A general method for C3 reductive alkylation of indoles

Article abstract of DOI:10.1016/S0040-4039(03)01010-4

General indole C₃ reductive alkylation conditions have been developed. The scope of this reaction includes C₂ unsubstituted indoles, aryl and alkyl aldehydes, as well as N-H and N-alkyl indole substrates.

Full text of DOI:10.1016/S0040-4039(03)01010-4

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 44 (2003) 4589–4591
A general method for C3 reductive alkylation of indoles
Anu Mahadevan,a,* Howard Sard,a Mario Gonzaleza and John C. McKewb,*
aOrganix Incorporated 240 Salem Street, Woburn, MA 01801, USA
bWyeth Research, Department of Chemical and Screening Sciences, 200 Cambridge Park Drive, Cambridge, MA 02140, USA
Received 10 January 2003; revised 15 April 2003; accepted 15 April 2003
Abstract—General indole C3 reductive alkylation conditions have been developed. The scope of this reaction includes C2
unsubstituted indoles, aryl and alkyl aldehydes, as well as N–H and N-alkyl indole substrates. © 2003 Published by Elsevier
Science Ltd.
In the context of a medicinal chemistry project the
convergent synthesis of C3 functionalized indoles
became a priority. One of the requirements for this
transformation was that it could be performed with the
indole nitrogen substituted or unsubstituted. There are
many methods to functionalize the C3 position of the
indole nucleus,1 but very few of them allow the use of
both N-alkylated and N–H indole skeletons. A reaction
reported by Steele2 allows the delivery of C3 alkylated
products with the desired flexibility at N1 (Scheme 1).
In this reaction it is crucial that the indolium ion 1
undergoes reduction before a second indole nucleus
adds to this electrophilic species to give the bis-indolyl-
methane. There are many examples of the formation of
bis-indolylmethanes3 and no other examples of the
intermolecular reaction where reduction occurs before
addition of a second indole moiety.4
There are two limitations stated in the published work:
indole and other C2 unsubstituted substrates do not
react cleanly, and alkyl aldehydes self-condense under
the reaction conditions before reacting in the desired
fashion. In this communication it is demonstrated that
both of these problems can be overcome by varying the
reaction conditions.
The first substrate that was investigated was 6-chloro-
indole. Treatment of this indole with TFA/Et3SiH and
aryl aldehyde 2b afforded the bis-indolylmethane 3 in
86% yield (Scheme 2). Since the isolated bis-indolyl-
Scheme 1.
* Corresponding authors. E-mail: jmckew@wyeth.com
Scheme 2.
0040-4039/03/$ - see front matter © 2003 Published by Elsevier Science Ltd.
doi:10.1016/S0040-4039(03)01010-4

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A. Mahade6an et al. / Tetrahedron Letters 44 (2003) 4589–4591
Table 1 lists several other examples of this reaction that
have been performed on indole N–H substrates. Elec-
tron withdrawing groups at either the 5 or 6 position of
the indole gave higher yields than electron donating
groups, cf. entries 3–5 and entry 6. Higher yields were
generally obtained with TFA as the acid source. The
low yield seen with 5,6-methylenedioxyindole, entry 6,
was due to difficulty in reducing the bis-indolylmethane
intermediate which required refluxing in dichloro-
ethane. The products resulting from these reductive
alkylations were then alkylated,5b using benzhydryl bro-
mide and sodium hydride, and subsequently hydrolyzed
using lithium hydroxide to yield the desired acids.
methane intermediate could be reduced to the desired
product 4, in 60% yield under the same reaction condi-
tions, it became clear that this could constitute a one-
pot transformation. This observation indicates that the
bis-indolylmethane acts as a product sink and that
reduction of intermediate 1 before further reaction is
not as important as initially reported. The reaction
conditions were adjusted so that an excess of indole was
used (1.6 equiv.), and these conditions proved to be
quite general for a variety of indoles.5 The excess indole
could be recovered after column chromatography but
was not routinely recycled.
A potential side product of this reaction would be
reduction of the indole to the indoline. This transfor-
mation has been reported6 using TFA/Et3SiH at 25°C,
but under the reaction conditions described no reduc-
tion was observed.
N-Substituted indoles were then examined. The benzo-
ylcarbamate of 6-methoxyindole gave no reaction with
TFA. When the reaction was performed with BF3·OEt2
or SnCl4, the aldehyde was reduced and the bis-indolyl-
methane was formed very slowly. The carbamate pro-
tecting group was deemed too deactivating for this
reaction. The SEM protecting group cleaved in the
presence of TFA in CH2Cl2. N-Alkylated indole sub-
strates proved to be quite reactive under these condi-
tions (Table 2).5,7 The benzyl derivative of 6-chloro-
indole gave the desired product under the standard
reaction conditions, entry 1. 6-Chloro-N-benzhydryl-
indole, 5, turned out to be a very effective substrate
for the reductive alkylation reaction. A variety of
aldehydes were examined with this substrate; alkyl alde-
hydes, hindered and non-hindered, react in moderate
to good yield (entries 2–3).
Table 1. Reductive alkylation of indole N–H substrates
with methyl 4-formyl-3-methoxybenzoate and methyl 4-
formylbenzoate
Table 2. Reductive alkylation of N-substituted indoles

A. Mahade6an et al. / Tetrahedron Letters 44 (2003) 4589–4591
4591
No competing reduction or self-condensation of these
aldehydes is seen. This is attributed to the speed at
which the initial reaction to generate the intermediate 1
occurs. The N-alkylated substrates were quite reactive
towards aryl aldehydes, examples 4–5, with the yield
decreasing as electron withdrawing groups were substi-
tuted on the ring. The use of this reaction to explore
further derivatives is under investigation.
(dd, J=1.7, 7.7 Hz, 1H), 7.41 (d, J=8.5 Hz, 1H), 7.35 (d,
J=1.7 Hz, 1H), 7.10 (d, J=7.7 Hz, 1H), 7.04 (dd, J=1.7,
8.5 Hz, 1H), 6.94 (d, J=2.5 Hz, 1H), 4.09 (s, 2H), 3.92 (s,
3H), 3.89 (s, 3H). Anal. calcd for C18H16ClNO3: C, 65.56;
H, 4.89; N, 4.25; Cl, 10.75. Found: C, 65.36; H, 4.91; N,
4.38; Cl, 10.86.
(b) Alkylation with benzhydryl bromide (general proce-
dure): To a solution of indole (1.0 equiv.) in DMF (0.2 M)
was added NaH (1.2 equiv.). After 15 min benzhydryl
bromide (1.1 equiv.) was added and the reaction mixture
was stirred for 18 h at ambient temperature. The reaction
was diluted with EtOAc and washed with water, brine,
dried over sodium sulfate and concentrated under reduced
pressure and purified by column chromatography using
EtOAc/hexanes as eluent to give the product.
References
1. (a) See for example: Sundberg, R. J. Indoles; Academic
Press: San Diego, CA, 1996; pp. 105–118; (b) Comins, D.
L.; Stroud, E. D. Tetrahedron Lett. 1986, 27, 1869–1872;
(c) Matassa, V. G.; Maduskuie, T. P.; Shapiro, H. S.;
Hesp, B.; Snyder, D. W.; Aharony, D.; Krell, R. D.;
Keith, R. A. J. Med. Chem. 1990, 33, 1781–1790; (d)
Katritzky, A. R.; Lan, X.; Lam, J. N. J. Org. Chem. 1991,
56, 4397–4403.
2. Appleton, J. E.; Dack, K. N.; Green, A. D.; Steele, J.
Tetrahedron Lett. 1993, 34, 1529–1532 and references cited
therein. For a related approach, see: Zhou, P.; Li, Y.;
Meagher, K. L.; Mewshaw, R. G.; Harrison, B. L. Tetra-
hedron Lett. 2001, 42, 7333–7335.
(c) Table 2, entry 1 (TFA) procedure: To a solution of
1-benzyl-6-chloro indole (4.0 g, 16.6 mmol) and 4-bromo-
butyraldehyde (2.5g, 16.6 mmol) in CH2Cl2 (165 mL) at
0°C was added triethylsilane (5.79 g, 47 mmol) followed by
trifluoroacetic acid (2.6 mL, 33 mmol). After 10 min at
0°C the ice bath was removed and the reaction was stirred
until the disappearance of the initially formed spot (bis-
indolylmethane intermediate) was observed by TLC analy-
sis. The reaction was worked up by addition of saturated
NaHCO3, diluted with CH2Cl2 and washed with saturated
NaHCO3, water, and brine, dried over sodium sulfate and
concentrated under reduced pressure. The crude mixture
was then purified by column chromatography using 1–5%
EtOAc/hexanes as eluent to give 3.78 g (48% yield) of the
3. (a) Maiti, A. M.; Bhattacharyya, P. J. Chem. Res. 1997,
424–425; (b) Noland, W. E.; Robinson, D. N. Tetrahedron
1958, 3, 68–72; (c) Bader, H.; Oroshnik, W. J. Am. Chem.
Soc. 1959, 81, 163–167.
4. For an intramolecular version, see: Martin, T.; Moody, C.
1
product as a white solid: H NMR (CDCl3, 300 MHz) l
J. J. Chem. Soc., Perkin Trans. 1 1998, 241–246.
7.48 (d, J=8.2 Hz, 1H), 7.29 (m, 3H), 7.23 (d, J=1.9 Hz,
1H), 7.07 (m, 2H), 7.06 (dd, J=1.7, 6.6 Hz, 1H), 6.89 (br
s, 1H), 5.22 (s, 2H), 3.43 (t, J=6.6 Hz, 2H), 2.75 (t, J=6.8
Hz, 2H), 1.91 (m, 4H). Anal. calcd for C19H19BrClN: C,
60.58; H, 5.08; N, 3.72. Found: C, 60.71; H, 5.08; N, 3.62.
6. (a) Ward, J. S.; Fuller, R. W.; Merritt, L.; Snoddy, H. D.;
Paschal, J. W.; Mason, N. R.; Horng, J. S. J. Med. Chem.
1988, 31, 1512–1519; (b) Lanzilotti, A. E.; Littel, R.;
Fanshawe, W. J.; McKenzie, T. C.; Lovell, F. M. J. Org.
Chem. 1979, 44, 4809–4813.
7. When the indole was N-alkylated under NaH/benzyl/benz-
hydryl bromide conditions approximately 15% of the N1C3
dialkylated indole was formed. The desired product was
difficult to separate from the dialkylated product and the
resulting mixture was used in the reductive alkylation
reaction. The yield on these reactions is thus based upon
the amount of aldehyde used.
5. (a) Table 1, entry 4 (BF3·Et2O procedure): To a solution of
methyl 4-formyl-3-methoxy-benzoate (2a, 4.52 g, 23 mmol)
in CH2Cl2 (212 mL) at −30°C was added BF3·Et2O (2.9
mL, 23 mmol) followed by dropwise addition of a solution
of 6-chloro indole (3.21 g, 21 mmol) in CH2Cl2. After a
few minutes triethylsilane (10.1 mL, 64 mmol) was added,
the cooling bath was removed after 15 min and the reac-
tion was stirred until the disappearance of the initially
formed spot (bis-indolylmethane intermediate) was
observed by TLC analysis. The reaction was worked up by
addition of 1N NaOH, diluted with CH2Cl2 and washed
with 1N NaOH, water, and brine, dried over sodium
sulfate and concentrated under reduced pressure. The
crude mixture was then purified by column chromatogra-
phy using 10–20% EtOAc/hexanes as eluent to give 3 g
(43% yield) of the product as a white solid: 1H NMR
(CDCl3, 300 MHz) l 7.96 (br s, 1H), 7.53 (br s, 1H), 7.52