Total synthesis and biological evaluation of marinopyrrole A and analogs

Article abstract of DOI:10.1016/j.tetlet.2010.09.059

A five-step total synthesis of the antibiotic marinopyrrole A (1) is described. The developed synthetic technology enabled the synthesis of several marinopyrrole A analogs whose antibacterial properties against methicillin-resistant Staphylococcus aureus TCH1516 were evaluated.

Full text of DOI:10.1016/j.tetlet.2010.09.059

Tetrahedron Letters 52 (2011) 2041–2043
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Total synthesis and biological evaluation of marinopyrrole A and analogs
K. C. Nicolaou a,b, , Nicholas L. Simmons a, Jason S. Chen a, Nina M. Haste c, Victor Nizet c,d
⇑
a Department of Chemistry and The Skaggs Institute for Chemical Biology, The Scripps Research Institute, 10550 North Torrey Pines Road, La Jolla, CA 92037, United States
b Department of Chemistry and Biochemistry, University of California, San Diego, 9500 Gilman Drive, La Jolla, CA 92093, United States
c Skaggs School of Pharmacy and Pharmaceutical Sciences, University of California, San Diego, 9500 Gilman Drive, La Jolla, CA 92093, United States
d Department of Pediatrics, University of California, San Diego, 9500 Gilman Drive, La Jolla, CA 92093, United States
a r t i c l e i n f o
a b s t r a c t
Article history:
A five-step total synthesis of the antibiotic marinopyrrole A (1) is described. The developed synthetic
technology enabled the synthesis of several marinopyrrole A analogs whose antibacterial properties
against methicillin-resistant Staphylococcus aureus TCH1516 were evaluated.
Ó 2010 Elsevier Ltd. All rights reserved.
Received 8 September 2010
Accepted 15 September 2010
Available online 11 October 2010
Dedicated to Harry Wasserman on the
occasion of his 90th birthday
Keywords:
Marine natural products
Total synthesis
Atropisomers
Clauson-Kaas reaction
Marinopyrroles A and B (1 and 2, Fig. 1) are two recently
reported alkaloids endowed with promising antibiotic activities
against methicillin-resistant Staphylococcus aureus (MRSA).1 Iso-
lated from an obligate marine Streptomyces (strain CNQ-418, col-
lected from the sea floor near La Jolla, CA), these structurally
unusual molecules exist as enantiopure M-(À)-atropisomers at
ambient temperature. The absolute structure of (À)-2 was estab-
lished through X-ray crystallographic analysis and (À)-1 through
spectroscopic comparisons. Due to their novel molecular struc-
tures and promising biological properties, the marinopyrroles have
attracted considerable attention. The preparation of several semi-
synthetic analogs,2 a study of the mode of action,3 and a total syn-
thesis4 of marinopyrrole A [( )-1] have been reported. A recent
evaluation of the pharmacological properties of marinopyrrole A
revealed potent anti-MRSA activity and favorable in vitro kinetics.5
We set out to develop a total synthesis that could deliver large
amounts of material and would be flexible enough to allow the
construction of a wide range of analogs for probing structure–
activity relationships (SARs). We report herein a short and efficient
total synthesis of marinopyrrole A (1) and a series of analogs, as
well as their biological evaluation.
alternative approach involving early construction of the bis-pyr-
role system followed by site-specific introduction of the benzenoid
rings and chlorine residues would allow easy access to a variety of
designed analogs. Scheme 1 summarizes the developed five-step
route to marinopyrrole (À)-1 (natural) and (+)-1 (unnatural) from
the readily available building blocks aminopyrrole 36 and 2,5-
dimethoxytetrahydrofuran (4; commercially available). Thus, a
PPTS-promoted Clauson-Kaas reaction7 between 3 and 4 in reflux-
ing 1,4-dioxane furnished bis-pyrrole 5 in 43% yield, establishing
the crucial C–N bond. The latter compound underwent smooth
mono-addition of lithiated anisole 6, in THF at À78 °C, to afford tri-
cycle 7 in 80% yield. Friedel–Crafts arylation of 7 with acid chloride
8, mediated by AlCl3 in CH2Cl2 at 0–25 °C, led to the marinopyrrole
core structure 9 in 64% yield. Despite multiple potential chlorina-
tion sites, compound 9 underwent selective pyrrole tetra-chlorina-
tion with 4.1 equiv of sulfuryl chloride8 (SO2Cl2) in CH2Cl2 at 0 °C to
Although a direct, late-stage dimerization of two fully elabo-
rated pyrrole units could be imagined, such a route might be lim-
iting in terms of substrate scope and coupling efficiency. An
⇑
Corresponding author. Tel.: +1 858 784 2400; fax: +1 858 784 2469.
E-mail address: kcn@scripps.edu (K.C. Nicolaou).
Figure 1. Marinopyrroles A (1) and B (2).
0040-4039/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2010.09.059

2042
K. C. Nicolaou et al. / Tetrahedron Letters 52 (2011) 2041–2043
O
O
O
OMe
O
MeO
OMe
H
N
H
N
H
N
O
OH
H
N
4 (1.4 eq)
OEt
OEt
BBr3 (4.0 eq)
PPTS, 1,4-dioxane,
reflux
N
NH2
O
N
O
N
5
3
CH2Cl2, 25 ºC
MeO
(43%)
HO
OMe
(91%)
9
Li
11
6 (6.0 eq)
THF, –78 ºC
(80%)
NBS (4.0 eq),
MeCN, 25 °C
(85%)
O
OMe
H
N
O
OH
H
N
OMe O
Cl
O
OMe
O
OMe
H
Br
Br
H
N
N
Br
Br
BBr3 (3.0 eq)
O
(1.2 eq)
8
N
Br
O
MeO
CH2Cl2, 25 ºC
N
N
Br
O
N
AlCl3 (1.3 eq), CH2Cl2,
0 ºC–25 ºC
HO
(92%)
MeO
Br
9
Br
(64%)
13
7
12
SO2Cl2 (4.1 eq),
CH2Cl2, 0 ºC
(80%)
O
OH
O
OAc
O
OH
H
N
H
N
H
N
Cl
Cl
Cl
Cl
O
OMe
Cl
Cl
H
Ac2O (2.5 eq),
NEt3 (3.4 eq), DMAP (cat.)
N
Cl
Cl
BBr3 (4.0 eq)
Cl
Cl
Cl
Cl
O
N
O
N
O
N
CH2Cl2, 0 ºC
HO
Cl
Cl
CH2Cl2, 25 ºC
AcO
HO
O
N
(90%)
Cl
Cl
(84%)
MeO
( )-1
14
( )-10
( )-1
OMe O
chiral
chiral
HPLC
HPLC
(+)-10
(–)-10
(+)-1
(–)-1
O
Cl
H
N
Cl
O
8
1.
(1.3 eq),
OEt
Cl
H
N
AlCl3 (1.3 eq), CH2Cl2,
25 °C (76%)
Scheme 1. Synthesis of marinopyrrole A [(+)-1 and (À)-1].
OEt
Cl
O
N
N
HO
2. NCS (4.0 eq), MeCN, 50 ºC (53%)
3. BBr3 (5.0 eq), CH2Cl2, 25 ºC (96%)
Cl
5
15
generate a dimethyl marinopyrrole A derivative ( )-10 in 80%
yield.4 Exposure of tetrachloride ( )-10 to additional SO2Cl2 or
other chlorination reagents [e.g., N-chlorosuccinimide (NCS)] led
to para-chlorination of the aryl moieties. Similarly, attempts to
brominate the remaining pyrrole position to prepare methyl-pro-
tected ( )-2 led only to para-bromination of the aryl rings. That
compound 10 exists as two stable atropisomers was confirmed
by chiral HPLC separation (4:1 hexanes:i-PrOH, ChiralcelÒ OD-H)
of the two enantiomers [(+)-10 and (À)-10]. These enantiomers
demonstrated remarkable thermal stability, showing no detectable
racemization in DMF at 120 °C after 24 h. In contrast, marinopyr-
role A (1) racemizes completely at that temperature.2 Finally,
cleavage of the methyl ethers of ( )-10 (BBr3, CH2Cl2, 0 °C, 90%
yield) delivered racemic marinopyrrole A [( )-1] in five steps and
16% overall yield from aminopyrrole 3. The two enantiomers of
( )-1 were separated by chiral HPLC under the published condi-
tions (19:1 hexanes:i-PrOH, ChiralcelÒ OD-H)1 to afford (+)-1 and
(À)-1.
O
Cl3C
Cl
(2.1 eq),
1.
O
OH
H
N
O
OMe
THF, reflux
2. NaOEt (4 eq), EtOH, 25 ºC
(29% over two steps)
H
N
Cl
Cl
Cl
Cl
O
N
N
3. NCS (4.0 eq), MeCN, 70 ºC (51%)
4. BBr3 (5.0 eq), CH2Cl2, 25 ºC (97%)
7
EtO
16
Scheme 2. Synthesis of marinopyrrole A analogs 11–16.
and demethylation with BBr3 in CH2Cl2 to afford 15 in 39% overall
yield. Similarly, 16 was prepared from 7 in 14% overall yield
though a four-step sequence involving C-acylation with trichloro-
acetylchloride in refluxing THF, trichloromethyl displacement with
NaOEt in EtOH, tetra-chlorination (NCS, MeCN, 70 °C), and cleavage
The developed technology for the total synthesis of marinopyr-
role A was employed for the synthesis of designed analogs 11–16
as summarized in Scheme 2. Thus, demethylation of compound 9
(BBr3, CH2Cl2, 91% yield) gave dehalogenated marinopyrrole A
11.4 Treatment of bis-pyrrole 9 with 4.0 equiv of N-bromosuccini-
mide (NBS) led to selective tetra-bromination on the pyrrole rings
to afford 12 in 85% yield. As observed with tetrachloride 10, expo-
sure of 12 to further amounts of NBS led to para-bromination of the
aryl moieties. Compound 12 was then demethylated (BBr3, CH2Cl2,
92% yield) to provide tetrabromomarinopyrrole 13. Acetylation of
the phenolic oxygens of ( )-1 proceeded readily with acetic anhy-
dride and NEt3 in the presence of catalytic amounts of DMAP, fur-
nishing diacetylmarinopyrrole A 14 in 84% yield.2,3 Mono-arylated
marinopyrrole 15 was prepared from bis-pyrrole 5 through a
three-step sequence involving a Friedel–Crafts C-arylation with
acid chloride 8, tetra-chlorination with NCS in MeCN at 50 °C,
Table 1
Antibacterial activities of synthetic marinopyrroles
a
Entry
Compound
MIC50
(l
g/mL)
1
2
3
4
5
6
7
8
9
( )-1
(+)-1
(À)-1
9
( )-10
11
12
13
14
15
0.375–0.750
0.189
0.189
>96
>96
48
>96
0.75
0.375
3
10
11
16
1.5
a
Tested against MRSA TCH1516 (ATCC BAA-1717).

K. C. Nicolaou et al. / Tetrahedron Letters 52 (2011) 2041–2043
2043
of the methyl ethers (BBr3, CH2Cl2). Although compounds 12–16
likely exist as enantiomeric atropisomers stable at room tempera-
ture, no chiral HPLC separation was undertaken prior to their test-
ing due to the potency equivalence of the two marinopyrrole A
enantiomers (see Table 1, entries 2 and 3).1
presence of human serum, suggesting utility in topical but not
systemic formulations. Further structural modifications, including
the design of prodrug-like compounds, may be necessary in order
to improve the pharmacological profile of the marinopyrroles.
The synthesized compounds [( )-1, (+)-1, (À)-1, ( )-10, and 11–
16] were evaluated for their antibacterial activities against
TCH1516, a strain representative of the current epidemic clone of
community-acquired MRSA.5 The results are shown in Table 1.
Thus, synthetic racemic [( )-1] and enantiopure [(+)-1 and (À)-1]
marinopyrroles (entries 1–3) exhibited antibacterial potencies
comparable to those of their naturally-derived counterparts.1
Interestingly, the tetrabrominated congener of marinopyrrole A
13 (entry 8) exhibited comparable potency to marinopyrrole A,
while the dehalogenated analog 11 (entry 6) was significantly less
active, indicating the importance of the halogen atoms for biolog-
ical activity. It was also clear that the free phenolic groups were
necessary for activity since dimethylated marinopyrrole deriva-
tives [9, ( )-10, and 12] showed no activity (entries 4, 5, and 7,
respectively). Bis-acetylated marinopyrrole 14 (entry 9) showed
similar antibacterial potency to marinopyrrole [( )-1] itself, possi-
bly due to in situ ester hydrolysis within the cell. Excision of one of
the two phenolic rings from the marinopyrrole structure led to ac-
tive, but less potent analogs, as demonstrated by compounds 15
and 16 (entries 10 and 11, respectively). When these compounds
were tested in the same assay, but in the presence of 20% normal
pooled human serum, they were found to lose antibacterial
activity, presumably due to protein adsorption.
Acknowledgments
Financial support for this work was provided by The Skaggs
Institute for Chemical Biology, a National Institutes of Health
(U.S.A.) grant and Ruth L. Kirschstein National Research Service
Award (NRSA) (to N.M.H.), and a National Science Foundation grad-
uate fellowship (to N.L.S.).
Supplementary data
Supplementary data associated (further experimental details
for the synthesis and biological evaluation of compounds as well
as selected physical properties of compounds) with this article
can be found, in the online version, at doi:10.1016/
j.tetlet.2010.09.059.
References and notes
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Am. Chem. Soc. 2009, 131, 12094.
4. Cheng, C.; Pan, L.; Chen, Y.; Song, H.; Qin, Y.; Li, R. J. Comb. Chem. 2010, 12, 541.
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8162.
In summary, a concise total synthesis of marinopyrrole A (1)
that allows for large scale preparation of this novel natural product
and its analogs is reported. The synthesized compounds were eval-
uated for activity against the clinically-important USA 300 clone of
MRSA, with several showing strong antibacterial potencies. How-
ever, all suffered complete loss of antibacterial activity in the
8. Kikuchi, H.; Sekiya, M.; Katou, Y.; Ueda, K.; Kabeya, T.; Kurata, S.; Oshima, Y. Org.
Lett. 2009, 11, 1693.