Annulative π-Extension (APEX) of Indoles to Pyrido[1,2- a]indoles Using 4-Oxo Peroxides as C4 Units

Using 4 ‑Oxo Peroxides as C4 Units

Letter

Annulative π‑Extension (APEX) of Indoles to Pyrido[1,2‑a]indoles

Xin Wang, Chenhao Lou, Leiyang Lv,* and Zhiping Li*

Cite This: Org. Lett. 2021, 23, 5978 −5982

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sı Supporting Information

ABSTRACT: Annulative π-extension (APEX) of 3-substituted

indoles to pyrido[1,2-a]indoles is developed by using 4-oxo

peroxides as π-extending reagents, which are employed as versatile

C4 building blocks. This transformation is initiated by Brønsted

acid-mediated Hock rearrangement of the peroxyl group. Notably,

the pyrido[1,2-a]indole products are obtained by elimination of

the indole moiety from the corresponding dihydropyrido[1,2-

a]indoles, which could be selectively formed at room temperature.

itrogen-fused heterocycles are important structural units

substituted indoles with 4-oxo peroxides may enable the

synthesis of analogous π-extended pyrido[1,2-a]indoles in one

step. Herein, we report our eﬀorts in the APEX of indoles to

prepare pyrido[1,2-a]indoles (Scheme 1d).

We initiated the study by investigating the reaction of 3-

methyl-1H-indole 1a and 4-oxo peroxide 2a to optimize the

reaction conditions (Table 1). Initially, the desired pyrido[1,2-

a]indole product 3a was not observed at 25 °C; instead, an

unexpected dihydropyrido[1,2-a]indole 4a was obtained in

prevalently found in the ﬁeld of medicinal chemistry and

material chemistry. Pyrido[1,2-a]indoles are known for

oﬃcinal candidates that possess a wide spectrum of

pharmaceutical activities and as building blocks to construct

various organic materials. Owing to their signiﬁcant

applications, various strategies have been developed to

synthesize such pyrido[1,2-a]indole motifs (Scheme 1). The

established methods have been mainly focused on the

construction of heteroatomic skeleton, that is, pyrrole ring

16% yield in the presence of 0.5 equiv of TfOH (entry 1). The

structure of 4a was established by NMR spectroscopy and

conﬁrmed by single-crystal X-ray analysis (Figure 1). The

yields of 4a were increased upon raising the loading of TfOH

or pyridine ring C (Scheme 1a). As is well-known,

annulative π-extension (APEX) enables rapid access to fused

aromatic molecules from simple aromatic compounds in one

step (Scheme 1b), state-of-the-art APEX transformation of

indoles represents the most appealing approach for the

synthesis of pyrido[1,2-a]indoles from the viewpoint of step-

economy and availability of starting materials (Scheme 1c). In

(

entries 2 and 3). The product 4a was obtained in 62% yield at

0 °C with 8% of 3a formed (entry 4). Further elevating the

reaction temperature to 85 °C resulted in the formation of 4a

and 3a in 35% and 45% yields, respectively (entry 5). To

ensure higher chemoselectivity, preparation of 4a was preferred

at 25 °C, although in moderate yield (48%). To our delight,

increasing the amount of TfOH to 2.0 equiv and the reaction

time to 3 h aﬀorded the target product 3a in 74% yield, with

no 4a detected (entry 6). Further increasing the loading of

TfOH to 3.0 equiv did not improve the reaction eﬃciency

this context, several π-extending reagents such as diazoenal,

ortho-haloarylalkyne, alkyne, etc. have been developed to

build such valuable scaﬀolds in the presence of metal catalysts.

However, the associated metal residues limit their application

in the pharmaceutical ﬁeld. Therefore, further improvements in

terms of reaction sustainability, diversity, and especially

developing new π-extending reagents are highly desirable.

A series of cyclocondensations of heteroarenes with 1,4-

dicarbonyl compounds have been established for the

(

entry 7). It is worth noting that 3a was obtained in 76% yield

with 3.0 equiv of 1a and 2.0 or 3.0 equiv of TfOH under 100

C for 5 h (entries 8 and 9). Lewis acid such as BF ·OEt

3 2

aﬀorded 3a in 49% yield (entry 10), while the reaction was

annulative π-extension. However, this protocol failed to

gain popularity due to the low selectivity and eﬃciency.

Received: June 20, 2021

Published: July 15, 2021

Recently, we have reported that 4-oxo tert-butyl peroxides

acted as versatile C4 building blocks for the selective synthesis

of furans, pyrroles, as well as indoles via Brønsted acid-

catalyzed Hock rearrangement of the peroxy group.

Continuing with our interest in the manipulations of

organoperoxides, we envisioned that APEX reactions of 3-

2021 American Chemical Society

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Org. Lett. 2021, 23, 5978−5982

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Letter

Scheme 1. Synthetic Strategies for the Pyrido[1,2-a]indoles

Figure 1. X-ray diﬀraction of 4a. Thermal ellipsoids are at the 50%

probability level.

a b

Scheme 2. Synthesis of Pyrido[1,2-a]indoles 3

Table 1. Optimization of the Reaction Conditions

entry

acid

T (°C)

3a (%)

4a (%)

TfOH (0.5 equiv)

TfOH (1.0 equiv)

TfOH (1.5 equiv)

TfOH (2.0 equiv)

TfOH (3.0 equiv)

TfOH (2.0 equiv)

TfOH (3.0 equiv)

BF ·OEt (2.0 equiv)

100

N.D.

Reaction conditions: 1 (0.3 mmol), 2 (0.1 mmol), TfOH (0.2

mmol), MeCN (2.0 mL), 100 °C, 5 h, under air. Reported yields

were based on 2 and determined by H NMR using an internal

standard; isolated ones were given in parentheses. Reactions were

carried out in 1.0 mmol.

N.D.

reacted smoothly with 2a to give the pyrido[1,2-a]indole

products 3a−3g in 47−82% yields, indicating that the 4-oxo

peroxide 2a plays as an eﬃcient C4 π-extending reagent. When

the reaction was carried out in 1.0 mmol, 3b was obtained in

FeCl (2.0 equiv)

N.D.

7% yield. It should be noted that electron-rich indoles are

Reaction conditions: 1a (0.5 mmol), 2a (0.1 mmol), MeCN (2.0

mL), 24 h, under air, unless otherwise noted. Reported yields were

based on 2a and determined by H NMR using CH Br as internal

standard. 1a (0.3 mmol), 3 h. 1a (0.3 mmol), 5 h.

more favorable in the current APEX reaction (3f and 3g).

Peroxide derived from β-keto ester also reacted with 1a to give

the corresponding product 3i. Importantly, monosubstituted

(

3j and 3k) and trisubstituted (3l) annulation products were

synthesized by application of the corresponding 4-oxo peroxide

2, albeit in relatively lower yields. We hypothesized that the

two former ones (3j and 3k) are due to the electronic eﬀect,

yet the exact reason for the latter (3l) is unclear at the current

stage.

Next, we explored the chemodivergent formation of the

dihydropyrido[1,2-a]indole products 4 in the presence of 1.5

messy and neither 3a nor 4a was detected in the case of FeCl₃

(

entry 11).

With the optimal reaction conditions in hand, we then

evaluated the reaction scope (Schemes 2 and 3). First, the

preparation of diverse pyrido[1,2-a]indole derivatives was

investigated (Scheme 2). A variety of 3-substituted indoles 1

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Org. Lett. 2021, 23, 5978−5982

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dihydropyrido[1,2-a ]indole 4 .

Letter

Scheme 3. Synthesis of Dihydropyrido[1,2-a]indole 4 ^,

a b

Scheme 4. A Tentative Reaction Mechanism

In conclusion, we have demonstrated 4-oxo tert-butyl

peroxides as new C4 π-extending reagents in the acid mediated

step-economy annulation of indoles to aﬀord N-fused

conjugated products. This chemodivergent transformation

proceeded in a stepwise manner; that is, the dihydropyrido-

Reaction conditions: 1 (0.5 mmol), 2 (0.1 mmol), TfOH (0.15

mmol), solvent (2.0 mL), 25 °C, 24 h, under air. Reported yields

were based on 2 and determined by H NMR using an internal

standard; isolated ones were given in parentheses.

[

1,2-a]indoles could be isolated at 25 °C, which readily

equiv of TfOH at 25 °C (Scheme 3). To our delight, 4a−4e

were obtained selectively under the standard conditions in

moderate to good yields. In contrast, 4i was obtained in 28%

yield along with 10% of 3i when peroxide that derived from β-

keto ester was tested with 1a under the same reaction

conditions.

A control experiment was carried out to gain the relationship

between dihydropyrido[1,2-a]indole 4 and pyrido[1,2-a]-

indole 3. For example, when 4a was treated with TfOH (1.0

equiv) in 100 °C for 1 h, both 3a and 3-methyl-1H-indole 1a

were obtained almost quantitatively (eq 1). This result clearly

demonstrates that the annulative π-extension product 3 is

formed via decomposition of the corresponding

rearomatized to form the pyrido[1,2-a]indoles promoted via

acid at high temperature by releasing the indole moiety.

ASSOCIATED CONTENT

■

sı Supporting Information

The Supporting Information is available free of charge at

https://pubs.acs.org/doi/10.1021/acs.orglett.1c02062.

Detailed experimental procedures and compound

characterization data, single-crystal X-ray structure of

4a, and NMR spectra of all new compounds (PDF)

Accession Codes

CCDC 2083477 contains the 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 Cam-

bridge Crystallographic Data Centre, 12 Union Road,

Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

AUTHOR INFORMATION

Corresponding Authors

■

Leiyang Lv − Department of Chemistry, Renmin University of

China, Beijing 100872, China; orcid.org/0000-0002-

2850-8952 ; Email: lvleiyang2008@ruc.edu.cn

Zhiping Li − Department of Chemistry, Renmin University of

China, Beijing 100872, China; orcid.org/0000-0001-

On the basis of the above experiment and literature

1−13

reports,

). Initially, the protonation of the peroxide 2 gives the

intermediate A, which undergoes the Hock rearrangement

1,2-migration of the phenyl group) to generate the key

a tentative mechanism was proposed (Scheme

7987-279X ; Email: zhipingli@ruc.edu.cn

(

oxonium intermediate B. Then addition of electron-rich 1a

toward B generates the intermediate C, which is protonated

and substituted by another molecule of indole to form

intermediate E. Subsequent intramolecular cyclocondensation

of E delivers the dihydropyrido[1,2-a]indole product 4.

Finally, high temperature (100 °C) and acid facilitate 4 to

rearomatize to aﬀord the pyrido[1,2-a]indole product 3

through intermediate F by release of 1.

Authors

Xin Wang − Department of Chemistry, Renmin University of

China, Beijing 100872, China

Chenhao Lou − Department of Chemistry, Renmin University

of China, Beijing 100872, China

Complete contact information is available at:

https://pubs.acs.org/10.1021/acs.orglett.1c02062

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Org. Lett. 2021, 23, 5978−5982

Organic Letters

Alkynes via C-H and N-H Bond Cleavages with Air as the Oxidant.

Letter

Notes

Org. Lett. 2010 , 12 , 2068 −2071. (b) Xia, Y. Q.; Dong, L.

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Sasagawa, A.; Yamazaki, K.; Ano, Y.; Chatani, N. Nickel-Catalyzed

Oxidative C-H/N-H Annulation of N-heteroaromatic Compounds

with Alkynes. Chem. Sci. 2019, 10, 3242−3248. (d) Liu, Y.; Yang, Z.;

Chauvin, R.; Fu, W.; Yao, Z.; Wang, L.; Cui, X. One-Pot Synthesis of

Furo[3,4-c ]indolo[2,1-a ]isoquinolines through Rh(III)-Catalyzed

Cascade Reactions of 2-Phenylindoles with 4-Hydroxy-2-alkynoates.

Org. Lett. 2020 , 22 , 5140 −5144. (e) Thavaselvan, S.; Parthasarathy,

K. Nickel-Catalyzed Cyclization Strategy for the Synthesis of

Pyrroloquinolines, Indoloquinolines, and Indoloisoquinolines. Org.

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The authors declare no competing ﬁnancial interest.

ACKNOWLEDGMENTS

■

This work was supported by the National Natural Science

Foundation of China (22071266). The research was also

supported by the Fundamental Research Funds for the Central

Universities, and the Research Funds of Renmin University of

China (21XNLG04).

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