Total Synthesis of Homo-and Heterodimeric Bispyrrolidinoindoline Dioxopiperazine Natural Products

Article abstract of DOI:10.1021/acs.jnatprod.0c01273

Total synthesis and structural confirmation of homo-and heterodimeric bispyrrolidinoindoline dioxopiperazine alkaloids isolated from fungi and bacteria, namely, ditryptoleucine A, ditryptoleucine B (11), the N,N′-bis-demethylated analogue (+)-12, (-)-dibrevianamide F (13), (-)-SF-5280-451 (14), tetratryptomycin A (15), (-)-Tryprophenaline (17), and (-)-SF-5280-415 (18), has been carried out starting from the corresponding bispyrrolidinoindolines derived from tryptophan. Our efforts to synthesize all possible diastereomers of the natural ditryptoleucine isolates uncovered structural factors that determine the rate and efficiency of dioxopiperazine ring formation, leading in some cases to mixtures of diastereomers by concomitant epimerization, to the formation of their putative monomeric dioxopiperazine dipeptide biogenetic precursors, and to the alternative formation of a dimer with a fused 1,3,5-Triazepan-6-one heterocycle.

Full text of DOI:10.1021/acs.jnatprod.0c01273

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Article
Total Synthesis of Homo- and Heterodimeric Bispyrrolidinoindoline
Dioxopiperazine Natural Products
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Andrea Areal, Marta Domínguez, Pim Vendrig, Susana Alvarez, Rosana Alvarez,* and Angel R. de Lera*
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ABSTRACT: Total synthesis and structural confirmation of homo- and
heterodimeric bispyrrolidinoindoline dioxopiperazine alkaloids isolated from
fungi and bacteria, namely, ditryptoleucine A, ditryptoleucine B (11), the N,N′-
bis-demethylated analogue (+)-12, (−)-dibrevianamide F (13), (−)-SF-5280-
451 (14), tetratryptomycin A (15), (−)-tryprophenaline (17), and (−)-SF-5280-
415 (18), has been carried out starting from the corresponding bispyrrolidi-
noindolines derived from tryptophan. Our efforts to synthesize all possible
diastereomers of the natural ditryptoleucine isolates uncovered structural factors
that determine the rate and efficiency of dioxopiperazine ring formation, leading
in some cases to mixtures of diastereomers by concomitant epimerization, to the
formation of their putative monomeric dioxopiperazine dipeptide biogenetic
precursors, and to the alternative formation of a dimer with a fused 1,3,5-
triazepan-6-one heterocycle.
2,5-Dioxopiperazines, commonly named 2,5-diketopiperazines
(DKPs), are representative members of the cyclic dipeptide
(CDP) family of natural products, which are mainly generated
by intramolecular condensation of dipeptide precursors.
Bioactive natural products with 2,5-dioxopiperazine scaffolds
are known to be produced by marine and terrestrial fungi,
bacteria, plants, and even animals.1
Within this family, the dimeric tryptophan-derived bispyrro-
lidinoindoline dioxopiperazine alkaloids, which have been
mainly isolated from Streptomyces sp. actinobacteria as well as
from Aspergillus/Eurotium fungal species,2,3 contain 2,5-
dioxopiperazines fused to hexahydropyrrolo[2,3-b]indole skel-
etons (Scheme 1 and Figure 1). The two subunits of these
dimeric alkaloids are connected through the C-3 and C-3′
atoms, forming a characteristic arrangement of two contiguous
quaternary stereogenic centers adjacent to two aminals (see 9,
Figure 1).
The biogenesis of symmetrical and nonsymmetrical C-3/C-
3′ bispyrrolidinoindoline dioxopiperazine alkaloids, as in-
dicated for (−)-ditryptophenaline 9 (Figure 1), involves as
the key step the coupling reaction, catalyzed by P450-
dependent oxidase enzymes, of indole-2,5-dioxopiperazine
units 4 derived from tryptophan-tRNA 2 (Scheme 1).2 The
radical generated at the dioxopiperazine ring of 4 evolves to
produce the pyrrolidinoindoline dioxopiperazine radical 5.
Radical recombination of dioxopiperazine units gives rise to C-
3/C-3′ homodimer (−)-ditryptophenaline 9.3−6 Additional
natural products, including 8 (Scheme 1), are alternatively
generated through connection of the monomeric pyrrolidi-
noindoline dioxopiperazine radical at C-3 of 6 to the N-1′
position (C-6′/C-3 or C-7′/C-3 heterodimeric positional
isomers have also been isolated)3,6 of the radical species
generated in 6 (and stabilized by resonance with the indole
ring as depicted in 7).3−8
The biosynthetic machineries of these microorganisms3
further contribute to diversify the already densely function-
alized alkaloid scaffold. Not only permutations on the amino
acids acting as biogenetic building blocks (Ala, Leu, Val, Phe,
Trp, Pro) that condense with the bispyrrolidinoindoline core
but also covalent modifications at the indole and dioxopiper-
azine nitrogen substituents are commonly found in the
structures of these dimeric natural products.3,6
Some representative members (9−18) of the family of
alkaloids relevant to the work described herein are depicted in
Figure 1. They are characterized by sharing homo- and
heterodimeric structures derived from Leu, Phe, Pro, and Trp,
the latter not only forming the bispyrrolidinoindoline core but
also appearing as the distal amino acid of the terminal
dioxopiperazine rings in tetratryptomycin A (15).6,8 A small
group of related homo- and heterodimeric bispyrrolidinoindo-
line dioxopiperazine natural products derived from Ala and Val
as terminal amino acid components are collected in the
Supporting Information (Figure S1).
Received: November 25, 2020
Published: May 21, 2021
© 2021 American Chemical Society and
American Society of Pharmacognosy
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a
Scheme 1. General Biogenesis of a Symmetrical C-3/C-3′ Bispyrrolidinoindoline Dioxopiperazine Alkaloid (9) and the C-3/
N-1′ Dioxopiperazine Analogue (8)3−6
a
Only one of the possible configurations is shown.
(−)-Ditryptophenaline (9) was the first alkaloid of this
group isolated from the mycelium of several strains of
Aspergillus obtained from contaminated food9 (and more
recently from other sources10−12), and the connectivity and
three-dimensional arrangement of the proposed structure was
confirmed by X-ray diffraction analysis9 and total synthesis.13,14
Additional symmetrical dioxopiperazine dimers include
(+)-WIN 64821 (10), isolated from an Aspergillus sp. SC319
culture originally extracted from soil,15 ditryptoleucines A and
B (11), isolated from Aspergillus oryzae (RIB40),16 (+)-12
from the fungus Aspergillus violaceofuscus present in a
Reniochalina sp. marine sponge,17 (−)-SF5280-451 (14),18,19
isolated from Aspergillus sydowii (MSX19583),18 and (−)-tet-
ratryptomycin A (15) obtained from a large-scale fermentation
of Streptomyces albus J1074 expressing a gene cluster8 from
Saccharopolyspora antimicrobica (Supporting Information,
Figure S2).20
Heterodimeric bispyrrolidinoindoline dioxopiperazine natu-
ral products in this series include (+)-WIN 64745 (16), which
was also isolated from Aspergillus sp. SC319 culture,15 and
(−)-SF5280-415 (18), which was obtained, together with
(−)-SF5280-451 (14), from the marine-derived Aspergillus sp.
SF-5280.19
Finally, as a result of the relaxed substrate specificity
exhibited by the P450 oxidase enzyme DtpC, homo- and
heterodimeric analogues termed (−)-dibrevianamide F (13)
and (−)-tryprophenaline (17), in addition to 9, were isolated
from A. flavus and shown to be biosynthetically generated
through radical mechanisms, as indicated in Scheme 1 for
parent 9.4,5
16,21 the inhibition of the LPS-induced expression of cytokine
I6-10 by (+)-12,17 and the inhibition of protein tyrosine
phosphatase PTP1B (the activation of which in the heart has
been associated with heart failure) by 18.19 Likewise,
intermediates used as precursors for the synthesis of 16 and
selected diastereomers have shown antifungal22 and antitumor
activities.23 Most likely, the complex dimeric skeletons might
play the role (in chemical space) of privileged structures24
through multipoint interactions with their biological targets,
which could explain the variety of biological activities of these
compounds.
The structural complexity of these natural products with six
stereocenters makes them challenging targets for organic
synthesis. Following putative biomimetic oxidative dimeriza-
tion reaction of dioxopiperazine biogenetic precursors (see 4,
Scheme 1), (−)-ditryptophenaline (9) (Figure 1) was first
synthesized, although in very low yield (3%),13 and more
recently, the sequence was adapted to the syntheses of 9−11
and 16 (Figure 1).21,25 Departing from the biomimetic
oxidative dimerization, synthetic efforts have mainly focused
on the construction of bispyrrolidinoindolines such as 9
(Figure 1) and ent-10 from bisoxindole diamines26 and of
(−)-9, (+)-10 (Figure 1), and a 1′-(2-phenylethylene)-
ditryptophenaline derivative27,28 by the Co(I)-mediated
dimerization of 3a-bromocyclotryptophans (see 21/26 in
Scheme 2).27−32 On the basis of the bioinspired Co(I)-
promoted dimerization we have also reported the synthesis of
(+)-10 (Figure 1) starting from C3-bromopyrrolidinoindoline
21 (Scheme 2) and extended the protocol to nonsymmetrical
dimeric alkaloids including (+)-16 (Figure 1).33,34
A wide range of biological activities and pharmacological
effects have been reported for these dimeric alkaloids, most
notably the inhibition of foam cell formation in macrophages
and the inhibition of ubiquitin-specific protease 7 (USP7) by
Given the structural similarities within this family of
alkaloids (Figure 1) and our previous approaches to sym-
metrical and nonsymmetrical congeners displaying the
congested dimeric scaffold,33,34 we set out to address the
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Figure 1. Homo- and heterodimeric bispyrrolidinoindoline dioxopiperazine alkaloids derived from Trp, Leu, Phe, and Pro.
total synthesis of the remaining members of the family of
homo- and heterodimeric natural products shown in Figure 1,
namely, 11−15, 17, and 18.
protected D-Trp derivative (R)-23 (Scheme 2), was selected
for the connection of the two monomeric units.27 The use of
appropriately protected D- or L-amino acids, as well as the
epimerization at C-2 of bispyrrolidinoindoline dioxopiper-
azines when required, would allow targeting of alkaloids shown
in Figure 1.
Moreover, as a synthesis tactic, these natural products
(Figure 1) were divided in two groups: A, homodimers that
incorporate the diastereoisomers (19) of the bispyrrolidinoin-
doline core structure of protected D-Trp, namely, (R)-23, and
each enantiomer of protected (N-Me-)L-Leu and (N-Me-)D-
Leu, toward the synthesis of 11 and 12 (exemplified for 11 in
Scheme 2A); and B, homo/heterodimers from L-Trp
derivatives that connect the corresponding protected amino
acids (L-Phe, L-Leu, L-Pro, and L-Trp) to bispyrrolidinoindoline
25 derived from protected L-Trp, namely, (S)-23, toward the
synthesis of 13−15, 17, and 18 (exemplified for 14 in Scheme
2B).
The exo-bromopyrrolidinoindolines 21 and 26 (Scheme 2)
required for the Co(I)-promoted dimerization27,36 were
independently synthesized in three-step sequences33 starting
from commercial D- or L-Trp (Supporting Information, Section
S6.2), namely, formation of the methyl ester, amine protection
as bis-Boc carbamate derivatives (R)-23 and (S)-23, and a
stereoselective bromocyclization reaction.33−35 Treatment of
(R)-23 or (S)-23 with N-bromosuccinimide (NBS) under the
optimized conditions (1 equiv of PPTS, CH2Cl2, 25 °C)34
RESULTS AND DISCUSSION
■
As illustrated in Scheme 2 A, our general retrosynthetic
analysis to these alkaloids, as indicated for 11, involves the late-
stage formation of the dioxopiperazine rings from precursors
such as 19, which would be generated by an amide
condensation reaction between the required amino acids and
the N-deprotected methyl esters of the bispyrrolidinoindoline
20. Two protecting groups (PGs) for the amine of the second
amino acid component, namely, Boc and Cbz, were initially
considered in synthetic planning, in particular with regard to
optimizing the reaction conditions for deprotection and release
of dipeptides 19 prior to or concomitant with dioxopiperazine
ring formation.21,25,33,34 Because the relative configuration of
the ring fusion carbons of the hexahydropyrrolo[2,3-b]indole
core is conserved during the dimerization process27 and this
can be traced back to the configuration of the protected
tryptophan derivative (23) used in the bromocyclization step,
an easy entry into the required diastereo- and enantiomeric
series of homo- and heterobispyrrolidinoindoline dioxopiper-
azine alkaloids should be feasible by adequate choice of the
synthetic partners (Scheme 2A).33,34 Thus, the Co(I)-induced
dimerization of bromopyrrolidinoindoline 21, which could be
generated by diastereoselective bromocyclization reaction35 of
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Scheme 2. Retrosynthetic Analysis of the Proposed Structures of Bispyrrolidinoindoline Dioxopiperazine Alkaloids (Figure 1)
Adapted to 11 and 14, Respectively: (A) from Protected D-Trp, (R)-23, and Coupling to Protected (N-Me-)L-Leu and (N-Me-)
D-Leu; (B) from Protected L-Trp, (S)-23, and Coupling to Protected L-Phe
afforded in 93% yield either 21 or 26 and their anti-
diastereomers (only 22 is shown) in a 94:6 ratio (Section
S6.2), a highly diastereoselective bromocyclization that was
justified through DFT calculations.35
diastereoisomers allowed the structural assignment of their
relative configurations, because the endo bispyrrolidinoindoline
shows a shielded signal (δH ∼3.1) relative to the exo
diastereomer (δH ∼3.7).37
Subsequent cleavage of the four N-Boc protecting groups in
20, 25, and 27 took place upon treatment with TMSI in
CH3CN at 0 °C (Scheme 3).39 Given the water solubility of
the resulting bispyrrolidinoindolines 28−30, a resin bearing
diisopropylamino groups was added to the reaction mixture40
followed by wet MeOH, with the purpose of quenching the
acidic media, which allowed isolation of these dimeric
compounds in high yields (93−95%).
The twofold coupling of endo-28 and exo-29 with N-Cbz-D-
Leu (31a) and N-Boc-D-Leu (31b) and with their enantiomers
derived from L-Leu was carried out in the presence of HATU
and Et3N in DMF to provide the corresponding dimers, 33a,b
and 34a,b and 35a,b and 37a,b, in variable yields (Schemes
4−6), which were purified but not fully characterized due to
the typical line broadening observed in their NMR spectra.
Nitrogen deprotection and dioxopiperazine formation2 were
then performed under alternative reaction conditions, namely,
catalytic hydrogenation for N-Cbz-protected precursors and
acidic treatment for N-Boc-protected analogues. Only the
Total Synthesis of Bispyrrolidinoindoline Dioxopiper-
azine Alkaloids Derived from L-Trp and Enantiomers of
1
Leu. As only H NMR and 13C NMR spectroscopic data, but
not relative and absolute configurations, were provided for
ditryptoleucine 11,16 in order to address the preparation of the
series of diastereomers and extend the protocol to the synthesis
of (+)-12 (Figure 1), the configuration at C-2/C-2′ in
homodimer 20 was inverted to efficiently generate 27 (Scheme
3). The selectivity is considered to be due to the
thermodynamic preference of exo-2-acylhexahydropyrrolo-
[2,3-b]indoles to place the acyl group at the endo position
under base-promoted equilibration conditions37 and thus
reduce torsional interactions around the formal
diazabicyclo[3.3.0]octane core.38 Thus, treatment of the exo
dimer 20 with 4 equiv of lithium hexamethyldisilazide
(LiHMDS) at −15 °C in THF and quenching of the
corresponding lithium ester enolates with MeOH at −78
°C37 afforded the endo diastereomer 27 in 93% yield (Scheme
3). The 1H NMR chemical shifts for the methyl esters of both
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In striking contrast, hydrogenolysis of the N-Cbz protecting
group of 33a (PG = Cbz) using Pearlman’s catalyst [Pd(OH)2
on C (20% wt loading)]44 was followed by dioxopiperazine
ring formation21 upon treatment with excess 28−30% aqueous
NH4OH in MeOH at rt for 2 h and afforded uneventfully the
endo-D-ditryptoleucine diastereomer 11b in 99% yield (Scheme
4B; see also Supporting Information, Scheme S2).
Scheme 3. (A) Synthesis of Diastereomeric
Bispyrrolidinoindolines (28, 29) Derived from D-Trp; (B)
Synthesis of Enantiomeric Bispyrrolidinoindoline (30)
Derived from L-Trp
Similarly, the synthesis of the exo-L-ditryptoleucine diaster-
eomer 11d (Scheme 5) by N-Cbz deprotection21 and ensuing
dioxopiperazine ring formation was carried out in an overall
94% yield upon stirring solutions of bispyrrolidinoindoline 36a
in MeOH (0.01 M) with Pd on C (10% wt loading) at rt for
15h.21
Lastly, hydrogenation of 35a (PG = Cbz) using Pd on C
(10% wt loading) but with excess aqueous ammonia (28%
NH4OH) in MeOH at rt for 4 h afforded exo-D-ditryptoleucine
11c in 67% yield accompanied not only by precursors resulting
from deprotection but also by diastereomer 11d (Scheme 5),
which confirmed the facile epimerization of diketopiperazine
rings under those reaction conditions (Supporting Informa-
tion, Scheme S4).
Interestingly, when either 35a (PG = Cbz) or the
deprotected derivative was treated with Pearlman’s catalyst
[Pd(OH)2 on C (20% wt loading)]44 in MeOH at rt under a
hydrogen atmosphere for 6 h, an unexpected compound was
obtained in high yield (88%). Interpretation of the
spectroscopic data and MS analysis suggested the formation
of a product with the dimeric structure 37 (Scheme 6). Instead
of the expected fused diketopiperazine, homodimer 37
contains a methyl octahydro-2a,5,6a-triazabenzo[a]cyclopenta-
[cd]azulene-2-carboxylate skeleton resulting from the gener-
ation of a fused 1,3,5-triazepan-6-one heterocycle with the
three nitrogen atoms, including those of the pyrrolidinoindo-
line fragment, as part of the new ring.45
Without discarding effects related to the undefined
composition of Pearlman’s catalyst,46 pyrrolidinone fragments
have been previously obtained upon deprotection of N- and O-
benzyl indole/phenol-containing pyrrolidine structures under
similar reaction conditions. An intriguing mechanistic proposal
(Supporting Information, Scheme S5) was further advanced in
order to justify the appearance of oxidation products under
formal hydrogenation reaction conditions.47 In accordance
with the proposed mechanism (Scheme S5), efforts to promote
the formation of the dioxopiperazine ring in 11c starting from
the deprotected precursor under the same conditions but in
the absence of Pd(OH)2 on C proved to be fruitless after 15 h
of stirring.
The contrasting behavior of the diastereomeric precursors
on dioxopiperazine ring formation has already been
noted.21,33,34 We have carried out computational studies
aimed to justify the structural requirement for cyclization of
the series of diastereomers 33−36 on route to the
ditryptoleucine family of natural products, and the results
can be found in the Supporting Information, Figure S6.
Comparison of the spectroscopic data with those reported16
for ditryptoleucines A and B (11) (Supporting Information,
Section 6.2.9; Tables S1−S4) suggested the assignment of the
relative configuration of ditryptoleucine A to diastereomer exo-
L 11d. Although the same conclusion was reached by Ishikawa
and co-workers after their synthesis of ditryptoleucine A,21 the
absolute configuration of exo-L 11d was assigned by structural
analogy with those of known analogues shown in Figure 1 and
the assumption that all these homo- and heterodimeric
procedures that provided the highest yields on the optimized
reaction conditions are shown in Schemes 4−6 (Supporting
Information, Section S6, contains a full description of the
experimental work), which differed slightly depending upon
the relative configuration of the bispyrrolidinoindoline
precursors.
Formation of the dioxopiperazines by condensation of the
amino and the methyl ester groups of 34 to afford endo-L-
ditryptoleucine 11a (Scheme 4A) was found to be the most
challenging of the diastereomeric series, as anticipated by prior
experience on the synthesis of 1633,34 and related dioxopiper-
azines.21 Hydrogenolysis of 34a (PG = Cbz) promoted by Pd
on C in MeOH as catalyst for 16 h led to cyclo-(L-Trp-L-Me-
Leu) 32,41,42 a putative N-Me derivative of natural cyclo-(L-
Trp-L-Leu), in quantitative yield (Supporting Information,
Section S2.9),41 in what could be formally considered as a
retrobiogenetic process.1 The formation of a related
dioxopiperazine, namely, cyclo-(L-Trp-L-Phe), was also re-
ported during the early efforts on the structure elucidation of
(+)-10 (Figure 1).15 The deprotection/cyclization of N-Boc-
protected dipeptide 34b required heating in toluene at 140 °C
with SiO2 in a microwave reactor (MW, 300 W power) for 1
h,43 which afforded homodimer 11a in 70% yield (Scheme 4)
together with additional byproducts (Supporting Information,
Scheme S1). The presence of impurities required further
purification by HPLC (Luna 5 μm PFP, 3 mL/min, 0.1%
HCO2H in 30:70 to 30:70 v/v, CH3CN/Milli-Q H2O
gradient; tR = 11.3 min) and afforded endo-L-ditryptoleucine
11a in 47% yield. Moreover, upon increasing the reaction
temperature from 140 °C to 180 °C, cyclo-(L-Trp-L-Me-Leu)
(32)41,42 was instead obtained in 65% yield (Supporting
Information, Section S6.2.9).
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a
Scheme 4. (A) Synthesis of endo-L-Ditryptoleucine 11a; (B) Synthesis of endo-D-Ditryptoleucine 11b
a
MW: microwave.
alkaloids in Nature were derived from L-Trp. However, the
recent isolation from Aspergillus versicolor 16F-11 (KM605199)
obtained from the marine sponge Phakellia fusca,48 and
structural assignment of (+)-asperflocin, the formal C-11′-
epimer of (+)-WIN 64821 (10) (Figure 1), suggested that it
could be biosynthesized by the fungus through the random
selection of L-Trp and D-Trp.48
Bispyrrolidinoindoline dioxopiperazine (+)-12,18 being
formally the N-demethylated analogue of endo-D-ditryptoleu-
cine 11d, was made following the same general synthetic
sequence (Scheme 7), namely, the condensation promoted by
HATU and Et3N of endo-28 with N-Cbz-D-Leu (R)-38a49 to
afford the bis-amide intermediate 39 in 48% yield and final
amine deprotection−dioxopiperazine ring formation upon
hydrogenation with Pearlman’s Pd(OH)2 on C catalyst (20%
wt loading)44 in MeOH, followed by treatment with 28−30%
aqueous NH4OH in MeOH (Scheme 7). In addition to (+)-12
(62% yield), the deprotected uncyclized tetrapeptide was also
obtained (Supporting Information, Section S6.2.11), thus
suggesting the more favorable DKP formation of the N-methyl
derivatives as shown for the synthesis of 11d. The absolute
configuration of natural product (+)-12 had been proposed,17
after confirmation of the presence of D-Leu by application of
1
Our synthetic sample of endo-D 11b showed also H NMR
data rather similar to those reported for natural ditryptoleucine
B16 and was assigned this structure despite some discrepancies
of the 13C NMR chemical shifts with those reported for the
natural product (Section 6.2.9; Tables S1−S4). As indicated
for exo-L 11d, neither specific rotation nor ECD spectra were
reported for these natural products, and therefore the absolute
configuration remains uncertain.16
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Homodimeric Bispyrrolidinoindoline Dioxopipera-
zines. For the synthesis of the homodimeric alkaloids, dimers
41, 43, and 46 (Scheme 8) were instead required. The former
was obtained in 70% yield upon coupling 30 with N-Cbz-L-Pro
40 (2.4 molar equiv) promoted by COMU and DIPEA in
DMF. Bis-amide 41 was deprotected by hydrogenolysis to the
bispyrrolidinoindoline derivative, which spontaneously under-
went cyclization to generate (−)-dibrevianamide F (13) in
almost quantitative yield (Scheme 8), in accordance with the
results of the exo diastereomers discussed before (Scheme 6).
The 1H NMR and 13C NMR spectra (Supporting Information,
Section 6.2.12; Table S6) and the specific rotation value of
synthetic (−)-13 ([α]24D −369 (c 0.5, CHCl3)) were similar to
Scheme 5. Synthesis of exo-L-Ditryptoleucine 11d
those reported for the natural product ([α]24 −483.91 (c
D
0.32, CHCl3)).4 The X-ray crystal structures of both (−)-13
and (−)-17 (Figure 1) already confirmed their absolute
configuration after isolation from natural sources.4
Similarly, (−)-ditryptophenaline (9) was also synthesized
(Section S6.2.13) in almost quantitative yield (94%) from 30
using protected N-Cbz-N-Me-L-Phe (2.4 molar equiv) as
reactant in the presence of COMU and DIPEA in DMF, and
the intermediate (73% yield) was deprotected by hydro-
genolysis catalyzed by Pd(OH)2 on C (20%) in MeOH
followed by 28% aqueous NH4OH at rt (Supporting
Information, Section S6.2.13, Table S7).
Because (−)-SF-5280−451 (14) is formally the bis-N,N′-
demethylated analogue of (−)-9, exchange of the amino acid
component was required for the coupling to 30. Whereas N-
Cbz-L-Phe 42a40 as coupling partner of 30 in the presence of
HATU and 2,6-lutidine in DMF provided a mixture of
homodimer 43a in 47% yield and monocoupled product 44a
in 23% yield (Scheme 8 B), N-Fmoc-L-Phe 42c under the same
conditions afforded 43c in 74% yield. Deprotection of the
dimeric structure 43c upon treatment with Et2NH in MeOH
Marfey’s method.50,51 Comparison of NMR data (Supporting
Information, Section S6.2.11; Table S5) and specific rotation
values of the synthetic compound ([α]24 +300 (c 0.34,
D
CHCl3)) with those of the natural product ([α]25 +530 (c
D
0.3, MeOH))18 confirmed the stereostructure of (+)-12.
Total Synthesis of Bispyrrolidinoindoline Dioxopiper-
azine Alkaloids Derived From L-Trp, L-Phe, and L-Pro.
Similarly, by choosing the enantiomer of the bispyrrolidinoin-
doline core, namely, exo-30, itself prepared from 25 (derived
from L-Trp; Supporting Information, Section S6.2) by
deprotection of the N-Boc groups (Scheme 3), another series
of natural products with the same configurations at C-11 and
C-15, but differing in the nature of the first and second
terminal amino acids, was prepared.
Scheme 6. Synthesis of exo-D-Ditryptoleucine 11c and Bispyrrolidinoindoline 1,3,5-Triazepan-6-one 37
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Scheme 7. Synthesis of Homodimeric Bispyrrolidinoindoline Dioxopiperazine (+)-12
1
for 15 h gave rise to (−)-14 in 78% yield. The H NMR and
13C NMR spectra (Section S6.2.13; Table S7) and the specific
rotation value ([α]24 −393 (c 0.13, CHCl3); cf. [α]24 −343
ucts resulting from the monocoupling of protected amino acid
peptide components when the reaction temperature was
maintained at −15 °C.33,34 However, in addition to the
heterodimeric bis-coupled reaction product, the homodimeric
analogues were also isolated in lower to comparable yields
depending upon the nature of the amino acid (cf. 10−15% for
L-Phe, 19% for D-Val, and 41% for D-Ala).58 Similar
observations were made by Ishikawa et al. during the
preparation of related alkaloids using an analogous strategy.21
In order to overcome some of these limitations, for the
nonsymmetrical bispyrrolidinoindoline dioxopiperazine natural
products (−)-tryprophenaline (17) and (−)-SF-5280−415
(18), the choice of sequential monocoupling reactions allowed
further improvement of the selectivity (Schemes 9 and 10).
Thus, Cbz-protected (S)-31a (1.3 molar equiv) was added to a
solution of bispyrrolidinoindoline 30 in DMF at 0 °C, followed
by COMU and DIPEA to provide, after 26 h, monocoupled
derivative 48 in 93% yield. Removal of the Cbz protecting
group using Pd(OH)2 on C as catalyst followed by treatment
with NH4OH gave rise to dioxopiperazine 49. A second
condensation of 49 with an excess of protected L-Pro 40 (2.4
equiv) afforded 50, and final hydrogenation of the latter
component with 10% Pd on C in MeOH provided (−)-17 in
77% yield (Scheme 9). Comparison of the NMR data
(Supporting Information, Section S6.2.17; Table S10) and
D
D
(c 0.04, CH2Cl2))18 were similar to those reported for the
natural product. The configuration of (−)-14 had previously
been proposed based on the similarity of the ECD spectrum to
that of (−)-9.18 (−)-SF-5280−451 (14) has also been recently
prepared,23 together with several diastereomers, using the same
strategy based on the epimerization of the bispyrrolidinoindo-
lines and peptide formation followed by DKP generation, but
its natural occurrence18 was not mentioned.
Similarly, the recently discovered (−)-tetratryptomycin A
(15)6 was more conveniently synthesized in 38% combined
yield by 2-fold condensation of 30 and N-Fmoc-L-Trp 45c in
the presence of DMT-MM in EtOH at room temperature and,
without further purification (Scheme 8C), deprotection of the
resulting intermediate 46c upon treatment with morpholine.
The 1H NMR and 13C NMR spectra (Supporting Information,
Section S6.2.15; Table S9) were similar to those reported for
the natural product. Although no absolute configuration was
reported for (−)-15,6 it has been assumed as indicated based
on the similarity of the ECD spectrum6 with that described for
analogues.52 When N-Cbz-L-Trp 45a was instead used for
coupling with 30, not only did the condensation reaction
promoted by BOPCl and Et3N in THF turn out to be less
efficient (22−36% yield), but the acyclic tetrapeptide 46a
underwent cleavage of the pyrrolidinoindoline fragment upon
treatment with H2 and Pd on C followed by aqueous ammonia
as described for 34a (Scheme 4A) and provided rather
complex reaction mixtures, from which cyclo-[L-Trp-L-Trp]
(47)53 (Scheme 8C) was the only product that could be
obtained (in 16% yield) and fully characterized (Supporting
Information, Section S6.2.16). Natural 47 has been previously
isolated during biogenetic studies aimed to discover P450
dimerization enzymes after heterologous expression of
candidate genes and gene clusters in Streptomyces albus
J1074, which led to the discovery of cWW synthases termed
TtpA1 and TtpA2.6 Additional cyclodipeptides leading
biogenetically to the same product42,54 and further involve-
ment of 47 in the biogenesis of related natural congeners have
recently been demonstrated.55−57
specific rotation values ([α]25 −419 (c 0.13, CHCl3); cf.
D
[α]27D −378.95 (c 0.43, CHCl3))4 allowed confirmation of the
structure of the natural product.4
A similar ordering of events was used for the synthesis of
(−)-18 (Scheme 10). First, condensation of 30 with N-Cbz-
Phe (S)-42a (1.3 mol equiv) in the presence of HATU and
anhydrous 2,6-lutidine (5 molar equiv) for 20 h at rt led
selectively to 44a. HPLC monitoring of the reaction progress
confirmed the selectivity of the monocoupling reaction.
Deprotection by hydrogenolysis afforded monoprotected
derivative 51 in 51% yield. Condensation of intermediate 51
with excess (2.6 molar equiv) N-Cbz-Leu (S)-38a (2.6 equiv)
promoted by COMU and DIPEA as indicated above, followed
by deprotection of 52 under catalytic hydrogenation
conditions, also resulted in DKP ring formation and afforded
(−)-18 in 74% yield (Scheme 10).
Heterodimeric Bispyrrolidinoindoline Dioxopipera-
zines. We have reported the selective one-pot formation of
bispyrrolidinoindoline dioxopiperazines as condensation prod-
Comparison of the NMR data (Supporting Information,
Section S6.2.18; Table S11) and specific rotation value ([α]25
D
−135 (c 0.055, MeOH); cf. [α]27 −261 (c 0.5, MeOH))19
D
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Scheme 8. (A) Synthesis of (−)-Dibrevianamide F (13); (B) Synthesis of (−)-SF-5280-451 (14); (C) Synthesis of
(−)-Tetratryptomycin A (15)
likewise confirmed the structure proposed for the natural
product. As indicated for symmetrical (−)-SF5280-451 (6),
the nonsymmetrical (−)-SF5280-415 (18) showed the same
absolute configuration as (−)-ditryptophenaline (9)19 but
different configurations on some of the stereocenters when
compared to (+)-WIN 64745 (16).15
dioxopiperazine ring formation allowed completion of the
synthesis of homodimeric (C2-symmetric) bispyrrolidinoindo-
line dioxopiperazine natural products. Control of peptide bond
formation by stepwise addition of the different amino acids
before diketopiperazine ring construction led instead to the
heterodimeric C1-nonsymmetric alkaloids. The base-induced
epimerization of the exo- to the endo-C2-acyl-
hexahydropyrrolo[2,3-b]indole further expanded the stereo-
chemical diversification of the family of dimeric alkaloids with
the synthesis of the C-11/C-11′ epimers. Having the more
congested dimeric scaffold, the exo-D-diastereomer under
hydrogenation conditions using Pearlman’s catalyst in MeOH
CONCLUSIONS
■
In summary, following the generation of the bispyrrolidinoin-
doline scaffold by using as a key step the Co(I)-induced
dimerization of the C3a-bromo-hexahydropyrrolo[2,3-b]indole
core to construct the C-3/C-3′ central bond, the subsequent
amide formation with Leu, Phe, Pro, and Trp and final
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Scheme 9. Synthesis of (−)-Tryprophenaline (17)
Scheme 10. Synthesis of (−)-SF-5280-415 18
led instead to the quantitative formation of a novel
bispyrrolidinoindoline-1,3,5-triazepan-6-one skeleton.
remains to be corroborated by comparison with the values of
natural ditryptoleucines,16 total synthesis continues to be the
ultimate proof for the confirmation and/or determination of
the three-dimensional structures of natural products.59
The sign of the specific rotation [α]D data for this family of
dimeric alkaloids is highly consistent with the configuration of
the hexahydropyrroloindole monomer. A positive [α]D value
would indicate the R configuration at C-2/C-2′ and C-3/C-3′
as in (+)-WIN 64821 (10), (+)-12, and (+)-WIN 64745 (16),
whereas a negative [α]D value would confirm the S
configuration for the same stereogenic centers in the series
of (−)-ditryptophenaline (9) and analogues, namely, (−)-di-
brevianamide F (13), (−)-SF-5280-451 (14), (−)-tetratrypto-
mycin A (15), (−)-tryprophenaline (17), and (−)-SF-5280-
415 (18). Although the absolute configuration of 11a and 11b
EXPERIMENTAL SECTION
■
General Experimental Procedures. Specific rotations were
obtained on a JASCO P-1020 polarimeter. IR spectra were obtained
on a JASCO IR 4200 spectrophotometer from a thin film deposited
onto NaCl glass. 1H NMR spectra were recorded in CDCl3, CD3CN,
or DMSO-d6 at ambient temperature (or the indicated temperature)
on a Bruker AMX-400 spectrometer at 400 (or 600 MHz) with
residual protic solvent as the internal reference (CDCl3, δH 7.26;
CD3CN, δH 1.94; DMSO-d6, δH 2.50). 13C NMR spectra were
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1
recorded in CDCl3, CD3CN, or DMSO-d6 at ambient temperature
unless otherwise indicated on the same spectrometer at 100 MHz,
with the central peak of CDCl3 (δC 77.16), CD3CN (δC 118.26), or
DMSO-d6 (δC 39.52) as the internal reference. DEPT135 and
bidimensional (COSY, HSQCed, HMBC, and NOESY) sequences
were used where appropriate to aid in the assignment of signals. Mass
spectra and HRMS (ESI+) were taken on an Apex III FT ICR MS
(Bruker Daltonics) apparatus.
(s); H NMR(400 MHz, CDCl3) δH 7.38 (d, J = 7.6 Hz, 2H, H5 +
H5′), 7.11 (t, J = 8.0 Hz, 2H, H7 + H7′), 6.79 (t, J = 8.0 Hz, 2H, H6 +
H6′), 6.60 (d, J = 7.9 Hz, 2H, H8 + H8′), 5.78 (s, 2H, 2×NH), 4.81 (s,
2H, H2 + H2′), 3.90 (app t, J = 10.0 Hz, 2H, H11 + H11′), 3.8−3.7 (br,
2H, H15 + H15′), 3.32 (dd, J = 13.5, 8.0 Hz, 2H, H12a + H12a′), 2.83 (s,
6H, 2×NCH3), 2.70 (dd, J = 13.5, 11.0 Hz, 2H, H12b + H12b′), 1.86
(ddd, J = 14.8, 7.9, 3.0 Hz, 2H, H17a + H17a′), 1.73 (dt, J = 14.8, 5.7
Hz, 2H, H17b + H17b′), 1.4−1.3 (m, 2H, H18 + H18′), 0.66 (d, J = 6.6
Hz, 6H, 2×CH3), 0.52 (d, J = 6.5 Hz, 6H, 2×CH3); 13C NMR (100
MHz, CDCl3) δC 166.8 (C), 166.6 (C), 149.3 (C), 130.8 (C), 129.9
(CH), 125.1 (CH), 120.5 (CH), 110.4 (CH), 80.5 (CH), 61.7 (CH),
59.9 (C), 56.8 (CH), 39.8 (CH2), 38.8 (CH2), 31.9 (CH3), 24.5
(CH), 23.5 (CH3), 22.7 (CH3); ESIMS m/z 625 [M + H]+;
HRESIMS (ESI+) m/z 625.3496 [M + H]+ (calcd for C36H45N6O4,
625.3493).
Synthesis of endo-D-Ditryptoleucine 11b. To a solution of the
dipeptide 33a (93 mg, 0.1 mmol, 1.0 equiv) in MeOH (0.7 mL, 0.01
M) was added activated Pd on C (69 mg, 10% wt loading, 1.05 g/
mmol). The argon atmosphere was replaced by hydrogen, which was
allowed to bubble into the solution for 5−10 min. The resulting
suspension was stirred at rt under hydrogen for 4 h. The reaction
mixture was filtered through Celite washing with EtOAc, and the
solvent was evaporated to afford 61.8 mg of the deprotected product,
which was used in the next step without further purification.
Procedure 1. The deprotected dimer (10 mg, 0.014 mmol, 1.0
equiv) was treated with a 28−30% v/v aqueous solution of ammonia
(52 μL, 3.68 mL/mmol) in MeOH (2.3 mL, 0.006 M). The resulting
mixture was stirred at rt for 72 h. The solvent was eliminated under
reduced pressure, and the residue was purified by flash-column
chromatography (silica gel, from 100:0 to 95:5 v/v CH2Cl2/MeOH)
to afford 8.1 mg (90% yield) of endo-D-ditryptoleucine 11b.
Solvents were dried according to published methods and distilled
before use. All reagents were commercial compounds of the highest
purity available. Reactions were carried out under an argon
atmosphere, unless indicated otherwise. Analytical TLC was
performed on aluminum plates with Merck Kieselgel 60F254 and
visualized by UV irradiation (254 nm) or by staining with an
ethanolic solution of phosphomolibdic acid. Flash-column chroma-
tography was carried out using Merck Kieselgel 60 (230−400 mesh)
under pressure.
General Procedure for Boc-Deprotection of Homodimeric
Bispyrrolidinoindolines. To a cooled (0 °C) solution of the protected
homodimer 27 (0.4 g, 0.48 mmol, 1.0 equiv) in CH3CN (11 mL, 0.05
M) was added TMSI (300 μL, 4.4 equiv). The resulting mixture was
stirred for 90 min until the reaction was completed, which was
followed by TLC. Diisopropylamine resin (400 mg) and MeOH (16
mL) were added to the mixture at 0 °C, the temperature was raised to
25 °C, and it was stirred for another 20 min. The suspension was
filtered, the resin was washed with MeOH and CH2Cl2, and the
solvent was eliminated under reduced pressure to afford the
deprotected tetraamine. When the product was used in homocoupling
reactions, the residue could be used without further purification
assuming a quantitative yield. If the product was used in
monocoupling reactions, the residue was purified by flash-column
chromatography (silica gel, from 100:0 to 90:10 v/v CH2Cl2/MeOH)
to afford 190 mg (93% yield) of deprotected bispyrrolidinoindoline
28.
General Procedure for Peptide Coupling of Leucine to
Homodimeric Bispyrrolidinoindolines. For the coupling reactions,
the two enantiomers of N-Boc- or N-Cbz-N-Me-Leu previously
synthesized (Section S6.4) were used. The protected L- or D-Leu
(0.33 mmol, 3.0 equiv) was added to a solution of the tetraamine
(0.05 g, 0.11 mmol, 1.0 equiv) in DMF (2 mL, 0.06 M) at 0 °C,
followed by HATU (0.13 g, 0.33 mmol, 3.0 equiv). After purging the
resulting mixture for 5−10 min, the inert gas was removed and
anhydrous Et3N (71 μL, 0.52 mmol, 4.5 equiv) was added. The
reaction mixture was stirred overnight at rt. H2O and EtOAc were
added, the aqueous layer was extracted with EtOAc (3×), the
combined organic layers were washed with H2O (4×), dried over
anhydrous Na2SO4, and filtered, and the solvent was evaporated
under reduced pressure. The residue was purified by flash-column
chromatography (silica gel, from 80:20 to 50:50 v/v hexane/EtOAc)
to afford the bis-coupled reaction products in yields ranging from 40%
to 70%. The compounds 32−36 could not be fully characterized
because of the complexity of the spectra, in most of the cases due to
the presence of rotamers.
Procedure 2. To a solution of the deprotected dipeptide (10 mg,
0.014 mmol, 1.0 equiv) in MeOH (1.4 mL, 0.01 M) were added
Pearlman’s catalyst (6 mg, 20% wt. loading, 0.41 g/mmol) and a 28−
30% v/v aqueous solution of ammonia (52 μL, 3.68 mL/mmol). The
resulting suspension was stirred at rt under hydrogen for 2 h, then
filtered through Celite washing with MeOH, and the solvent was
evaporated. The resulting residue was purified by flash-column
chromatography (silica gel, 98:2 v/v CH2Cl2/MeOH) to afford 8.7
mg (99% yield for the two steps) of endo-D-ditryptoleucine 11b.
[α]23 +120 (c 0.17, CHCl3); UV (MeOH) λmax (nm) 243, 300; IR
D
(NaCl) νmax (cm−1) 3600−3100 (br, N−H), 2960 (m, C−H), 2873
1
(w, C−H), 1663 (s, CO), 1598 (w, CO), 1465 (m); H NMR
(400 MHz, CDCl3) δH 7.34 (d, J = 7.5 Hz, 2H, H5 + H5′), 7.14 (td, J
= 7.7, 1.1 Hz, 2H, H7 + H7′), 6.79 (td, J = 7.5, 1.0 Hz, 2H, H6 + H6′),
6.63 (dd, J = 7.9, 0.7 Hz, 2H, H8 + H8′), 5.63 (s, 2H, 2×NH), 4.96 (s,
2H, H2 + H2′), 4.02 (app t, J = 8.9 Hz, 2H, H11 + H11′), 3.67 (dd, J =
7.9, 5.8 Hz, 2H, H15 + H15′), 3.22 (dd, J = 14.0, 9.0 Hz, 2H, H12a
+
H
12a′), 2.83 (s, 6H, 2×NCH3), 2.9−2.8 (m, 2H, H12b + H12b′), 1.6−
1.5 (m, 2H, H18 + H18′), 1.5−1.4 (m, 4H, H17a + H17a′ + H17b
H
+
17b′), 0.88 (d, J = 6.6 Hz, 6H, 2×CH3), 0.82 (d, J = 6.5 Hz, 6H,
2×CH3); 13C NMR (100 MHz, CDCl3) δC 168.3 (C), 167.2 (C),
149.1 (C), 130.5 (C), 129.9 (CH), 125.2 (CH), 120.0 (CH), 110.4
(CH), 80.7 (CH), 63.5 (CH), 60.6 (C), 56.6 (CH), 40.1 (CH2), 38.3
(CH2), 33.2 (CH3), 24.8 (CH), 23.3 (CH3), 22.5 (CH3); MS ESIMS
m/z 625 [M + H]+; HRESIMS m/z 625.3497 [M + H]+ (calcd for
C36H45N6O4, 625.3493).
Synthesis of the Four Stereoisomers of Ditryptoleucine 11.
Synthesis of endo-L-Ditryptoleucine 11a (Scheme 4). To a solution
of the dipeptide 34b (40 mg, 0.045 mmol, 1.0 equiv) in toluene (2.2
mL, 0.02 M) was added silica gel (585 mg, 13 g/mmol), and the
resulting suspension was heated in a microwave reactor at 140 °C for
1 h. The silica gel was washed with a 90:10 v/v CH2Cl2/MeOH
solution, and the solvent was evaporated. Purification by flash-column
chromatography (silica gel, 95:5 to 90:10 v/v CH2Cl2/MeOH)
afforded 18 mg of impure ditryptoleucine. The product was further
purified by HPLC-UV (Luna 5 μm PFP, 254 and 280 nm, 3 mL/min,
0.1% HCO2H in 30:70 to 30:70 v/v CH3CN/Milli-Q H2O gradient;
tR = 11.3 min) to obtain 7.6 mg (47%) of a white solid, which was
characterized as endo-L-ditryptoleucine 11a. The purification by
HPLC was required because the reaction conditions led to some
Synthesis of exo-D-Ditryptoleucine 11c. To a solution of the
dipeptide 35a (15 mg, 0.016 mmol, 1.0 equiv) in MeOH (1.6 mL,
0.01 M) was added activated Pd on C (16 mg, 10% wt loading, 1.05
g/mmol). The argon atmosphere was replaced by hydrogen, which
was bubbled into the solution for 5 min. The resulting suspension was
stirred at rt under hydrogen for 2 h, and the hydrogen was then
replaced by argon and a 28−30% v/v aqueous solution of ammonia
(59 μL, 3.68 mL/mmol). After 2.5 h of stirring, the reaction mixture
was filtered through Celite washing with EtOAc and the solvent was
evaporated. The residue was purified by flash-column chromatog-
raphy (silica gel, from 100:0 to 90:10 v/v CH2Cl2/MeOH) to afford
6.7 mg (67% yield for the two steps) of exo-D-ditryptoleucine 11c.
epimerization of the substrate. [α]23 +110 (c 0.6, CHCl3); UV
D
(MeOH) λmax (nm) 240, 299; IR (NaCl) νmax (cm−1) 3600−3100
(br, N−H), 2956 (m, C−H), 2869 (w, C−H), 1658 (s, CO), 1460
[α]25 +271 (c 0.34, CHCl3); UV (MeOH) λmax (nm) 244, 303; IR
D
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(NaCl) νmax (cm−1) 3600−3100 (br, N−H), 2957 (m, C−H), 2870
pressure to afford 7.5 mg (88% yield) of compound 37 as a white
1
(w, C−H), 1658 (s, CO), 1605 (w, CO), 1457 (s); H NMR
solid: [α]25 +131 (c 0.16, CHCl3); UV (MeOH) λmax (nm) 252,
D
(400 MHz, CDCl3) δH 7.24 (d, J = 7.7 Hz, 2H, H5 + H5′), 7.09 (td, J
= 7.6, 1.2 Hz, 2H, H7 + H7′), 6.73 (td, J = 7.5, 1.0 Hz, 2H, H6 + H6′),
6.55 (d, J = 7.8 Hz, 2H, H8 + H8′), 5.45 (s, 2H, 2×NH), 5.16 (s, 2H,
H2 + H2′), 3.95 (app ddd, J = 11.4, 5.7, 1.9 Hz, 2H, H11 + H11′), 3.86
(dd, J = 5.3, 2.5 Hz, 2H, H15 + H15′), 2.86 (s, 6H, 2×NCH3), 2.8−2.7
(m, 4H, H12a + H12a′ + H12b + H12b′), 1.7−1.6 (m, 2H, H18 + H18′),
1.4−1.2 (m, 4H, H17a + H17a′ + H17b + H17b′), 0.83 (d, J = 6.6 Hz, 6H,
2×CH3), 0.54 (d, J = 6.6 Hz, 6H, 2×CH3); 13C NMR (100 MHz,
CDCl3) δC 166.4 (C), 165.2 (C), 151.3 (C), 130.3 (CH), 126.4 (C),
125.9 (CH), 119.5 (CH), 110.0 (CH), 79.1 (CH), 61.2 (CH), 59.5
(C), 58.7 (CH), 38.5 (CH2), 37.7 (CH2), 32.3 (CH3), 24.6 (CH),
23.8 (CH3), 22.8 (CH3); ESIMS m/z 625 [M + H]+; HRESIMS m/z
625.3497 [M + H]+ (calcd for C36H45N6O4, 625.3493).
Synthesis of exo-L-Ditryptoleucine 11d. Procedure 1. To a
solution of the dipeptide 36a (93 mg, 0.09 mmol, 1.0 equiv) in
MeOH (9 mL, 0.01 M) was added activated Pd on C (112 mg, 10%
wt loading, 1.05 g/mmol). The argon atmosphere was replaced by
hydrogen, which was allowed to bubble into the solution for 3−5 min.
The resulting suspension was stirred at rt under hydrogen overnight.
The reaction was filtered through Celite washing with EtOAc, and the
solvent was evaporated under vacuum to afford 62.4 mg of exo-L-
ditryptoleucine 11d (94% yield for the two transformations). Further
purification was not required.
306; IR (NaCl) νmax (cm−1) 2953 (m, C−H), 2867 (w, C−H), 1745
1
(s, CO), 1651 (s, CO), 1601 (w, CO), 1464 (s); H NMR
(400 MHz, CDCl3) δH 7.20 (td, J = 7.8, 1.2 Hz, 2H, H7 + H7′), 7.09
(d, J = 7.4 Hz, 2H, H5 + H5′), 6.77 (t, J = 7.4 Hz, 2H, H6 + H6′), 6.70
(d, J = 8.0 Hz, 2H, H8 + H8′), 5.28 (s, 2H, H2 + H2′), 4.79 (d, J = 14.8
Hz, 2H, H16a + H16a′), 4.40 (d, J = 14.8 Hz, 2H, H16b + H16b′), 3.99
(dd, J = 10.0, 6.9 Hz, 2H, H11 + H11′), 3.7−3.6 (m, 2H, H15 + H15′),
3.69 (s, 6H, 2×CO2Me), 2.50 (dd, J = 12.7, 10.1 Hz, 2H, H12a
+
H
12a′), 2.40 (dd, J = 12.7, 7.0 Hz, 2H, H12b + H12b′), 2.15 (s, 6H, H21
+ H21′), 1.7−1.5 (m, 4H, H17a + H17a′ + H18 + H18′), 1.4−1.3 (m, 2H,
H17b + H17b′), 0.92 (d, J = 6.5 Hz, 6H, 2×CH3), 0.89 (d, J = 6.4 Hz,
6H, 2×CH3); 13C NMR (100 MHz, CDCl3) δC 173.1 (C), 173.0 (C),
150.1 (C), 130.6 (CH), 126.9 (C), 126.0 (CH), 118.9 (CH), 107.3
(CH), 82.6 (CH), 67.9 (CH2), 62.7 (CH), 59.8 (C), 59.1 (CH), 52.8
(CH3), 37.0 (CH2), 35.4 (CH3), 34.0 (CH2), 24.6 (CH), 24.0
(CH3), 22.3 (CH3); ESIMS m/z 713 [M + H]+; HRESIMS m/z
713.4028 [M + H]+ (calcd for C40H53N6O6, 713.4021).
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.jnatprod.0c01273.
■
sı
Procedure 2. To a solution of the dipeptide 36a (70 mg, 0.07
mmol, 1.0 equiv) in MeOH (7 mL, 0.01 M) was added Pearlman’s
catalyst (28.7 mg, 20% wt loading, 0.41 g/mmol). The argon
atmosphere was replaced by hydrogen, which was allowed to bubble
into the solution for 10 min. The resulting suspension was stirred at
room temperature under hydrogen for 5 h. The reaction was filtered
through Celite washing with MeOH, and the solvent was evaporated.
The residue was purified by flash-column chromatography (silica gel,
from 100:0 to 90:10 v/v CH2Cl2/MeOH) to afford 36.5 mg (85%
Structures of additional homodimeric and heterodimeric
bispyrrolidinoindoline dioxopiperazine natural products,
further efforts on the synthesis of ditryptoleucines,
mechanistic proposal for formation of 37, experimental
procedures, spectroscopic characterization, and copies of
NMR spectra for synthetic intermediates and final
products, and computational studies on bispyrrolidi-
noindoline dioxopiperazine ring formation (PDF)
yield for the two steps) of exo-L-ditryptoleucine 11d: [α]25 +418 (c
D
0.38, CHCl3); UV (MeOH) λmax (nm) 244, 301; IR (NaCl) νmax
(cm−1) 3600−3100 (br, N−H), 2957 (m, C−H), 2870 (w, C−H),
1608 (s, CO), 1464 (s); 1H NMR (400 MHz, CDCl3) δH 7.23 (d, J
= 7.6 Hz, 2H, H5 + H5′), 7.15 (t, J = 7.5 Hz, 2H, H7 + H7′), 6.80 (t, J
= 7.5 Hz, 2H, H6 + H6′), 6.63 (t, J = 7.8 Hz, 2H, H8 + H8′), 5.25 (s,
2H, H2 + H2′), 5.00 (s, 2H, 2×NH), 3.82 (dd, J = 10.4, 6.7 Hz, 2H,
H11 + H11′), 3.74 (t, J = 6.9 Hz, 2H, H15 + H15′), 2.92 (s, 6H,
2×NCH3), 2.7−2.6 (m, 4H, H12a + H12a′ + H12b + H12b′), 1.7−1.6 (m,
2H, H18 + H18′), 1.48 (t, J = 7.0 Hz, 4H, H17a + H17a′ + H17b + H17b′),
0.87 (d, J = 6.5 Hz, 12H, 4×CH3); 13C NMR (100 MHz, CDCl3) δC
167.4 (C), 167.0 (C), 150.4 (C), 130.3 (CH), 127.0 (C), 125.9
(CH), 119.7 (CH), 110.5 (CH), 79.0 (CH), 63.5 (CH), 59.3 (C),
58.3 (CH), 41.1 (CH2), 36.5 (CH2), 33.5 (CH3), 24.9 (CH), 23.2
(CH3), 22.8 (CH3); 1H NMR (400 MHz, DMSO-d6) δH 7.23 (d, J =
7.4 Hz, 2H, H5 + H5′), 7.05 (td, J = 7.6, 1.2 Hz, 2H, H7 + H7′), 6.71
(s, 2H, 2×NH), 6.66 (td, J = 7.9, 1.0 Hz, 2H, H6 + H6′), 6.61 (dd, J =
7.9, 1.0 Hz, 2H, H8 + H8′), 5.09 (s, 2H, H2 + H2′), 3.9−3.8 (m, 2H,
H15 + H15′), 3.8−3.7 (m, 2H, H11 + H11′), 2.81 (s, 6H, 2×NCH3),
AUTHOR INFORMATION
■
Corresponding Authors
́
Rosana Alvarez − Departamento de Química Orgánica,
CINBIO, and Instituto de Investigacións Biomédicas de Vigo
(IBIV), Universidade de Vigo, 36310 Vigo, Spain;
orcid.org/0000-0001-5608-7561; Phone: 34 986
812316; Email: qolera@uvigo.es; Fax: 34 986 812770
́
Angel R. de Lera − Departamento de Química Orgánica,
CINBIO, and Instituto de Investigacións Biomédicas de Vigo
(IBIV), Universidade de Vigo, 36310 Vigo, Spain;
orcid.org/0000-0001-6896-9078; Phone: 34 986
812632; Email: rar@uvigo.es; Fax: 34 986 812770
Authors
Andrea Areal − Departamento de Química Orgánica,
CINBIO, and Instituto de Investigacións Biomédicas de Vigo
(IBIV), Universidade de Vigo, 36310 Vigo, Spain
Marta Domínguez − Departamento de Química Orgánica,
CINBIO, and Instituto de Investigacións Biomédicas de Vigo
(IBIV), Universidade de Vigo, 36310 Vigo, Spain
Pim Vendrig − Departamento de Química Orgánica,
CINBIO, and Instituto de Investigacións Biomédicas de Vigo
(IBIV), Universidade de Vigo, 36310 Vigo, Spain
Susana Alvarez − Departamento de Química Orgánica,
CINBIO, and Instituto de Investigacións Biomédicas de Vigo
(IBIV), Universidade de Vigo, 36310 Vigo, Spain
2.6−2.4 (m, 4H, H12a + H12a′ + H12b + H12b′), 1.6−1.5 (m, 2H, H18
+
H
18′), 1.5−1.4 (m, 4H, H17a + H17a′ + H17b + H17b′), 0.84 (d, J = 6.5
Hz, 6H, 2×CH3), 0.81 (d, J = 6.4 Hz, 6H, 2×CH3); 13C NMR (100
MHz, DMSO-d6) δC 166.7 (C), 165.5 (C), 151.5 (C), 129.7 (CH),
127.2 (C), 125.3 (CH), 118.0 (CH), 109.4 (CH), 77.7 (CH), 62.2
(CH), 58.8 (C), 57.5 (CH), 40.0 (CH2), 37.0 (CH2), 32.5 (CH3),
24.4 (CH), 23.3 (CH3), 22.4 (CH3); ESIMS m/z 625 [M + H]+;
HRESIMS m/z 625.3497 [M + H]+ (calcd for C36H45N6O4,
625.3493).
Synthesis of Bispyrrolidinoindoline 1,3,5-Triazepan-6-one 37. To
a solution of the bis-coupled product 35a (10 mg, 0.01 mmol, 1.0
equiv) in MeOH (1 mL, 0.01 M) was added Pearlman’s catalyst (4
mg, 20% wt loading, 0.41 g/mmol). The argon atmosphere was
replaced by hydrogen, which was allowed to bubble into the solution
for 5 min. The resulting suspension was stirred at rt under hydrogen
for 5 h. The reaction mixture was filtered through Celite washing with
CHCl3 and MeOH, and the solvent was evaporated under reduced
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.jnatprod.0c01273
Notes
The authors declare no competing financial interest.
1736
https://doi.org/10.1021/acs.jnatprod.0c01273
J. Nat. Prod. 2021, 84, 1725−1737

Journal of Natural Products
pubs.acs.org/jnp
Article
(28) Movassaghi, M.; Ahmad, O. K.; Lathrop, S. P. J. Am. Chem. Soc.
2011, 133, 13002−13005.
ACKNOWLEDGMENTS
■
This work was supported by funds from the Spanish MINECO
(SAF2016-77620-R-FEDER), Xunta de Galicia (Consolida-
ción GRC ED431C 2017/61 from DXPCTSUG; ED-431G/02
INBIOMED-FEDER “Unha maneira de facer Europa”). We
are grateful to Dr. P. Villar for computations of the minimized
dimeric structures shown in Figure S6. We thank Centro de
Apoio Científico-Tecnolóxico á Investigación (C.A.C.T.I.) for
invaluable help on structural elucidation. P.V. was an
internship student from the TUE Eindhoven.
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