Rhodium-Catalyzed Stereoselective Cyclization of 3-Allenylindoles and N-Allenyltryptamines to Functionalized Vinylic Spiroindolenines

Article abstract of DOI:10.1021/acs.orglett.1c01234

Herein, we report a highly enantio- and diastereoselective rhodium-catalyzed cyclization of N-allenyltryptamines and 3-allenylindoles to 6-membered spirocyclic indolenines. This allylic addition methodology offers the advantage of using a comparably cheap commercially available ligand with low loadings of an affordable rhodium precursor. The products can be converted into functionalized spirooxindoles and spiroindolines, which are regarded as important building blocks for the synthesis of a lot of natural products with biological activities.

Full text of DOI:10.1021/acs.orglett.1c01234

pubs.acs.org/OrgLett
Letter
Rhodium-Catalyzed Stereoselective Cyclization of 3‑Allenylindoles
and N‑Allenyltryptamines to Functionalized Vinylic Spiroindolenines
Antonia Becker, Christian P. Grugel, and Bernhard Breit*
Cite This: Org. Lett. 2021, 23, 3788−3792
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ABSTRACT: Herein, we report a highly enantio- and diastereoselective
rhodium-catalyzed cyclization of N-allenyltryptamines and 3-allenylindoles
to 6-membered spirocyclic indolenines. This allylic addition methodology
offers the advantage of using a comparably cheap commercially available
ligand with low loadings of an affordable rhodium precursor. The products
can be converted into functionalized spirooxindoles and spiroindolines,
which are regarded as important building blocks for the synthesis of a lot of
natural products with biological activities.
ith more than 1000 known compounds, indole alkaloids
represent an essential group of natural products. Many
of these natural products exhibit a wide range of biological
activities. A shared feature of many of these alkaloids is the
amine moiety with a C2-linker attached to the indole 3-
position, derived from the naturally occurring amino acid L-
tryptophane.1
In this work, specifically the molecular scaffold of spirocyclic
indolenines and their closely related structures are relevant.
Three prominent representatives thereof are shown in Figure
1. Stryvomicine shows neuroprotective activity, and perophor-
noteworthy concerning the field of intramolecular, stereo-
selective formation of spiroindolenines are the contributions by
the You group. Using chiral phosphoramidite ligands, they
developed an Ir-catalyzed asymmetric allylic substitution of
indole-3-yl allylic carbonates with a broad scope.6−9 There are
also several accounts of Au-, Ag-, or Cu-catalyzed procedures
using tethered terminal or internal alkynes, which lead to the
aforementioned products in a stereoselective manner.10
In recent years, we reported a series of rhodium-catalyzed
chemo-, regio-, and stereoselective allylic addition reactions,
coupling allenes, as well as alkynes as allylic precursors with
several different pronucleophiles.11 These hydrofunctionaliza-
tions are useful tools to form new branch-selective C−
heteroatom12 and C−C13 bonds. Especially the selective
formation of C−C bonds by allylic addition still represents
an ongoing challenge for organic synthesis. In this respect, we
recently reported a rhodium-catalyzed enantioselective cycliza-
tion of 3-allenylindoles to functionalized tetrahydrocarbazoles
(Scheme 1A).14 Following the confirmation that this
cyclization proceeds via a transient spirocyclic intermediate
as previously described by the You group,7 the corresponding
spiroindolines could be obtained via in situ reduction with
Hantzsch ester.15
W
Figure 1. Selected biologically active natural products containing a
spiroindoline core.
amidine acts cytotoxic against specific cancer cells.2 Koumine
is an alkaloid of Gelsemium elegans Benth., which shows anti-
inflammatory properties.3 Given their ability to be easily
oxidized at the imine moiety, spirocyclic indolenines allow for
a convenient synthesis of spirooxindoles. This gives access to
an entire group of compounds with a plethora of
pharmacological effects.4
To construct these valuable building blocks in an efficient
and stereoselective way, new synthetic methodologies need to
be established. An overview on the synthetic developments
thus far, regarding the topic of spirocyclic indolenines is
outlined in a recent review by Zheng and You.5 Especially
These findings lead to the work portrayed in this article.
Compared to the aforementioned 5-membered spirocycles, the
homologue 6-membered spirocyclic indolenines are less prone
to isomerization because of their lower ring strain and thus,
Received: April 12, 2021
Published: April 26, 2021
© 2021 The Authors. Published by
American Chemical Society
https://doi.org/10.1021/acs.orglett.1c01234
Org. Lett. 2021, 23, 3788−3792
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a
Scheme 1. Previous Work by the Breit Group
Scheme 2. Reaction Scope of Tryptamine Derivatives
a
All yields are isolated yields. er was determined by chiral HPLC, and
1
dr was determined by H NMR analysis of the crude product. The
absolute configuration of the products was determined by crystal
structure analysis of 2a and assigned by analogy to all other
compounds. ORTEP structure with 50% ellipsoid probability.
higher intrinsic stability. This should allow for their selective
synthesis and would be an improvement regarding atom
economy compared to previous protocols using allylic
substitution.6,16 Since many natural products are based on
the abundantly present precursor tryptamine, we decided to
incorporate this in the model substrate.
Subjecting the tosyl-protected model substrate 1a to the
synthetic protocol from the tetrahydrocarbazole synthesis (see
Scheme 1A) gave the desired product, although the yield and
stereoselectivity were moderate after 24 h (70%, 82:18 er, 2:1
dr). Therefore, further optimization of the ligand toward
optimal reaction conditions for tryptamine-containing sub-
strates was carried out. Because of the extent of preceding
optimizations, it was refrained from testing different classes of
privileged ligands, but rather use axial chiral ligands derived
from the BINAP and related scaffolds. Both, (R)-Segphos and
(R)-BINAP gave excellent selectivities and the previously
prolonged reaction time could be reduced to 16 h overnight
reaction with increased temperature. The optimized reaction
conditions are shown in Scheme 2.17
The scope of the N-allenyltryptamines was extended by
applying different protecting groups as well as substituents in
5-position of the indole core as shown in Scheme 2. All
aromatic sulfonyl amide protecting groups gave very good
results with yields of up to 97% and enantiomeric ratios of over
95:5, though the diastereoselectivity remained mediocre. The
carbamate protection group Cbz gave slightly inferior results,
whereas several oxygen-based functional groups in 5-position
were well-tolerated. Additionally, the tryptophane-based
substrate 1i was prepared and successfully converted into the
corresponding spirocyclic indolenine with excellent yield and a
satisfying diastereoselectivity.
of compound 2a and assigned to all other derivatives by
analogy.16 The absolute configuration was determined only
after enantiomerically pure crystals of compound 2a were
obtained. The acquired X-ray crystal structure is depicted in
Scheme 2 as ORTEP representation and corresponds to the
(S,S)-enantiomer.
Reactions with model substrate 3a derived from the
preceding investigations leading to 5-membered spirocyclic
products, gave good results under the catalytic conditions
previously published, as shown in Scheme 3. Applying a lower
temperature results in an increased enantioselectivity but is
accompanied by a slight drop in yield (30 °C, 70% yield, 95:5
er, 13:1 dr).
For a small scope of this 3-allenylindole cyclization, a few
selected functional groups were introduced to the indole core.
All of the applied derivatives 3a−f showed a satisfactory
outcome under the shown catalytic conditions (Scheme 3). In
case of 5-bromo- and 5-iodo-substituted spiroindolenines (4b
and 4c) centrosymmetric crystal structures could be obtained,
which are further proof of the observed diastereoselectivity.
In analogy to the postulated catalytic cycle for the 5-
membered spirocycle and tetrahydrocarbazole formation, we
would expect this transformation to proceed via formation of a
rhodium(III) hydride complex with PPTS, hydrometalation of
the allene to give an allyl-rhodium(III) intermediate, which
then undergoes a nucleophilic attack by the indole 3-
position.14 Interestingly, formation of the 6-membered spiro-
cycle results in an inverted diastereoselectivity compared to the
5-membered ring formation. The differences of the transition
state (TS) geometry seems to be a possible rationale: While
The relative configuration of the cyclization products 2 was
1
first determined by H NMR comparison with literature data
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Scheme 3. Reaction Scope of Malonate-Tethered Vinylic
Spiroindolenines
Table 1. Robustness Screening
a
a
b
c
entry
additive
benzaldehyde
benzoic acid
4-bromopyrazole
ethanol
phenol
penta-3,4-dien-1-ylbenzene
prop-1-yn-1-ylbenzene
2-TMS-buta-1,3-diene
(non flame-dried flask)
yield (%)
dr
er
1
2
3
4
5
6
88
83
62
83
84
29
83
71
84
81
7.5:1
5:1
6:1
7.5:1
6.5:1
6.5:1
6:1
5:1
9:1
6:1
98.5:1.5
93:7
75:25
96.5:3.5
97:3
97:3
97:3
95:5
95.5:4.5
96:4
d
7
8
9
10
water
a
b
Isolated yield. Determined by 1H NMR analysis of the isolated
c
d
compound. Determined by chiral HPLC. 70% unreacted alkyne
determined via crude H NMR.
1
a
All yields are isolated yields. er was determined by chiral HPLC, dr
was determined by 1H NMR analysis of the crude product. The
absolute configuration of the products was assigned by analogy to 2a.
ORTEP structures with 50% ellipsoid probability.
resulted in a lower yield. It could be shown that an internal
alkyne, a known intermediate in rhodium-catalyzed allylic
additions, cannot compete with the intramolecular C−C-bond
formation (see entry 7).11 Addition of 2-TMS-buta-1,3-diene
resulted in only a slight decrease in the yield. Therefore, a
coordination to the catalyst resulting in a blockage thereof
could be ruled out. As previously shown for the formation of
tetrahydrocarbazoles, this system is highly insensitive when it
comes to moisture.14 The reaction could be carried out in a
non-flame-dried flask and even the addition of one equivalent
of water did not significantly influence the reaction outcome.
The scalability of this methodology was demonstrated by
performing a reaction at a 1 gram scale with substrate 1b,
achieving a comparable yield and stereoselectivity. The
obtained spirocyclic indolenine undergoes versatile trans-
formations, as shown in Scheme 4. Selective reduction of the
imine moiety was accomplished by the use of sodium
cyanoborohydride in methanol, while harsher reaction
conditions lead to reduction of imine and terminal alkene.
When using the p-nosyl protecting group, the nitro-group was
reduced to the corresponding amine, which could be avoided
with the MBS protecting group. The corresponding oxindole 7
was obtained after selective oxidation using PINNICK oxidation
conditions.18 Furthermore, reaction with p-thiocresole and
potassium carbonate in DMF gave the deprotected spiroindo-
lenine 8.19 As expected for this ring size, application of the
conditions for acid-catalyzed migration did not lead to an
annulated 7-membered heterocycle.8 In all of the above-
mentioned transformations no loss of optical purity was
observed.
the 6-membered ring formation might occur through a
chairlike TS, the 5-membered spirocycle likely is formed via
an envelope transition state geometry, each favoring a
pseudoequatorial orientation of the allyl rhodium fragment
(Figure 2).
Figure 2. Postulated transition states for 5-membered versus 6-
membered spirocycle formation as possible explanation for the
observed diastereoselectivity.
To show the robustness of this transformation, several
experiments with carefully chosen additives have been
performed. This selection is comprised of competing
nucleophiles and electrophiles, each bearing potentially
reactive functional groups. Moreover, the reaction was carried
out without precaution to moisture and even with the addition
of water. The results are listed in Table 1.
The competing nucleophiles benzaldehyde, benzoic acid,
ethanol and phenol (entries 1, 2, 4, and 5) could be added to
the reaction mixture without significant differences in the
reaction outcome. When using one of the best nucleophiles in
allylic additions, 4-bromopyrazole, yield and enantioselectivity
of 2b dropped (entry 3). Nevertheless, the intramolecular
reaction is still somewhat favored over an intermolecular
attack. Addition of a competing terminal allene (entry 6)
In conclusion, we herein present a method to convert 3-
tethered allenyl indoles 1 and 3 to 6-membered spirocyclic
indolenines 2 and 4 via rhodium-catalyzed allylic addition,
giving access to valuable products with high stereoselectivities.
Aside from the general advantage of using allylic addition
compared to substitution reactions regarding the higher atom
economy, the low loadings of commercially available,
affordable ligand and the robustness of the reaction are
benefits of this protocol. The versatility was demonstrated by
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Scheme 4. Gram-Scale Catalysis and Selected
Transformations of Spiroindolenine 2b
AUTHOR INFORMATION
Corresponding Author
■
z
Bernhard Breit − Institut fu
̈
r Organische Chemie, Albert-
Ludwigs-Universität Freiburg, 79104 Freiburg, Germany;
orcid.org/0000-0002-2514-3898;
Email: bernhard.breit@chemie.uni-freiburg.de
Authors
Antonia Becker − Institut fu
Ludwigs-Universität Freiburg, 79104 Freiburg, Germany
r Organische Chemie, Albert-
̈
r Organische Chemie, Albert-
Christian P. Grugel − Institut fu
̈
Ludwigs-Universität Freiburg, 79104 Freiburg, Germany
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.orglett.1c01234
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by Deutsche Forschungsgemein-
schaft (DFG) [BR 1646/18-1]. The authors would like to
thank the entire analytics department of organic chemistry
(University Freiburg), Dr. M. Keller for NMR experiments, C.
Warth for MS measurements, and especially J. Emmerich for
difficult HPLC separations. We thank Dr. D. Kratzert
(University Freiburg) for X-ray crystal structure analyses. G.
Ciplak (University Freiburg) is acknowledged for technical
support.
z
1
All yields given as isolated yields. dr was determined by H NMR
analysis of the isolated product. er was determined by chiral HPLC.
Reactions and conditions: (a) NaBH3CN (1.0 equiv), MeOH, 0 °C to
rt, 16 h; (b) Pd/C (5 wt %, 10 mol % Pd), H2 (1 atm, via balloon),
MeOH, rt, 16 h, (ABS = 4-aminobenzenesulfonyl); [a] was synthesized
from 2e; (c) 2-methyl-2-butene (10 equiv), NaClO2 (5.0 equiv),
NaH2PO4 (aq. 1 M, 1.5 equiv), THF, rt, 7 h; (d) p-thiocresole (1.5
equiv), K2CO3 (3.0 equiv), DMF, 50 °C, 16 h; (b) er determined
after conversion to derivative 2a; (e) TsOH (30 mol %), THF, rt to
60 °C, 4 h; (f) HCl (g) in THF (4.5 M), rt, 16 h.
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ASSOCIATED CONTENT
■
sı
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.orglett.1c01234.
Experimental details and analytical data for the
synthesized compounds (PDF)
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nines by Mechanistically-Driven Reaction Development. Acc. Chem.
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supplementary crystallographic data for this paper. These
data can be obtained free of charge via www.ccdc.cam.ac.uk/
data_request/cif, or by emailing data_request@ccdc.cam.ac.
uk, or by contacting The Cambridge Crystallographic Data
Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44
1223 336033.
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