Reaction of Indole Carboxylic Acid/Amide with Propargyl Alcohols: [4 + 3]-Annulation, Unexpected 3- to 2- Carboxylate/Amide Migration, and Decarboxylative Cyclization

Article abstract of DOI:10.1021/acs.orglett.9b01686

1-Methylindole-3-carboxamides react with substituted propargyl alcohols to afford lactams by [4 + 3]-annulation and carboxamide group migration to the indole-2-position. In contrast, indole-2-carboxylic acids/amides form fused seven-membered lactones/lact

Full text of DOI:10.1021/acs.orglett.9b01686

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Reaction of Indole Carboxylic Acid/Amide with Propargyl Alcohols:
[4 + 3]-Annulation, Unexpected 3- to 2- Carboxylate/Amide
Migration, and Decarboxylative Cyclization
Karuppu Selvaraj, Shubham Debnath, and K. C. Kumara Swamy*
School of Chemistry, University of Hyderabad, Hyderabad 500 046, Telangana, India
S
* Supporting Information
ABSTRACT: 1-Methylindole-3-carboxamides react with substituted propargyl alcohols to afford lactams by [4 + 3]-annulation
and carboxamide group migration to the indole-2-position. In contrast, indole-2-carboxylic acids/amides form fused seven-
membered lactones/lactams (oxepinoindolones/azepinoindolones) upon treatment with substituted propargyl alcohols using
catalytic Cu(OTf)2. Decarboxylative cyclization of 1-methylindole-2- or indole-3-carboxylic acids with substituted propargyl
alcohols under Lewis (for 1-methylindole-2-carboxylic acid) or Brønsted (for 1-methylindole-3-carboxylic acid) acid catalysis
gives the same 3,4-dihydrocyclopentaindoles, demonstrating 3- to 2-carboxylate migration in the latter case.
ndole-fused seven-membered ε-lactams are often found as
core structural motifs in natural products and biologically
active compounds (Figure 1), but reported methods for their
gold-catalyzed cyclization of indoles with an electron-deficient
allene at the 3-position to obtain 3,3-disubstituted cyclopenta-
[b]indoles.6 Very recently, Wang’s group reported the
synthesis of substituted 3,4-dihydrocyclopenta[b]indoles via
3-alkenylation of indole with propargyl alcohol in the presence
of triflic acid.7
I
From another perspective, decarboxylative cyclization has
emerged as a powerful tool for the synthesis of acyclic/
polycyclic compounds.8 Most of these decarboxylation
reactions are assisted by expensive metal catalysts and reagents,
while reports on metal-free decarboxylation reactions are
rather limited.9 To develop the metal-free Lewis acid mediated
decarboxylative cyclization, a variety of carboxylic acid
substrates are used.10 However, this strategy has limitations
in terms of substrate scope and affords products in poor yields
in many cases. Relative to the well-established [1,2]-migration
of alkyl, aryl, or other electron-donating groups (EDG),11
migration of strongly electron-withdrawing groups (EWG) is
less frequent.12 Carboxylate or amide group migration is
interesting, but only a handful of examples are available in the
literature because the relative order of the migrating aptitude is
reverse (EDG < EWG).13 As a part of our studies on reactions
Figure 1. Selected examples of natural products, biologically active
lactams, and 3,4-dihydrocyclopenta[b]indoles.
synthesis are very limited.1 The corresponding indole-fused ε-
lactones are not common,2 but their decarboxylated products,
cyclopentaindoles, are frequently found in natural products
(Figure 1).3 The latter class of compounds with an indole-
fused cyclopentane ring can be synthesized by many methods.4
Even for 3,3-disubstituted cyclopenta[b]indole, several elegant
methods have been reported.5 Ma and co-workers reported
Received: May 14, 2019
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.9b01686
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Letter
involving functionalized indoles and propargylic alcohols,14 we
herein report (i) the direct [4 + 3] annulation reaction of
indole-2-carboxylic acids/amides with propargyl alcohols and
(ii) unexpected [3,2]-carboxylate/amide migration [1,2-shift]
followed by decarboxylative cyclization of 1-methylindole-3-
carboxylic acids/amides with propargyl alcohols. The resulting
products are seven-membered lactones/lactams; further, the
lactones thus obtained undergo decarboxylative cyclization to
form 3,4-dihydrocyclopenta[b]indoles.
Scheme 1. Scope of the Reaction of Indole-2-carboxylic
Acids 1 with Propargylic Alcohols 5
a
The precursors used in the present study are shown in
Figure 2. The initial reaction was performed between 1-
a
Reaction conditions: indole-2-carboxylic acid 1 (0.57 mmol),
propargylic alcohol 5 (0.62 mmol), solvent (DCM, 10 mL), 6 h, 25
b
c
°C. Yield of the isolated product. 0 °C was used.
Figure 2. Precursors used in the present study.
Me; 6ab−6ac) at the para-position of the aromatic ring
provided slightly higher yields than that bearing electron-
withdrawing substituent (Cl; 6ad). With R2 = Ph and R3 =
biphenyl, ε-lactone 6ae could be obtained as a single product
in 67% yield. Other unsymmetrical propargylic alcohols 5f−5h
also gave products 6af−6ah in good to excellent yields.
Propargylic alcohols with an electron-donating para-substitu-
ent on the phenyl ring (tert-Bu, Me; 6ag−6ah) gave a higher
yield than that with electron-withdrawing group at the meta-
position (F; 6ai). The reaction also worked well when R4 was
thiophenyl (5j) giving the product 6aj [cf. Figure S2 (X-ray)].
The 5-substituted indole-2-carboxylic acids (R1 = OMe, Me,
Cl, Br, 1b, 1c, 1d, 1e) with electron-donating or electron-
withdrawing groups led to products 6ba, 6ca, 6da, and 6ea in
good yields. Lastly, 1-ethyl-indole-2-carboxylic acid 1f (R = Et)
provided the ε-lactone 6fa in 76% yield. The parent 1H-indole-
2-carboxylic acid 1g (R = H) also reacted with the propargylic
alcohol 5a at 0 °C to afford the product 6ga in good yield.
In the above reaction, the CO2 moiety is intact in the ε-
lactones 6 and the decarboxylated products 7 are either minor
or trace products during optimization. However, when we used
the Lewis acid BF3·OEt2 in DCM at rt for 12 h, 7aa was
obtained in 81% yield from 1-methylindole-2-carboxylic acid
1a via decarboxylation followed by cyclization. Although
TfOH, MsOH, and p-TSA also worked (yields: 71%, 64%,
and 76%, respectively), BF3·OEt2 was the best and 3,4-
dihydrocyclopenta[b]indoles 7ac−7ag and 7ak−7al [cf. Figure
S3 for 7al (X-ray)] were readily obtained in good to excellent
yields (Scheme 2). Propargylic alcohol 5s bearing an alkyl
substituent (R2 = Me, R3 = Ph) gave 7as in 75% yield.
Propargylic alcohols with R2, R3 = Me, H (5t, 5u) and R4 = n-
methylindole-2-carboxylic acid 1a and propargyl alcohol 5a in
the presence of Sc(OTf)3. The oxepinoindolone (ε-lactone)
6aa [cf. Figure S1 (X-ray)] via direct [4 + 3]-annulation and
the unexpected dihydrocyclopentaindole 7aa via decarbox-
ylative cyclization of the ε-lactone were obtained in 74% and
10% yields, respectively, after 12 h (Table S1, entry 1). Based
on this result, we screened the reaction conditions, catalyst,
solvent, reaction temperature, reaction time, and amount of
catalyst. First, the effectiveness of other catalysts Zn(OTf)2,
In(OTf)3, Bi(OTf)3, AgOTf, NaOTf, and FeCl3 was checked.
Products 6aa and 7aa were obtained in moderate yields in
these cases. The sole product 6aa in slightly increased yield
was obtained using Yb(OTf)3 or Fe(OTf)3. The highest yield
of 6aa (93%) was observed by conducting the reaction by
using Cu(OTf)2 as the catalyst in DCM solvent for 6 h (Table
S1, entry 10). Increasing the reaction time to 12 h also
afforded the same yield. Conducting the reaction at 0 or 40 °C
gave a lower yield. Other solvents like dichloroethane,
chloroform, 1,4-dioxane, acetonitrile, tetrahydrofuran, toluene
and dimethyl sulfoxide afforded lower yields. No reaction
occurred in the absence of the catalyst. Increasing the catalyst
loading 10 mol % to 15 and 20 mol %, did not improve yield.
Thus, Cu(OTf)2 in DCM at rt (25 °C) was found to be
optimal for obtaining 6aa.
For the substrate scope, we checked a wide variety of indole-
2-carboxylic acids 1 and propargylic alcohols 5 to obtain
substituted indole fused ε-lactones 6 (Scheme 1). First, a series
of substituted propargyl alcohols 5b−5e were reacted with 1a
to examine the substituent effect (e.g., 6ab−6ae). Propargylic
alcohols 5 bearing an electron-donating substituent (OMe,
B
DOI: 10.1021/acs.orglett.9b01686
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Letter
Scheme 2. Scope of BF3·OEt2 Mediated Decarboxylative
Scheme 3. Synthesis of the Allenamide 8aa and ε-Lactams
9aa−9ai
a
Cyclization of 1-Methylindole-2-carboxylic Acids 1 and
a
Propargyl Alcohols 5
a
Reaction conditions: Indole-2-carboxamide 2 (0.57 mmol), prop-
argylic alcohol 5 (0.62 mmol), solvent (DCM, 10 mL), 6 h, 60 °C.
b
Yields of the isolated products.
Scheme 4. p-TSA-Mediated Reaction of 1-Methylindole-3-
a,b
carboxylic Acid 3a with Propargylic Alcohols 5
a
Reaction conditions: 1-methylindole-2-carboxylic acid 1 (0.100 g,
0.57 mmol), propargylic alcohol 5 (0.62 mmol), solvent (DCM, 10
b
c
mL), 12 h, 25 °C. Yield of the isolated product. 0.39 mmol was
1
used; key peaks in the H NMR: 3.54(OCH3), 3.91 (NCH3), 6.46
(PhC = CH) 6.80−7.86 (Ar-H). LCMS 428 [M + 1].
butyl, cyclopropyl (5v, 5w) did not yield any isolable products.
The reaction worked well when R2 was thiophenyl (5x) giving
the product 7ax in 79% yield. By using 1h, we could isolate
compound I in ca. 75% purity in rather low yield (ca. 10%
1
LCMS, H NMR).
Interestingly, under the above Cu(OTf)2-catalyzed con-
ditions, use of 1-methylindole-2-carboxamide 2a and prop-
argylic alcohol 5a afforded only the allene product 8aa in 89%
yield. This allene underwent nucleophilic attack of the −NH2
group in the presence of 10 mol % of Cu(OTf)2 in refluxing
DCM for 6 h to afford the ε-lactam 9aa in 85% yield. Hence,
we surmised that direct one-pot treatment of 2a with 5a using
Cu(OTf)2 catalyst should give 9aa, which was proved to be
correct (Scheme 3). Pleasingly, the other ε-lactams 9ac−9ae
(cf. Figure S4 for 9ac), 9ag−9ai and 9ba−9ca were obtained
in good yields.
With the expectation that 3-substituted ε-lactone and 1,4-
dihydrocyclopenta[b]indole will be formed when the carboxyl
group is at the indole-3-position, we treated 1-methylindole-3-
carboxylic acid 3a with propargylic alcohol 5a under the above
conditions (cf. Schemes 1 and 2), but the reaction did not
occur. Surprisingly, in the presence of p-TSA, the unexpected
3,4-dihydrocyclopenta[b]indole 7aa, rather than the expected
1,4-dihydrocyclopenta[b]indole 10aa (Scheme 4), was ob-
tained in 69% yield. Analogous [3,2]-carboxylate migration
cum decarboxylative cyclization products 7ac−7ad and 7ak−
7ap [cf. Figure S5 for 7an (X-ray)] were similarly prepared in
good yields. In this reaction, we could not isolate the ε-lactone
intermediate. The 1H-indole-3-carboxylic acid 3b with
a
Reaction conditions: 1-methylindole-3-carboxylic acid 3a (0.100 g,
0.57 mmol), propargylic alcohol 5 (0.62 mmol), solvent (DCM, 10
b
mL), 12 h, 25 °C. Yield of the isolated products.
propargylic alcohol 5a did not afford an isolable product
under these conditions.
Pleasingly though, 1-methylindole-3-carboxamide 4a upon
treatment with propargyl alcohol 5a in the presence of p-TSA
afforded the ε-lactam 9aa [cf. Figure S6 (X-ray)] in 43% yield
via [3,2]-carboxamide migration (Scheme 5). The lower yield
suggests a side reaction; the reaction mixture showed a pink
colored compound (TLC) that could not be characterized.
Compounds 9al−9am, 9ao−9ar, and 9bs [Scheme 5; see
Figure S7 for 9ap] could be similarly isolated. For
unambiguous proof of [3,2]-carboxylate/carboxamide migra-
tion, we have determined the X-ray structures of 4a (Figure
S8) and 9aa/9ap. Thus, we conclude that the formation of 7aa
C
DOI: 10.1021/acs.orglett.9b01686
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Lewis acid to give compound 7. The ε-lactam 9 is formed in a
manner similar to that of 6.
The reaction using indole-3-carboxylic acid/carboxamide
must take place by the migration of the carboxylate/
carboxamide group from indole-3- to indole-2-position (1,2-
migration; Scheme 7). Some literature is available on
Scheme 5. Scope of p-TSA-Mediated 3 to 2-Amide
Migration of 1-Methylindole-3-carboxamides 4 and
a
Propargyl Alcohols 5
Scheme 7. Proposed Pathway for the p-TSA-Mediated [1,2]-
Carboxylate or Amide Migration and Formation of 7/9
a
Reaction conditions: 1-methylindole-3-carboxamide 4 (0.57 mmol),
propargylic alcohol 5 (0.62 mmol), solvent (DCM, 10 mL), 12 h, 25
b
°C. Yield of the isolated products.
from 3a and 5a occurs via carboxylate migration from 3- to 2-
position on indole, followed by decarboxylative cyclization as
shown in Scheme 4. We also performed the reaction to obtain
9aa using catalytic BF3·OEt2 or Cu(OTf)2; in the former case,
the reaction did not occur and in the latter case the yield of 9aa
was lower (30%).
A possible pathway for the formation of ε-lactone 6,
dihydrocyclopenta[b]indole 7, and ε-lactam 9 based on the
literature15 is shown in Scheme 6. First, the propargylic alcohol
somewhat different reactions.5l,12,13 First, in the presence of
p-TSA, the propargylic alcohol 5 is converted to the allene
intermediate E, which would then undergo Friedel−Crafts-
type reaction with 1-methylindole-3-carboxylic acid 3 or amide
4 to form the allene intermediate F. This intermediate F is
transformed into the six-membered spirocycle G via intra-
molecular nucleophilic attack of OH/NH2 group. The
resulting N-heterocycles contain an indolinium ion, which
can undergo a carboxylate or amide migration to produce H/
H′. Finally, aromatization of intermediate H′ affords ε-lactone
6 or ε-lactam 9. The indole fused ε-lactone 6 undergoes
decarboxylative cyclization in the presence of p-TSA to give
dihydrocyclopentaindoles 7 as described above.
Scheme 6. Proposed Catalytic Cycle for the Lewis Acid
Mediated Formation of 6 and 7 (Similarly 9)
In summary, [4 + 3]-annulation of indole-2-carboxylic
acids/amides with propargyl alcohols under mild reaction
conditions leads to ε-lactones/ε-lactams. Rather unexpectedly,
carboxylate/amide migrates from indole-3 to indole-2 position
in the reaction of 1-methylindole-3-carboxylic acid/amide with
propargyl alcohols. The ε-lactones so obtained undergo facile
acid-catalyzed (metal-free) decarboxylative cyclization to
afford 3,4-dihydrocyclopenta[b]indoles. Future work may
involve chiral version leading to dihydrocyclopenta[b]indoles.
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.9b01686.
Experimental details, characterization data, X-ray
crystallographic data, ORTEPs of compounds 6aa, 6aj,
7al, 9ac, 7an, 9aa, 9ap and 4a (Figures S1−S8), and
1H/ 13C NMR spectra (PDF)
5 is converted to the intermediate A in the presence of
Cu(OTf)2. Species A undergoes Friedel−Crafts-type reaction
with indole-2-carboxylic acid 1 to form the allene intermediate
B. Subsequently, B is transformed to intermediate C and then
to D via intramolecular nucleophilic attack of −OH group of
carboxylic acid. The product 6 and the regenerated catalyst
Cu(OTf)2 are formed by subsequent protonation. This step is
supported by the isolation of 6 (cf. Scheme 1). This ε-lactone
6 undergoes decarboxylative cyclization in the presence of
Accession Codes
CCDC 1915530−1915537 contain the supplementary crys-
tallographic 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
D
DOI: 10.1021/acs.orglett.9b01686
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Organic Letters
Letter
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Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
Corresponding Author
■
*E-mail: kckssc@uohyd.ac.in; kckssc@yahoo.com.
ORCID
(6) Chen, B.; Fan, W.; Chai, G.; Ma, S. Org. Lett. 2012, 14, 3616.
(7) Zhang, L.; Zhu, Y.; Yin, G.; Lu, P.; Wang, Y. J. Org. Chem. 2012,
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K. C. Kumara Swamy: 0000-0002-7617-706X
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
■
We thank the Department of Science & Technology (DST,
New Delhi) for the single-crystal X-ray diffractometer and
HRMS facility (PURSE, IRHPA, and FIST grants). We also
thank UGC for the UPE-II and NRC programs. K.C.K.S.
thanks SERB (SR/S2/JCB-53/2010) for funding. K.S. thanks
DST-SERB-NPDF (PDF/2017/000044), and S.D. thanks
DST-INSPIRE (IF160731) for a fellowship.
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DOI: 10.1021/acs.orglett.9b01686
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