Total Synthesis of Seongsanamide B

Article abstract of DOI:10.1021/acs.orglett.0c01642

The first total synthesis of the bicyclic depsipeptide natural product seongsanamide B is described. The successful approach employed solid-phase peptide synthesis of a core heptapeptide, incorporating on-resin esterification, followed by solution-phase macrolactamization and a late stage intramolecular Evans-Chan-Lam coupling to generate the biaryl ether of the isodityrosine unit.

Full text of DOI:10.1021/acs.orglett.0c01642

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Total Synthesis of Seongsanamide B
Sadegh Shabani and Craig A. Hutton*
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ABSTRACT: The first total synthesis of the bicyclic depsipeptide natural product seongsanamide B is described. The successful
approach employed solid-phase peptide synthesis of a core heptapeptide, incorporating on-resin esterification, followed by solution-
phase macrolactamization and a late stage intramolecular Evans−Chan−Lam coupling to generate the biaryl ether of the
isodityrosine unit.
he seongsanamides A−F (Figure 1) were isolated by
TChoi and co-workers in 2018 from a Bacillus safensis
strain and were shown to exhibit antiallergenic properties on
bone-marrow-derived mast cells (BMMCs).1 The increasing
prevalence of allergic diseases, such as asthma, in recent
decades has led to increased interest in the discovery of novel
antiallergy agents.2,3 Seongsanamides A−D exhibited dose-
dependent inhibition of β-hexosaminidase release and were
noncytotoxic at 20 μM. Further, seongsanamide A was orally
active in a mouse model with antiallergenic activity comparable
to known H1 blocker fexofenadine.
Seongsanamides A−D are bicyclic peptides biosynthesized
by nonribosomal peptide synthetases (NRPSs). Monocyclic
seongsanamide E is the putative precursor to seongsanamide B,
lacking the biaryl ether linkagegenerated by CYP-catalyzed
oxidative couplingthat forms the isodityrosine unit and
Figure 1. Structures of seongsanamides A−F.
second ring. In addition to the biaryl ether linkage between
Tyr-3 and Tyr-8, seongsanamides A−D also contain an ester
The main challenges in the total synthesis of seongsanamide
B include the formation of the biaryl ether linkage that forms
the isodityrosine unit, together with construction of the
macrolactone, in addition to the issues that frequently plague
peptide macrocyclizations.8−12 A number of approaches to the
synthesis of isodityrosines have been reported, including
linkage between the hydroxyl group of Thr-4 and the C-
terminal carboxylate of Tyr-8, designating these compounds as
bicyclic depsipeptides. The biosynthesis of seongsanamide F 6
is presumed to proceed through a truncation during the NRPS
pathway. The presence of three epimerase domains in the
NRPSs correspond to the presence of D-residues at positions
Ala-2, Tyr-3, and Leu-5.
Received: May 14, 2020
To date, none of the seongsanamides has succumbed to total
synthesis. The intriguing biological activity of the seongsana-
mides, together with their unusual structures and our long-
standing interest in the synthesis of tyrosine cross-linked cyclic
peptide natural products,4−7 prompted our investigations into
the synthesis of seongsanamide B.
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© XXXX American Chemical Society
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ester of 10 was removed to give acid 12, which was then
coupled to Leu-Leu-Ile-OtBu 13 to afford the corresponding
isodityrosine-containing pentapeptide. Deprotection of the N-
and C-termini with TFA, followed by macrolactamization at
the Ile-7-Dopa-8 junction, gave the corresponding cyclic
peptide, which was treated with LiOH to effect hydrolysis of
the benzyl ester to afford 14 (Scheme 2). Transannular
lactonization of 14 was investigated; however, all attempts
toward generation of the ester bridge were again unsuccess-
ful.39,40
Scheme 1. Early-Stage Macrolactonization Approach
With both early and late macrolactonization strategies
toward seongsanamide B unsuccessful, we elected to
investigate a more traditional approach toward the depsipep-
tide component, incorporating an early-stage ester formation
with a late-stage construction of the biaryl ether. Accordingly,
Scheme 3. SPPS of Linear Peptide Containing Ester Link
Ullman13−19 and Evans−Chan−Lam20−25 coupling reac-
tions,26 Dotz benzannulation, and nucleophilic aromatic
27
̈
substitution (SNAr) reactions.28−34
We first attempted an early biaryl ether construction,
envisaging late-stage macrolactamization to generate the
bicycle (Scheme 1). An Evans−Chan−Lam coupling of
phenylalalanine boronic acid and DOPA derivatives 7 and 8
was chosen to generate protected isodityrosine 9. Cleavage of
the three Boc groups and tert-butyl ester was followed by
reprotection of the DOPA amine. Subsequent coupling of Thr-
OtBu to the tyrosine carboxylate generated peptide 10, with
hydrolysis of the benzyl ester then providing acid 11.
Attempted macrolactonization of 11 under a variety of
coupling conditions was unsuccessful. Related macrolactamiza-
tions to generate isodityrosine-containing cyclic peptides have
proven difficult,5,6 as have macrolactonizations to generate
cyclic depsipeptides.35−38
Next, we envisaged that cyclization to generate the lactone
may be facilitated if the alcohol and acid groups were already
constrained in a macrolactam, and thus, a late-stage macro-
lactonization strategy was pursued. Accordingly, the tert-butyl
Scheme 2. Late-Stage Macrolactonization Approach
B
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resin-bound tetrapeptide 15 was prepared through standard
Fmoc-SPPS and then a fragment coupling with tripeptide 16
generated resin-bound heptapeptide 17 incorporating a
phenylalanine-boronic acid residue. The protected DOPA 18
was prepared from 8 and then coupled to the Thr-4 side chain
hydroxyl group to generate the ester-linked adduct 19, which
was cleaved from the resin to furnish 20 (Scheme 3). Attempts
to purify 20 by HPLC resulted in partial protodeborylation of
the Phe-Bpin residue, so the crude peptide was carried into the
next step.
At this stage, the two key cyclizations were the final steps to
be completed. Macrolactamization was effected by treatment
of 20 with HATU, HOAt and DIPEA in a dilute (0.8 mM)
solution of DMF/CH2Cl2 (1:1), which provided the cyclized
protected depsipeptide 21 after 12 h (Scheme 4). Removal of
the allyl group with (Ph3P)4Pd and subsequent oxidative
cleavage of the pinacolboronate ester afforded peptide 22
possessing phenolic and arylboronic acid groups. Intra-
molecular Evans−Cham−Lam coupling was successful, gen-
erating the bicyclic depsipeptide 23. Not surprisingly, it is
noted that the successful route to the bicyclic structure
matches the proposed biosynthetic pathway.
Benzyl ether deprotection of 23 was performed to afford
seongsanamide B (2). The overall yield of seongsanamide B
was 3.6% over 16 steps (from chlorotrityl resin) with only two
HPLC purifications required. The NMR spectra (1H, 13C,
COSY, NOESY and ROESY) of the synthetic material were all
in agreement with those reported for the natural product
(note; several misassigned peaks were corrected upon
correlation with seongsanamides A, C and D, and the synthetic
material, see the SI), indicating successful synthesis and
confirmation of the structure of seongsanamide B. Interest-
1
ingly, the resonance for the Thr-4 γ-methyl group in the H
NMR spectra of seongsanamides A−D occurs remarkably
upfield (δ ∼0.38 ppm) compared with the typical position of
such resonances. For example, in seongsanamide E the
equivalent resonance occurs at δ 1.12,1 a typical shift observed
in numerous depsipeptides.41−43 Presumably, the additional
conformational constraints in the bicyclic seongsanamides
imposed by the biaryl ether linkage position the Thr-4 methyl
group in close proximity to the Tyr-3 aromatic ring, such that
anisotropic effects result in a significant upfield shift. The
potential for atropisomers in the seongsanamides, a feature not
unusual in biaryl ether cross-linked peptides,44 was also briefly
examined. No mention of atropisomers was noted in the
original report of the isolation of these natural products.
Heating of seongsanamide B to 130 °C for 2 h resulted in no
change in the NMR spectrum, indicating either a low barrier to
rotation about the biaryl bond such that discrete atropisomers
do not exist or a high barrier such that interconversion does
not occur.
Scheme 4. Synthesis of Seongsanamide B 2
In conclusion, we have completed the first synthesis of
seongsanamide B employing a combination of solid-phase and
solution-phase synthetic strategies and a key late-stage Evans−
Chan−Lam coupling. The work described here lays the
foundation for generating analogues of the seongsanamides
with a view to development of candidates with improved
antiallergic activity.
ASSOCIATED CONTENT
* Supporting Information
■
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The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.orglett.0c01642.
Experimental procedures and NMR and MS data (PDF)
AUTHOR INFORMATION
■
Corresponding Author
Craig A. Hutton − School of Chemistry and Bio21 Molecular
Science and Biotechnology Institute, The University of
Melbourne, Melbourne, Victoria 3010, Australia; orcid.org/
0000-0002-2353-9258; Email: chutton@unimelb.edu.au
Author
Sadegh Shabani − School of Chemistry and Bio21 Molecular
Science and Biotechnology Institute, The University of
Melbourne, Melbourne, Victoria 3010, Australia; orcid.org/
0000-0003-4542-9858
C
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Complete contact information is available at:
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The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the Australian Research Council
(DP180101804). S.S. acknowledges an award from the Albert
Shimmins Fund, University of Melbourne. The authors
acknowledge the support of the Bio21 Molecular Science
and Biotechnology Institute MSP and Peptide Technologies
facilities.
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