Concise total synthesis and structural revision of (+)-pestalazine B

Article abstract of DOI:10.1039/c0ob00531b

A convergent synthesis of the proposed structure of (+)-pestalazine B has been achieved in 4 steps using the N-alkylation of an unprotected tryptophan diketopiperazine with a 3a-bromopyrrolidinoindoline as the key step. Although its structure was confirmed by X-ray analysis, the spectroscopic data did not match those of the natural product. The versatility of the methodology allowed the preparation of several diastereomers, and the database generated led to the proposal of an isomeric structure for the natural alkaloid where the d-leucine and d-phenylalanine residues exchanged positions, which was corroborated by total synthesis

Full text of DOI:10.1039/c0ob00531b

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PAPER
www.rsc.org/obc | Organic & Biomolecular Chemistry
Concise total synthesis and structural revision of (+)-pestalazine B†
´
Carlos Pe´rez-Balado and Angel R. de Lera*
Received 2nd August 2010, Accepted 12th August 2010
DOI: 10.1039/c0ob00531b
A convergent synthesis of the proposed structure of (+)-pestalazine B has been achieved in 4 steps using
the N-alkylation of an unprotected tryptophan diketopiperazine with a 3a-bromopyrrolidinoindoline
as the key step. Although its structure was confirmed by X-ray analysis, the spectroscopic data did not
match those of the natural product. The versatility of the methodology allowed the preparation of
several diastereomers, and the database generated led to the proposal of an isomeric structure for the
natural alkaloid where the D-leucine and D-phenylalanine residues exchanged positions, which was
corroborated by total synthesis
two disconnections; i) the final condensation of D-phenylalanine
Introduction
with the corresponding hexahydropyrrolo[2,3-b]indole to form
the fused diketopiperazine ring, and ii) the N-alkylation of the
tryptophan/leucine diketopiperazine subunit with 3a-bromo-2-
methylcarboxylate-hexahydropyrrolo[2,3-b]indole. The expedient
construction of C–N bonds between the latter compound and
indole derivatives has been reported by Espejo and Rainier,4 who
have proposed that the ester enolate initially formed under basic
conditions displaces the bromine atom to afford a transient cyclo-
propane that is then trapped by the indole nitrogen. Subsequent
kinetic protonation results in the endo isomer of the substituted
indoline.5 Therefore, besides the formation of the key C–N bond,
the required configuration at C11 of 2 is expected to be established
in the same step.
Pestalazines A and B (Fig. 1) are heterodimeric diketopiperazine
alkaloids isolated from the pathogenic fungus Pestalotiopsis theae
that contain two tryptophan units, one of them as a pyrrolidi-
noindoline fused to the diketopiperazine ring. In contrast to other
dimeric structures, joined through the C3–C3¢ bond, both natural
products show an unsymmetrical connection (C3–C24 and C3–
N30, respectively, for pestalazine A 1 and pestalazine B 2, Fig.
1) between the indoline core and the indole unit.1 Attracted by
the striking architecture of 2 and the recent synthetic advances
in the preparation of tryptophan-based dimeric alkaloids,2,3 we
undertook a program directed towards the total synthesis of the
proposed structure of pestalazine B.
Results and discussion
In order to assess the feasibility of the synthetic plan, diketopiper-
azine 3 and protected 3a-bromopyrrolidinoindoline 4 were pre-
pared in multi-gram scale. Condensation between L-tryptophan
methyl ester and N-Fmoc-D-leucine in the presence of EDC (N-
(3-dimethylaminopropyl)-N¢-ethylcarbodiimide hydrochloride) as
coupling agent afforded the corresponding L-Trp-D-Leu dipeptide,
which underwent the removal of the Fmoc group and the concomi-
tant ring-closure under basic conditions (Et2NH in MeOH) to give
3 (55% yield over the two steps). Diketopiperazine 3 has been pre-
viously isolated from the fungus Penicillium brevicompactum and
shown to regulate plant growth.6 3a-Bromopyrrolidinoindoline 4
was synthesised in one step from bis-BOC-D-tryptophan methyl
ester and NBS (N-bromosuccinimide) following our reported
procedure.3a
Fig. 1 Proposed structures of pestalazines A and B and main disconnec-
Since the presence in 3 of three NH moieties could produce
mixtures of regioisomers on its reaction with 4, we first studied the
reactivity of 3 towards silicon electrophiles to explore an eventual
protection of the amide NH as silyl enol ethers. Interestingly,
treatment of 3 with TIPSCl (2 equiv.) and Et3N in CH2Cl2 afforded
exclusively the indole N-silylated product. To extrapolate this
finding to the alkylation with 3a-bromo-hexahydropyrroloindole
4, we assayed the conditions reported by Espejo and Rainier,4 by
adding a 1 M solution of KOtBu in THF to a suspension of 3 and 4
in CH3CN. Despite the poor solubility of 3 in CH3CN, the desired
coupling product 5 was obtained in modest yields (25–30%).
tions of pestalazine B.
According to our retrosynthetic analysis, 2 could be prepared
in a quite concise and convergent manner on the basis of
Departamento de Qu´ımica Orga´nica, Facultade de Qu´ımica, Universidade
de Vigo, Lagoas-Marcosende, 36310 Vigo, Spain. E-mail: qolera@uvigo.es;
Fax: +34 986 811940; Tel: +34 986 812316
† Electronic supplementary information (ESI) available: Copies of 1H and
13C-NMR spectra, CD spectra and X-ray details. CCDC reference numbers
767367. For ESI and crystallographic data in CIF or other electronic
format see DOI: 10.1039/c0ob00531b
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Table 1 N-Alkylation of tryptophan derivative
bromopyrrolidinoindoline 4
3
with 3-exo-
Entry
Base (equiv.)
Conditions
Yield of 5 (%)
1
2
3
4
5
KOtBu (2.0)
KOtBu (2.0)
KOtBu (2.5)
NaH (2.0)
CH3CN, 12 ◦
C
31
23
18
25
15
CH3CN–THF (5 : 1), 10 ◦
C
C
CH3CN–DMF (5 : 1), 10 ◦
CH3CN, 25 ◦
THF–DMF (1 : 1), 25 ◦
C
NaH (2.0)
C
Nevertheless, other regioisomers could not be identified, and the
starting materials 3 and 4 were partially recovered. Although the
solubility of 3 improved using THF and DMF as co-solvents, the
yields dropped. Alternatively, the use of NaH either in acetonitrile
or in a THF–DMF mixture did not afford better yields and
more complex mixtures were observed. The expected inversion
Scheme 1 Reagents and conditions: (a) TMSI (2.2 equiv.), CH3CN, 0 ◦C,
91%. (b) N-Fmoc-D-phenylalanine (1.1 equiv.), HATU (1.1 equiv.), Et3N
(2.2 equiv.), DMF, 0 to 25 ◦C. (c) Et2NH (55 equiv.), MeOH, 25 ◦C, 56%
combined.
stereocenter. This was assigned as S by the similarity of the CD
(circular dichroism) spectra of 2 and pestalazine A (1), whereas the
latter was correlated with that of (+)-asperazine,11 a close analog of
pestalazine A in which both diketopiperazine subunits are formed
with phenylalanine residues. In the absence of other proofs for
the absolute configuration at C34 in pestalazine B, we decided to
prepare the C34 epimer of 2.
Utilising the same sequence of steps, epimer 11 was as-
sembled in a straightforward manner using the epimeric cis-
diketopiperazine 8 (readily prepared using D-tryptophan methyl
ester and N-Fmoc-D-leucine in an analogous manner to 3) and
3a-bromopyrrolidinoindoline 4 (Scheme 2). Disappointingly the
spectroscopic data of 11 did not match those of the natural product
either. Incidentally, we found that cis diketopiperazine 8 and the
intermediates containing fragment 8, including the final product
11, show a characteristic signal in the 1H NMR spectrum between
d 0.0 and d -0.5 ppm for one of the diastereotopic C40 protons
(pestalazine numbering), which is attributed to the shielding effect
of the indole ring.12 The absence of this signal in the spectrum of
pestalazine B is a good indication that it does not contain a cis
diketopiperazine.
We then considered the opposite configuration of the indoline
ring-fusion. We have recently revised the structure of the related
dimeric diketopiperazine alkaloid (+)-asperdimin3c and showed
that the indoline ring-fusion stereostructure cannot be safely
determined based on the comparison of NMR data of analogous
compounds.3c A reverse 2S, 3R and 11R configuration would still
comply with the results of Marfey’s analysis and the NOE signals
observed for the natural product. Diastereoisomer 15 was there-
fore prepared from enantiomeric 3a-bromopyrrolidinoindoline
123b and diketopiperazine 3 using the same protocol (Scheme 2).
Unfortunately, the NMR data of 15 also differed from those of the
natural product. Furthermore, the specific optical rotation ([a]D23
1
of configuration at C2 was confirmed in all cases by H NMR,
on the basis of the characteristic chemical shift of the endo methyl
ester at d ~3.2 ppm (Table 1).7
The rapid access to advanced intermediate 5, despite the room
for improvement in the yield of this step,8 prompted us to complete
the synthesis of the natural product. Hence, the BOC groups of
5 were removed using TMSI (trimethylsilyl iodide) to afford 6
(91% yield, Scheme 1), which was subsequently condensed with N-
Fmoc-D-phenylalanine in the presence of HATU as coupling agent
and Et3N to give the tetrapeptide 7.3b This was not characterised
due to severe NMR line broadening, with rotameric effects
arising from the carbamate protecting group and the peptide
bond. The deprotection of the Fmoc group and the ring-closure
to the diketopiperazine was effected in the same step upon
addition of excess Et2NH in MeOH to 7 (56% yield over the
1
two steps). Much to our disappointment the H and 13C NMR
spectra of 2 in acetone-d6 (or acetone-d6–DMSO-d6 mixtures)9
did not match those reported for the natural product (Scheme
1).1 Recrystallisation of 2 from MeOH and hexane led to the
formation of prism-shaped crystals. X-Ray analysis confirmed the
stereochemistry of the final product and ruled out configurational
scrambling of any of the four enolizable chiral centers along the
sequence (Fig. 2). These results showed the need to revise the
structure of pestalazine B.
Taking into account the versatility of this synthetic route
that allows the preparation of diverse diastereomers of 2, we
tried to identify which stereogenic centers could be involved
in the misassignment of the structure of pestalazine B. Since
the R absolute configuration at C15 and C37 was secured
by amino acid identification using Marfey’s method,10 and the
relative configuration of the C2–C3–C11 triad was established by
NOESY experiments, we initially directed our attention to the C34
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Fig. 2 ORTEP representation of synthetic 2.
Table 2 Comparison of selected 1H-NMR chemical shifts for natural
pestalazine B and synthetic diastereoisomers of 2
the isobutyl methyl groups differently to the diketopiperazine
ring of the diastereomers of 2.14 Compound 21 was assembled
from diketopiperazine 1715 and 3a-bromopyrrolidinoindoline 4
followed by the final construction of the fused diketopiperazine
ring using a D-leucine derivative (Scheme 4). Much to our delight,
the spectroscopic data (including optical rotation, [a]2D3 +194
(c 0.1, MeOH)) of 21 matched those of the natural product,16
thus confirming the connectivity and absolute stereostructure of
(+)-pestalazine B.
600 MHz
pestalazine B
Ass. d (ppm)
400 MHz 400 MHz 400 MHz 400 MHz
2
11
d (ppm)
15
d (ppm)
16
d (ppm)
d (ppm)
C15 3.47
C34 3.62
C42 0.95
C43 0.90
4.21
4.30
0.76
0.52
4.12
4.35
0.60
0.34
4.76
4.25
0.68
0.39
4.53
4.22
0.78
0.59
In summary, a convergent and versatile synthetic route aimed
to the total synthesis of (+)-pestalazine B has led to its structural
revision to isomer 21. The rapid modular assembly of the two
subunits with minimal use of protecting groups has provided
a series of diastereomers that greatly facilitated the structural
revision. The analysis of their spectroscopic data showed the
inconsistencies with the natural product and suggested a structure
with exchange of the phenylalanine and leucine residues on the
diketopiperazine rings, which finally explained the spectroscopic
discrepancies. The structural revision of (+)-pestalazine B is yet
another example of the fundamental role of total synthesis in the
validation of the proposed structures of natural products.17
-77 (c 0.09, MeOH)) and CD spectrum13 were very different to
those reported for pestalazine B ([a]D +199 (c 0.1, MeOH)).1
Thorough examination of the NMR data of the synthetic
compounds 2, 11 and 15 revealed contrasting differences in
chemical shifts of key hydrogens when compared to the natural
product. Remarkably the hydrogens attached to the stereogenic
centres C15 and C34 are found to be deshielded in the synthetic
series (Table 2). Moreover, the chemical shift of H34 is rather
insensitive to changes not only in the relative configurations of
the southern pyrrolidinoindoline unit stereocenters but also of
C34. The chemical shift of H15 shows greater variation (d 4.12–
4.76 ppm) in the diastereomeric series (Table 2), but it was always
found downfield relative to the natural product (d = 3.47 ppm)
even in the C15-epimer 16 (d = 4.56 ppm), which was easily
prepared by condensation of the advanced intermediate 6 with
N-Fmoc-L-phenylalanine in two steps (Scheme 3). Conversely,
the isobutyl methyl groups were found shielded in the synthetic
series relative to the natural product (Table 2). Taken together, the
spectroscopic analysis led us to propose for the natural product
the structure of an isomer with the same relative and absolute
configurations of 2 in which the residues of phenylalanine and
leucine are interchanged. This proposal would be fully compatible
with the spectroscopic data and the degradation analysis described
for natural pestalazine B. A benzyl substituent at C37 with the R
configuration could shield H34, whereas the pyrrolidinoindoline
diketopiperazine subunit would affect the chemical shift of
Experimental Section
(3S,6R)-3-((1H-Indol-3-yl)methyl)-6-isobutylpiperazine-2,5-
dione (3)
EDC (N-(3-dimethylaminopropyl)-N¢-ethylcarbodiimide hydro-
chloride) (0.74 g, 3.87 mmol, 1.0 equiv) was added to a solution
of L-tryptophan methyl ester (0.85 g, 3.87 mmol) and N-Fmoc-D-
leucine (1.37 g, 3.87 mmol) in CH2Cl2 (24 mL) and the resulting
solution was stirred overnight at 25 ◦C. The reaction mixture
was diluted with CH2Cl2 (20 mL) and washed with H2O (2¥).
The organic layer was dried over Na2SO4 and the solvents were
removed under reduced pressure. The residue was purified by flash
chromatography (silica gel, 90 : 10 CH2Cl2–MeOH) to afford the
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Scheme
3
Reagents and conditions: (a) N-Fmoc-L-phenylalanine
(1.1 equiv.), HATU (1.1 equiv.), Et3N (2.2 equiv.), DMF, 0 to 25 ◦C.
(b) Et2NH (55 equiv.), MeOH, 25 ◦C, 56% combined.
Scheme 4 Reagents and conditions: (a) KOtBu (2.0 equiv.), CH3CN, 12 ◦C,
30%. (b) TMSI (2.2 equiv.), CH3CN, 0 ◦C, 85%. (c) 1). N-Fmoc-D-leucine
(1.1 equiv.), HATU (1.1 equiv.), Et3N (2.2 equiv.), DMF, 0 to 25 ◦C. 2)
Et2NH (55 equiv.), MeOH, 25 ◦C, 57% combined.
Scheme 2 Reagents and conditions: (a) KOtBu (2.0 equiv.), CH3CN, 12 ◦C,
9, 31%; 13, 31%. (b) TMSI (2.2 equiv), CH3CN, 0 ◦C, 10, 84%; 14, 91%.
(c) 1). N-Fmoc-D-phenylalanine (1.1 equiv.), HATU (1.1 equiv.), Et3N
(2.2 equiv.), DMF, 0 to 25 ◦C. 2) Et2NH (55 equiv.), MeOH, 25 ◦C, 11,
66%; 15, 55% combined.
1H), 0.75 (d, J = 6.5 Hz, 3H), 0.62 (d, J = 6.5 Hz, 3H) ppm. 13
C
NMR (100 MHz, CD3OD) d 171.7 (s, CO), 171.5 (s, CO), 138.1
(s), 128.9 (s), 126.2 (d), 122.7 (d), 120.3 (d), 119.9 (d), 112.3 (d),
109.2 (s), 57.6 (d) 53.4 (d), 41.9 (t), 31.1 (t), 25.2 (d), 23.3 (q, CH3),
22.1 (q, CH3) ppm. IR (NaCl) n 3400–3050 (br, N–H), 2956 (w,
C–H), 2926 (w, C–H), 2869 (w, C–H), 1666 (s, C O), 1456 (s),
1318 (m), 1098 (w), 1010 (w), 740 (s) cm-1. MS (ESI+) m/z (%)
332 ([M+K]+, 77), 322 ([M+Na]+, 99), 300 ([M+1]+, 100). HRMS
(ESI+) calcd for C17H22N3O2 ([M+1]+), 300.1706; found, 300.1798.
[a]2D4 +69 (c 0.16, MeOH).
corresponding dipeptide L-Trp-D-Leu (1.48 g, 70% yield) as a
white solid. Et2NH (5 mL, 48.33 mmol, 22 equiv.) was added to
a solution of the dipeptide obtained above (1.23 g, 2.22 mmol)
in MeOH (75 mL) and the mixture was stirred overnight at
25 ◦C. Acetonitrile (100 mL) was added to the suspension and
the resulting precipitate was filtered and washed with acetonitrile.
The white solid obtained was dried under vacuum, identified as
the title compound 3 (0.52 g, 79% yield) and used in the next
reaction without further purification. M.p. (CH3CN): 105–107 ◦C.
1H NMR (400 MHz, CD3OD) d 7.61 (d, J = 8.0 Hz, 1H, ArH),
7.33 (d, J = 8.0 Hz, 1H, ArH), 7.1–7.0 (m, 2H, ArH), 7.06 (s, 1H,
ArH), 7.00 (t, J = 7.1 Hz, 1H, ArH), 4.22 (t, J = 3.9 Hz, 1H), 3.46
(dd, J = 14.6, 4.5 Hz, 1H), 3.15 (dd, J = 14.6, 4.5 Hz, 1H), 2.63 (dd,
J = 7.1, 4.7 Hz, 1H), 1.6–1.5 (m, 1H), 1.5–1.4 (m, 1H), 1.4–1.3 (m,
(2S,3aS,8aS)-1,8-Di-tert-butyl-2-methyl-3a-(3-(((2S,5R)-5-
isobutyl-3,6-dioxopiperazin-2-yl)methyl)-1H-indol-1-yl)-3,3a-
dihydropyrrolo[2,3-b]indole-1,2,8(2H,8aH)-tricarboxylate (5)
A solution of KOtBu (400 mL, 1.0 M in THF, 0.40 mmol, 2.0 equiv)
was added dropwise over a period of 15 min to a suspension
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of the diketopiperazine 3 (60 mg, 0.20 mmol, 1.0 equiv) and
the bromoindoline 4 (150 mg, 0.30 mmol, 1.5 equiv) in CH3CN
(15 mL) at 12 ◦C. The cooling bath was removed and stirring was
continued for 1 h. A saturated NaHCO3 aqueous solution (7 mL)
was added and the mixture was extracted with CH2Cl2 (2¥). The
combined organic layers were dried over Na2SO4 and the solvents
were removed under reduced pressure. The residue was purified
by flash chromatography (silica gel, 95 : 5 CH2Cl2–MeOH) to give
45 mg of the title compound (32% yield) as a white solid. M.p.
(MeOH): 99–100 ◦C. 1H NMR (400 MHz, CD3OD) d 7.8–7.6 (m,
2H, ArH), 7.38 (t, J = 7.8 Hz, 1H, ArH), 7.32 (d, J = 7.5 Hz, 1H,
ArH), 7.2–7.0 (m, 5H, ArH), 6.69 (s, 1H), 4.93 (d, J = 8.7 Hz, 1H),
4.19 (t, J = 4.0 Hz, 1H), 3.55 (dd, J = 12.8, 9.2 Hz, 1H), 3.40 (dd,
J = 14.7, 3.6 Hz, 1H), 3.23 (s, 3H, CO2CH3), 3.2–3.0 (m, 2H), 2.53
(dd, J = 7.3, 4.4 Hz, 1H), 1.57 (s, 9H), 1.48 (s, 9H), 1.5–1.3 (m, 3H),
0.74 (d, J = 6.4 Hz, 3H), 0.46 (d, J = 6.4 Hz, 3H) ppm. 13C NMR
(100 MHz, CD3OD) d 172.5 (s, CO2CH3), 171.6 (s, CO), 171.3 (s,
CO), 153.8 (s, 2xOCON), 144.8 (s), 136.8 (s), 132.0 (d), 131.3 (s),
131.1 (s), 128.0 (d), 125.2 (d), 123.5 (d), 121.5 (d, 2x), 121.1 (d),
112.4 (d, 2x), 110.3 (s), 83.7 (d), 81.4 (s), 74.1 (s), 60.9 (d), 57.3 (d),
53.5 (d), 52.9 (q, CO2CH3), 41.9 (t, 2x), 30.8 (t), 28.8 (q, 3x), 28.7
(q, 3x), 25.0 (d), 23.4 (q), 21.9 (q) ppm. IR (NaCl) n 3400–3050
(br, N–H), 2956 (m, C–H), 2931 (m, C–H), 2871 (w, C–H), 1720
(s, C O), 1679 (s, C O), 1456 (m), 1393 (s), 1158 (s), 1015 (m),
856 (m), 738 (s) cm-1. MS (ESI+) m/z (%) 738 ([M+Na]+, 9), 716
([M+1]+, 25) 616 ([M–CO2tBu]+, 90), 560 (100), 516 (93). HRMS
(ESI+) calcd for C39H50N5O8 ([M+1]+), 716.3654; found, 716.3664.
[a]2D1 +95 (c 0.1, MeOH).
(s, C O), 1457 (m), 1317 (m), 743 (m) cm-1. MS (ESI+) m/z (%)
516 ([M+1]+, 100). HRMS (ESI+) calcd for C29H34N5O4 ([M+1]+),
516.2605; found, 516.2614. [a]2D4 +50 (c 0.14, MeOH).
Proposed structure of (+)-pestalazine B (2)
N-Fmoc-D-phenylalanine (17 mg, 0.04 mmol, 1.1 equiv) was
added to a solution of diamine 6 (20 mg, 0.03 mmol, 1.0 equiv)
in DMF (1.0 mL). The mixture was cooled down to 0 ◦C and
Et3N (13 mL, 0.08 mmol, 2.0 equiv) was added, followed by
HATU (16 mg, 0.04 mmol, 1.1 equiv). The cooling bath was
removed and the mixture was stirred for 16 h at 25 ◦C. The
resulting mixture was diluted with EtOAc (10 mL) and washed
with H2O (3 ¥ 15 mL). The combined organic layers were dried over
Na2SO4 and the solvents were removed under reduced pressure.
The residue was purified by flash chromatography on silica gel
(95 : 5 CH2Cl2–MeOH) to give 20 mg (60% yield) of the coupled
tetrapeptide which was used directly in the next step. Et2NH
(125 mL, 1.21 mmol, 55 equiv) was added to a solution of the
dipeptide obtained above (20 mg, ◦0.02 mmol) in MeOH (2 mL)
and the mixture was stirred at 25 C for 8 h (TLC monitoring).
The solvents were removed under reduced pressure and the residue
was purified by flash chromatography (silica gel, 95 : 5 CH2Cl2–
MeOH) to give 13 mg of the title compound (93% yield) as a white
solid. This solid was recrystallised from MeOH–hexane to afford
colourless prism-shaped crystals. 1H NMR (400 MHz, (CD3)2CO)
d 7.62 (d, J = 7.8 Hz, 1H, H27), 7.48 (d, J = 4.0 Hz, 1H, H14), 7.29
(s, 1H, H31), 7.22 (t, J = 7.3 Hz, 1H, H21), 7.14 (br s, 1H), 7.13 (t,
J = 7.2 Hz, 1H, H7), 7.1–7.0 (m, 4H, H19/H20/H22/H23), 6.98
(t, J = 7.8 Hz, 1H, H26), 6.89 (t, J = 7.8 Hz, 1H, H25), 6.86 (d, J =
7.9 Hz, 1H, H5), 6.8–6.7 (m, 2H, H8/H38), 6.66 (d, J = 8.0 Hz,
1H, H24), 6.60 (t, J = 7.2 Hz, 1H, H6), 6.59 (d, J = 3.6 Hz, 1H, H1),
5.86 (d, J = 3.6 Hz, 1H, H2), 4.30 (dd, J = 4.8, 2.3 Hz, 1H, H34),
4.21 (dt, J = 5.7, 4.4 Hz, 1H, H15), 3.50 (dd, J = 14.5, 5.1 Hz, 1H,
H33A), 3.4–3.2 (m, 3H, H11/H12A/H33B), 3.14 (dd, J = 13.6,
6.1 Hz, 1H, H17A), 2.99 (dd, J = 13.6, 6.1 Hz, 1H, H17B), 2.82
(m, 1H, H37), 2.19 (dd, J = 15.8, 13.3 Hz, 1H, H12B), 1.7–1.6 (m,
1H, H41), 1.6–1.5 (m, 1H, H40A), 1.4–1.3 (m, 1H, H40B), 0.76
(d, J = 6.5 Hz, 3H, H42), 0.52 (d, J = 6.5 Hz, 3H, H43) ppm.
13C NMR (100 MHz, (CD3)2CO) d 171.5 (s, C36), 169.7 (s, C39),
168.8 (s, C13), 167.7 (s, C16), 148.9 (s, C9), 137.0 (s, C18), 136.5 (s,
C29), 130.8 (d, C19/C23), 130.7 (s, C28), 129.6 (s, C4), 129.4 (d,
C20/C22), 128.0 (d, C7), 126.9 (d, C31), 123.8 (d, C21), 122.5 (d,
2x, C5/C25), 120.6 (s, C27), 120.5 (d, C26), 119.7 (d, C6), 112.8
(s, C24), 111.0 (d, C8), 109.8 (s, C32), 83.2 (d, C2), 74.1 (s, C3),
59.8 (d, C15), 56.8 (d, C11), 56.5 (d, C34), 53.2 (d, C37), 42.3 (t,
C40), 41.2 (t, C12), 40.5 (t, C17), 30.4 (t, C33), 24.5 (d, C41), 23.4
(q, C42), 21.7 (q, C43) ppm. IR (NaCl) n 3600–3100 (br, N–H),
2954 (m, C–H), 2924 (w, C–H), 2869 (w, C–H), 1673 (s, C O),
1455 (m), 1317 (m), 1106 (m), 743 (s) cm-1. MS (ESI+) m/z (%)
631 ([M+1]+, 100). HRMS (ESI+) calcd for C37H39N6O4 ([M+1]+),
631.3027; found, 631.3018. [a]2D4 +87 (c 0.1, MeOH).
(2S,3aS,8aS)-Methyl-3a-(3-(((2S,5R)-5-isobutyl-3,6-
dioxopiperazin-2-yl)methyl)-1H-indol-1-yl)-1,2,3,3a,8,8a-
hexahydropyrrolo[2,3-b]indole-2-carboxylate (6)
Iodotrimethylsilane (33 mL, 0.23 mmol, 2.2 equiv) was added
dropwise to a solution of the protected indoline 5 (75 mg,
0.11 mmol) in acetonitrile (2.5 mL) at 0 ◦C. The resulting
yellow solution was stirred at 0 ◦C for 30 min. Polymer-bound
benzyldiisopropylamine (210 mg, 50–90 mesh, Aldrich) and then
wet MeOH (3 mL) were added. The cooling bath was removed and
the suspension was stirred for 15 min. The resin was filtered and
washed with acetonitrile (10 mL) and MeOH (10 mL), the solvents
of the collected filtrate were removed under reduced pressure and
the residue was purified by flash chromatography on silica gel
(90 : 10 CH2Cl2–MeOH) to afford 49 mg (91% yield) of the title
compound as a white solid. M.p. (MeOH): 115–118 ◦C. 1H NMR
(400 MHz, CD3OD) d 7.6–7.5 (m, 1H, ArH), 7.5–7.4 (m, 1H,
ArH), 7.35 (s, 1H, ArH), 7.1–7.0 (m, 1H, ArH), 7.0–6.9 (m, 3H,
ArH), 6.64 (d, J = 7.9 Hz, 1H, ArH), 6.61 (td, J = 7.4, 0.9 Hz,
1H, ArH), 5.45 (s, 1H), 4.3–4.2 (m, 1H), 4.18 (dd, J = 7.7, 3.0 Hz,
1H), 3.50 (dd, J = 14.5, 3.1 Hz, 1H), 3.4–3.3 (m, 1H), 3.34 (s, 3H,
CO2CH3), 3.11 (dd, J = 10.8, 3.9 Hz, 1H), 3.07 (dd, J = 8.8, 3.9 Hz,
1H), 2.4–2.3 (m, 1H), 1.5–1.2 (m, 3H), 0.70 (d, J = 6.3 Hz, 3H),
0.31 (d, J = 6.3 Hz, 3H) ppm. 13C NMR (100 MHz, CD3OD) d
175.0 (s, CO2CH3), 172.0 (s, CO), 171.7 (s, CO), 151.8 (s), 137.4 (s),
131.2 (d), 130.7 (s), 129.6 (s), 128.1 (d), 125.6 (d), 123.0 (d), 120.9
(d), 120.4 (d), 120.1 (d), 113.2 (d), 111.1 (d), 109.2 (s), 83.4 (d), 77.4
(s), 61.1 (d), 57.5 (d), 53.3 (d), 52.6 (q, CO2CH3), 43.0 (t), 41.4 (t),
31.1 (t), 24.9 (d), 23.4 (q), 21.6 (q) ppm. IR (NaCl) n 3400–3050
(br, N–H), 2954 (w, C–H), 2871 (w, C–H), 1735 (m, C O), 1673
(3R,6R)-3-((1H-Indol-3-yl)methyl)-6-isobutylpiperazine-2,5-
dione (8)
According to the general procedure described for 3, diketopiper-
azine 8 was prepared in 73% yield. M.p. (MeOH): 123–125 ◦C.
1H NMR (400 MHz, CD3OD) d 7.59 (d, J = 8.0 Hz, 1H, ArH),
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7.31 (d, J = 8.0 Hz, 1H, ArH), 7.1–7.0 (m, 2H, ArH), 7.05 (s, 1H,
ArH), 7.00 (t, J = 7.5 Hz, 1H, ArH), 4.26 (t, J = 4.0 Hz, 1H), 3.57
(dd, J = 9.9, 4.2 Hz, 1H), 3.48 (dd, J = 14.6, 3.5 Hz, 1H), 3.15
(dd, J = 14.6, 4.6 Hz, 1H), 1.2–1.0 (m, 1H), 0.66 (ddd, J = 13.8,
9.6, 4.2 Hz, 1H), 0.60 (d, J = 6.5 Hz, 3H), 0.49 (d, J = 6.5 Hz,
3H), -0.1– -0.2 (m, 1H) ppm. 13C NMR (100 MHz, CD3OD) d
170.7 (s, CO), 170.0 (s, CO), 138.0 (s), 129.4 (s), 126.0 (d), 122.6
(d), 120.3 (d), 120.2 (d), 112.5 (d), 109.6 (s), 57.6 (d) 54.3 (d), 45.2
(t), 30.8 (t), 24.7 (d), 23.4 (q, CH3), 21.4 (q, CH3) ppm. IR (NaCl)
n 3500–3050 (br, N–H), 2954 (w, C–H), 1664 (s, C O), 1457 (s),
1325 (m), 1093 (w), 1011 (w), 840 (m), 741 (s) cm-1. MS (ESI+)
m/z (%) 332 ([M+K]+, 100), 322 ([M+Na]+, 38), 300 ([M+1]+, 76).
HRMS (ESI+) calcd for C17H22N3O2 ([M+1]+), 300.1706; found,
300.1705. [a]2D6 -2 (c 0.16, MeOH).
(d), 113.4 (d), 111.5 (d), 109.0 (s), 83.3 (d, C8a), 77.4 (s, C3a), 61.4
(d), 57.1 (d), 54.3 (d), 52.7 (q, CO2CH3), 45.3 (t), 42.3 (t), 30.6 (t),
24.8 (d), 23.6 (q), 21.4 (q) ppm. IR (NaCl) n 3500–3050 (br, N–H),
2953 (m, C–H), 2922 (w, C–H), 2870 (w, C–H), 1734 (m), 1670 (s,
C
O), 1608 (m), 1457 (s), 1319 (m), 1212 (m), 742 (s) cm-1. MS
(ESI+) m/z (%) 516 ([M+1]+, 100), 217 (22). HRMS (ESI+) calcd
for C29H34N5O4 ([M+1]+), 516.2605; found, 516.2623. [a]2D4 +105
(c 0.20, CHCl3).
C34 Epimer of 2 (11)
According to the general procedure described for 2, epimer 11 was
prepared in 66% yield. 1H NMR (400 MHz, (CD3)2CO) d 7.71 (s,
1H), 7.61 (d, J = 7.6 Hz, 1H), 7.55 (d, J = 1.6 Hz, 1H), 7.38 (d, J =
4.3 Hz, 1H), 7.33 (dd, J = 7.8, 1.4 Hz, 2H), 7.3–7.1 (m, 4H), 7.09
(d, J = 1.9 Hz, 1H), 6.99 (t, J = 7.0 Hz, 1H), 6.93 (t, J = 7.0 Hz,
1H), 6.9–6.8 (m, 2H), 6.7–6.6 (m, 3H), 6.07 (d, J = 4.0 Hz, 1H),
4.76 (dd, J = 12.1, 5.7 Hz, 1H), 4.4–4.3 (m, 1H), 4.12 (dt, J = 11.4,
4.0 Hz, 1H), 3.7–3.5 (m, 4H), 3.23 (dd, J = 14.4, 5.4 Hz, 1H), 3.05
(dd, J = 13.6, 3.9 Hz, 1H) 2.38 (dd, J = 14.4, 12.2 Hz, 1H), 1.2–1.1
(m, 1H), 0.96 (ddd, J = 13.3, 11.0, 2.6 Hz, 1H), 0.60 (d, J = 6.5 Hz,
3H), 0.34 (d, J = 6.5 Hz, 3H), -0.49 (ddd, J = 13.3, 11.0, 2.6 Hz,
1H) ppm. 13C NMR (100 MHz, (CD3)2CO) d 170.2 (s), 169.0 (s),
167.9 (s), 167.9 (s), 148.9 (s), 138.2 (s), 136.2 (s), 131.3 (s), 130.8
(d, 2x), 130.8 (d), 129.6 (s), 129.3 (d, 3x), 127.6 (d), 127.2 (d), 123.5
(d), 122.4 (d), 120.9 (d), 120.7 (d), 119.8 (d), 113.1 (d), 111.1 (d),
109.6 (s), 84.1 (d), 74.3 (s), 60.1 (d), 57.5 (d), 56.5 (d), 54.0 (d), 44.1
(t), 41.6 (t), 39.9 (t), 30.7 (t), 24.1 (d), 23.9 (q), 21.0 (q) ppm. IR
(NaCl) n 3500–3050 (br, N–H), 2956 (m, C–H), 2925 (m, C–H),
2870 (w, C–H), 1672 (s, C O), 1456 (s), 1320 (m), 741 (s) cm-1.
MS (ESI+) m/z (%) 653 ([M+Na]+, 98), 631 ([M+1]+, 100). HRMS
(ESI+) calcd for C37H39N6O4 ([M+1]+), 631.3027; found, 631.3052.
[a]2D0 +139 (c 0.16, MeOH).
(2S,3aS,8aS)-1,8-Di-tert-butyl-2-Methyl 3a-(3-(((2R,
5R)-5-isobutyl-3,6-dioxopiperazin-2-yl)methyl)-1H-indol-1-yl)-
3,3a-dihydropyrrolo[2,3-b]indole-1,2,8(2H, 8aH)-
tricarboxylate (9)
According to the general procedure described for 5, substitu◦ted
indoline 9 was prepared in 31% yield. M.p. (MeOH): 91–92 C.
1H NMR (400 MHz, CD3OD) d 7.8–7.6 (m, 1H, ArH), 7.63 (d,
J = 7.8 Hz, 1H, ArH), 7.5–7.3 (m, 2H, ArH), 7.25 (d, J = 7.9 Hz,
1H, ArH), 7.2–7.1 (m, 2H, ArH), 7.1–7.0 (m, 1H, ArH), 6.91 (br
s, 1H, ArH), 6.75 (s, 1H), 4.94 (d, J = 9.0 Hz, 1H), 4.3–4.2 (m,
1H), 3.7–3.5 (m, 2H), 3.46 (dd, J = 14.7, 3.1 Hz, 1H), 3.23 (s, 3H,
CO2CH3), 3.00 (d, J = 12.9 Hz, 1H), 2.93 (dd, J = 14.6, 5.1 Hz,
1H), 1.6–1.4 (m, 18H), 1.2–1.0 (m, 1H), 0.92 (ddd, J = 13.5, 10.0,
3.7 Hz, 1H), 0.61 (d, J = 6.5 Hz, 3H), 0.41 (d, J = 6.5 Hz, 3H),
-0.2– -0.3 (m, 1H) ppm. 13C NMR (100 MHz, CD3OD) d 172.6
(s, CO2CH3), 171.0 (s, CO), 169.8 (s, CO), 153.8 (s, 2xOCON),
145.0 (s), 136.2 (s), 132.2 (d), 131.8 (s), 131.1 (s), 128.5 (d, 2x),
125.2 (d), 123.5 (d), 121.5 (d, 2x), 112.6 (d, 2x), 109.9 (s), 83.6 (s),
81.0 (d), 74.0 (s), 61.0 (d), 56.9 (d), 54.4 (d), 52.9 (q, CO2CH3),
45.4 (t, 2x), 30.4 (t), 28.8 (q, 3x), 28.6 (q, 3x), 24.8 (d), 23.9 (q),
21.4 (q) ppm. IR (NaCl) n 3400–3050 (br, N–H), 2956 (m, C–H),
2931 (m, C–H), 2870 (w, C–H), 1720 (s, C O), 1679 (s, C O),
1456 (s), 1393 (s), 1325 (m), 1158 (s), 1015 (m), 856 (m), 738 (s)
cm-1. MS (ESI+) m/z (%) 738 ([M+Na]+, 100), 716 ([M+1]+, 28)
616 ([M–CO2tBu]+, 17). HRMS (ESI+) calcd for C39H49N5NaO8
([M+Na]+), 738.3473; found, 738.3486. [a]2D1 -13 (c 0.08, MeOH).
(2R,3aR,8aR)-1,8-Di-tert-butyl-2-methyl-3a-(3-(((2S,5R)-5-
isobutyl-3,6-dioxopiperazin-2-yl)methyl)-1H-indol-1-yl)-3,3a-
dihydropyrrolo[2,3-b]indole-1,2,8(2H,8aH)-tricarboxylate (13)
According to the general procedure described for 5, substituted
indoline 13 was prepared in 30% yield. M.p. (MeOH): 101–102 ◦C.
1H NMR (400 MHz, CD3OD) d 7.8–7.6 (m, 1H, ArH), 7.66 (d, J =
7.5 Hz, 1H, ArH) 7.5–7.4 (m, 2H, ArH), 7.3–7.1 (m, 4H, ArH),
6.93 (br s, 1H, ArH), 6.76 (s, 1H), 4.94 (d, J = 9.1 Hz, 1H), 4.20 (t,
J = 3.7 Hz, 1H), 3.59 (dd, J = 12.6, 9.4 Hz, 1H), 3.45 (dd, J = 14.6,
3.2 Hz, 1H), 3.24 (s, 3H, CO2CH3), 3.1–2.9 (m, 2H), 2.51 (dd, J =
6.2, 5.0 Hz, 1H), 1.6–1.3 (m, 21H), 0.78 (d, J = 6.5 Hz, 3H), 0.64
(d, J = 6.5 Hz, 3H) ppm. 13C NMR (100 MHz, CD3OD) d 172.6 (s,
CO2CH3), 171.9 (s, CO), 171.2 (s, CO), 153.8 (s, 2x OCON), 145.0
(s), 136.3 (s), 132.2 (d), 131.5 (s), 130.8 (s), 128.5 (d, 2x), 125.2 (d),
123.5 (d), 121.5 (d), 121.2 (d), 112.5 (d, 2x), 109.9 (s), 83.6 (s), 81.0
(d), 74.0 (s), 60.8 (d), 57.2 (d), 53.6 (d), 52.9 (q, CO2CH3), 42.0 (t,
2x), 30.7 (t), 28.8 (q, 3x), 28.6 (q, 3x), 25.2 (d), 23.3 (q), 22.3 (q)
ppm. IR (NaCl) n 3500–3050 (br, N–H), 2976 (m, C–H), 2932 (m,
C–H), 2871 (w, C–H), 1718 (s, C O), 1680 (s, C O), 1481 (s),
1394 (s), 1327 (m), 1157 (s), 1016 (m), 856 (m), 737 (s) cm-1. MS
(ESI+) m/z (%) 738 ([M+Na]+, 83), 716 ([M+1]+, 62), 480 (100).
HRMS (ESI+) calcd for C39H50N5O8 ([M+1]+), 716.3654; found,
716.3664. [a]2D1 +61 (c 0.08, MeOH).
(2S,3aS,8aS)-Methyl-3a-(3-(((2R,5R)-5-isobutyl-3,6-
dioxopiperazin-2-yl)methyl)-1H-indol-1-yl)-1,2,3,3a,8,8a-
hexahydropyrrolo[2,3-b]indole-2-carboxylate (10)
According to the general procedure described for 6, diamine 10
was prepared in 84% yield. M.p. (MeOH): 130–133 ◦C. 1H NMR
(400 MHz, CD3OD) d 7.58 (dd, J = 7.0, 1.7 Hz, 1H, ArH), 7.44 (d,
J = 7.4 Hz, 1H, ArH), 7.21 (s, 1H, ArH), 7.1–6.9 (m, 4H, ArH),
6.7–6.6 (m, 2H, ArH), 5.43 (s, 1H, H8a), 4.3–4.2 (m, 2H), 3.61 (dd,
J = 10.0, 3.9 Hz, 1H), 3.44 (m, 2H), 3.33 (s, 3H, CO2CH3), 3.02
(dd, J = 14.6, 4.8 Hz, 1H), 2.93 (dd, J = 12.9, 2.9 Hz, 1H), 1.2–1.1
(m, 1H), 0.82 (ddd, J = 13.6, 9.7, 4.1 Hz, 1H), 0.56 (d, J = 6.6 Hz,
3H), 0.38 (d, J = 6.6, Hz, 3H), -0.13 (ddd, J = 13.6, 10.0, 4.7 Hz,
1H) ppm. 13C NMR (100 MHz, CD3OD) d 175.3 (s, CO2CH3),
170.9 (s, CO), 169.8 (s, CO), 152.0 (s), 136.7 (s), 131.5 (d), 131.4
(s), 128.9 (s), 128.3 (d), 126.6 (d), 122.8 (d), 120.9 (d, 2x), 119.9
5184 | Org. Biomol. Chem., 2010, 8, 5179–5186
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(2R,3aR,8aR)-Methyl-3a-(3-(((2S,5R)-5-isobutyl-3,6-
dioxopiperazin-2-yl)methyl)-1H-indol-1-yl)-1,2,3,3a,8,8a-
hexahydropyrrolo[2,3-b]indole-2-carboxylate (14)
2H), 3.02 (dd, J = 14.6, 7.6 Hz, 1H), 2.41 (dd, J = 14.6, 11.2 Hz,
1H), 1.8–1.6 (m, 1H), 1.56 (ddd, J = 13.5, 8.8, 4.5 Hz, 1H), 1.7–1.6
(m 1H), 0.78 (d, J = 6.5 Hz, 3H), 0.59 (d, J = 6.5 Hz, 3H) ppm.
13C NMR (100 MHz, (CD3)2CO) d 169.9 (s), 169.8 (s), 169.6 (s),
168.2 (s), 148.9 (s), 138.1 (s), 136.5 (s), 130.9 (s), 130.8 (d), 130.5
(d, 2x), 129.8 (s), 129.4 (d, 2x), 127.6 (d), 126.8 (d), 123.7 (d), 122.5
(d), 120.7 (d), 120.4 (d), 119.8 (d), 113.0 (d), 111.1 (d), 110.1 (s),
83.1 (d), 74.9 (s), 58.6 (d), 57.2 (d), 56.7 (d), 53.3 (d), 42.4 (t), 40.6
(t), 36.2 (t), 30.4 (t), 24.7 (d), 23.4 (q), 21.9 (q) ppm. IR (NaCl)
n 3500–3050 (br, N–H), 2956 (m, C–H), 2927 (m, C–H), 2870 (w,
C–H), 1682 (s, C O), 1456 (s), 1434 (s), 1319 (m), 740 (s) cm-1.
MS (ESI+) m/z (%) 631 ([M+1]+, 100). HRMS (ESI+) calcd for
C40H37N6O4 ([M+1]+), 631.3027; found, 631.3035. [a]2D8 +167 (c
0.08, MeOH).
According to the general procedure described for 6, diamine 14
was prepared in 91% yield. M.p. (MeOH): 117–118 ◦C. 1H NMR
(400 MHz, CD3OD) d 7.7–7.5 (m, 1H, ArH), 7.4–7.3 (m, 1H,
ArH), 7.24 (s, 1H, ArH), 7.10 (td, J = 7.7, 1.2 Hz, 1H, ArH),
7.1–6.9 (m, 3H, ArH), 6.7–6.6 (m, 2H, ArH), 5.43 (s, 1H), 4.24
(dd, J = 7.8, 3.4 Hz, 1H), 4.21 (t, J = 3.4 Hz, 1H), 3.45 (dd, J =
14.5, 3.4 Hz, 1H), 3.4–3.3 (m, 1H), 3.37 (s, 3H, CO2CH3), 3.06
(dd, J = 14.6, 4.6 Hz, 1H), 2.94 (dd, J = 13.1, 3.4 Hz, 1H), 2.50
(dd, J = 6.4, 4.7 Hz, 1H), 1.7–1.2 (m, 3H), 0.73 (d, J = 6.5 Hz,
3H), 0.59 (d, J = 6.5 Hz, 3H) ppm. 13C NMR (100 MHz, CD3OD)
d 175.3 (s, CO2CH3), 171.8 (s, CO), 171.3 (s, CO), 151.8 (s), 136.9
(s), 131.4 (d), 131.1 (s), 129.2 (s), 128.2 (d), 126.2 (d), 122.9 (d),
121.0 (d), 120.6 (d), 119.9 (d), 113.3 (d), 111.5 (d), 109.0 (s), 83.8
(d), 77.6 (s), 61.4 (d), 57.5 (d), 53.5 (d), 52.7 (q, CO2CH3), 42.6
(t), 42.0 (t), 30.9 (t), 25.2 (d), 23.3 (q), 22.3 (q) ppm. IR (NaCl)
n 3500–3050 (br, N–H), 2954 (m, C–H), 2870 (w, C–H), 1733 (m,
(3S,6R)-3-((1H-Indol-3-yl)methyl)-6-benzylpiperazine-2,5-
dione (17)
According to the general procedure described for 3, diketopiper-
azine 17 was prepared in 68% yield. M.p. 198–200 ◦C (CH3CN).
1H NMR (400 MHz, CD3OD) d 7.51 (d, J = 8.0 Hz, 1H, ArH),
7.32 (d, J = 8.0 Hz, 1H, ArH), 7.2–7.1 (m, 3H, ArH), 7.1–7.0 (m,
3H, ArH), 7.02 (s, 1H, ArH), 7.98 (ddd, J = 7.9, 7.1, 1.0 Hz, 1H,
ArH), 3.53 (td, J = 4.4, 1.0 Hz, 1H), 3.36 (td, J = 4.5, 1.0 Hz, 1H),
3.26 (dd, J = 14.7, 4.6 Hz, 1H), 3.1–3.0 (m, 2H), 2.79 (dd, J = 14.0,
4.7 Hz, 1H) ppm. 13C NMR (100 MHz, CD3OD) d 170.8 (s), 169.9
(s), 138.1 (s), 136.7 (s), 131.3 (d, 2x), 129.5 (d, 2x), 128.9 (s), 128.3
(d), 126.0 (d), 122.7 (d), 120.2 (d), 119.9 (d), 112.3 (d), 109.2 (s),
56.6 (d) 56.5 (d), 39.5 (t), 30.5 (t) ppm. IR (NaCl) n 3400–3050
(br, N–H), 2966 (w, C–H), 2881 (w, C–H), 1673 (s, C O), 1456
(s), 1321 (m), 1090 (w), 1010 (w), 743 (s) cm-1. MS (ESI+) m/z (%)
334 ([M+1]+, 100). HRMS (ESI+) calcd for C20H20N3O2 ([M+1]+),
334.1550; found, 334.1547. [a]2D4 -5 (c 0.12, MeOH). [a]D24 -5 (c
0.12, MeOH).
C
O), 1674 (s, C O), 1609 (m), 1458 (m), 1316 (m), 1211 (m),
742 (s) cm-1. MS (ESI+) m/z (%) 516 ([M+1]+, 100). HRMS (ESI+)
calcd for C29H34N5O4 ([M+1]+), 516.2605; found, 516.2614. [a]1D0
-24 (c 0.08, MeOH).
Diastereoisomer 15 of 2
According to the general procedure described for 2, diastereoiso-
mer 15 was prepared in 55% yield. 1H NMR (400 MHz, (CD3)2CO)
d 7.66 (s, 1H), 7.61 (d, J = 7.8 Hz, 1H), 7.4–7.3(m, 3H), 7.3–7.2
(m, 2H), 7.11 (s, 1H), 7.0–6.9 (m, 1H), 6.9–6.8 (m, 3H), 6.7–6.6
(m, 1H), 6.7–6.5 (m, 3H), 5.92 (d, J = 3.9 Hz, 1H), 4.97 (dd, J =
11.7, 6.2 Hz, 1H), 4.76 (dd, J = 7.5, 4.8 Hz, 1H), 4.3–4.2 (m, 1H),
3.6–3.5 (m, 2H), 3.41 (dd, J = 14.4, 4.7 Hz, 1H), 3.15 (dd, J = 14.4,
5.0 Hz, 1H), 3.00 (dd, J = 14.8, 7.6 Hz, 1H), 2.45 (dd, J = 14.8,
11.8 Hz, 1H), 2.13 (dd, J = 8.5, 4.2 Hz, 1H), 1.6–1.5 (m, 1H), 1.39
(ddd, J = 13.5, 9.0, 4.3 Hz, 1H), 1.27 (ddd, J = 13.5, 8.4, 5.7 Hz,
1H), 0.68 (d, J = 6.5 Hz, 3H), 0.39 (d, J = 6.5 Hz, 3H) ppm. 13
C
(2S,3aS,8aS)-1,8-Di-tert-butyl-2-methyl-3a-(3-(((2S,5R)-5-
benzyl-3,6-dioxopiperazin-2-yl)methyl)-1H-indol-1-yl)-3,3a-
dihydropyrrolo[2,3-b]indole-1,2,8(2H,8aH)-tricarboxylate (18)
NMR (100 MHz, (CD3)2CO) d 171.0 (s), 170.5 (s), 169.6 (s), 168.7
(s), 148.9 (s), 138.5 (s), 136.7 (s), 130.8 (d), 130.7 (s), 130.3 (d,
2x), 129.5 (s), 129.4 (d, 2x), 127.8 (d), 127.5 (d), 123.5 (d), 122.6
(d), 120.8 (d), 120.7 (d), 119.8 (d), 113.2 (d), 111.3 (d), 109.9 (s),
83.6 (d), 75.2 (s), 58.7 (d), 57.0 (d), 56.9 (d), 52.7 (d), 41.4 (t), 40.2
(t), 35.7 (t), 30.8 (t), 24.3 (d), 23.4 (q), 21.2 (q) ppm. IR (NaCl)
n 3500–3050 (br, N–H), 2956 (m, C–H), 2927 (m, C–H), 2870 (w,
C–H), 1673 (s, C O), 1456 (s), 1320 (m), 741 (s) cm-1. MS (ESI+)
m/z (%) 653 ([M+Na]+, 37), 631 ([M+1]+, 100). HRMS (ESI+)
calcd for C40H37N6O4 ([M+1]+), 631.3027; found, 631.3013. [a]2D3
-79 (c 0.09, MeOH).
According to the general procedure described for 5, substituted
indoline 18 was prepared in 30% yield. 1H NMR (400 MHz,
CD3OD) d 7.8–7.6 (m, 1H, ArH), 7.53 (d, J = 7.6 Hz, 1H, ArH),
7.4–7.3 (m, 2H, ArH), 7.2–7.1 (m, 3H, ArH), 7.1–6.9 (m, 7H,
ArH), 6.69 (s, 1H), 5.0–4.9 (m, 1H), 3.57 (dd, J = 12.9, 9.3 Hz,
1H), 3.5–3.4 (m, 1H), 3.23 (s, 3H, CO2CH3), 3.3–3.1 (m, 2H), 3.09
(d, J = 12.9 Hz, 1H), 2.99 (dd, J = 13.9, 4.6 Hz, 2H), 2.80 (dd,
J = 13.9, 4.6 Hz, 1H), 1.55 (s, 9H), 1.49 (s, 9H) ppm. 13C NMR
(100 MHz, CD3OD) d 172.6 (s), 170.6 (s), 169.9 (s), 153.8 (s), 144.9
(s), 136.8 (s), 136. 6 (s), 132.2 (d), 131.4 (s), 131.2 (d, 2x), 131.1
(s), 129.5 (d, 3x), 128.3 (d), 126.6 (d), 127.7 (d), 124.9 (d), 123.5
(d), 121.4 (d), 121.0 (d), 112.4 (d), 110.2 (s), 83.7 (s), 81.3 (d), 74.1
(s), 60.8 (d), 56.6 (d), 56.5 (d), 52.9 (q), 39.5 (t, 2x), 30.2 (t), 28.7
(q, 6x) ppm. IR (NaCl) n 3400–3050 (br, N–H), 2977 (m, C–H),
2930 (m, C–H), 1716 (s, C O), 1681 (s, C O), 1455 (m), 1393
(s), 1157 (s), 1015 (m), 855 (m), 741 (s) cm-1. MS (ESI+) m/z (%)
772 ([M+Na]+, 43), 750 ([M+1]+, 23), 650 ([M–CO2tBu]+, 100).
HRMS (ESI+) calcd for C42H48N5O8 ([M+1]+), 750.3497; found,
750.3499. [a]2D6 +30 (c 0.18, MeOH).
C15-Epimer (16)
According to the general procedure described for 2, diastereoiso-
mer 16 was prepared in 56% yield. 1H NMR (400 MHz, (CD3)2CO)
d 7.69 (s, 1H), 7.62 (d, J = 7.7 Hz, 1H), 7.4–7.3 (m, 2H), 7.3–7.2
(m, 2H), 7.2–7.1 (m, 4H), 7.0–6.8 (m, 4H), 6.86 (d, J = 8.0 Hz,
1H), 6.79 (d, J = 8.3 Hz, 1H), 6.65 (t, J = 7.4 Hz, 1H), 6.62 (d, J =
3.7 Hz, 1H), 6.06 (d, J = 3.4 Hz, 1H), 4.83 (dd, J = 11.6, 5.9 Hz,
1H), 4.53 (dd, J = 6.8, 5.2 Hz, 1H), 4.3–4.2 (m, 1H), 3.68 (dd,
J = 14.6, 6.1 Hz, 1H), 3.41 (dd, J = 14.6, 4.5 Hz, 1H), 3.3–3.2 (m,
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Org. Biomol. Chem., 2010, 8, 5179–5186 | 5185
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(2S,3aS,8aS)-Methyl-3a-(3-(((2S,5R)-5-benzyl-3,6-
dioxopiperazin-2-yl)methyl)-1H-indol-1-yl)-1,2,3,3a,8,8a-
hexahydropyrrolo[2,3-b]indole-2-carboxylate (19)
Acknowledgements
We acknowledge financial support from the European Union
(EPITRON, LSHC-CT-2005-518417), Xunta de Galicia (IN-
BIOMED) and the MICIIN (SAF07-63880, FEDER). We are
thankful to Prof. Y. Che (Beijing) for confirming the identification
of the amino acids, and to Dr Roberto R. Gil (Carnegie Mellon)
and Prof. Nina Berova (Columbia) for fruitful discussions on
NMR and CD spectroscopy, respectively.
According to the general procedure described for 6, diamine 19
was prepared in 85% yield. 1H NMR (400 MHz, CD3OD) d 7.49
(d, J = 7.4 Hz, 1H, ArH), 7.42 (d, J = 7.8 Hz, 1H, ArH), 7.29 (s,
1H, ArH), 7.2–7.1 (m, 3H, ArH), 7.1–6.9 (m, 6H, ArH), 6.7–6.6
(m, 2H, ArH), 5.45 (s, 1H, H8a), 4.17 (dd, J = 7.7, 3.0 Hz, 1H),
3.66 (t, J = 3.9 Hz, 1H), 3.5–3.3 (m, 2H), 3.44 (m, 2H), 3.34 (s,
3H, CO2CH3), 3.1–2.9 (m, 4H), 2.78 (dd, J = 14.0, 4.5 Hz, 1H)
ppm. 13C NMR (100 MHz, CD3OD) d 175.1 (s), 171.0 (s), 172.0
(s), 151.8 (s), 137.2 (s), 136.7 (s), 131.4 (d), 131.1 (d, 2x), 130.9
(s), 129.6 (s), 129.5 (d, 2x), 128.2 (d), 127.8 (d), 125.5 (d), 123.0
(d), 120.9 (d), 120.4 (d), 120.0 (d), 113.2 (d), 111.2 (d), 109.1 (s),
83.4 (d), 77.4 (s), 61.1 (d), 56.8 (d), 56.3 (d), 52.7 (q), 42.8 (t),
39.0 (t), 30.6 (t) ppm. IR (NaCl) n 3500–3050 (br, N–H), 2950 (m,
C–H), 2926 (w, C–H), 1736 (m, C O), 1672 (s, C O), 1607 (m),
1457 (m), 1319 (m), 1209 (m), 741 (s) cm-1. MS (ESI+) m/z (%)
550 ([M+1]+, 100). HRMS (ESI+) calcd for C32H32N5O4 ([M+1]+),
550.2448; found, 550.2438. [a]2D4 +227 (c 0.10, MeOH).
References
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4 V. R. Espejo and J. D. Rainier, J. Am. Chem. Soc., 2008, 130, 12894.
5 For the exo/endo nomenclature in hexahydropyrrolo[2,3-b]indoles see:
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7 The 2-methylcarboxylate group of the endo indoline exhibits a remark-
ably upfield signal at d ~3.1 ppm, whereas the exo shows usually a more
common resonance at d ~3.7 ppm.
(+)-Pestalazine B (21)
According to the general procedure described for 2, isomer 21 was
prepared in 57% yield. 1H NMR (600 MHz, (CD3)2CO) d 7.81 (d,
J = 4.3 Hz, 1H), 7.66 (s, 1H), 7.52 (d, J = 7.8 Hz, 1H), 7.2–7.1
(m, 7H), 7.0–6.9 (m, 4H), 6.85 (d, J = 7.9 Hz, 1H), 6.78 (d, J =
8.2 Hz, 1H), 6.7–6.6 (m, 2H), 6.05 (d, J = 3.3 Hz, 1H), 4.84 (dd,
J = 11.7, 6.0 Hz, 1H), 3.89 (dt, J = 9.6, 4.8 Hz, 1H), 3.69 (dd, J =
14.8, 6.0 Hz, 1H), 3.7–3.6 (m, 1H), 3.50 (br t, J = 4.7 Hz, 1H),
3.21 (app. d, J = 3.2 Hz, 2H), 3.06 (dd, J = 13.9, 5.4 Hz, 1H),
2.96 (dd, J = 13.9, 4.6 Hz, 1H), 2.46 (dd, J = 14.8, 11.7 Hz, 1H),
1.9–1.8 (m, 1H), 1.8–1.7 (m, 1H), 1.52 (ddd, J = 13.6, 8.5, 5.3 Hz,
8 Very recently, Rainier and coworkers have reported an improved
method for the N-alkylation of 3-bromoindolines: V. R. Espejo,
X.-B. Li and J. D. Rainier, J. Am. Chem. Soc., 2010, 132, 8282.
9 Following the suggestion of the authors (ref. 1), DMSO-d6 was used as
co-solvent in an attempt to reproduce the exact conditions in which the
NMR spectra of the natural product was recorded.
1H), 0.95 (d, J = 6.5 Hz, 3H), 0.90 (d, J = 6.5 Hz, 3H) ppm. 13
C
10 P. Marfey, Carlsberg Res. Commun., 1984, 49, 591.
11 (a) M. Varoglu, T. H. Corbett, F. A. Valeriote and P. Crews, J. Org.
Chem., 1997, 62, 7078; (b) S. P. Govek and L. E. Overman, J. Am.
Chem. Soc., 2001, 123, 9468.
NMR (100 MHz, (CD3)2CO) d 169.0 (s, 2x), 168.6 (s, 2x), 148.9
(s), 137.1 (s), 136.6 (s), 131.0 (d, 2x), 130.8 (s), 130.8 (d), 129.1 (d,
2x), 127.7 (d), 126.6 (d), 123.6 (d), 122.4 (d), 120.5 (d), 120.4 (d),
119.7 (d), 113.0 (s), 111.0 (d), 110.0 (s), 83.5 (d), 74.4 (s), 57.3 (d),
56.9 (d), 56.3 (d), 55.8 (d), 42.9 (t), 41.3 (t), 39.1 (t), 30.4 (t), 25.1
(d), 23.3 (q), 21.8 (q) ppm. IR (NaCl) n 3500–3050 (br, N–H), 2956
(m, C–H), 2925 (m, C–H), 2870 (w, C–H), 1682 (s, C O), 1457
(s), 1321 (m), 741 (s) cm-1. MS (ESI+) m/z (%) 631 ([M+1]+, 100).
HRMS (ESI+) calcd for C40H37N6O4 ([M+1]+), 631.3027; found,
631.3027. [a]2D8 +194 (c 0.1, MeOH).
12 T. Shiba and K. Nunami, Tetrahedron Lett., 1974, 15, 509.
13 See the ESI† for CD spectra of compounds 2, 11, 15, 16 and 21.
14 See the ESI† for a detailed comparison of the NMR data of all final
compounds.
15 Diketopiperazine 17 was prepared from L-tryptophan and D-
phenylalanine as described for 3 and 8.
16 Only the NH signals in the 1H NMR spectra differed, most likely due
to the recording conditions of the different samples. See ref. 9.
17 K. C. Nicolaou and S. A. Snyder, Angew. Chem., Int. Ed., 2005, 44,
1012.
5186 | Org. Biomol. Chem., 2010, 8, 5179–5186
This journal is
The Royal Society of Chemistry 2010
©