Regioselective allylic rearrangement of 3-alkyl-n-allylindoles to 3-alkyl-2-allylindoles

Full text of DOI:10.1002/bkcs.10890

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DOI: 10.1002/bkcs.10890
BULLETIN OF THE
H. R. Moon et al.
KOREAN CHEMICAL SOCIETY
Regioselective Allylic Rearrangement of 3-Alkyl-N-allylindoles to
3-Alkyl-2-allylindoles
Hye Ran Moon,† Sangku Lee,‡ Su Yeon Kim,† and Jae Nyoung Kim†,
*
†Department of Chemistry and Institute of Basic Science, Chonnam National University, Gwangju 500-
757, Korea. *E-mail: kimjn@chonnam.ac.kr
‡Targeted Medicine Research Center, Korea Research Institute of Bioscience and Biotechnology,
Cheongwon, Chungbuk, 363-883, Korea
Received May 26, 2016, Accepted July 8, 2016, Published online August 24, 2016
Keywords: Allylic rearrangement, Morita–Baylis–Hillman adducts, Indole, Polyphosphoric acid
Morita–Baylis–Hillman (MBH) adducts have been used
extensively for the synthesis of various cyclic and acyclic
compounds.1 Both the primary and secondary positions of
MBH adducts could be selectively attached at the N-1 posi-
tion of indoles to produce 1 and 2, as shown in Scheme 1.2,3
In addition, a regioselective introduction of MBH adduct at
the C-3 position of indoles to form 3 and 4 has also been
reported.4,5 However, an introduction of MBH adduct at the
C-2 position of indoles to form 5 and 6 has not been
reported, to the best of our knowledge.
2-Benzylindoles have been synthesized from N-
benzylindoles by rearrangement of the benzyl group under
acidic conditions.6 As an example, N-benzylindole was
converted to 2-benzylindole in polyphosphoric acid (PPA)
in moderate yield.6a,b As compared to N-benzylindoles,
the rearrangement of N-allylic indoles produced more
complex mixtures of products depending on N-allylic moi-
ety and the reaction conditions.7 Casnati et al. reported
the rearrangement of N-crotyl or N-prenyl group of 3-
substituted indoles.7a,b They observed the rearrangement
of the allylic moieties to the C-2 position with partial
allylic rearrangement. Later, Okazaki and coworkers sug-
gested the reaction mechanism would involve a charge-
induced [3,3]-shift of the allylic moiety from N-1 to C-3
position, following a [1,5]-shift, and subsequent enamina-
tion process.7d
In this respect, we decided to examine the allylic rear-
rangement of 1a and 2a under the influence of PPA, as
shown in Scheme 2. The starting materials 1a and 2a were
prepared from MBH bromide or carbonate according to the
reported methods.2,3 The rearrangement of 1a occurred
smoothly in PPA at 70ꢀC to afford 6a in good yield (73%)
along with 5a in low yield (5%).8 The reaction using BF3
etherate (10 equiv) in 1,2-dichloroethane (reflux, 2 h)7e or
trifluoroacetic acid (reflux, 2 h)7a,b showed a similar result
to that of PPA. The reaction in PPA at higher temperature
(100ꢀC) was less efficient due to the formation of many
intractable side products. In contrast to the selective forma-
tion of 6a from 1a, the reaction of 2a afforded 5a in good
yield (64%) along with 6a in low yield (8%).
Encouraged by the successful results, 1b–1f and 2b–2f
were prepared according to the reported methods,2,3 and the
allylic rearrangement was examined. The results are sum-
marized in Table 1. The reaction of ethyl ester 1b (entry 3)
afforded 6b as a major product in good yield (77%) along
with a low yield of 5b (4%). The reaction of 2b (entry 4)
gave 5b as a major product (65%) along with 6b (7%).
Likewise, the reactions of p-chlorophenyl derivatives 1c
(entry 5) and 2c (entry 6) or 3-ethylindole derivatives 1d
(entry 7) and 2d (entry 8) showed similar results. For the
reaction of naphthyl derivative 1e (entry 9), the rearranged
products 5e and 6e could not be separated. Thus, 5e and 6e
were obtained together as a mixture, and the ratio of 5e/6e
1
(1:7) was determined based on its H nuclear magnetic res-
onance (NMR) spectrum. In addition, somewhat unusual
product ratio (5e/6e = 2:3) was observed in the reaction of
2e (entry 10); however, the reason is not clear at this stage.
The thiophene derivatives 1f and 2f were decomposed
under a typical reaction condition (PPA, 70ꢀC). Thus, the
reactions of 1f and 2f were carried out in a mixed solvent
(ClCH2CH2Cl/PPA, 1:1) at lower temperature (60ꢀC) in
short time (30 min). It is interesting to note that primary
product 5f was obtained as a sole product in moderate
yields (42 and 45%) irrespective of the starting materials 1f
and 2f (entries 11 and 12).
Based on the result of 1f (entry 11), we assumed that pri-
mary product 5f could be a thermodynamically favored
one.7a,b In this context, we examined the reaction of 1f at
lower temperature (30ꢀC) in a mixed solvent (ClCH2CH2Cl/
PPA, 1:1). Surprisingly, the allylic rearrangement proceeded
even at 30ꢀC for 2 h in this case, and the secondary product
6f was formed successfully in moderate yield (38%) along
with 5f (32%), as shown in Scheme 3.9,10
A plausible reaction mechanism for the rearrangement of
1a to the major product 6a is proposed in Scheme 4. As
suggested by Okazaki and coworkers,7d a charge-induced
[3,3]-shift of the allylic moiety from N-1 to C-3 position to
form 3,3-disubstituted indolenine intermediate I, following
a [1,5]-shift to form II,11 and a subsequent enamination
process would produce 6a.
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The reaction of 3-unsubstituted indole derivative 1g failed
under a typical reaction condition, as shown in Scheme 5.
Most of 1g remained and the formation of some side products
was observed. The reason for the sluggish reactivity of 1g is
not clear at this stage. It is interesting to note that N,3-diallylic
compound 7 could be isolated in a reasonable yield (47%)
when we used AlCl3 as an acid catalyst in refluxing benzene
(3 h).7d,f Compound 7 might be formed via a Friedel–Crafts
type alkylation at the C-3 position of 1g by the N-allylic moi-
ety of another molecule 1g which is activated by AlCl3.
As a synthetic application of the synthesized products,
we examined the synthesis of pyrrolo[1-2-a]indol-3-one
derivative 812 from 6a in the presence of K2CO3 in DMF
(120ꢀC), as shown in Scheme 6. Compound 8 was obtained
in moderate yield (49%) in a short time (40 min).
2o position
N
of MBH adduct
OR'
N
+
2o
R
2
1o
R
1
MeOOC
R
COOMe
R
COOMe
COOMe
(R' = H, Ac, Boc)
or
2o
N
H
1o
COOMe
+
Conditions
R
COOMe
R
N
H
N
H
4
3
Br
MeOOC
MeOOC
R
1o position
2o
R
+
1o
of MBH adduct
N
N
H
H
6
5
Scheme 1. Various attachments of MBH adduct to indole.
Me
Me
COOMe
Ph
COOMe
Br
3-methylindole
PPA
Ph
Ph
+
N
5a (5%)
70 oC, 2 h
Cs2CO3 (2.0 equiv)
CH3CN, rt, 8 h
N
H
Ph
6a (73%)
COOMe
Me
1a (73%)
Me
OBoc
3-methylindole
PPA
70 oC, 4 h
COOMe
+
6a (8%)
Ph
DABCO (10 mol%)
ClCH2CH2Cl, 50 o
N
N
H
C
MeOOC
Ph
20 h
5a (64%)
COOMe
2a (92%)
Scheme 2. Regioselective synthesis of C-2 substituted indoles 5a and 6a.
Table 1. Regioselective rearrangement of 3-substituted N-allylindoles.a
R1
R1
R1
R1
EWG
PPA
R2
+
or
70 °C, 2-4 h
R2
N
N
N
N
R2
R2
H
H
EWG
5a-5f
6a-6f
1a-1f
2a-2f
EWG
EWG
Entry
Substrate
Time (h)
Product (%)
1
2
3
4
5
6
7
8
1a (R1 = Me, R2 = Ph, EWG = COOMe)
2a (R1 = Me, R2 = Ph, EWG = COOMe)
1b (R1 = Me, R2 = Ph, EWG = COOEt)
2
4
3
4
3
3
3
3
2
4
0.5c
0.5c
5a (5) + 6a (73)
5a (64) + 6a (8)
5b (4) + 6b (77)
5b (65) + 6b (7)
5c (4) + 6c (81)
5c (73) + 6c (6)
5d (6) + 6d (70)
5d (61) + 6d (2)
5e/6e (66, 1:7)b
5e/6e (50, 2:3)b
5f (42) + 6f (–)d
5f (45) + 6f (–)d
2b (R1 = Me, R2 = Ph, EWG = COOEt)
1c (R1 = Me, R2 = 4-ClC6H4, EWG = COOMe)
2c (R1 = Me, R2 = 4-ClC6H4, EWG = COOMe)
1d (R1 = Et, R2 = Ph, EWG = COOMe)
2d (R1 = Et, R2 = Ph, EWG = COOMe)
9
1e (R1 = Me, R2 = 2-naphthyl, EWG = COOMe)
2e (R1 = Me, R2 = 2-naphthyl, EWG = COOMe)
1f (R1 = Me, R2 = 2-thienyl, EWG = COOMe)
2f (R1 = Me, R2 = 2-thienyl, EWG = COOMe)
10
11
12
a
Conditions: substrate 1 or 2 (0.5 mmol), PPA, 70ꢀC, given time (h).
A mixture of 5e/6e and the ratio determined based on 1H NMR.
Carried out at 60ꢀC in a mixed solvent (PPA/ClCH2CH2Cl).
The formation of 6f was not observed.
b
c
d
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CH3
CH3
Me
Me
S
COOMe
Conditions
Ph
1f
+
K2CO3 (1.0 equiv)
Ph
N
N
N
6a
DMF, 120 oC
40 min
N
H
H
MeOOC
S
Me
O
6f
5f
O
8 (49%)
(i) PPA/ClCH2CH2Cl, 60 oC, 30 min (Entry 11, Table 1): 5f (42%) + no 6f
(ii) PPA/ClCH2CH2Cl, 30 oC, 2 h: 5f (32%) + 6f (38%)
Scheme 6. Synthesis of pyrrolo[1,2-a]indol-3-one derivative 8.
Scheme 3. Trial for the synthesis of 6f.
Compound 5a. 64%; white solid, mp 102–104ꢀC; IR (KBr)
3413, 1699, 1462, 1271 cm−1; 1H NMR (CDCl3,
500 MHz) δ 2.13 (s, 3H), 3.90 (s, 3H), 3.94 (s, 2H), 7.08
(t, J = 7.5 Hz, 1H), 7.13 (t, J = 7.5 Hz, 1H), 7.28 (d,
J = 7.5 Hz, 1H), 7.35–7.44 (m, 5H), 7.49 (d, J = 7.5 Hz,
1H), 7.90 (s, 1H), 8.53 (br s, 1H); 13C NMR (CDCl3,
125 MHz) δ 8.68, 24.75, 52.70, 106.97, 110.61, 118.32,
118.99, 121.33, 128.82, 129.02, 129.29, 129.35, 129.91,
132.36, 135.25, 135.37, 141.90, 169.94; ESIMS m/z
306 [M++H]. Anal. Calcd. for C20H19NO2: C, 78.66; H,
6.27; N, 4.59. Found: C, 78.73; H, 6.41; N, 4.37.
Compound 6a. 73%; white solid, mp 128–130ꢀC; IR (KBr)
3403, 1721, 1460, 1252 cm−1; 1H NMR (CDCl3,
500 MHz) δ 2.28 (s, 3H), 3.74 (s, 3H), 5.54 (s, 1H), 5.63
(s, 1H), 6.48 (s, 1H), 7.08–7.41 (m, 8H), 7.57 (t,
J = 6.6 Hz, 1H), 7.85 (br s, 1H); 13C NMR (CDCl3,
125 MHz) δ 8.70, 45.03, 52.36, 108.48, 110.81, 118.60,
119.28, 121.57, 127.20, 128.08, 128.23, 128.95, 129.46,
133.27, 135.32, 140.11, 141.62, 167.18; ESIMS m/z
306 [M++H]. Anal. Calcd. for C20H19NO2: C, 78.66; H,
6.27; N, 4.59. Found: C, 78.79; H, 6.24; N, 4.40.
Compound 5d. 61%; white solid, mp 100–102ꢀC; IR (KBr)
3413, 1701, 1487, 1274 cm−1; 1H NMR (CDCl3,
500 MHz) δ 1.13 (t, J = 7.5 Hz, 3H), 2.59 (q, J = 7.5 Hz,
2H), 3.87 (s, 3H), 3.98 (s, 2H), 7.06 (t, J = 8.0 Hz, 1H),
7.11 (t, J = 8.0 Hz, 1H), 7.28 (d, J = 8.0 Hz, 1H),
7.35–7.44 (m, 5H), 7.54 (d, J = 8.0 Hz, 1H), 7.90 (s, 1H),
8.41 (br s, 1H); 13C NMR (CDCl3, 125 MHz) δ 15.47,
17.51, 24.59, 52.64, 110.76, 114.00, 118.60, 118.98,
121.22, 128.41, 128.83, 129.09, 129.35, 129.87, 131.62,
135.23, 135.55, 141.80, 169.72; ESIMS m/z 320 [M++H].
Anal. Calcd. for C21H21NO2: C, 78.97; H, 6.63; N, 4.39.
Found: C, 78.73; H, 6.82; N, 4.55.
Compound 6d. 70%; pale yellow solid, mp 119–121ꢀC; IR
(KBr) 3402, 1720, 1458, 1260 cm−1; 1H NMR (CDCl3,
500 MHz) δ 1.21 (t, J = 7.5 Hz, 3H), 2.76 (q, J = 7.5 Hz,
2H), 3.72 (s, 3H), 5.55 (s, 1H), 5.61 (s, 1H), 6.46 (s, 1H),
7.09 (t, J = 7.5 Hz, 1H), 7.13 (t, J = 7.5 Hz, 1H), 7.18 (d,
J = 7.5 Hz, 2H), 7.23–7.28 (m, 2H), 7.30–7.34 (m, 2H),
7.60 (d, J = 7.5 Hz, 1H), 7.87 (br s, 1H); 13C NMR
(CDCl3, 125 MHz) δ 15.50, 17.59, 44.89, 52.36, 110.94,
115.20, 118.87, 119.24, 121.53, 127.15, 128.13, 128.27,
128.53, 128.91, 132.73, 135.57, 140.45, 142.13, 167.19;
ESIMS m/z 320 [M++H]. Anal. Calcd. for C21H21NO2: C,
78.97; H, 6.63; N, 4.39. Found: C, 79.04; H, 6.79; N, 4.32.
Scheme 4. Proposed mechanism for the formation of 6a.
In summary, a regioselective introduction of MBH
adduct at the C-2 position of 3-substituted indoles was car-
ried out in a two-step method, namely initial N-allylation
followed by PPA-mediated allylic rearrangement process.
Experimental
Typical Procedure for the Synthesis of 5a and 6a.
A
stirred solution of 1a (153 mg, 0.5 mmol) in PPA (1.5 g)
was heated to 70ꢀC for 2 h. After appropriate aqueous
extractive workup and column chromatographic purification
process (hexanes/Et2O, 10:1) compounds 5a (8 mg, 5%)
and 6a (112 mg, 73%) were obtained as white solids. Other
compounds were synthesized similarly, and the selected
spectroscopic data of 5a, 6a, 5d, 6d, and 5f are as follows.
H
Ph
COOMe
Conditions
N
N
Ph
Ph
COOMe
1g
COOMe
7
PPA, 70oC, 3 h: no reaction
AlCl3 (1.0 equiv), benzene, reflux, 3 h: 7 (47%)
Compound 5f. 45%; white solid, mp 117–119ꢀC; IR (KBr)
3402, 1701, 1275, 1209 cm−1; 1H NMR (CDCl3,
Scheme 5. The reaction of 3-unsubstitued indole 1g.
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(c) P. Goswami, A. J. Borah, P. Phukan, J. Org. Chem. 2015,
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500 MHz) δ 2.35 (s, 3H), 3.83 (s, 3H), 4.18 (s, 2H), 7.07
(t, J = 7.5 Hz, 1H), 7.10 (t, J = 7.5 Hz, 1H), 7.12 (dd,
J = 5.0, 3.5 Hz, 1H), 7.22 (d, J = 7.5 Hz, 1H), 7.34 (d,
J = 3.5 Hz, 1H), 7.48–7.51 (m, 2H), 8.00 (s, 1H), 8.11
(br s, 1H); 13C NMR (CDCl3, 125 MHz) δ 9.02, 25.46,
52.60, 108.03, 110.55, 118.28, 119.07, 121.32, 125.91,
127.88, 129.45, 129.85, 130.96, 133.09, 133.55, 135.35,
137.69, 169.17; ESIMS m/z 312 [M++H]. Anal. Calcd. for
C18H17NO2S: C, 69.43; H, 5.50; N, 4.50. Found: C,
69.77; H, 5.68; N, 4.29.
5. For preparation and synthetic applications of compounds 4,
see: (a) S. Ma, S. Yu, Z. Peng, H. Guo, J. Org. Chem. 2006,
71, 9865; (b) Z. Shafiq, L. Liu, Z. Liu, D. Wang, Y.-J. Chen,
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Y. Zheng, D. Wang, Y.-J. Chen, Synlett 2009, 2965;
(d) Z. Liu, D. Wang, Y. Chen, Lett. Org. Chem. 2011, 8, 73.
6. For migration of N-benzylindole to 2-benzylindole, see:
(a) N. Barbero, R. Sanmartin, E. Dominguez, Tetrahedron
Lett. 2009, 50, 2129; (b) P. Wiedenau, S. Blechert, Synth.
Commun. 1997, 27, 2033; (c) P. Wiedenau, B. Monse,
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Acknowledgments. This work was supported by the
National Research Foundation of Korea (NRF) grant
funded by the Korea government (MSIP) (NRF-
2015R1A4A1041036). Spectroscopic data were obtained
from the Korea Basic Science Institute, Gwangju branch.
Supporting Information. Additional supporting informa-
tion is available in the online version of this article.
7. For rearrangement of N-allylic moieties of 3-substituted
indoles to the C-2 position, see: (a) G. Casnati, A. Pochini,
Chem. Commun. 1970, 1328; (b) G. Casnati, R. Marchelli,
A. Pochini, J. Chem. Soc., Perkin Trans. 1 1974, 754;
(c) K. J. Baird, M. F. Grundon, D. M. Harrison,
M. G. Magee, Heterocycles 1981, 15, 713; (d) S. Inada,
K. Nagai, Y. Takayanagi, M. Okazaki, Bull. Chem. Soc. Jpn.
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Biomol. Chem. 2006, 4, 3966; (f ) A. S. Cardoso,
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8. To our delight, an acid-catalyzed intramolecular Friedel-Crafts
cyclization of 1a either at the C-2 or C-7 position of 1a was
not observed. For similar intramolecular Friedel-Crafts cycliza-
tion of N-substituted isatins and indoles, see: (a) H. R. Moon,
S. Y. Kim, J. W. Lim, J. N. Kim, Bull. Korean Chem. Soc.
2015, 36, 2773; (b) D. V. Patil, M. A. Cavitt, P. Grzybowski,
S. France, Chem. Commun. 2012, 48, 10337.
9. The reaction of 2f afforded 5f (50%) and 6f (11%) under the
same reaction condition (30ꢀC, 2.5 h) in a mixed solvent
(ClCH2CH2Cl/PPA, 1:1).
10. The reaction of 1a was also examined in a mixed solvent
(ClCH2CH2Cl/PPA, 1:1); however, the reaction was very
sluggish at 30ꢀC and a small amount of 6a (<5%) was formed
even after 15 h. The reaction was not completed even at 50ꢀC
for 15 h, and 6a (61%) and 5a (3%) were isolated along with
recovered 1a (17%).
11. Formation of the corresponding 3-allylic-2-methyl derivative
was not observed, and the result stated that a [1,5]-shift of an
allylic moiety of I proceeded preferentially over than that of
the methyl group.
12. For synthesis of pyrrolo[1,2-a]indol-3-one derivatives, see:
(a) H. Ikemoto, T. Yoshino, K. Sakata, S. Matsunaga,
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