Natural products in parallel synthesis: Triazole libraries of nonactic acid

Article abstract of DOI:10.1016/j.bmcl.2008.06.020

The synthesis of a library of nonactic acid-derived triazoloamide derivatives and their evaluation as antimicrobial agents is described.

Full text of DOI:10.1016/j.bmcl.2008.06.020

Bioorganic & Medicinal Chemistry Letters 18 (2008) 3946–3949
Contents lists available at ScienceDirect
Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
Natural products in parallel synthesis: Triazole libraries of nonactic acid
Sarah B. Luesse a, Gregg Wells a, Abhijit Nayek b, Adrienne E. Smith c, Brian R. Kusche c,
c
c
c
Stephen C. Bergmeier b, , Mark C. McMills , Nigel D. Priestley , Dennis L. Wright
*
a Promiliad Biopharma, 340 W. State Street Athens, OH 45701, USA
b Department of Chemistry & Biochemistry, Ohio University, Clippinger Laboratory, Athens, OH 45701, USA
c Promiliad Biopharma, PO Box 10, Alberton, MT 59820, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
The synthesis of a library of nonactic acid-derived triazoloamide derivatives and their evaluation as anti-
microbial agents is described.
Received 23 May 2008
Accepted 5 June 2008
Available online 10 June 2008
Ó 2008 Elsevier Ltd. All rights reserved.
Keywords:
Antimicrobial
Natural products
The development of pharmaceutical agents often relies on
compounds prepared from natural products or structures in-
spired by natural products.1,2 We have been interested in
straightforward synthetic methods that combine the stereochem-
ical and topological complexity of natural product scaffolds with
the diverse analogue generation available from combinatorial
synthesis to produce diverse, pharmacologically interesting
compounds.
1,2,3-Triazoles are a significant class of heterocyclic com-
pounds, generally prepared from a 1,3-dipolar addition of terminal
alkynes and azides. Recent reports have described the synthesis
and testing of series of 1,2,3-triazoles,11–15 and significant biologi-
cal activities have been observed for several triazole ring-based
O
Nonactin3 (1) is a macrotetrolide, antibiotic, natural product
produced by Streptomyces griseus. The macrocyclic ring of nonactin
is composed of two units of (+)-nonactate and two units of
(À)-nonactate in an alternating arrangement of (+)-(À)-(+)-(À),
such that nonactin is achiral.4–9
O
O
O
O
O
O
O
O
O
O
O
1
nonactin,
Methanolysis of nonactin followed by resolution using Rhodo-
coccus under aerobic and anaerobic conditions provides both enan-
tiomers of methyl nonactate (Fig. 1).10 We have been interested in
the monomeric (+) and (À)-methyl nonactate (2) units as complex
natural product scaffolds for combinatorial compound libraries.
Incorporation of the nonactic acid building block in diversity-ori-
ented synthesis can efficiently generate a highly diverse library
with relatively complex stereochemistry. Given the biological
activity of the precursor, we hypothesized that synthetic deriva-
tives of methyl nonactate would be an excellent place to identify
new molecules with interesting pharmacological activity. We pro-
posed to examine these compounds in a series of antimicrobial as-
says to test this hypothesis.
( )-2
OH
OH
CO2CH3
CO2CH3
O
O
2
2
methyl (+)-nonactate, (+)- methyl (−)-nonactate, (−)-
Figure 1. Nonactin and the key starting component methyl nonactate.
OH
N3
a, b
CO2CH3
CO2R
O
O
(−)-2
3
4
R = CH3
R = H
c
* Corresponding author. Tel.: +1 17405178462; fax: +1 17405930148.
E-mail address: bergmeis@ohio.edu (S.C. Bergmeier).
Scheme 1. Reagents and conditions: (a) p-toluenesulfonyl chloride, pyridine; (b)
NaN3, DMF, 50 °C, 66% (2 steps); (c) LiOHÁH2O, THF/MeOH/H2O (2:1:1), 95%.
0960-894X/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2008.06.020

S. B. Luesse et al. / Bioorg. Med. Chem. Lett. 18 (2008) 3946–3949
3947
dently synthesized to increase the diversity of substituted
triazoloamide products. Propargyl alcohol or amine and the requi-
site chloroformate were used to prepare alkynes 7{20} and 7{27}.
The necessary isocyanate was reacted with propargyl alcohol or
amine to prepare alkynes 7{28–30}. Alkynes 7{19} and 7{31–35}
were prepared from propargyl bromide and substituted phenols
or amines.
The general triazole synthesis was modified from previously
published one-pot procedures17,19,20 and optimized for our system.
The best general conditions were found to be 100 mol% alkyne,
100 mol% CuSO4, and 33 mol% sodium ascorbate, relative to azi-
doamide 6, in tert-butanol/H2O (1:1) submitted to microwave irra-
diation at 100 °C for 5 min.21 All compounds were obtained by a
simple aqueous work-up followed by silica plug purification.
While a lower copper catalyst loading did provide the desired
product, a full equivalent was most efficient for driving all varia-
tions of alkyne building blocks to completion. Higher levels of
CuSO4 were most significant for reaction completion in the cases
of nitrogen-containing alkynes, specifically 7{23–30}.
Table 1 presents the results for the synthesis of 161 triazoloa-
mide library compounds (27-97%). Library members are identified
using the ‘Chemset’ brace numbering system. Compound 8{2,4} re-
fers to use of amine 5{2} and alkyne7{4}. For example, N-benzylm-
ethylamine (Fig. 2, compound 5{2}) is used to produce the
corresponding amide 6{2}. Reaction of amide 6{2} with 4-eth-
ynylbiphenyl (Fig. 3, 7{4}) provides triazoloamide 8{2,4}.
All library members were analyzed by LC/MS and one-fifth of
the isolated products were structurally verified by 1H NMR analy-
sis. In all cases, a single regioisomeric product was observed by 1H
NMR corresponding to the 1,4-disubstituted addition product, as
expected for the Cu-catalyzed reaction.15,17
Three alkynes (7{9}, 7{13}, 7{14}) were observed to consistently
result in low conversion to triazoloamide and recovery of signifi-
cant azide starting material (6). While there is no clear explanation
for the failure of the reactions using these alkynes, solubility prob-
lems were encountered using standard reaction conditions.
With a set of nonactic acid derivatives in hand, we examined
their activity against a small panel of Gram-positive and Gram-
negative bacteria as well as yeast/fungi (Table 2). All compounds
were initially screened at a single concentration (1 mM) using
the Alamar Blue dye reduction assay. Those compounds showing
more than marginal activity were then assayed at a range of dilu-
tions to measure the minimal inhibitory concentration (MIC; low-
est concentration at which no dye reduction is observed).
Fourteen compounds (9%) showed weak to moderate activity
against one or more microorganisms. In general, most of the anti-
microbial activity was focused on the Gram-positive bacteria and
N3
N3
O
a, b
R3
CO2H
R1
O
4
N
O
R2
6
N
N
c
N
O
R1
N
O
8
R2
Scheme 2. Reagents and conditions: (a) oxalyl chloride, benzene, 50 °C; (b) 5,
poly(vinylpyridine), CH2Cl2, 6{1}, 54%; 6{2}, 66%; 6{3}, 79%; 6{4}, 73%; 6{5}, 53%;
(c) 7, CuSO4, sodium ascorbate, MW, 100 °C, 5 min, 27–97%.
derivatives. Our goal was to integrate the nonactic acid scaffold
with the potential pharmacophore of a substituted triazole ring.
The copper-catalyzed azide-alkyne 1,3-dipolar cycloaddition is
an efficient one-pot method to access triazoles.15–17 Given our
interest in triazole-substituted nonactic acid, azide-substituted
nonactic acid (4) was targeted as the starting scaffold for nonactic
acid-based libraries. Azide 4 has previously been synthesized from
methyl nonactate via a double inversion procedure through a bro-
mide intermediate.18
Our straightforward synthesis of azido acid 4 from hydroxyester
2 is shown in Scheme 1. (À)-Methyl nonactate ((À)-2) was con-
verted to the tosylate ester followed by substitution with sodium
azide (DMF, 50 °C, 5 h) to provide azidoester 3. Ester hydrolysis
provided azido-nonactic acid (4) in excellent yield.
As shown in Scheme 2, synthesis of a library of triazoloamides
began with conversion of nonactic acid 4 to the corresponding
amide 6 through formation of the acid chloride, followed by reac-
tion with amines 5 and poly(vinylpyridine). The triazoloamide 8
was produced through a microwave assisted copper-catalyzed
1,3-dipolar addition of amide 6 with terminal alkyne 7.
Five amine building blocks (Fig. 2) were used to produce a series
of amides. One primary amine (5{5}) and four secondary amines
were selected.
To produce the 161-member triazoloamide library, 32 terminal
alkynes (7{1–35}) were selected (Fig. 3). While many terminal al-
kynes are commercially available, several alkynes were indepen-
Bn
N
HN
N
H
4
N
H
1
HN
H2N
O
Me
5
2
3
Figure 2. Amine building blocks 5{1–5} for the synthesis of amide 6.
O
O
PhO
Ph
F3C
PhO
O
O
1
2
3
4
5
6
7
8
9
MeO
NO2
MeO
MeO
BnO
Ph
O
O
10
OMe
11
12
13
14
15
16
17
18
O
O
OMe
MeO
Ph
N
H
NC
O
Me2N
22
N
Bu2N
25
BnO
Ph
N
H
27
O
N
O
19
20
21
23
24
26
O
F3C
Br
O
O
N
N
N
N
H
N
H
N
N
H
N
H
N
H
O
O
O
Ph
28
29
30
31
32
33
34
35
Figure 3. Alkyne building blocks (7{1–35}) used in triazoloamide library synthesis.

3948
S. B. Luesse et al. / Bioorg. Med. Chem. Lett. 18 (2008) 3946–3949
Table 1
Nonactic acid-derived triazoloamide library21
Compound
Yielda (%)
Compound
Yielda (%)
Compound
Yielda (%)
Compound
Yielda (%)
Compound
Yielda (%)
8{1,1}
8{1,2}
8{1,3}
8{1,4}
8{1,5}
8{1,6}
8{1,7}
8{1,8}
58
36
43
72
53
96
72
63
—
8{2,1}
8{2,2}
8{2,3}
8{2,4}
8{2,5}
8{2,6}
8{2,7}
8{2,8}
50
60
90
86
97
84
90
48
—
8{3,1}
8{3,2}
8{3,3}
8{3,4}
8{3,5}
8{3,6}
8{3,7}
8{3,8}
62
38
36
57
34
30
52
38
—
8{4,1}
8{4,2}
8{4,3}
8{4,4}
8{4,5}
8{4,6}
8{4,7}
8{4,8}
68
72
64
57
59
63
58
66
—
8{5,1}
8{5,2}
8{5,3}
8{5,4}
8{5,5}
8{5,6}
8{5,7}
8{5,8}
60
62
53
54
57
60
66
33
—
8{1,9}
8{2,9}
8{3,9}
8{4,9}
8{5,9}
8{1,10}
8{1,11}
8{1,12}
8{1,13}
8{1,14}
8{1,15}
8{1,16}
8{1,17}
8{1,18}
8{1,19}
8{1,20}
8{1,21}
8{1,22}
8{1,23}
8{1,24}
8{1,25}
8{1,26}
8{1,27}
8{1,28}
8{1,29}
8{1,30}
8{1,31}
8{1,32}
8{1,33}
8{1,34}
8{1,35}
48
59
57
—
8{2,10}
8{2,11}
8{2,12}
8{2,13}
8{2,14}
8{2,15}
8{2,16}
8{2,17}
8{2,18}
8{2,19}
8{2,20}
8{2,21}
8{2,22}
8{2,23}
8{2,24}
8{2,25}
8{2,26}
8{2,27}
8{2,28}
8{2,29}
8{2,30}
8{2,31}
8{2,32}
8{2,33}
8{2,34}
8{2,35}
48
82
51
—
8{3,10}
8{3,11}
8{3,12}
8{3,13}
8{3,14}
8{3,15}
8{3,16}
8{3,17}
8{3,18}
8{3,19}
8{3,20}
8{3,21}
8{3,22}
8{3,23}
8{3,24}
8{3,25}
8{3,26}
8{3,27}
8{3,28}
8{3,29}
8{3,30}
8{3,31}
8{3,32}
8{3,33}
8{3,34}
8{3,35}
45
48
28
—
8{4,10}
8{4,11}
8{4,12}
8{4,13}
8{4,14}
8{4,15}
8{4,16}
8{4,17}
8{4,18}
8{4,19}
8{4,20}
8{4,21}
8{4,22}
8{4,23}
8{4,24}
8{4,25}
8{4,26}
8{4,27}
8{4,28}
8{4,29}
8{4,30}
8{4,31}
8{4,32}
8{4,33}
8{4,34}
8{4,35}
62
72
55
—
8{5,10}
8{5,11}
8{5,12}
8{5,13}
8{5,14}
8{5,15}
8{5,16}
8{5,17}
8{5,18}
8{5,19}
8{5,20}
8{5,21}
8{5,22}
8{5,23}
8{5,24}
8{5,25}
8{5,26}
8{5,27}
8{5,28}
8{5,29}
8{5,30}
8{5,31}
8{5,32}
8{5,33}
8{5,34}
8{5,35}
67
62
63
—
—
—
68
44
51
53
44
43
27
51
43
50
94
53
35
60
42
76
58
35
53
49
43
68
—
—
70
50
69
78
58
45
83
72
69
56
73
72
57
64
73
82
100b
73
58
71
78
65
55
89
89
87
41
34
97
56
90
85
62
45
100b
49
77
76
61
56
61
62
61
72
36
52
61
63
60
60
69
40
67
64
54
92
78
76
55
63
76
73
68
52
59
62
60
53
57
65
60
55
51
45
56
57
71
55
77
67
63
68
73
68
a
Compounds were determined to be >90% pure by 1H NMR or LC/MS analysis.
Compounds exhibited a purity level of >80% by 1H NMR or LC/MS.
b
Table 2
Minimum inhibitory concentrationsa,b
(lg/mL)
Compound
Gram-positive bacteria
Gram-negative bacteria
Escherichia Pseudomonas
Yeast/fungi
Bacillus
anthracis
Bacillus
cereus
Bacillus
subtilis
Staphylococcus
aureus
Candida
albicans
Cryptococcus
neoformans
Saccharomyces
cerevisiae
coli
aeruginosa
8{2,2}
430
640
440
430
219
430
220
8{2,15}
8{2,23}
8{2,24}
8{2,25}
8{3,3}
440
>430c
250
250
250
240
250
250
240
560
820
790
880
820
500
500
120
500
250
480
500
>480c
120
480
240
590
240
290
8{3,4}
8{3,7}
8{3,13}
8{3,17}
8{3,25}
8{3,29}
8{4,19}
8{4,28}
263
260
150
>590c
290
480
570
700
a
Measured by Alamar Blue dye reduction assay. A blank indicates that no inhibition was observed at the maximum test concentration of 1 mM.
Only compounds from the total set of 161 that showed activity in at least one assay are included in the table. Compounds not listed were found to be inactive in all assays.
‘>’ shows that some inhibition was seen at this concentration although at this highest test concentration, some growth was observed and so a MIC value could not be
b
c
reliably estimated.
yeast or fungi. The only compound which showed activity against
Gram-negative bacteria was 8{3,3}, which showed a very broad
chemistry. More importantly, this compound library has been used
to identify novel antimicrobial leads.
spectrum of activity with MIC values of 120 lg/mL against Staphy-
lococcus aureus and Escherichia coli.
Acknowledgments
In summary, we have shown that the nonactic acid building
block can be readily incorporated into a parallel synthetic scheme.
This provides a diverse library with relatively complex stereo-
We thank the National Science Foundation for support through
a Partnerships for Innovation grant (EHR-0227907) and Ohio Uni-

S. B. Luesse et al. / Bioorg. Med. Chem. Lett. 18 (2008) 3946–3949
3949
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21. General experimental. Azidoamide 6 (1.0 equiv) and alkyne 7 (1.0 equiv) were
combined in a 2.0 mL microwave vial and suspended in t-BuOH/H2O (1:1,
0.6 mL). CuSO4Á5H2O (1.0 M in H2O, 1.0 equiv) and sodium ascorbate (0.5 M in
H2O, 0.33 equiv) were added. The reaction was submitted to microwave
irradiation (100 °C for 5 min). 1.0 mL 50% NH4OH was added, followed by
1.0 mL CH2Cl2. Organic extracts were collected using a Biotage phase separator,
washed with H2O, and collected through the phase separator. Compounds were
purified via a silica plug. A forerun of hexanes removed nonpolar impurities;
desired products were eluted with 10% MeOH in CH2Cl2 to provide 8.
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