Efficient oxidation of N-protected tryptophan and tryptophanyl-dipeptides by in situ generated dimethyldioxirane provides hexahydropyrroloindoline-containing synthons suitable for peptide synthesis and subsequent tryptathionylation

Article abstract of DOI:10.1007/s00726-016-2364-3

A series of hydroxypyrroloindoline (Hpi) containing dipeptides along with the corresponding monomeric Hpi-α-amino acid (Hpi-2-carboxylate), were prepared by reacting a series of N^α-protected-tryptophans in aqueous or biphasic [water/cyclopentyl

Full text of DOI:10.1007/s00726-016-2364-3

Amino Acids
DOI 10.1007/s00726-016-2364-3
SHORT COMMUNICATION
Efficient oxidation of N‑protected tryptophan
and tryptophanyl‑dipeptides by in situ generated
dimethyldioxirane provides hexahydropyrroloindoline‑containing
synthons suitable for peptide synthesis and subsequent
tryptathionylation
Antoine Blanc1 · Fan Xia1 · Mihajlo Todorovic1 · David M. Perrin1
Received: 7 September 2016 / Accepted: 10 November 2016
© Springer-Verlag Wien 2016
Abstract A series of hydroxypyrroloindoline (Hpi) con-
taining dipeptides along with the corresponding monomeric
Hpi-α-amino acid (Hpi-2-carboxylate), were prepared by
reacting a series of Nα-protected-tryptophans in aqueous
or biphasic [water/cyclopentyl methyl ether (CPME)] solu-
tions containing Oxone® (potassium peroxymonosulfate)
and acetone. This procedure avoids the tedious distillation
of unstable dimethyldioxirane (DMDO), which is com-
monly used to oxidize indoles. Monomers Nα-Boc-Hpi-OH
and Nα-Fmoc-Hpi-OH were readily incorporated by solid-
phase peptide synthesis (SPPS) into a peptide containing a
cysteine; in trifluoroacetic acid (TFA), the Hpi underwent
intramolecular dehydrative condensation with the cysteine
thiol to afford the anticipated tryptathionine crosslink. This
eco- and user-friendly oxidative methodology greatly sim-
plifies the synthesis of Hpi derivatives while enabling the
synthesis of tryptathionine crosslinks characteristic of phal-
loidin and amanitin, two potent peptide toxins of present
interest.
Introduction
The 3a-hydroxypyrrolo[2,3-b]indoline (Hpi) (Fig. 1) motif
arises from selective oxidation of tryptophan at position-3,
represents a key toxicophore, and is found predominantly in
the syn-cis configuration, in a large number of peptide and
peptide-derived natural products (Ruiz-Sanchis et al. 2011;
Roche et al. 2015; Lopez et al. 2008; Xu et al. 2014; New-
house et al. 2010; Zhao et al. 2012; Ishikura et al. 2015;
Deng et al. 2014, 2015; Zhu et al. 2015; Kamenecka and
Danishefsky 1998a, b, 2001; Greenman et al. 2004; Buchel
et al. 1998). The Hpi moiety can be constructed by treating
tryptophan and its derivatives with several oxidants, most
notably DMDO. Selective indole oxidation methods are
thus of interest as they enable further elaboration to a host
of tryptophan-derived natural products.
Hpi formation typically requires protection of the
COOH to avoid oxindole formation. In some cases protec-
tion also affords highly diastereoselective oxidation that
favors the syn-cis configuration, particularly in the case
of a sterically bulky tert-butyl ester (Kamenecka and Dan-
ishefsky 2001; Savige and Fontana 1980). While protec-
tion with sterically bulky groups, e.g., a tert-butyl ester
promotes high diastereoselectivity, multiple protecting
group interconversions were required to incorporate Hpi
into various natural products (Kamenecka and Danishef-
sky 2001). Such manipulations significantly reduce the
overall yield in procuring a single diastereomer that can
otherwise be obtained by chromatographic resolution from
a non-diastereoselective oxidation. Indeed, to circumvent
such interconversions and obviate the need for C-terminal
protection, we explored the synthesis of Hpi-dipeptides (2)
and (3) (May et al. 2005). These were procured by oxidiz-
ing Nα-Tr-Trp-Xaa-OMe (1) dipeptides with freshly dis-
tilled DMDO in dichloromethane (Scheme 1) (Kamenecka
Keywords Dimethyldioxirane · Tryptophan oxidation ·
3a-Hydroxypyrrolo[2,3-b]indoline · Peptide synthesis
Handling Editor: J. Bode.
Electronic supplementary material The online version of this
article (doi:10.1007/s00726-016-2364-3) contains supplementary
material, which is available to authorized users.
* David M. Perrin
dperrin@chem.ubc.ca
1
Department of Chemistry, University of British Columbia,
Vancouver, BC, Canada
1 3

A. Blanc et al.
and Danishefsky 2001; Mikula et al. 2013; Roche et al.
2015). Following saponification of the methyl ester, the
N-protected Hpi dipeptides were compatible with standard
peptide synthesis protocols.
Under acidic conditions, the Hpi (4) reacts with a
cysteine thiol (5) to afford tryptathionine crosslink (indolyl-
2-thiol ether) (6), that is found in amatoxin (7) and phal-
lotoxin (8), two peptide toxins family of enduring interest
(Scheme 2) (Wieland and Faulstich 1991; May and Perrin
2007; Anderl et al. 2012; Moldenhauer et al. 2012; Zhao
et al. 2015). Because stereochemistry is abolished upon
Fig. 1 Structures of anti-cis and syn-cis diastereomers of
3a-hydroxypyrrolo[2,3-b]indoline (Hpi) (May et al. 2005)
Scheme 1 N-1-Tr-3a-hydroxypyrrolo [2,3-b] indolyl amino acid methyl ester (Tr-Hpi-Xaa-OMe) syn-cis (2) and anti-cis (3) synthesis
Scheme 2 Savige-Fontana reaction to afford a tryptathionine bridge
(S-(2-tryptophanyl)cysteine) embedded in the synthetic preparation
of amatoxin (7) and phallotoxin (8). Various derivatives are found
in naturally occurring toxins with R1=NH2 or OH; R2=OH or H;
R3=CH3 or CO2H; R4=CH3 or CH(CH3)2
1 3

Efficient oxidation of N-protected tryptophan and tryptophanyl-dipeptides by in situ…
tryptathionine formation, an unresolved mixture of syn-cis
and anti-cis Hpi diastereomers may be used.
procedure can be found in the supporting info. Both syn-cis
and anti-cis diastereomers were simply collected as a mix-
ture of diastereomers. Once purified by flash chromatogra-
phy, acetic acid was not removable by standard procedure,
and thus an additional work was done as follows. The dry
mixture was dissolved in EtOAc, washed with saturated
KH2PO4(aq) (0.33 volume) and the brine (0.33 volume).
The organic layer was dried over anhydrous MgSO4, fil-
tered and evaporated to dryness under reduced pressure to
yield the pure carboxylic acid Hpi.
Materials and methods
General procedure A: synthesis of Hpi derivatives
A solution of 12 mM Nα-Tr-Trp-Xaa-OMe dipeptide (1
equivalent) was prepared in cyclopentyl methyl ether. Then
two volumes of Milli-Q water and one volume of HPLC
grade acetone were added relative to 1 equivalent volume
of CPME followed by a saturating amount of NaHCO3
(140 equivalents) to keep pH at a value of approximately
8. To the vigorously stirred mixture maintained in an ice/
water bath was added dropwise an aqueous solution of
Oxone® (0.138 M) in three portions of 0.48 equivalents,
0.32 equivalents and 0.16 equivalents at 45 min intervals.
Upon completion of reaction, the mixture was diluted with
one volume of ethyl acetate (EtOAc). The organic layer
was washed with saturated NaHCO3 (aq) (3 × 0.5 vol-
ume) and brine (3 × 0.5 volume). The organic layer was
dried over anhydrous MgSO4, filtered out and evaporated
to dryness under reduced pressure. The crude product was
purified by flash chromatography on silica pretreated with
triethylamine (TEA) using dry loading. A detailed proce-
dure can be found in the supplementary material electronic
file. Both syn-cis and anti-cis diastereomers were collected
unless otherwise specified. All N-1-Tr-Hpi-Xaa-OMe com-
pounds had been previously reported and were briefly char-
acterized by 1H-NMR and HR-ESI–MS and found to be in
accordance with our previous report.(May et al. 2005).
General procedure C: saponification of methyl ester
A 0.09 M solution of N-1-Tr-Hpi-Xaa methyl ester (1
equivalent) in DCM was cooled in ice/water bath and a
solution of 2.6 M NaOH (15 equivalents) in MeOH was
added. The reaction was vigorously stirred and let warm
up to RT for 1 h. The mixture was evaporated to dryness
under reduced pressure. The product was purified by flash
chromatography using dry loading on silica pretreated with
TEA.
General solid‑phase peptide synthesis (SPPS) procedure
Manual solid-phase peptide synthesis was carried out in
desalt Zeba™ spin column (Pierce) with vortex mixing on
2-chlorotrityl chloride (CTC) resin using standard Fmoc
chemistry. Loading procedure: The vacuum-P2O5 dried
resin (0.200 g) was resuspended in a solution of vacuum-
P2O5 dried Nα-Fmoc-Cys(Trt)-OH (4 equivalents, 0.15 M)
in 1:1:3 (v/v/v) dry dimethylformamide (DMF)/dry ace-
tonitrile (MeCN)/dry EtOAc and diisopropylethylamine
(DIPEA) (9 equivalents). The mixture was shaken 5 h and
30 min, then MeOH (300 equivalents) was added and fur-
ther shaken for 1 h. All soluble materials in solvent were
recovered by filtration. The resin was washed with 1:1:3
(v/v/v) dry DMF/dry MeCN/dry EtOAc (10 ml) and then
resuspended in 1:1:3 (v/v/v) dry DMF/dry MeCN/dry
EtOAc (5 mL), shaken, drained by filtration and proce-
dure was repeated six more times with fresh solvent. The
filtration and shaking procedure was then repeated using
1:1 ethanol (EtOH)/EtOAc, dry MeCN, dry EtOAc and
dry DCM. A loading test was performed to give on aver-
age a loading of 0.9 mmol/g. Fmoc deprotection proce-
dure: If the previous step was not done in DMF then the
resin was shaken 3 x 10 min in dry DMF and drained by
filtration each time. The resin (0.200 g) was resuspended in
20% (v/v) piperidine in 0.5 M Oxyma in dry DMF (5 mL),
shaken for 5 min, drained by filtration and procedure was
repeated. The resin was washed with dry DMF (15 ml). The
resin was resuspended in dry DMF (5 mL), shaken, drained
by filtration and procedure was repeated six more times
with fresh solvent. The filtration and shaking procedure
General procedure B: synthesis of Hpi derivatives
A 20 mM aqueous solution of Nα-Boc-tryptophan (1 equiv-
alent) or Nα-Fmoc-tryptophan (1 equivalent) was dissolved
in 22% (v/v) HPLC grade acetone in Milli-Q water and
NaHCO3 (20 equivalents) and retained in an ice/water bath.
To the vigorously stirred mixture was added dropwise an
aqueous solution of Oxone® (40 mM) in three portions of
0.36 equivalents, 0.36 equivalents and 0.30 equivalents at
45 min intervals in ice/water bath where pH 8 all along.
Upon completion of reaction, the mixture was diluted with
saturated aqueous KH2PO4 and 2% (v/v) aqueous H2PO4
until pH 3 was reached. The aqueous layer was extracted
with EtOAc (7 × 0.15 volume) and the organic layers
were pooled. The organic layer was washed with brine
(3 × 0.17 volume), dried over anhydrous MgSO4, filtered
out and evaporated to dryness under reduced pressure.
The crude reaction was subjected to flash chromatography
on silica (acetic acid (AcOH)/dichloromethane (DCM)/
EtOAc/Heptane or Pet Ether) using dry loading. A detailed
1 3

A. Blanc et al.
Scheme 3 Typical Hpi preparation in biphasic solvent system
was repeated using 1:1 (v/v) EtOH/EtOAc, dry MeCN,
dry EtOAc, dry DCM. A Kaiser test was performed.
For peptide coupling, then the resin was further washed
with 1:1:3 (v/v/v) dry DMF/dry MeCN/dry EtOAc. Nα-
Fmoc-protected amino acid coupling procedure: the resin
(0.200 g) was resuspended in solution made of Nα-Fmoc-
protected amino acid (4 equivalents, 0.15 M), Oxyma (4
equivalents), (1-cyano-2-ethoxy-2-oxoethylidenaminooxy)
previous filtrate. The resin was resuspended in 1:1 (v/v)
DCM/toluene (5 mL), shaken and drained by filtration
where the filtrate was pooled with the previous one. The
shaking procedure was repeated six more times with fresh
solvent. The shaking procedure was repeated ten times with
each of the following solvents: DCM, MeCN, and EtOAc
where all filtrates were pooled and evaporated to dryness
under reduced pressure. The crude solid was dissolved in
CPME and 10% (v/v) AcOH in water. The organic layer
was extracted with 10% (v/v) AcOH in water and all aque-
ous layers were pooled. The resulting aqueous mixture was
evaporated to dryness under reduced pressure and manual
reverse phase column chromatography was performed on
Sep-Pak (C18) gel (Waters, Delaware). The tryptathionine
cyclic peptide was further purified by RP-HPLC (C18)
with gradient of 20–100% solvent B over 30 min.
dimethylamino-morpholino-carbenium
hexafluorophos-
phate (COMU) (4 equivalents), DIPEA (12 equivalents)
in 1:1:3 (v/v/v) dry DMF/dry MeCN/dry EtOAc. The resin
was shaken for a maximum of 1 h and a half. Upon com-
pletion of the reaction estimated by Kaiser test, the resin
was drained by filtration. The resin was filtered with 1:1:3
(v/v/v) dry DMF/dry MeCN/dry EtOAc (15 ml). The resin
was resuspended in 1:1:3 (v/v/v) dry DMF/dry MeCN/dry
EtOAc (5 mL), shaken, drained by filtration and procedure
was repeated six more times with fresh solvent. The filtra-
tion and shaking procedure was repeated using dry MeCN,
dry EtOAc, dry DCM. A Kaiser test and loading test were
performed. If the following step was Nα-Fmoc deprotection,
then the resin was further washed with dry DMF. Resin
capping procedure: following each coupling step, a capping
step was performed as follows. The resin (0.200 g) was
resuspended in 1:2:2 (v/v/v) Ac2O/TMP/dry EtOAc (5 mL)
and shaken for 1 h, then drained by filtration. The resin was
washed as done for Fmoc-protected amino acid coupling
procedure. Hpi coupling procedure: the resin (0.200 g) was
resuspended in solution made of Hpi derivative (2 equiva-
lents, 0.07 M), Oxyma (2 equivalents), COMU (2 equiva-
lents), DIPEA (6 equivalents) in 1:1:3 (v/v/v) dry DMF/dry
MeCN/dry EtOAc. The resin was shaken overnight in dark
and drained by filtration. The resin was washed as done for
Nα-Fmoc-protected amino acid coupling procedure.
Results and discussion
Production of DMDO typically requires careful distillation,
but only recently has a method been developed to improve
large scale procedure for DMDO preparation and stor-
age (Mikula et al. 2013). However, in our hands, we have
observed a need for periodic titration to assess a continu-
ously waning concentration and rapid decrease in quality
upon storage when used to oxidize tryptophan derivatives.
Such considerations led us to investigate a simpler method
for the preparation of Hpi-containing amino acids and
dipeptides. Inspired by previous work in other laboratories,
here we report the use of a simple admixture of Oxone in
acetone in a biphasic reaction set-up which obviates the
need for DMDO distillation (Scheme 3) (Hashimoto and
Kanda 2002; Cheshev et al. 2006; Hussain et al. 2013; Zhu
et al. 2015).
General Savige‑Fontana tryptathionylation reaction
procedure from solid‑phase
Initially, a cyclopentyl methyl ether (CPME) solution of
12 mM of Nα-Tr-Trp-Xaa-OMe was diluted in an admixture
of 1:1:2 (v/v/v) acetone/saturated aqueous sodium bicarbo-
nate/water (pH 8.5). CPME was chosen based on its resist-
ance to peroxide oxidation (Mouret et al. 2014). We found
that a biphasic solution provided better results than the
use of a homogeneous solution such as water/MeCN and
The dry resin (0.200 g) was resuspended in TFA (5 mL)
and shaken for 4 h. The supernatant was collected by fil-
tration. The resin was drained by filtration with 1:1 (v/v)
DCM/toluene (20 mL) and the filtrate was pooled with the
1 3

Efficient oxidation of N-protected tryptophan and tryptophanyl-dipeptides by in situ…
Table 1 N-1-Tr-Hpi-Xaa-OMe (a: syn-cis; b: anti-cis) dipeptide
yield from in situ DMDO oxidation
having a significant excess of acetone and bicarbonate was
more critical than their actual concentrations. More impor-
tant was the dropwise addition of aqueous Oxone solution
in batches of 0.33 equivalents at 0–4 °C and pH >8. Fol-
lowing each addition, the reaction was deliberately stalled
for 30–45 min; we found that addition of the Oxone all
at once generated additional oxidized by-products, which
were difficult to remove. Notably, without acetone, no Hpi
is produced, suggesting that Oxone reacts with acetone to
form DMDO in situ at a steady state rate. The reaction was
retained at 0–4 °C to suppress oxindole formation along
with other oxidized by-products, which proved difficult
to separate from the desired Hpi products. In some cases,
we deliberately added less than one equivalent of Oxone
to avoid the formation of these difficult to separate oxida-
tion products; while in such cases, the reaction did not go
to completion; the desired Hpi compound could be cleanly
separated from starting materials. Interestingly, tetrabutyl-
ammonium hydrogensulfate (TBAHS), added as a phase
transfer catalyst, failed to improve either the yield or the
apparent reaction rate (Lafont et al. 2011).
No.
Name
Yield (%)
10a:b
Tr-Hpi-Tyr(t-Bu)-OMe
Tr-Hpi-Phe-OMe
Tr-Hpi-Ala-OMe
Tr-Hpi-Leu-OMe
Tr-Hpi-Val-OMe
Tr-Hpi-Ile-OMe
30:30
29:21
69a
12a:b
13a + b
14a:b
54:27
83a
68a
15a + b
16a + b
17a
Tr-Hpi-Pro-OMe
Tr-Hpi-Gly-OMe
Tr-Hpi-Thr(t-Bu)-OMe
29
56a
18a + b
19a:b
26:21
a
Purified as a diastereomeric mixture
The reaction proved effective on a variety of dipeptide
substrates, yielding an expected mixture of diastereoiso-
mers that slightly favored the syn-cis configuration, and
in overall yields comparable to those that we previously
reported when using freshly distilled DMDO (Table 1;
Supplementary information). Consistent with previous
results, we observed highly selective oxidative cyclization
of Nα-Tr-Trp-Pro-OMe to the syn-cis diastereomer (17a)
(Zhao et al. 2012). Overall, the oxidation method was suffi-
ciently clean that the crude Hpi-dipeptide could be directly
saponified to the corresponding carboxylate. For example,
following basic work-up, the crude diastereomeric mixture
N-1-Tr-Hpi-Tyr(t-Bu)-OMe (10) was saponified in hydrox-
ide methanol (MeOH)/DCM solution to N-1-Tr-Hpi-Tyr(t-
Bu)-OH (11) in 34% yield over two steps as a diastere-
omeric mixture (Theodorou et al. 2007). Using the same
saponification method, but from a diastereomeric mixture
(10) purified by flash chromatography, N-1-Tr-Hpi-Tyr(t-
Bu)-OH (11) was prepared in 73% yield.
Gratified with the facility of this approach to indole
oxidation within a dipeptide sequence, we extended this
method to afford an Hpi synthon directly from mono-
meric tryptophan without protection of the COOH; such a
monomer may find general use in peptide coupling, and in
particular, for the synthesis of dipeptides such as Hpi-Trp
and Hpi-Met, whose side chains are readily oxidized by
DMDO (Savige and Fontana 1980). Thus, with bicarbonate
buffered aqueous solution of Nα-Boc-Trp-OH and acetone,
a dropwise addition of Oxone gave N-1-Boc-Hpi-OH (20)
in 32% yield as a diastereomeric mixture (Scheme 4).
Accordingly, N-1-Fmoc-Hpi-OH (21) and N-1-Fmoc-d-
Hpi-OH (21′) were prepared in 46% and 33% yield from
Scheme 4 Synthesis of N-1-Boc-Hpi-OH (20) in 32% yield,
N-1-Fmoc-Hpi-OH (21) in 46% yield and N-1-Fmoc-d-Hpi-OH (21′)
in 33% yield
Nα-Fmoc-Trp-OH and Nα-Fmoc-d-Trp-OH, respectively.
Although yields were low, this method allowed the oxida-
tion of over a gram of Nα-protected tryptophan with a free
carboxylic acid group. Herein, the method led to the slight
preference for the syn-cis diastereomer over the anti-cis
in most Nα-protected tryptophanyl-dipeptides and tryp-
tophans, an observation that is consistent with our previ-
ous reports on the modest diastereoselectivity when using
DMDO in dichloromethane.
With an interest in forming tryptathionine crosslinks and
to show by chemical correlation that this method afforded
tryptathionine linked peptides, we prepared a diastere-
omeric mixture of N-1-Boc-Hpi-Tyr(t-Bu)-Cys(Tr) (24a)
and N-1-Fmoc- Hpi-Tyr(t-Bu)-Cys(Tr) (24b) by standard
SPPS on a chlorotrityl resin (Scheme 5).
Using COMU and Oxyma in a high proportion of ethyl
acetate (60% in EtOAc), all peptide coupling reactions pro-
ceeded cleanly (MacMillan et al. 2013). Acetonitrile and
DMF were added to improve solubility of the reagents.
Oxyma was added to the piperidine solution during Fmoc
deprotection to lower the risk of base-mediated side reac-
tions.(Subiros-Funosas et al. 2012) An important applica-
tion of N-1-Fmoc-Hpi was the ability to incorporate it into
a peptide sequence by standard SPPS, followed by Fmoc
deprotection with piperidine and then subsequent peptide
1 3

A. Blanc et al.
Scheme 5 SPPS of tryptathionine peptide precursors and Sav-
ige-Fontana cyclization. Reagents and conditions:
Nα-Fmoc-
Cys(Tr)-OH, DIPEA, 1:1:3 MeCN/DMF/EtOAc, overnight; b 20%
piperidine in 0.5 M oxyma in DMF, 1 × 5 min, 1 × 10 min; c Nα-
Fmoc-Tyr(t-Bu)-OH, COMU, Oxyma, DIPEA, 1:1:3 MeCN/DMF/
EtOAc, 1 h 30 min; d repeated step b; e N-1-Boc-Hpi-OH (20) or
N-1-Fmoc-Hpi-OH (21), COMU, Oxyma, DIPEA, 1:1:3 MeCN/
DMF/EtOAc, overnight; f TFA, 4 h, after purification yield 25; g
repeated step b; h Nα-Fmoc-d-Phe-OH, COMU, Oxyma, DIPEA,
1:1:3 MeCN/DMF/EtOAc, 1 h 30 min; i repeated step b; j TFA, 4 h,
after purification yield 27
a
coupling as exemplified by tetrapeptide (26). Fmoc-d-Phe
was chosen over other possible amino acids because the
resulting tetrapeptide (26) was of interest in the develop-
ment of additional peptide synthesis, currently ongoing in
our laboratory. In all cases, tryptathionylation proceeded
smoothly, with concomitant acid-mediated deprotection of
side chains and the liberation of the peptide from the resin
which corroborated previous results that an acylated Hpi
can also undergo tryptathionylation. (May and Perrin 2008)
Consequently, the procedure yielded H-(Trp-Tyr-Cys)c-OH
(25) and H-D-Phe-(Trp-Tyr-Cys)c-OH (27) in good purity
following Sep-Pak (C18) manual purification but were fur-
ther purified by RP-HPLC (C18). The 16% yield of cyclic
tripeptide (25) and 22% yield of cyclic tetrapeptide (27)
were similar to previous reports on cyclic tryptathionine
(May et al. 2005).
1 3

Efficient oxidation of N-protected tryptophan and tryptophanyl-dipeptides by in situ…
Hashimoto N, Kanda A (2002) Practical and environmentally friendly
Conclusion
epoxidation of olefins using oxone. Org Proc Res Dev 6(4):405–
406. doi:10.1021/op025511f
In summary, herein we report a simplified method for Hpi
preparation that uses cheap, commercially available, and
environmental friendly reagents. Oxone by-products are
easily neutralized into nontoxic sulfate, while DMDO is
presumably produced in situ at a steady state rate. Salient
points herein are that this method: (a) obviates the poten-
tially hazardous, tedious, and often low yielding distil-
lation of DMDO; (b) is amenable to scale up; (c) avoids
COOH protection, albeit with loss of diastereoselectivity;
and (d) in the case of water-soluble tryptophan derivatives,
such as Nα-Boc/Fmoc-Trp-OH, might be incorporated
into a flow-based peptide synthesis system. As shown in
scheme 5, the facile synthesis of monomeric N-1-Boc and
N-1-Fmoc-Hpi-OH, provides a suitably protected mono-
meric Hpi that can be treated as a standard amino acid for
coupling, which in turn allows access to Hpi-containing
peptides that can otherwise contain oxidant-sensitive side
chains (e.g., indoles, thioether). Evidence of this asser-
tion is provided by the preparation of resin-bound peptide
23, which contains a thioether, i.e., the tritylated cysteine.
Hence, we suggest that this method merits consideration in
procedures where DMDO or other oxidants, would be oth-
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Complaince with ethical standards
Conflict of interest The authors declare that they have no conflicts
of interest.
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