Organic Letters
Letter
cocktail, then the Z-dehydrotryptophan was completely
reduced to the corresponding Trp. This phenomenon was
also noted by Chen et al. during their synthesis of CDA3a.4
CDA3a and CDA4a contain other uncommon amino acids
such as D-hydroxyphenylglycine (D-HOPhGly) and D-erythro-
hydroxyasparagine (D-eHOAsn). CDA4a also has the un-
common amino acid (2S,3R)-3-methylglutamate (MeGlu) at
position 10, whereas CDA3a has Glu at this position.16−18 All
of the CDAs contain an exocyclic Ser residue that is attached
to Thr2 and acylated with (2S,3R)-3-propyloxirane-2-carbox-
ylic acid.
As previously mentioned, the synthesis of CDA3a has
recently been reported using tripeptide 3 as a building block
for Fmoc SPPS.4 An off-resin cyclization strategy was used to
prepare the cyclic core, which, after high-performance liquid
chromatography (HPLC) purification, was acylated using an
off-resin serine ligation reaction followed by HPLC purification
to obtain CDA3a and CDA3a analogs containing different lipid
tails. The researchers forwent subjecting the protected peptide
containing the lipid tail to acidic deprotection conditions due
to apprehensions about the epoxide moiety’s stability to acid.
However, Radzey et al. demonstrated that similar epoxides are,
in fact, capable of withstanding 95:5 v/v TFA/H2O.19 Keeping
this in mind, we decided to adopt an on-resin cyclization
strategy of a protected, lipidated peptide, similar to what we
used for the total synthesis of daptomycin.20
1
The H and 13C NMR spectra were consistent with those
Next, we turned our attention to the preparation of
sclerotide A. The sclerotides are a pair of ΔTrp-containing
hexapeptides originally isolated from Aspergillus sclerotiorum, a
marine-derived halotolerant fungal strain, that show moderate
antifungal, antibacterial, and cytotoxic activity.13 Both
sclerotides are composed of six amino acids: ΔTrp, D-Phe, D-
Ser, anthranilic acid (Ant), L-Ala, and L-Thr. The two peptides
differ only in the stereochemistry of the ΔTrpthe Z-isomer
in sclerotide A (Figure 1) and the E-isomer in sclerotide B.
The configuration of the ΔTrp residue of the two peptides was
found to rapidly interconvert when exposed to light, with the
Z-isomer of sclerotide A being the favored isomer.13 Neither
sclerotide has ever been prepared by chemical synthesis.
The synthesis of sclerotide A is outlined in Scheme 5. Resin-
bound Ala was extended to the Ant residue using Fmoc SPPS
Scheme 5. Synthesis of Sclerotide A
The synthesis began with the coupling of Fmoc-D-
HOPhGly(OTBS)-OH to the α-NH2 group of resin-bound
dipeptide 12 (Scheme 6).20 Because HOPhGly is somewhat
racemization-prone during coupling, this residue was installed
using COMU/2,6-lutidine, as Liang et al. have shown that
these conditions minimize this problem.21 The resulting
peptide 13 was elongated to Ser1 using standard Fmoc SPPS
with DIC/HOBt as coupling agents and 4-MP for Fmoc
removal to give peptide 14. Acylation of the N-terminus of 14
with (2S,3R)-3-propyloxirane-2-carboxylic acid22 and DIC/
HOBt gave peptide 15. Building block 4 was incorporated into
peptide 15 using DIC/DMAP in dichloromethane (DCM) for
24 h to give peptide 16.
The next two residues, Fmoc-MeGlu(OtBu)-OH23 or Fmoc-
Glu(OtBu)-OH, and Fmoc-D-eHOAsn(Trt)-OH24,25 were
installed using DIC/HOBt to give peptides 17 and 18. 2-
Methylpiperidine (2-MP) was used to remove the Fmoc group
to minimize the possibility of aminolysis of the ester bond.
This was followed by the removal of the allyl group using 1,3-
dimethylbarbituric acid (DMBA) and cat. Pd(PPh3)4, the
removal of the Fmoc group using 2-MP in dimethylformamide
(DMF), and cyclization using PyAOP/HOAt/2,4,6-collidine
to give peptides 19 and 20. The last step, global deprotection/
resin cleavage, required some optimization because the epoxide
group is acid-sensitive. Using 95:5 TFA/H2O at room
temperature (rt) for 60 min gave a significant amount of the
epoxide-hydrolyzed peptides. Reducing the amount of water to
2% gave similar results. Performing the reaction in the absence
of water (such as with 9:1 TFA/PhMe) resulted in no
detectable epoxide hydrolysis, the removal of the peptide from
the resin, and the removal of all of the protecting groups with
the exception of the TBS protecting group of HOPhGly6. We
found that for peptide 19, epoxide hydrolysis could be kept to
∼10% using ice-cold 95:5 TFA/H2O, which was then allowed
to react for 75 min at rt. After precipitation in ice-cold ether
and centrifugation, the pellet was dissolved in ice-cold 1:1
phosphate buffer/MeCN and stored overnight at 0 °C, during
which CO2 adducts were hydrolyzed. CDA3a was obtained in
an overall 6% yield (based on resin loading) after purification
by reversed-phase (RP)-HPLC. We found that the residue at
to give peptide 9. Coupling to the amino group in Ant can be
challenging due to the reduced nucleophilicity of the amino
group compared with α-amino acids. Masuda et al. have
reported that coupling to Ant residues during SPPS can be
achieved using triphosgene/2,4,6-colidine as coupling agents.14
These conditions were also successful for coupling compound
4 to peptide 9 to give peptide 10, although the reaction was
sluggish, and it required 19 h to go to completion. The Thr
residue was coupled to peptide 10 to give peptide 11. Treating
peptide 11 with TFA/H2O (95:5) for 3 h gave the cleaved,
linear ΔTrp-containing peptide, which was cyclized using
PyAOP/HOAt/2,4,6-collidine to give sclerotide A in 24%
1
overall yield. Sclerotide B was not detected. The H and 13C
NMR spectra matched closely with those reported in the
Next, we tested our methodology on more challenging
targets, namely, CDA3a and CDA4a. These two peptides are
part of a family of antibiotics called the CDAs isolated from
Streptomyces coelicolor.10,11 Like all of the CDAs, CDA3a and
CDA4a contain a ten amino acid macrocycle closed via an
ester (depsi) bond (Figure 1). In addition to a ΔTrp residue,
3050
Org. Lett. 2021, 23, 3048−3052