The enantioselective synthesis and biological evaluation of chimeric promysalin analogs facilitated by diverted total synthesis

Full text of DOI:10.1038/ja.2016.4

The Journal of Antibiotics (2016), 1–3
2016 Japan Antibiotics Research Association All rights reserved 0021-8820/16
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&
NOTE
The enantioselective synthesis and biological
evaluation of chimeric promysalin analogs facilitated
by diverted total synthesis
Kyle W Knouse and William M Wuest
The Journal of Antibiotics advance online publication, 10 February 2016; doi:10.1038/ja.2016.4
The biosynthesis and subsequent modification of fatty acids plays a and pharmacological properties of migrastatin9 and epothilone,10
major role in the primary and secondary metabolism of both leading to viable drug candidates. In a similar fashion, the Reddy
terrestrial and marine organisms. One such metabolite, myristic group applied DTS toward the synthesis of hybrid hermitamides,
acid (1) is the biosynthetic precursor of a number of secondary derived from the coupling of lyngbic acid, a marine metabolite, with
metabolites, four of which are depicted in Figure 1 (2–5). These amines from the terrestrial amides of the Piperaceae family resulting in
molecules have each been shown to possess a broad range of compounds with in vitro cytotoxicity against human cancer cell lines.11
antibacteral and antivirulence activity ranging from the inhibition of Taking inspiration from these examples, we sought to utilize DTS
biofouling and biofilm formation to the disruption of quorum-sensing through the coupling of a readily available precursor from our
networks (vide). Our laboratory has recently completed the first total promysalin synthesis, ( − )-6S, with the structurally similar alcohol
synthesis of promysalin ((− )-2), which confirmed both its absolute variants of lyngbic acid and the hermitamides. Herein, we report the
stereochemistry and potent species-specific antibacterial activity (sub- biological evaluation and an enantioselective synthesis of both
micromolar activity against P. aeruginosa).1,2 In 1978 Moore et al.3 enantiomers of lyngbic acid and hermatidamides A and B, and a
identified lyngbic acid, ((− )-3) as a major component of the lipid focused library of chimeric promysalin derivatives.
extracts from the marine cyanobacteria Lyngbya majuscula. Gerwick
The synthesis of lyngbic acid began with the asymmetric allylation
later studied the biological properties of this secondary metabolite and of octanal with allyltributylstannane mediated by a titanium-BINOL
reported that ( − )-3 displays antimicrobial activity toward Gram- complex to afford either stereoisomer of allylic alcohol 7 (Scheme 1).12
positive bacteria (Staphylococcus aureus and Bacillus subtilis); however, Methylation of 7 followed by homologation via cross metathesis with
Gram-negative strains were not evaluated.4 Additionally, ( − )-3 was 4-pentenoic acid affords both enantiomers of lyngbic acid (3) in an
shown to interfere with the CqsS-mediated signaling pathway that is overall yield of 32%. The total syntheses of hermitamide A and B
responsible for quorum sensing in Vibrio harveyi5 and inhibit the was accomplished by HATU-mediated coupling of 3 and phenylethy-
growth of marine fungi Fusarium sp., Lindra thalassiae and Dendry- lamine to afford 4 or tryptamine to provide 5 in a longest linear
phiella salina.6 In 2000, Gerwick et al.7 reported the isolation of two sequence of four steps.
aromatic amide derivatives of lyngbic acid, hermitamides A ((− )-4)
and B ((− )-5). These compounds were shown to be inhibitors of the molecules which required intermediate ( − )-6S, which was prepared
human voltage-gated sodium channel,8 cytotoxic to neuro-2a neuro- as previously reported,4 and seco-acid
(available via cross
We next focused our attention toward the DTS of chimeric
8
blastoma cells and active in a brine shrimp toxicity assay.7 As with metathesis). Compound 8 was either treated with (2-(trimethylsilyl)
lyngbic acid, these compounds were not evaluated against Gram- ethoxy)methyl chloride to afford the protected ester or HATU and the
negative pathogens. Motivated by our earlier work on promysalin and respective amine to furnish the corresponding amides (10) and (11).
the dire need for the discovery of novel therapeutics to combat The alcohols were then coupled to ( − )-6S using Shiina conditions13
Pseudomonas infections, we sought to investigate the potential of these and globally deprotected to provide the three sets of chimeric
molecules as species-specific antibacterial agents.
molecules: promysalin–lyngbic acid (12), promysalin–hermitamide A
Similar to diversity-oriented synthesis, diverted total synthesis (13) and promysalin–hermitamide B (14) in a longest linear sequence
(DTS) is a method by which one can leverage achievements in total of five steps.
synthesis to access structural space, which is not possible through
The antibacterial activity of the six classes of molecules (3–5, 12–14)
biosynthetic means. Pioneering work by the Danishefsky group against a panel of four Gram-negative bacteria (two strains of
demonstrated that DTS could be used to improve both the efficacy P. aeruginosa (PAO1, PA14), P. fluorescens, and P. putida) at
Department of Chemistry, Temple University, Philadelphia, PA, USA
Dedicated to Professor Amos B Smith, III, in recognition of his outstanding contributions to the fields of natural products and total synthesis.
Correspondence: Professor WM Wuest, Department of Chemistry, Temple University, 1901 N. 13th Street, Philadelphia, PA 19122, USA.
E-mail: wwuest@temple.edu
Received 10 December 2015; revised 12 January 2016; accepted 14 January 2016

Enantioselective synthesis and evaluation of chimeric promysalin analogs
KW Knouse and WM Wuest
2
concentrations ranging from 455 nM–900 μM was next investigated (see
Supplementary Table S1). To our surprise, only one analog ((+)-12R,
S) displayed antibacterial activity against all strains tested with an IC50
of 455 μM, ~1000-fold less active than ( − )-2. As one might expect,
the absolute stereochemistry of (+)-12R,S is identical to that of
promysalin, reconfirming the role of those stereocenters in antibacter-
ial activity. Curiously, during the course of our assays
(at 48 h) we noticed a pronounced, non-lethal species-specific
phenotype against PA14 (the strain that is most sensitive to the
activity of promysalin) when dosed with compounds ( − )-13R,S and
( − )-14S,S (Supplementary Figure S1). This may hint at a disparate
mechanism of action unrelated to antibacterial activity and is currently
being investigated in our laboratory.
O
N
HO
C7H15
OH
OH
O
Myristic Acid (1)
O
O
H2N
C6H13
Promysalin ((-)-2)
O
O
OMe
C7H15
R
OH
Lyngbic Acid ((-)-3): R = OH
Hermitamide A ((-)-4): R =
(-)-6S
N
Ph
NH
N
O
H
O
Hermitamide B ((-)-5): R =
N
H
OSEM
In summary, we have reported a concise, enantioselective total
synthesis of the marine secondary metabolites lyngbic acid,
hermitamide A and hermitamide B. Furthermore, we utilized DTS
to construct a focused library of six chimeric promysalin analogs
to further evaluate the structure–activity relationship of this
species-specific natural product. Our biological assays show, for the
Diverted Total
Synthesis
Will chimera molecules retain
antimicrobial activity?
Figure 1 Natural products derived from myristic acid and proposed diverted
total synthesis of chimera analogs.
Ph
HATU, DIEA,
Amine
O
OMe
C7H15
O
OMe
C7H15
HN
N
H
Ph
NH2
NH
NH2
HN
Hermitamide B
Hermitamide A
(-)-5S: 42%
(+)-5R: 64%
(-)-4S: 84%
(+)-4R: 86%
1) NaH, MeI
2) 4-Pentenoic Acid,
Grubbs II
O
OMe
OH
HO
C7H15
C7H15
Lyngbic Acid
(-)-7 S
(+)-7R
(-)-3S: 45% over 2 steps
(+)-3R: 38% over 2 steps
4-Pentenoic Acid,
Hoveyda-Grubbs II
1) SEMCl, Et3N
2) (-)-6S, MNBA,
DMAP, Et3N
N
O
OH
N
O
TBAF,
DMPU
HO
O
O
SEMO
O
O
HO
C7H15
O
O
O
(+)-8S: 72%
(-)-8R: 67%
C7H15
HO
C7H15
SEMO
Promysalin-Lyngbic Acid
(-)-12S,S: 74%
(-)-9S,S: 55% over 2 steps
(+)-9R,S: 21% over 2 steps
(+)-12R,S: 89%
N
HATU, DIEA,
Ph
1) (-)-6S, MNBA,
DMAP, Et3N
2) TBAF, DMPU
O
OH
HO
O
O
Ph
O
O
NH2
HN
C7H15
(+)-10S: 84%
C7H15
Ph
N
H
HATU, DIEA,
NH2
(-)-10 86%
:
R
Promysalin-Hermitamide A
(-)-13S,S: 60% over 2 steps
(-)-13R,S: 49% over 2 steps
NH
O
OH
C7H15
1) (-)-6 S, MNBA,
DMAP, Et3N
2) TBAF, DMPU
N
HN
HN
N
HO
O
O
O
O
(+)-11 : 42%
S
(-)-11 R: 64%
C7H15
N
H
Promysalin-Hermitamide B
H
(-)-14 S,S: 32% over 2 steps
(+)-14R,S: 28% over 2 steps
Scheme 1 Synthesis of lyngbic acid (3), hermitamides A (4) and B (5) and chimeric compounds. For clarity only one stereoisomer is depicted and
represented by the top compound name.
The Journal of Antibiotics

Enantioselective synthesis and evaluation of chimeric promysalin analogs
KW Knouse and WM Wuest
3
first time, that lyngbic acid and the hermitamides do not possess any
significant antibacterial activity against a panel of Gram-negative
bacteria. Finally, we have demonstrated that subtle structural changes
to the promysalin side chain significantly reduce the biological activity
of the natural product.
3
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7
CONFLICT OF INTEREST
The authors declare no conflict of interest.
ACKNOWLEDGEMENTS
8
9
This work was funded by the National Science Foundation under
CHE-1454116 and Temple University through the Undergraduate Research
Program. The authors thank Dr CW Ross, Dr C DeBrosse and MC Jennings
(Temple University) for invaluable assistance and Materia, Inc., for olefin
metathesis catalysts and Professor GA O’Toole (Dartmouth Medical School) for
the generous donation of strains.
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Supplementary Information accompanies the paper on The Journal of Antibiotics website (http://www.nature.com/ja)
The Journal of Antibiotics