepared
tails, see the Supporting Information). The structures of the in a similar manner (Scheme S4).
other cycloadducts were assigned in a similar manner. The
The antibacterial activity of the series of compounds was
reaction sequence of 5g–i with BocHN(CH2)3NHOH (Boc evaluated,[16] and the results are summarized in Table 2.
= tert-butoxycarbonyl) was also successful to provide highly Overall, these analogues exhibited moderate activity against
functionalized lactam-fused isoxazolidines 8g–i and 9g–i a range of Gram-positive drug-resistant bacterial pathogens
(Table 1, entries 7–9). However, the reaction of 4i gave an including S. aureus SR3637 (MRSA) and E. faecium
inseparable mixture of four diastereomers, which was not SR7917 (VRE) with MIC values of 8–64 μgmL–1. There is
used for further transformation.
a trend that type II compounds show higher activity than
Lactam-fused isoxazolidines 8g,h and 9g were converted type I compounds. Compound 14a was the most potent an-
into two types of analogues. One has a linear substituent alogue, the activity of which was comparable to that of 1
linking a lipophilic side chain and the accessory arginine within only a 2–4-fold decrease. Compound 14a did not ex-
group (type I), and the other is a branched version with hibit significant cytotoxicity against human liver hepatocel-
each substituent on the lactam ring (type II). These were lular (HepG2) cells (IC50 Ͼ 50 μgmL–1).
synthesized within five steps from 8g,h and 9g. For example,
Table 2. Antibacterial activity of lactam-fused isoxazolidine ana-
logues.
Scheme 2. Reagents and conditions: (a) tBuOK (5 equiv.), THF,
r.t., 79%; (b) H2, Pd/C, MeOH, r.t.; (c) 11 (1 equiv.), EDCI
(1.5 equiv.), HOBt (1.5 equiv.), CH2Cl2, r.t., (d) 80% aq. TFA, r.t.,
70% over 3 steps; (e) H2, Pd/C, MeOH, r.t.; (f) palmitoyl chloride
(1.2 equiv.), Et3N (1.5 equiv.), CH2Cl2, r.t., 55% over 2 steps;
[a] MICs were determined by a microdilution broth method as re-
commended by the National Committee for Clinical Laboratory
Standards (NCCLS) with cation-adjusted Mueller–Hinton broth
(g) H2,
(h) BocHNC(=NBoc)NHTf (2 equiv.), NaHCO3 (excess), THF/
H2O, r.t.; (i) 80% aq. TFA, r.t., 69% over 3 steps. EDCI = 1-[3- CFU (CFU = colony-forming unit) of each strain in a volume of
PtO2,
Me3N·HCl
(1 equiv.),
MeOH,
r.t.; (CA-MHB). Serial twofold dilutions of each compound were made
in appropriate broth, and the plates were inoculated with 5ϫ104
(dimethylamino)propyl]-3-ethylcarbodiimide hydrochloride, HOBt
= 1-hydroxybenzotriazole. TFA = trifluoroacetic acid.
0.1 mL. Plates were incubated at 35 °C for 20 h and then MICs
were scored. [b] VCM = vancomycin.
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T. Tanino, M. Yamaguchi, A. Matsuda, S. Ichikawa
SHORT COMMUNICATION
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Conclusions
In conclusion, the FOS of liponucleoside antibiotics was
investigated to lead to the discovery of lactam-fused isox-
azolidine derivatives as lead compounds active against
drug-resistant bacterial pathogens. It is important to sim-
plify the core structures of natural products to reduce the
size of the molecules and to stabilize the chemically labile
structure. Lactam-fused isoxazolidine derivatives 12 and 14
possess a simple chemical structure with a molecular weight
half of that of the parent natural products, which can be
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Acknowledgments
This research was supported by the Japan Society for the Pro-
motion of Science through a Grant-in-Aid for Challenging Explor-
atory Research (S.I., grant number 22659020), Scientific Research
on Innovative Areas “Chemical Biology of Natural Products” (S.I.,
grant number 24102502), and Scientific Research (B) (S.I., grant
number 25293026). The authors thank Ms. S. Oka and Ms. A. Tok-
umitsu (Center for Instrumental Analysis, Hokkaido University)
for measurement of the mass spectra. We are also thankful to Ms.
M. Okane and K. Uotani (Shionogi Techno Advance Research Co.,
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Published Online: February 19, 2014
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