Published on Web 03/20/2007
Total Synthesis of Plusbacin A3: A Depsipeptide Antibiotic Active Against
Vancomycin-Resistant Bacteria
Aaron Wohlrab, Ryan Lamer, and Michael S. VanNieuwenhze*
Department of Chemistry and Biochemistry, UniVersity of California at San Diego, La Jolla, California 92093
Received November 24, 2006; E-mail: msv@ucsd.edu
Plusbacin A3 is a lipodepsipeptide isolated from a fermentation
broth of Pseudomonas sp. PB-6250 obtained from a soil sample
collected in the Okinawa Prefecture, Japan (Figure 1).1 The structure
of plusbacin A3 was established in 1992. It is a member of a family
of lipodepsipeptide natural products that differ either in the structure
of their respective fatty acid side chains or in substitution of
L-proline for 3-hydroxy-L-proline residues.2
In a recent evaluation, plusbacin A3 displayed strong antibiotic
activity against methicillin-resistant Staphylococcus aureus and
VanA-type vancomycin-resistant enterococci with minimum inhibi-
tory concentration (MIC) values from 0.78 to 3.13 µg/mL.3
Plusbacin A3 also inhibited incorporation of N-acetylglucosamine
into staphylococcal cell wall peptidoglycan with a 50% inhibitory
concentration (IC50) that was close to its MIC value. Like
vancomycin, plusbacin A3 was found to inhibit nascent peptidogly-
can formation; however, unlike vancomycin, plusbacin was also
found to inhibit the formation of the lipid intermediates utilized in
bacterial cell wall biosynthesis. Interestingly, the activity of
plusbacin A3 was not antagonized by the presence of N-acetyl-L-
Lys-D-Ala-D-Ala, a tripeptide mimic of the binding domain for
vancomycin,3 suggesting that if plusbacin A3 achieves its biological
activity through binding to the lipid intermediates or to nascent
peptidoglycan it does so at a site on these precursors that is not
Figure 1. Structure of plusbacin A3 and its retrosynthetic analysis.
utilized by vancomycin itself. As a result, plusbacin A3 possesses
significant promise for use in the treatment of vancomycin-resistant
infections.
could influence the intervening residues to adopt a conformation
that would facilitate the final macrocyclization event.
The amino acid sequences of the plusbacins were established
through Edman degradation of their deacylated products and
supported by mass spectrometric studies. Degradation experiments
also suggested a lactone linkage between an L-threo-â-hydroxyas-
partic acid residue and a 3-hydroxy fatty acid subunit. In the case
of plusbacin A3, the fatty acid component is reported to be
3-hydroxyisopentadecanoic acid, although the stereochemical con-
figuration at the hydroxyl stereocenter has yet to be assigned.
Plusbacin A3 also has several non-proteinogenic amino acids
embedded in its peptide backbone. In addition to the L-threo-â-
hydroxyaspartic acid residue mentioned above, other non-natural
amino acids contained in plusbacin A3 include D-threo-â-hydroxy-
aspartic acid, D-allo-threonine, and trans-3-hydroxy-L-proline. The
presence of these non-natural amino acids, coupled with the base
sensitivity of the lactone linkage, renders plusbacin A3 a challenging
target for total synthesis.
Our retrosynthetic analysis for plusbacin A3 with our selected
disconnections is presented in Figure 1. We chose to divide the
target molecule into four fragments of approximately equal
complexity. This was done in order to provide a measure of
flexibility over the amide bond to be made in the final macrolac-
tamization step since there was no solution conformation data
available to guide our selection. Our analysis was further guided
by a hypothesis that the hydroxyproline residues located at each
end of plusbacin A3 may enforce a â-turn type conformation that
Upon closer examination of the structure of plusbacin A3, one
is drawn to the possibility that the fatty acid side chain and the
arginine residue may play important roles in its biological activity.
For example, the fatty acid side chain may be important for
membrane localization, while the terminal guanidine group of the
arginine residue could be important for binding interactions with
diphosphate or carboxylate groups present in the lipid intermediates
and/or nascent peptidoglycan. Thus, we wished to devise a synthetic
strategy that would easily accommodate variations in the lipophilic
side chain and arginine/ornithine residues in order to gain a greater
insight into the roles each may have in the biological activity of
the plusbacins.
The preparation of fragments 2-4 is presented in Scheme 1.
Dipeptide 2 was prepared via coupling of Boc-D-Ser(OBn)-OH
with the allyl ester obtained from commercially available trans-3-
hydroxy-L-proline (EDCI, DIEA, HOAt, 89%). The C-terminal allyl
ester was then cleanly removed (PdCl2(PPh3)3, Ph3SiH, CH2Cl2,
85%) under standard deprotection conditions to provide the
C-terminal acid 2 in high overall yield.
Synthesis of fragment 3 required an independent synthesis of
an orthogonally protected D-threo-hydroxyaspartic acid derivative
7. This compound was prepared in eight steps from the Garner’s
aldehyde derived from Boc-L-Ser.4 Coupling of this precursor with
Boc-L-Orn(Fmoc)-OH (EDCI, DIEA, HOAt, 93%) provided the
corresponding dipeptide. Cleavage of the N-terminal Boc protective
9
10.1021/ja068455x CCC: $37.00 © 2007 American Chemical Society
J. AM. CHEM. SOC. 2007, 129, 4175-4177
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