High yielding synthesis of 3a-hydroxypyrrolo[2,3-6]indoline dipeptide methyl esters: Synthons for expedient introduction of the hydroxypyrroloindoline moiety into larger peptide-based natural products and for the creation of tryptathionine bridges

Article abstract of DOI:10.1021/jo051105t

This work describes a rapid and high yielding oxidation of 14 tryptophanylated amino acid methyl esters to the corresponding 3a-hydroxypyrrolo[2,3-6]indoline (Hpi) amino acids with generally facile separation of syn-cis and anti-cis diastereomers. Structu

Full text of DOI:10.1021/jo051105t

High Yielding Synthesis of 3a-Hydroxypyrrolo[2,3-b]indoline
Dipeptide Methyl Esters: Synthons for Expedient Introduction of
the Hydroxypyrroloindoline Moiety into Larger Peptide-Based
Natural Products and for the Creation of Tryptathionine Bridges.
†
†,‡
†
†
Jonathan P. May, Pierre Fournier, Jonathan Pellicelli, Brian O. Patrick, and
David M. Perrin*,
†
Department of Chemistry, University of British Columbia, Vancouver, B.C. V6T 1Z1, Canada
dperrin@chem.ubc.ca
Received June 1, 2005
This work describes a rapid and high yielding oxidation of 14 tryptophanylated amino acid methyl
esters to the corresponding 3a-hydroxypyrrolo[2,3-b]indoline (Hpi) amino acids with generally facile
separation of syn-cis and anti-cis diastereomers. Structural X-ray diffraction data are presented
for both diastereomers of Tr-Hpi-Gly-OMe, which allow for a putative assignment of the other 13
1
pairs of diastereomers reported herein, based on correlations with H NMR chemical shifts. Selective
and high yielding deprotection at either the N or C terminus is described, allowing the Hpi motif
to be introduced efficiently into potential targets with minimal protecting group manipulation.
Two tripeptides containing Hpi and cysteine were prepared and treated with acid in the Savige-
Fontana reaction to produce a cyclic tryptathionine linkage, characteristic of both amatoxins and
phallotoxins.
Introduction
The 3a-hydroxypyrrolo[2,3-b]indoline (Hpi) structural
motif is of pharmaceutical interest, as it is found in a1
variety of natural products that include the sporidesmins,
2
3,4
5
brevianamides, okaramines, phakellistatins, gypse-
6
7-9
tins, and himastatins (Figure 1).
It is also closely
related to several other pharmacophores by replacement
FIGURE 1. The structure of the syn-cis and anti-cis diaster-
eomers of the 3a-hydroxypyrrolo[2,3-b]indoline (Hpi) core.
*
Address correspondence to this author. Fax: +1 604 822 2847.
Phone: +1 604 822 0567.
†
University of British Columbia.
Present address: Institute of Pharmacy and Molecular Biotech-
‡
10,11
of the 3a-hydroxyl with either a proton
or an alkyl
nology, University of Heidelberg, Im Neuenheimer Feld 364, 69120
Heidelberg, Germany
12,13
14
group (e.g., roquefortines,
flustramines, and amau-
15
(
1) Francis, E.; Rahman, R.; Safe, S.; Taylor, A. J. Chem. Soc., Perkin
romines ), or by insertion of an oxygen atom into the
Trans. 1 1972, 470-472.
16
indole C2-N1 bond, as in paeciloxazine. Accordingly,
(
2) Fukui, Y.; Somei, M. Heterocycles 2001, 55, 2055-2057.
(
3) Roe, J. M.; Webster, R. A. B.; Ganesan, A. Org. Lett. 2003, 5,
2
2
2
825-2827.
(8) Kamenecka, T. M.; Danishefsky, S. J. Angew. Chem., Int. Ed.
1998, 37, 2995-2998.
(
4) Hewitt, P. R.; Cleator, E.; Ley, S. V. Org. Biomol. Chem. 2004,
, 2415-2417.
5) Greenman, K. L.; Hach, D. M.; Van Vranken, D. L. Org. Lett.
004, 6, 1713-1716.
6) Schkeryantz, J. M.; Woo, J. C. G.; Siliphaivanh, P.; Depew, K.
M.; Danishefsky, S. J. J. Am. Chem. Soc. 1999, 121, 11964-11975.
7) Kamenecka, T. M.; Danishefsky, S. J. Angew. Chem., Int. Ed.
998, 37, 2993-2995.
(9) Kamenecka, T. M.; Danishefsky, S. J. Chem. Eur. J. 2001, 7,
41-63.
(
(10) Crich, D.; Bruncko, M.; Natarajan, S.; Teo, B. K.; Tocher, D. A.
Tetrahedron 1995, 51, 2215-2228.
(
(11) Crich, D.; Huang, X. J. Org. Chem. 1999, 64, 7218-7223.
(12) Chen, W.-C.; Joullie, M. M. Tetrahedron Lett. 1998, 39, 8401-
8404.
(
1
10.1021/jo051105t CCC: $30.25 © 2005 American Chemical Society
8424
J. Org. Chem. 2005, 70, 8424-8430
Published on Web 09/15/2005

3a-Hydroxypyrrolo[2,3-b]indoline Dipeptide Methyl Esters
there has been considerable interest in developing syn-
thetic methods that permit facile and high yielding
introduction of this motif into these structures and
related analogues (vide infra).
17
18
Shioiri and Christophersen used the photosensitizer
Rose Bengal in the presence of molecular oxygen to obtain
Hpi methyl esters and dipeptides, albeit with rather low
yields. In the synthesis of phakellistatin 3, Van Vranken
and co-workers used this method to induce Hpi formation
in a prefabricated peptide, but again they experienced
both low yields and difficult separation of their final
products. The most direct route to 3a-hydroxypyrroloin-
doles is by a one-pot oxidation of the indole 2-3 double
bond of tryptophan and intramolecular attack of the
FIGURE 2. General structures of amatoxins (left) and phal-
lotoxins (right). R is either an OH or an NH , R and R
represent hydroxylated and oxidized variants found in natu-
rally occurring toxins and synthetic analogues.
1
2
2
3
5
2
R-NH at the indole 2-position. In the past this was
19
achieved by using mCPBA, but the yields were poor,
several byproducts were produced, and purification proved
troublesome.
Danishefsky and co-workers described a more success-
9
ful approach en route to himastatin. In their report,
tryptophan, when suitably protected, was oxidized using
dimethyldioxirane (DMDO). Numerous combinations of
protecting groups were investigated with regards to
achieving high yields and diastereomeric excess, but
oxidation was most successful with tryptophan protected
FIGURE 3. The acid-catalyzed Savige-Fontana reaction
affording a tryptathionine linkage.
at the R-NH
2
and R-COOH by a trityl and a tert-butyl
ester, respectively. With proper protection, DMDO oxida-
tion produced exclusively a syn-cis stereochemistry in the
resultant Hpi. Acid deprotection of the trityl moiety
afforded the Hpi-tert-butyl ester over two steps (55% total
yield). In their synthesis, selective deprotection of the tert-
butyl ester required several more steps to avoid destruc-
tion of the acid-labile Hpi moiety. These included tran-
5
10 products was obtained. In summary, the generation
of Hpi often has required a rather elaborate and exten-
sive use of protecting groups.
Whereas considerable effort has been devoted to meth-
ods for preparing and manipulating the Hpi core en route
to the synthesis of the aforementioned highly bioactive
compounds, it is worth noting that the Hpi core also
sient silylation of the 3a-OH and reprotection of N
b
following detritylation, ultimately enabling incorporation
of Hpi into himastatin.
Most recently, Ley and co-workers described the el-
egant synthesis of 3a-hydroxypyrroloindoles via oxidative
deselenation of the corresponding 3a-phenylselenopyr-
roloindole. This was prepared by oxidative phenylselena-
provides an entr e´ e into two other classes of highly potent
natural products: the amatoxins2
1,22
and phallotoxins
(Figure 2). These are chemically similar bicyclic octa- and
heptapeptides that bind extremely tightly to unrelated
protein folds of RNA polymerase and actin, respec-
tively.2
3,24
A significant degree of this extraordinary
tion from fully protected tryptophan (N
OMe) with concomitant cyclization to the pyrroloindole.
R
-Z, Nindole-Boc, C2
R
-
potency is derived from a pre-ordered rigidity imparted
by the characteristic tryptathionine cross-link.
0
Yields and stereocontrol depended on the N-protecting
groups employed. Oxidative deselenation with mCPBA
subsequently afforded several different N-protected Hpi
methyl ester products (over 4-6 steps depending on the
extent of protection).
Van Vranken and co-workers attempted a similar
oxidative phenylselenation of tryptophan contained within
a heptapeptide as a means of directly generating the Hpi
motif in phakellistatin-like structures, but a mixture of
This linkage may be created under acidic conditions
by intramolecular condensation of a thiol (e.g., Cys) with
the Hpi motif via the Savige-Fontana reaction (Figure
19,25
3).
Interest in such peptides is underscored in a recent
publication from the Guy lab that described a synthesis
of phallotoxins. Instead of employing the Savige-Fon-
tana condensation following introduction of the Hpi core,
they used Wieland’s electrophilic condensation of tryp-
tophan with cysteine sulfenyl chloride to create the
desired tryptathionine. This necessitated four-way or-
thogonal protection for directional introduction in peptide
synthesis.24 Previously, Zanotti and co-workers have
discussed the solution phase peptide synthesis of aman-
(
13) Schiavi, B. M.; Richard, D. J.; Joullie, M. M. J. Org. Chem. 2002,
7, 620-624.
14) Bruncko, M.; Crich, D.; Samy, R. J. Org. Chem. 1994, 59, 5543-
549.
15) Marsden, S. P.; Depew, K. M.; Danishefsky, S. J. J. Am. Chem.
6
(
5
(
Soc. 1994, 116, 11143-11144.
16) Schwaebisch, D.; Tchabanenko, K.; Adlington, R. M.; Cowley,
A. M.; Baldwin, J. E. Chem. Commun. 2004, 2552-2553.
17) Sakai, A.; Tani, H.; Aoyama, T.; Shioiri, T. Synlett 1998, 257-
58.
18) Anthoni, U.; Christophersen, C.; Nielsen, P. H.; Christoffersen,
(21) Wieland, T.; Faulstich, H. Experientia 1991, 47, 1186-1193.
(22) Schmitt, W.; Zanotti, G.; Wieland, T.; Kessler, H. J. Am. Chem.
Soc. 1996, 118, 4380-4387.
(
(
(23) Zanotti, G.; Falcigno, L.; Saviano, M.; D’Auria, G.; Bruno, B.
M.; Campanile, T.; Paolillo, L. Chem. Eur. J. 2001, 7, 1479-1485.
(24) Anderson, M. O.; Shelat, A. A.; Guy, R. K. J. Org. Chem. 2005,
70, 4578-4584.
2
(
M. W.; Sorensen, D. Acta Chem. Scand. 1998, 52, 958-960.
(
19) Savige, W. E. Aust. J. Chem. 1975, 28, 2275-2287.
(25) Savige, W. E.; Fontana, A. J. Chem. Soc. Chem. Commun. 1976,
600-601. Savige, W. E.; Fontana, A. Int. J. Peptide Protein Res. 1980,
15, 102-112.
(
20) Ley, S. V.; Cleator, E.; Hewitt, P. R. Org. Biomol. Chem. 2003,
1
, 3492-3494.
J. Org. Chem, Vol. 70, No. 21, 2005 8425

May et al.
SCHEME 1a
itin and phalloidin derivatives by means of Savige-
Fontana chemistry.2
2,26
Our interest in these bicyclic peptide structures,
particularly with regards to a combinatorial array of
such, led us to consider the synthesis of Hpi in the context
of functionalized peptide structures and its ultimate use
in the Savige-Fontana condensation. Herein we report
a robust synthesis of 3a-hydroxypyrrolo[2,3-b]indoline
derivatives prepared from Tr-Trp-Xaa-OMe dipeptides,
by oxidation with DMDO. This reaction was shown to be
compatible with dipeptides where Xaa could be any one
of a number of amino acids, which were cleanly oxidized
to the desired tricyclic products in high yield. Charac-
terization of syn-cis and anti-cis diastereomers for each
product was possible with X-ray crystallography and
NMR. Efficient deprotection and subsequent coupling at
both the N and C termini provides a reliable method for
introducing the Hpi moiety into various pharmaceutical
targets, either in solution phase or via linear solid-phase
synthesis. Finally, Savige-Fontana cyclizations were
performed on Tr-Hpi-Xaa-Cys(Tr)-OMe tripeptides to
yield a cyclic tryptathionine, demonstrating the use of a
tryptathionine bridge as a general rigidification strat-
a
Reagents and conditions: (i) DMDO/acetone, DCM, -78 °C,
t
∼
1 h. Where Xaa ) Ala, Asn(Tr), Glu( Bu), His(Tr), Ile, Leu,
Lys(Boc), Phe, Pro, Ser( Bu), Thr( Bu), Tyr( Bu), and Val.
t
t
t
2
7
egy. In contrast to the aforementioned investigations,
our method avoids the extensive protecting group ma-
nipulations that are needed to install either an Hpi or a
tryptathionine linkage.
Results and Discussion
Based on the results of Danishefsky’s work6-9 we opted
for DMDO oxidation to prepare the Hpi moiety from Tr-
t
Trp-O Bu because it was found to provide the highest
chemical yields. Nevertheless, to render the Hpi moiety
compatible with standard peptide synthesis protocols,
deprotection of the trityl and tert-butyl groups would be
required. We were not inclined to reprotect the detrity-
FIGURE 4. Crystal structures of the Tr-Hpi-Gly-OMe 1a syn-
cis (left) and 1b anti-cis (right) diastereomers. Crystals were
grown from EtOAc/hexanes.
b
lated N of the pyrroloindoline in order to cleanly reveal
glycinamide NH might have competed with the more
sterically hindered NH-trityl to give a six-membered ring.
In such a scenario, the two observed products would have
been constitutional isomers rather than diastereomers.
X-ray diffraction data showed that only diastereomeric
Hpi products were observed. Thus for the reaction herein,
the nucleophilicity of an NH-trityl predominates over its
considerable steric bulk.
In the case of Tr-Trp-Gly-OMe the oxidation proceeded
in good yield, but with poor diastereoselectivity, giving
both syn-cis (1a) and anti-cis (1b) diastereomers. Al-
though separation of these diastereomers by silica gel
the R-COOH. In accord with Kamenecka’s extensive
9
study of different protecting groups, it was apparent that
b
several additional steps (i.e., reprotection of N and
transient silylation of the 3a-OH) would be required to
afford a properly protected Hpi synthon suitable for
peptide synthesis.
With the goal being to introduce the Hpi into a linear
peptide fragment, ultimately for the creation of amatoxin
and phallotoxin derivatives, we examined whether a
tryptophanylglycine methyl ester (Tr-Trp-Gly-OMe) would
cleanly undergo DMDO oxidation to afford the corre-
sponding Tr-Hpi-Gly-OMe, thus obviating the need for
R-COOH protection of tryptophan. Since glycine is found
at the adjacent position in amanitin, DMDO oxidation
of the dipeptide would presumably conform to a linear
peptide synthesis strategy. As anticipated, DMDO treat-
ment of Tr-Trp-Gly-OMe at low temperature (-78 °C)
afforded clean conversion to Tr-Hpi-Gly-OMe in 70% yield
for the two diastereomers 1a and 1b (Scheme 1).
chromatography proved rather difficult (∆R < 0.05,
f
EtOAc/hexane 1:1), pure samples of each were obtained
and crystal structures were acquired (Figure 4). Both
diastereomers could be used without resolution for the
Savige-Fontana reaction because the stereochemistry is
abolished during the synthesis of a tryptathionine link-
age. Nevertheless, in light of the high yields we obtained
along with feasible separation, we characterized these
diastereomers further, given the general interest in
preparing various Hpi-containing targets.
R
Given precedence for diketopiperazines and N -Boc-
Trp to undergo cyclization with attack of the Boc-NH to
6
form the pyrrolidine ring, we were concerned that the
These structures show the contrasting conformations
of each diastereomer, both 1a and 1b have cis geometry
about the C3a-C8a fused bond, but the orientation of the
indole moiety appears to be considerably more sterically
congested in the anti-cis conformation. In the syn-cis
(
26) Falcigno, L.; Costantini, S.; D’Auria, G.; Bruno, B. M.; Zobeley,
S.; Zanotti, G.; Paolillo, L. Chem. Eur. J. 2001, 7, 4665-4673.
27) Zanotti, G.; Beijer, B.; Wieland, T. Int. J. Peptide Protein Res.
987, 30, 323-329.
(
1
8426 J. Org. Chem., Vol. 70, No. 21, 2005

3a-Hydroxypyrrolo[2,3-b]indoline Dipeptide Methyl Esters
TABLE 1. Tr-Hpi-Xaa-OMe Dipeptidesa
large groups at the C-terminus of Hpi will favor the less
constrained syn-cis diastereomer (see the Supporting
Information for this structure). In this case, the chemical
no.
name
yield (%)
H8a
NHCO
1
a
Tr-Hpi-Gly-OMe
35
35
28
25
59
38
20
25
60
0
29
61
40
30
35
40
11
32
84
0
42
40
30
45
26
21
48
44
5.54
5.36
5.52
5.39
5.54
5.35
5.53
5.40
5.42
-
5.64
5.30
5.60
5.35
5.55
5.43
5.66
5.31
5.49
-
5.68
5.28
5.68
5.43
4.92
5.29
5.62
5.33
5.95
9.21
5.64
9.12
6.65
9.51
6.09
9.18
9.82
-
5.74
8.92
5.60
9.00
6.11
9.12
5.62
9.11
N/A
-
6.04
9.12
5.95
9.09
5.45
7.91
5.85
8.97
shifts and R
f
values were consistent with the observed
1
2
2
3
3
4
4
5
5
6
6
7
7
8
8
9
9
1
1
1
1
1
1
1
1
1
1
b
a
Tr-Hpi-Ala-OMe
trend that guided the assignment. The single diastere-
omer of Tr-Hpi-His(Tr)-OMe (5a) formed was also thought
to have syn-cis geometry, but this cannot be undeniably
confirmed without X-ray crystallography data. Possible
ways to control the stereospecificity of this reaction were
briefly considered, but attempts at using chiral equiva-
b
a
Tr-Hpi-Asn(Tr)-OMe
Tr-Hpi-Glu(tBu)-OMe
Tr-Hpi-His(Tr)-OMe
Tr-Hpi-Ile-OMe
b
a
b
a
b
lents of DMDO, such as those described by Shi et al., were
a
28,29
unsuccessful (data not shown).
Further studies were
b
not pursued because both diastereomers were equally
valuable for the Savige-Fontana reaction.
a
Tr-Hpi-Leu-OMe
b
a
Tr-Hpi-Lys(Boc)-OMe
Tr-Hpi-Phe-OMe
Not only did we wish for a high yielding synthesis of
the Hpi moiety, but we also sought an efficient incorpora-
tion of this motif into linear peptides with a view to
coupling the Hpi-Xaa dipeptide at either terminus. The
protecting groups we chose allow for high yielding and
orthogonal deprotection of the Tr-Hpi-Xaa-OMe dipep-
tide. Efficient deprotection of the trityl moiety with HFIP
b
a
b
0a
0b
1a
1b
2a
2b
3a
3b
4a
4b
Tr-Hpi-Pro-OMe
Tr-Hpi-Ser(tBu)-OMe
Tr-Hpi-Thr(tBu)-OMe
Tr-Hpi-Tyr(tBu)-OMe
Tr-Hpi-Val-OMe
(
hexafluoro-2-propanol) in DCM (1:4) for 15 min (Scheme
2) yielded the diastereomers 15a (40%) and 15b (30%),
which were easily separated by silica gel chromatogra-
phy. Unfortunately, the yield was a little lower than what
is normally expected for a deprotection due to the
inherent acid sensitivity of the Hpi moiety. This mild
deprotection removed the trityl protection on the Hpi
selectively over other acid labile groups, even when other
trityl protected amino acid side chains are present in the
molecule (e.g., Cys(Tr) and Asn(Tr), data not shown).
These H-Hpi-Xaa-OMe dipeptides could then be extended
a
Isolated yields of cyclization and characteristic 1H chemical
shifts for the syn-cis (a) and anti-cis (b) diastereomers.
conformation there is a hydrogen bond between the 3a-
hydroxyl and the carbonyl oxygen of the amide bond,
whereas the orientation of the anti-cis amide is twisted
through 180° away from the pyrroloindole core. These
dramatic differences may in part account for the apparent
b
by coupling amino acids at N . Although similar couplings
20
have been reported to be problematic, coupling of Fmoc-
Gly-OH with PyBOP yielded a clean tripeptide product
1
trends of the amide NH shift in the H NMR (vide infra).
We also investigated the generality of this reaction by
examining a variety of tryptophanylated amino acids.
Appropriately protected dipeptides (Tr-Trp-Xaa-OMe)
were synthesized and each was exposed to the DMDO
reaction conditions described. We observed good conver-
sion for all dipeptides studied (compounds 2-14, 40-99%
for two diastereomers). For the majority of the dipeptides,
a mixture of diastereomers was observed, but a clear
preference for a single diastereomer was found in two
1
6 in good yield from 15a. Nevertheless, the amide bond
introduced at this position displayed strong cis-trans
1
atropisomerism, and hence H NMR spectra in CDCl
displayed severe line-broadening, consistent with similar
structures previously reported by Ley et al.
3
20
Saponification of the C-terminal methyl ester is rapid
and efficient with minimal decomposition (Scheme 3);
compounds 1a/b and 12a were saponified with LiOH in
quantitative yield to give the free acids, which were
purified as triethylammonium salts (17a/b and 18a).
Coupling of S-trityl cysteine methyl ester to the C-
terminus of 17a/b and 18a with DCC/HOBt afforded the
τ
instances when Xaa ) Pro or His(N Tr) (Table 1).
Separation of all diastereomers was considerably easier
than previously with 1a/1b, and all products were well
resolved during purification. Although this approach
appears to be fairly general, we would assume DMDO
oxidation is incompatible in cases where Xaa ) Trp, Cys-
two Tr-Hpi-Xaa-Cys(Tr)-OMe tripeptides with Xaa ) Gly
t
(
conformationally flexible) and Thr( Bu) (sterically en-
cumbered) as the variable amino acid (19a/b and 20).
This demonstrated the feasibility for functionalizing the
C-terminus of the Tr-Hpi-Xaa-OH, while also forming a
precursor for the Savige-Fontana cyclization. Subse-
quent treatment with TFA at room temperature removed
trityl protecting groups and produced the corresponding
cyclic products (21, 22), both of which displayed a λmax of
(Tr), and Met, as these side chains are readily oxidized
by DMDO. Not investigated in this study were three
other dipeptides where Xaa ) Arg, Asp, and Gln, due to
acknowledged difficulty with the solubility of the former,
and a generally recognized functional group redundancy
of the two latter amino acids as Glu (4) and Asn (3) were
investigated.
2
92 nm characteristic of the tryptathionine linkage. The
Assignment of each dipeptide as either the syn-cis or
yields for these cyclic structures were low (16-54%), but
are similar to previous reports on cyclic tryptathionine
the anti-cis diastereomer was possible by comparison of
1
the R
f
, H NMR chemical shift, and crystal structures of
the Tr-Hpi-Gly-OMe dipeptides. In the case of Tr-Trp-
Pro-OMe the syn-cis diastereomer was given exclusively
(
28) Tu, Y.; Wang, Z.-X.; Shi, Y. J. Am. Chem. Soc. 1996, 118, 9806-
807.
29) Wang, Z.-X.; Tu, Y.; Frohn, M.; Zhang, J.-R.; Shi, Y. J. Am.
Chem. Soc. 1997, 119, 11224-11235.
9
(
10a). Some indication of steric congestion was found in
(
the crystal structures of this dipeptide, suggesting that
J. Org. Chem, Vol. 70, No. 21, 2005 8427

May et al.
SCHEME 2a
Direct oxidation of the Trp-Xaa dipeptide obviates mul-
tiple protections and deprotections needed to afford an
Hpi. In just three steps from unprotected tryptophan,
good to excellent yields of both the syn-cis and anti-cis
diastereomers of Hpi can be isolated in pure form.
Although stereocontrol was not investigated, protecting
group use is minimal, yields for each Hpi-Xaa diastere-
omer are relatively high, and products are easily sepa-
rated. Thus, the approach herein represents a simple
synthesis of Hpi synthons suitable for incorporation into
larger peptides or natural products. Efficient, orthogonal
deprotections are described for both the N- and C-termini
of the Hpi containing dipeptide. Subsequent peptide
coupling of these Hpi-Xaa dipeptides is described for both
N- and C-termini, yielding protected tripeptides. Two Tr-
Hpi-Xaa-Cys(Tr)-OMe tripeptides were also prepared and
Savige-Fontana cyclizations were performed to yield
cyclic tryptathionine containing structures. The apparent
generality of the Savige-Fontana reaction would suggest
utility for this biomimetic rigidification strategy in the
generation of a combinatorial array of bicyclic peptides,
inspired by two of Nature’s extremely potent toxins:
amanitin and phalloidin.
a
Reagents and conditions: (i) HFIP/CH2Cl2 (1:4), rt, 30 min
(
15a: 40%, plus other diastereomer; 15b: 30%); (ii) Fmoc-Gly-
OH, PyBOP, HOBt, DIPEA, DMF, rt, 4 h (44%).
Experimental Section
SCHEME 3a
General Method for Preparation of Hpi Derivatives.
To a solution of the appropriate side chain protected Tr-Trp-
Xaa-OMe (0.10 mmol) in dry CH Cl (1 mL) at -78 °C was
2 2
31
added a solution of DMDO in acetone (0.11 mmol, 1.1 equiv).
After 15 min the mixture was concentrated to dryness under
reduced pressure at room temperature. The crude material
was purified by column chromatography on pretreated (NEt
3
)
silica gel (hexanes/EtOAc/NEt 80:20:1 to 60:40:1) to give fast-
3
running product a (syn-cis) and a slow running product b (anti-
cis).
N1-Trityl-3a-hydroxypyrrolo[2,3-b]indolylglycine Meth-
f
yl Ester (syn-cis) (1a). R (hexane/EtOAc, 1:1) 0.4, white solid,
1
0
.02 g, 35% yield. H NMR (CDCl ) δ: 7.50 (d, 6H, J ) 6.8 Hz,
3
Tr
6
ArH ), 7.48-7.18 (m, 10H, ArH), 7.07 (t, 1H, J ) 7.6 Hz, CH ),
5
7
6
5
.73 (t, 1H, J ) 7.6 Hz, CH ), 6.38 (d, 1H, J ) 7.8 Hz, CH ),
.95 (dd, 1H, J ) 5.0, 4.9 Hz, NHCO), 5.54 (d, 1H, J ) 3.6 Hz,
8
a
2
CH ), 5.49 (s, 1H, OH), 3.81 (d, 1H, J ) 9.3 Hz, CH ), 3.65 (s,
a
Reagents and conditions: (i) LiOH, dioxane/H2O (2:1), rt, 1 h;
OMe
R
3
1
H, CH
3
), 3.59 (dd, 1H, J ) 18.5, 5.0 Hz, CH ), 3.52 (dd,
(
ii) H-Cys(Tr)-OMe, DCC, HOBt, NEt3, DCM, 0 °C f rt, 15 h; (iii)
TFA, rt, 5 h.
R
H, J ) 18.5, 4.9 Hz, CH ), 3.01 (d, 1H, J ) 3.6 Hz, NH), 2.48
3
3
(
dd, 1H, J ) 13.9, 9.3 Hz, CH ), 2.28 (d, 1H, J ) 13.9 Hz, CH ).
1
3
C NMR (CDCl
3
) δ: 176.6 (C), 169.8 (C), 146.9 (C), 143.7 (C),
compounds and comparable with other non-main chain
1
31.1 (CH), 130.1 (C), 128.8 (CH), 127.5 (CH), 127.1 (CH),
2
7,30
cyclizations.
122.7 (CH), 118.8 (CH), 110.0 (CH), 92.3 (CH), 86.8 (C), 75.6
+
(
5
C), 64.5 (CH), 52.3 (CH
3
), 42.8 (CH
2
), 41.0 (CH
2
). ES /MS:
+
+
+
56.2 (M + Na) . HRMS (ES ) for C33
H
31
N
3
O
4
(M + H) :
Conclusion
calculated 534.2393, found 534.2390. UV/vis (MeOH): 239 (1.5
6
2
-1
6
2
-1
An efficient, high yielding, and versatile synthesis of
Tr-Hpi-Xaa-OMe dipeptides is described where Xaa ) 14
different L-amino acids. In certain cases, high diastere-
omeric excess is observed in the oxidation with DMDO,
but the majority form readily separable mixtures of the
syn-cis and anti-cis diastereomers in relatively equal
amounts. These studies have demonstrated that this
oxidation is compatible with a large range of natural
amino acids, excepting those with indoles and sulfur
atoms, and highlights the generality of this reaction for
the synthesis of an array of Hpi containing peptides.
× 10 cm mol ), 297 nm (0.3 × 10 cm mol ).
N1-Trityl-3a-hydroxypyrrolo[2,3-b]indolylglycine Meth-
f
yl Ester (anti-cis) (1b). R (hexane/EtOAc, 1:1) 0.4, white
1
solid, 0.02 g, 35% yield. H NMR (CDCl
3
) δ: 9.25-9.19 (br,
Tr
1
9
H, NHCO), 7.71 (d, 6H, J ) 7.6 Hz, ArH ), 7.48-7.25 (m,
6,4
H, ArH), 7.13-6.98 (m, 2H, CH ), 6.73 (t, 1H, J ) 7.4 Hz,
5
7
CH ), 6.62 (t, 1H, J ) 7.4 Hz, CH ), 5.36 (d, 1H, J ) 5.5 Hz,
8
a
2
CH ), 4.89 (d, 1H, J ) 5.5 Hz, NH), 4.25-4.19 (m, 2H, CH ,
R
OMe
CH ), 3.80 (s, 3H, CH
3
), 3.18 (dd, 1H, J ) 18.3, 3.2 Hz,
3
R
CH ), 2.35 (d, 1H, J ) 13.4 Hz, CH ), 1.08 (dd, 1H, J ) 13.4,
3
13
9
.8 Hz, CH ). C NMR (CDCl
C), 144.8 (C), 130.1 (C), 129.3 (CH), 128.8 (CH), 128.3 (CH),
26.8 (CH), 124.5 (CH), 120.7 (CH), 110.3 (CH), 90.7 (C), 89.6
CH), 78.6 (C), 66.7 (CH), 52.2 (CH ), 44.9 (CH ), 41.3 (CH ).
3
) δ: 174.3 (C), 169.8 (C), 148.5
(
1
(
3
2
2
(30) Reid, R. C.; Abbenante, G.; Taylor, S. M.; Fairlie, D. P. J. Org.
Chem. 2003, 68, 4464-4471. Baran, P. S.; Corey, E. J. J. Am. Chem.
Soc. 2002, 124, 7904-7905. Berthelot, A.; Piguel, S.; Le Dour, G.; Vidal,
J. J. Org. Chem. 2003, 68, 9835-9838.
(31) Adam, W.; Bialas, J.; Hadjiarapoglou, L. Chem. Ber. 1991, 124,
2377.
8428 J. Org. Chem., Vol. 70, No. 21, 2005

3a-Hydroxypyrrolo[2,3-b]indoline Dipeptide Methyl Esters
+
+
+
ES /MS: 556.1 (M + Na) . HRMS (ES ) for C33
H
31
N
3
O
4
(M +
ture. Following consumption of the starting material by TLC
(CH Cl /MeOH, 9:1), the reaction mixture was evaporated to
dryness and the residue was purified by a short silica plug,
eluting with CH Cl /MeOH/NEt (90:10:1). Fractions were
combined to yield a white solid as the triethylamine salt of
the two diastereomers, 0.13 g (85%). Due to the acid sensitivity
of the Tr-Hpi-Gly-OH, purification and comprehensive char-
acterization was avoided and the mixture was reacted further
to form the tripeptide 19.
+
H) : calculated 534.2393, found 534.2396. UV/vis (MeOH):
2
2
6
2
-1
6
2
-1
2
35 (1.8 × 10 cm mol ), 292 nm (0.2 × 10 cm mol ).
a-Hydroxypyrrolo[2,3-b]indolylglycine Methyl Ester
15a, 15b). A 1:1 diastereomeric mixture of Tr-Hpi-Gly-OMe
0.20 g, 0.3 mmol) was dissolved in CH Cl (20 mL) and
3
2
2
3
(
(
2
2
hexafluoro-2-propanol (5 mL) was added slowly, with stirring
at room temperature. The solution turned red/orange and TLC
CH Cl /MeOH, 9:1) showed disappearance of starting material
2 2
(
after 15 min. The solution was evaporated to dryness and the
crude material was purified by silica gel column chromatog-
17a/b: R (CH Cl /MeOH, 9:1) 0.2. 17a/b: mixture of diaster-
f 2 2
1
eomers, but H NMR was possible to be assigned separately.
1
raphy (CH
2
Cl
2
/MeOH, 95:5) to yield the two diastereomers,
17a: H NMR (d
4
-MeOH) δ: 9.59-9.53 (br, 1H), 7.51 (d,
each as a pale orange foam: upper 0.04 g (40%); lower 0.03 g
6H, J ) 7.8 Hz), 7.27-7.11 (m, 10H), 7.01-6.45 (m, 2H), 6.25
(t, 1H, J ) 7.9 Hz), 5.49 (s, 1H), 4.21 (d, 1H, J ) 9.5 Hz), 3.14-
3.09 (m, 2H), 2.25 (dd, 1H, J ) 13.8, 9.5 Hz), 2.18 (d, 1H, J )
13.8 Hz).
(
30%).
5a: R
1
1
f
(CH
Cl
2 2
/MeOH, 9:1) 0.3. H NMR (d
4
-MeOH) δ: 7.23
4
6
(
(
(
(
d, 1H, J ) 7.6 Hz, CH ), 7.02 (t, 1H, J ) 7.6 Hz, CH ), 6.71
5
7
17b: 1H NMR (d
t, 1H, J ) 7.6 Hz, CH ), 6.38 (d, 1H, J ) 7.6 Hz, CH ), 4.92
4
-MeOH) δ: 9.55-9.48 (br, 1H), 7.75 (d,
8a
R
OMe
s, 1H, CH ), 3.91 (s, 2H, CH ), 3.70 (s, 3H, CH
3
), 3.64
6H, J ) 7.8 Hz), 7.40-7.25 (m, 10H), 7.01-6.45 (m, 3H), 5.26
(s, 1H), 3.98 (d, 1H, J ) 10.1 Hz), 3.77 (d, 1H, J ) 17.6 Hz),
2.81 (d, 1H, J ) 17.6 Hz), 2.07 (d, 1H, J ) 13.0 Hz), 1.21 (dd,
1H, J ) 13.0, 10.1 Hz).
2
dd, 1H, J ) 10.6, 6.1 Hz, CH ), 2.55 (dd, 1H, J ) 12.2, 6.1
3
3
13
Hz, CH ), 2.28 (dd, 1H, J ) 12.2, 10.6 Hz, CH ). C NMR (d
MeOH) δ: 173.8 (C), 170.4 (C), 149.4 (C), 130.0 (CH), 129.9
C), 124.1 (CH), 119.9 (CH), 110.5 (CH), 88.5 (C), 84.8 (CH),
4
-
1
3
(
17a/b: mixture of diastereomers. C NMR (d -MeOH) δ:
4
+
6
0.4 (CH), 52.3 (CH
3
), 44.3 (CH
2
), 40.7 (CH
2
). ES /MS: 292.1
180.0, 176.0 (C), 175.7 (C), 156.3, 151.3 (C), 146.4, 145.4 (C),
130.9, 129.9 (CH), 130.3 (C), 129.4 (CH), 129.2, 128.8 (CH),
125.1 (CH), 120.0, 119.8 (CH), 111.0 (CH), 94.8, 93.6 (CH),
+
+
+
(
2
M + H) . HRMS (ES ) for C14
92.1297, found 292.1300.
5b: R (CH Cl /MeOH, 9:1) 0.2. H NMR (CDCl
s, 1H, NH), 7.15 (d, 1H, J ) 7.6 Hz, CH ), 7.03 (t, 1H, J ) 7.6
H N O (M + H) : calculated
18 3 4
1
1
f
2
2
3
) δ: 7.79
2
90.7, 90.2 (C), 80.1 (C), 68.0, 66.0 (CH), 56.4, 54.9 (CH ), 46.6
4
-
-
(
(NEt
3
), 44.8, 44.4 (CH
2
), 7.8 (NEt
28 3 4
3
). ES /MS: 518.4 (M - H) .
6
5
-
-
Hz, CH ), 6.68 (t, 1H, J ) 7.6 Hz, CH ), 6.49 (d, 1H, J ) 7.6
HRMS (ES ) for C32
H N O (M - H) : calculated 518.2080,
7
8a
Hz, CH ), 4.99 (s, 1H, CH ), 4.02 (dd, 1H, J ) 8.5, 5.3 Hz,
found 518.2062.
2
R
CH ), 3.81 (dd, 1H, J ) 18.4, 6.0 Hz, CH ), 3.70 (s, 3H,
Compound 18 was prepared using the same method as 17a/
b, but crude product was reacted directly to form the tripeptide
20, characterized below.
OMe
R
CH
3
), 3.43 (dd, 1H, J ) 18.4, 4.7 Hz, CH ), 2.55 (dd, 1H, J
3 3 13
)
13.3, 8.5 Hz, CH ), 2.28 (dd, 1H, J ) 13.3, 5.3 Hz, CH ). C
NMR (CDCl
3
) δ: 173.9 (C), 170.6 (C), 149.9 (C), 129.9 (CH),
General Method for the Synthesis of Hpi Tripeptides
1
28.6 (C), 123.9 (CH), 119.6 (CH), 110.2 (CH), 89.5 (C), 86.2
(19a/b, 20). Tr-Hpi-Xaa-OH‚NEt (1.0 mmol), DCC (1.1 mmol),
3
+
(
2
CH), 61.6 (CH), 52.2 (CH
3
), 42.9 (CH
2
), 40.7 (CH). ES /MS:
and HOBt (1.1 mmol) were dissolved in dry CH Cl and stirred
2
2
+
+
+
92.1 (M + H) . HRMS (ES ) for C14
H
17
N
O
3 4
Na (M + Na) :
under inert atmosphere at 0 °C for 10 min. A mixture of
H-Cys(Tr)-OMe (1.1 mmol) and triethylamine (1.2 mmol) was
added to this and the vessel was allowed to warm slowly to
room temperature. Stirring was continued for 15 h and
calculated 314.1117, found 314.1114.
N-(9H-Fluoren-9-ylmethoxy)carbonylglycyl(3a-hydroxy-
pyrrolo[2,3-b]indolyl)glycine Methyl Ester (16). To a
solution of Fmoc-Gly-OH (0.60 g, 0.2 mmol) in dry DMF (8
mL) was added PyBOP (0.11 g, 0.2 mmol), H-Hpi-Gly-OMe
2 2
followed by TLC (CH Cl /MeOH, 9:1). On consumption of
starting material the mixture was evaporated to dryness and
redissolved in EtOAc and the white precipitate was filtered
off. The filtrate was washed with solutions of citric acid (5%),
(
0.06 g, 0.2 mmol), and DIPEA (20 µL, 0.2 mmol). The mixture
was stirred for a period of 2 h at room temperature before the
solvent was evaporated to dryness. The mixture was purified
by flash chromatography on silica gel (hexane:EtOAc 1:1) to
3 2 4
NaHCO (sat.), and NaCl (sat.), then dried over Na SO
(anhydrous). Evaporation in vacuo gave the crude tripeptides
as pale yellow oils. These were purified by silica gel column
chromatography (hexane/EtOAc, 2:1 to 1:1) to afford the pure
tripeptides.
yield product as a white solid, 0.05 g (44%). R
f
(CH
2
Cl
2
/MeOH,
1
Fmoc
9
7
:1) 0.5. H NMR (CDCl
.55 (d, 2H, J ) 7.4 Hz, ArH
3
) δ: 7.75 (d, 2H, J ) 7.4 Hz, ArH
),
Fmoc
), 7.35 (t, 2H, J ) 7.4 Hz,
Fmoc
Fmoc
5
ArH
), 7.30-7.20 (m, 3H, ArH
), 7.12-6.95 (m, /
3
H,
N1-Trityl-3a-hydroxypyrrolo[2,3-b]indolylglycyl-S-tri-
tyl-L-cysteine Methyl Ester (19a and 19b) (mixture of
f
5
1
NHCO′ + ArH ), 6.85 (br, /
3
H, NHCO′), 6.80-6.69 (m, 1H,
6
7
ArH ), 6.57-6.48 (m, 1H, ArH ), 5.59-5.72 (br, 1H, NHCO),
diastereomers). R (hexane/EtOAc, 1:1) 0.2. White solid, 0.06
2
8a
1
1
5
.63 (br, /
3
H, J ) 2.8 Hz, CH ), 5.55 (br, /
3
H, NH), 5.42 (br,
g, 24% yield. H NMR (CDCl ) δ: 9.23-9.14 (br, 1H), 7.69 (d,
3
2
1
8a
1
2
/
3
H, NH), 5.25 (s, /
3
H, CH ), 4.85 (d, /
3
H, J ) 7.0 Hz, CH ),
6H, J ) 7.8 Hz), 7.60-6.92 (m, 24H), 6.74-6.67 (m, 1H), 6.65-
6.58 (m, 1H), 6.33-6.19 (m, 1H), 5.35-5.28 (m, 1H), 4.91-
4.80 (m, 1H), 4.63-4.48 (m, 1H), 4.12-4.07 (m, 1H), 3.89-
3.78 (m, 1H), 3.73-3.65 (m, 3H), 3.64-3.53 (m, 2H), 3.11-
2
2
11
Fmoc
4
.54 (d, /
3
H, J ) 8.7 Hz, CH ), 4.35-4.06 (m,
/
3
H, CH
,
R
2
R
CH ), 4.00 (dd, /
3
H, J ) 18.1, 6.5 Hz, CH ), 3.88-3.60 (m,
5
2
2
R
R′
OMe
/
/
/
3
3
3
H, CH , CH ), 3.57-3.51 (m, 3H, CH
3
), 3.49-3.38 (m,
R′
1
R′
13
H, CH ), 3.24 (dd, /
3
H, J ) 18.1, 4.0 Hz, CH ), 3.09 (dd,
2.88 (m, 1H), 2.72-2.54 (m, 2H), 2.36-0.77 (m, 2H). C NMR
R
1
H, J ) 18.1, 4.0 Hz, CH ), 2.86 (dd, /
3
H, J ) 13.4, 9.6 Hz,
(CDCl ) δ: 174.8, 170.2 (C), 169.0 (C), 148.2 (C), 144.7, 144.1
3
3
3
2
CH ), 2.80-2.71 (m, 1H, CH ), 2.59 (dd, /
3
H, J ) 13.4, 9.6
) δ: 172.1, 172.0 (C), 171.4, 171.3
C), 170.7 (C), 158.4 (C), 149.8, 149.6 (C), 145.2, 145.1, 142.7
C), 131.5 (C), 132.0, 131.8 (CH), 129.2, 128.5, 126.5 (CH),
25.7, 125.5 (CH), 122.4, 122.0 (CH), 121.4 (CH), 113.0, 112.5
CH), 87.7 (C), 84.7, 84.6 (C), 68.9, 68.7 (CH ), 62.7 (CH), 54.9
CH ), 53.7, 53.6 (CH), 48.4 (CH), 43.8 (CH ), 42.6, 42.4 (CH ).
Na
(C), 129.9 (C), 129.8 (CH), 129.4, 129.2, 128.2, 127.9, 126.8
(CH), 126.8 (CH), 120.4 (CH), 110.2 (CH), 90.5 (CH), 86.9 (C),
3
13
Hz, CH ). C NMR (CDCl
3
(
(
1
78.6, 77.5 (C), 66.7 (CH), 52.6 (CH), 51.3 (CH ), 47.2 (CH),
3
+
+
43.6 (CH ), 33.6 (CH ). ES /MS: 901.5 (M + Na) . HRMS
(ES ) for C H N O S (M + Na) : calculated 901.3400, found
55 50 4 5
2
2
+
+
(
(
2
901.3405.
N1-Trityl-3a-hydroxypyrrolo[2,3-b]indolyl-O- butyl-L-
threonyl-S-trityl-L-cysteine Methyl Ester (20). R (hexane/
EtOAc, 1:1) 0.3. White solid, 0.03 g, 25% yield (2 steps). H
NMR (CDCl ) δ: 7.65 (d, 1H, J ) 8.0 Hz), 7.55 (d, 6H, J ) 7.3
t
3
2
3
+
+
+
ES /MS: 593.0 (M + Na) . HRMS (ES ) for C31
30 4 7
H N O
f
+
(
M + Na) : calculated 593.2012, found 593.2023.
N1-Trityl-3a-hydroxypyrrolo[2,3-b]indolylglycine Tri-
1
3
ethylammonium Salt (17a, 17b). To a solution of Tr-Hpi-
Gly-OMe (mixture of diastereomers) (0.13 g, 0.2 mmol) in
dioxane/water (15 mL, v:v 2:1) was added LiOH (0.05 g, 10
equiv) and the reaction was stirred for 1 h at room tempera-
Hz), 7.48-7.11 (m, 24H), 6.98 (t, 1H, J ) 7.5 Hz), 6.71 (t, 1H,
J ) 7.3 Hz), 6.36-6.28 (m, 2H), 6.18 (d, 1H, J ) 5.2 Hz), 5.74
(d, 1H, J ) 2.7 Hz), 4.53-4.48 (m, 1H), 4.14-4.06 (m, 1H),
3.87-3.82 (m, 1H), 3.67 (s, 3H), 3.42 (d, 1H, J ) 9.0 Hz), 2.82
J. Org. Chem, Vol. 70, No. 21, 2005 8429

May et al.
1
(
d, 1H, J ) 2.7 Hz), 2.54 (dd, 1H, J ) 12.0, 5.8 Hz), 2.48 (dd,
White solid, 20% yield. H NMR (d
4
-MeOH) δ: 7.66 (d, 1H, J
4
7
1
H, J ) 12.0, 4.7 Hz), 2.23 (dd, 1H, J ) 13.6, 9.0 Hz), 2.01 (d,
H, J ) 13.6 Hz), 1.28 (s, 9H), 0.48 (d, 3H, J ) 6.2 Hz), 0.12
) 7.7 Hz, CH ), 7.29 (d, 1H, J ) 7.7 Hz, CH ), 7.14 (t, 3H, J
6
5
1
) 7.7 Hz, CH ), 7.09 (t, 1H, J ) 7.7 Hz, CH ), 4.76 (t, 1H, J )
1
3
R cys
â thr
R thr
(
s, 1H). C NMR (CDCl
3
) δ: 171.6 (C), 170.2 (C), 169.0 (C),
3.9 Hz, CH
(m, 2H, CH
), 4.21 (d, 1H, J ) 4.6 Hz, CH
), 4.03-3.87
R trp
1
48.2 (C), 144.2 (C), 131.2 (C), 130.9 (CH), 129.4 (CH), 128.7
CH), 127.7 (CH), 126.8 (CH), 118.7 (CH), 110.1 (CH), 92.7
CH), 86.9 (C), 80.1 (C), 75.5 (C), 66.5 (CH), 65.7 (CH), 65.1
, CH
), 3.76 (s, 3H, OCH ), 3.61 (dd, 1H, J )
3
(
(
(
14.5, 4.5 Hz, CHâ cys), 3.49 (dd, 1H, J ) 14.6, 3.6 Hz, CHâ cys),
â
trp
3.43-3.31 (m, 1H, CH
), 3.04 (dd, 1H, J ) 14.2, 3.0 Hz,
â trp
γ thr 13
CH), 57.2 (CH), 52.4 (CH
3
), 43.3 (CH
2 2 3
), 33.9 (CH ), 28.2 (CH ),
CH
), 1.02 (d, 1H, J ) 6.4 Hz, CH
4
). C NMR (d -MeOH)
+
+
+
1
6.1 (CH
for C61
001.4283.
General Method for the Synthesis of Cyclic Trypta-
3
). ES /MS: 1001.5 (M + Na) . HRMS (ES )
δ: 178.0 (C), 172.9 (C), 171.0 (C), 139.6 (C), 128.4 (C), 128.2
C), 123.7 (CH), 120.7 (CH), 119.5 (CH), 113.8 (C), 112.0 (CH),
5.3 (CH ) 59.1 (CH), 55.2 (CH), 54.1 (CH), 53.3 (CH ), 36.8
CH ), 33.2 (CH
M + Na) . HRMS (ES ) for C19
+
62 4 6
H N O S (M + Na) : calculated 1001.4288, found
(
1
6
2
3
+
+
(
(
2
2 2
), 20.2 (CH ). ES /MS: 421.1 (M + H) , 443.0
2
7
thionines. To protected tripeptide methyl ester (0.5 mmol)
was added trifluoroacetic acid (2 mL) and the yellow solution
was stirred for 5 h. Methanol was added and the solvent was
removed in vacuo. This was repeated twice more, and then
the residue was purified by silica gel column chromatography
+
+
+
24 4 5
H N O S (M + H) : calculated
6
2
4
21.1546, found 421.1550. UV/vis (MeOH): 284 (7.1 × 10 cm
-1 6 2 -1 6 2 -1
mol ), 292 (8.1 × 10 cm mol ), 301 nm (7.5 × 10 cm mol ).
Acknowledgment. J.P.M. received a postdoctoral
fellowship from the Royal Society, U.K.; J.P. was
awarded a graduate fellowship from NSERC; and
D.M.P. received a Junior Career Award form the
Michael Smith Foundation for Health Research in B.C.
This work was supported by UBC start-up funds and
funding from the Michael Smith Foundation for Health
Research in B.C. and PENCE Inc.
(
CH
-Mercapto-L-tryptophanylglycyl-L-cysteine Cyclic Sul-
fide Methyl Ester (21). R (CH Cl /MeOH, 9:1) 0.4. White
-MeOH) δ: 7.60 (d, 1H, J ) 7.7
2 2
Cl /MeOH 9.5:0.5).
2
f
2
2
1
solid, 16% yield. H NMR (d
4
4
7
Hz, CH ), 7.30 (d, 1H, J ) 7.7 Hz, CH ), 7.16 (t, 3H, J ) 7.7
6
5
Hz, CH ), 7.08 (t, 1H, J ) 7.7 Hz, CH ), 4.89-4.84 (m, 1H,
R cys
R gly
CH
CH
CH
), 4.25 (d, 1H, J ) 15.0 Hz, CH
), 4.03-3.95 (m, 1H,
R trp
â cys
OMe
), 3.82 (s, 3H, CH
), 3.47 (dd, 1H, J ) 14.5, 4.5 Hz, CH
3
), 3.64 (dd, 1H, J ) 14.5, 2.6 Hz,
â cys
), 3.39 (dd, 1H,
â trp
R gly
J ) 13.5, 12.0 Hz, CH
.09 (dd, 1H, J ) 13.5, 2.6 Hz, CHâ trp). C NMR (d
δ: 178.3 (C), 173.0 (C), 172.9 (C), 141.0 (C), 130.1 (C), 129.9
C), 125.2 (CH), 122.2 (CH), 121.0 (CH), 114.7 (C), 113.5 (CH),
), 3.12 (d, 1H, J ) 15.0 Hz, CH
),
13
3
4
-MeOH)
Supporting Information Available: Supporting syn-
thetic procedures and characterization for compounds 2-14,
(
5
1
13
including H and C NMR data and ES/HRMS data; crystal-
6.5 (CH), 55.2 (CH), 54.8 (CH
3
), 46.4 (CH), 39.3 (CH
2
), 33.6
lography data for compounds 1a, 1b, and 10a; copies of the
+
+
+
(
CH
2
). ES /MS: 377.1 (M + H) . HRMS (ES ) for C17
H
21
N
4
O
4
S
1
13
H and C NMR spectra for compounds 1a, 1b, and 15-22
+
(
M + H) : calculated 377.1284, found 377.1292. UV/vis
1
and H NMR for 2-14; 2D COSY spectra for compounds 1a/
b, 15a/b, 16, 21, and 22. This material is available free of
charge via the Internet at http://pubs.acs.org.
6
2
-1
6
2
-1
(MeOH): 284 (6.9 × 10 cm mol ), 292 (7.8 × 10 cm mol ),
6
2
-1
3
01 nm (7.0 × 10 cm mol ).
-Mercapto-L-tryptophanyl-L-threonyl-L-cysteine Cy-
(CH Cl /MeOH, 9:1) 0.3.
2
clic Sulfide Methyl Ester (22). R
f
2
2
JO051105T
8430 J. Org. Chem., Vol. 70, No. 21, 2005