Antibacterial activity of (-)-deoxypseudophrynaminol versus its racemate and derivatives

Article abstract of DOI:10.1016/j.bmcl.2006.01.093

(-)-Deoxypseudophrynaminol 1 possesses 43-fold greater antibacterial potency than the racemate toward Staphylococcus aureus, indicating that the (-)-enantiomer is the biologically active isomer in this assay. Comparison of the percent growth inhibition by derivatives of 1 indicates that prenylation of N⁸ and replacement of N¹-methyl by methyl carbamate are detrimental to antibacterial potency. (-)-1 is a promising lead structure for the development of the novel hexahydropyrrolo[2,3-b]indole class of antibacterial agents.

Full text of DOI:10.1016/j.bmcl.2006.01.093

Bioorganic & Medicinal Chemistry Letters 16 (2006) 2522–2524
Antibacterial activity of (ꢀ)-deoxypseudophrynaminol versus
its racemate and derivatives
a
a
b
a,
*
Andrew V. Dix, Carly M. Meseck, Adam J. Lowe and Miguel O. Mitchell
a
Department of Chemistry, Henson School of Science and Technology, Salisbury University, Salisbury, MD 21801, USA
b
Department of Biology, Henson School of Science and Technology, Salisbury University, Salisbury, MD 21801, USA
Received 22 December 2005; revised 17 January 2006; accepted 19 January 2006
Available online 7 February 2006
Abstract—(ꢀ)-Deoxypseudophrynaminol 1 possesses 43-fold greater antibacterial potency than the racemate toward Staphylococcus
aureus, indicating that the (ꢀ)-enantiomer is the biologically active isomer in this assay. Comparison of the percent growth inhibi-
8
1
tion by derivatives of 1 indicates that prenylation of N and replacement of N -methyl by methyl carbamate are detrimental to
oa nf t ai bn at icbt ae rc it ae lr i pa lo ta egne cn yt .s .(ꢀ)-1 is a promising lead structure for the development of the novel hexahydropyrrolo[2,3-b]indole class
Ó 2006 Elsevier Ltd. All rights reserved.
(
±)-Deoxypseudophrynaminol 1 (Fig. 1) is a known
antibacterial agent with MIC of 20–40 lg/mL versus
vancomycin-resistant Enterococci and methicillin-resis-
tant Staphylococcus aureus. However, the antibacterial
3a
N
3
1
8
N
activity of the individual enantiomers of 1 is unreported.
Other hexahydropyrrolo[2,3-b]indoles, natural products
isolated from the marine bryozoan Flustra foliacea, also
H
8
H
CH
3
a
(
+)-1
2
display antibacterial activity. To improve our under-
Figure 1. (±)-Deoxypseudophrynaminol.
standing of the structural requirements for antibacterial
activity among the hexahydropyrrolo[2,3-b]indoles, we
decided to obtain (ꢀ)-1 via resolution of (±)-1, to pre-
pare two derivatives of (±)-1, and to perform a colony
count based assessment of antibacterial potency versus
S. aureus.
a
b
NH2
NH
OCH3
NHCH3
N
H
N
H
N
H
O
2
3
4
(
±)-1 was prepared by a three-step synthesis from trypt-
c
amine 2 (Scheme 1). Acylation of 2 with methyl chloro-
formate furnished carbamate 3 in 81% yield. Reduction
of 3 with 10 eq lithium aluminum hydride under reflux
d
(+)-1
N
N
H
3
H
CH
3
followed a literature procedure with the important
modification of simple extraction with ethyl acetate to
increase the yield of 4 from 50% to 79%. Alkylative
cyclization of the indolyl Grignard of 4 with prenyl bro-
mide provided (±)-1 in 25% yield. When we attempted
to resolve (±)-1 by combining it with equimolar
dibenzoyl-D-tartaric acid to form solid diastereomeric
(-)-1
Scheme 1. Reagents and conditions: (a) ClCO
81%); (b) LiAlH (10 equiv), THF, reflux (79%); (c) MeMgBr, then
O (25%); (d) dibenzoyl-D-tartaric acid then flash
2
Me, DIEA, THF
(
4
4
prenyl bromide, Et
chromatography.
2
salts of differing solubility, as per a known method for
5
Keywords: Indoles; Hexahydropyrrolo[2,3-b]indoles; Antibacterial;
Deoxypseudophrynaminol; Debromoflustramine.
resolving hexahydropyrrolo[2,3-b]indoles, only an
intractable oil resulted in all tested solvents (hexanes,
*
Corresponding author. E-mail: momitchell@salisbury.edu
0
960-894X/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2006.01.093

A. V. Dix et al. / Bioorg. Med. Chem. Lett. 16 (2006) 2522–2524
2523
THF, CH Cl , CHCl , acetone, CH CN, and Et O). To
3
there was little enantiopreference (ca. 4% ee by optical
8
rotation ), so 6 was nearly racemic.
2
2
3
2
our surprise, the diastereomeric salts separated cleanly
by flash chromatography (2:1 EtOAc/CH Cl to remove
2
2
the dibenzoyl-D-tartrate salt of (+)-1, followed by meth-
anol to isolate the dibenzoyl-D-tartrate salt of (ꢀ)-1).
Unfortunately, the less polar dibenzoyl-D-tartrate salt
of (+)-1 was missed during UV monitoring of collected
fractions due to extensive dilution by band broadening.
The dibenzoyl-D-tartrate salt in the more polar metha-
nolic fractions was concentrated in vacuo, basified with
Figure 2 illustrates the effectiveness of each compound,
averaging the data (see Table 1 in Supplementary Mate-
rial). (±)-6 has modest activity, reducing bacteria growth
over 30 percent at 40 lg/mL. Carbamate (±)-5 had no
antibacterial effect. (±)-1 had good antibacterial potency,
reducing growth by almost 85 percent at 40 lg/mL but
failed to have any significant effect at 10 lg/mL. (ꢀ)-1
produced the most promising results, with over 99%
reduction of bacteria at 40 lg/mL and 50% reduction
at 10 lg/mL. Note that the micromolar concentration
range for these different molecular weight compounds
is narrow (0.113–0.165 lM at 40 mg/mL) and unlikely
to contribute to the dramatic effects observed.
1
0% NaOH (aq), and extracted with ethyl acetate to
20
obtain (ꢀ)-1 ½aꢁ ꢀ32.3 (c 0.0296, CHCl ) spectroscop-
D
ically identical to its racemate. H NMR spectra of the
3
4
1
6
(
R)-MTPA amide of (ꢀ)-1 versus that of (±)-1 showed
that (ꢀ)-1 was homochiral (>95% ee). The absolute con-
figuration assigned to (ꢀ)-1 is consistent with that of all
2
,5,7–9
known (ꢀ)-hexahydropyrrolo[2,3-b]indoles.
In this study, (±)-1 produced moderate inhibition of the
growth of S. aureus, while (ꢀ)-1 gave an impressive 43-
fold greater inhibition (see Table 2 in Supplementary
Material). (ꢀ)-1 is the eutomer. For antibacterial poten-
From 3, two derivatives of (±)-1 were also prepared
Scheme 2). Alkylative cyclization of 3 to furnish (±)-5
(
was based on a quinine (35% yield) or quinidine (53%
yield) modification of Ganesan’s original approach with
1
cy, hydrophobic substitutions at N appear to be pre-
1
0
DIEA, which did not work in our hands. Reduction of
±)-5 with 10 equiv of lithium aluminum hydride
ferred over hydrophilic ones, both in this study and in
1
,2
(
previous work. In contrast, much greater antibacterial
potency was obtained when N was unsubstituted (a
1
0
8
produced racemic debromoflustramine B, (±)-6, in
8–74% yield. As expected at room temperature, even
though quinine and quinidine are homochiral amines,
6
hydrogen bond donor/acceptor). Future studies should
also explore the relevance of substitutions on the ben-
zene ring, the effect of different substituents at 3a, hu-
man cell toxicity, and the scope of antibacterial activity.
a
b
Acknowledgments
3
N
N
N
N
H
H
CO2CH3
CH3
We are indebted to the Department of Biological Scienc-
es at Salisbury University for their advice and material
contribution to the microbiological studies in this pro-
ject. We particularly thank Julie Meeks for her assistance
with microbiological studies. A.V.D. and C.M.M. also
received financial support from a Henson Student
Research Grant. We also thank Dr. Fred Kundell
(
+)-5
(+)-6
Scheme 2. Reagents and conditions: (a) prenyl bromide, Bu
Zn(OTf) , quinine (35%) or quinidine (53%), toluene; (b) LiAlH
10 equiv), THF, reflux (68–74%).
4
NI,
2
4
(
Figure 2. Percent inhibition of the growth of Staphylococcus aureus colonies at 10 and 40 lg/mL concentrations of inhibitors relative to DMSO
control (100% growth).

2524
A. V. Dix et al. / Bioorg. Med. Chem. Lett. 16 (2006) 2522–2524
(Dept. of Chemistry, Salisbury University) for assistance
with the polarimeter and Dr. Mustafa Guzel (Principal
4. Mitchell, M. O.; Dorroh, P. A facile synthesis of (± ±-
deoxypseudophrynaminol. Tetrahedron Lett. 1991, 32,
1
Scientist, Transtech Pharma) for H NMR and LC–
7
641.
5
. Yu, Q.; Pei, X.; Holloway, H. W.; Greig, N. H. Total
syntheses and anticholinesterase activities of (3aS)-N(8)-
norphysostigmine, (3aS)-N(8)-norphenserine, their antip-
odal isomers and other N(8)-substituted analogues.
J. Med. Chem. 1997, 40, 2895.
MS analysis of the MTPA amides of (±)-1 and (ꢀ)-1.
Supplementary Data
6
. Mickelson, J. W.; Belonga, K. L.; Jacobsen, E. J.
Asymmetric synthesis of 2,6-methylated piperazines.
J. Org. Chem. 1995, 60, 4177.
. Carle, J. S.; Christophersen, C. Bromo-substituted alka-
loids from the marine bryozoan Flustra foliacea, flustr-
amine C and flustraminol A and B. J. Org. Chem. 1981,
Supplementary data associated with this article can be
found, in the online version, at doi:10.1016/
j.bmcl.2006.01.093.
7
References and notes
4
6, 3440.
8
. Holst, P. B.; Anthoni, U.; Christophersen, C.; Nielsen, P.
H. Two alkaloids, flustramine E and debromoflustramine
B from the marine bryozoan Flustra foliacea. J. Nat. Prod.
1994, 57, 997.
1
. Hodgson, J. W.; Mitchell, M. O.; Thomas, M. L.; Waters,
K. F. Deoxypseudophrynaminol: a novel antibacterial
alkaloid. Bioorg. Med. Chem. Lett. 1995, 5, 2527.
. Peters, L.; Konig, G. M.; Wright, A. D.; Pukall, R.;
Stackebrandt, E.; Eberl, L.; Riedel, K. Secondary metab-
olites of Flustra foliacea and their influence on bacteria.
J. Org. Chem. 2003, 69, 3469.
2
9. Smith, B. P.; Tyler, M. J.; Garraffo, H. M.; Spande, T. F.;
Daly, J. W. Evidence for biosynthesis of pseudophryn-
amine alkaloids by an Australian Myobatrachid frog
(Pseudophryne± and for sequestration of dietary pumilio-
toxins. J. Nat. Prod. 2002, 65, 439.
10. Tan, G. H.; Zhu, X.; Ganesan, A. Total synthesis of
debromoflustramine B via biomimetic alkylative cycliza-
tion. Org. Lett. 2003, 5, 1801.
3
. Benson, S. C.; Lee, L.; Yang, L.; Snyder, J. K. Intramo-
lecular inverse electron demand Diels–Alder reactions of
tryptamine with tethered heteroaromatic azadienes.
Tetrahedron 2000, 56, 1165.