Conversion of 2-alkylcinnamaldehydes to 2-alkylindanones via a catalytic intramolecular Friedel-Crafts reaction

Article abstract of DOI:10.1021/jo070952o

(Chemical Equation Presented) The preparation of indanones by the intramolecular acylation of 3-arylpropanoic acids or halides requires the use of non-catalytic acid promoters. In the presence of 5-10 mol % FeCl₃, in situ generated dimethyl acetals of (E)-2-alkylcinnamaldehydes cyclize to 1-methoxy-2-alkyl-1H-indenes in good-to-high yields. The 1-methoxyindenes were converted in high yield into 2-alkylindanones by treatment with triethylamine, to effect isomerization to the isomeric enol ethers, followed by acid-catalyzed hydrolysis. Thus, a catalytic, intramolecular Friedel-Crafts reaction leading to 2-alkylindanones from 2-alkylcinnamaldehydes was developed.

Full text of DOI:10.1021/jo070952o

the direct preparation of 1-indanones.4 In all cases, however,
these transformations generally require the use of at least an
equivalent amount of either a Lewis or Brønsted acid to be
effective, and the reaction mixtures often must be heated. These
methods are extremely harsh and generate large amounts of
acidic waste for disposal. Because of these concerns, alternative
methods for the preparation of indanones that avoid strongly
acidic conditions, such as transition metal-catalyzed carbocyli-
zations5a-c and isomerization of arylpropargyl alcohols,5d-f
continue to be of interest. Another approach is the development
of a catalytic version of the intramolecular electrophilic substitu-
tion reaction. Indanones were obtained by treating 3-arylpro-
panoic acids with 5-20 mol % Tb(OTf)3 at 250 °C.6 Opting to
modify the substrate to achieve milder reaction conditions,
Fillion et al. found that catalytic quantities of some metal triflates
were effective at cyclizing 5-benzyl derivatives of Meldrum’s
acid to yield 1-indanones.7 The intramolecular acylation, which
liberates acetone and carbon dioxide, was performed using 10
mol % Sc(OTf)3 in nitromethane at 100 °C.
Conversion of 2-Alkylcinnamaldehydes to
2-Alkylindanones via a Catalytic Intramolecular
Friedel-Crafts Reaction
Gary B. Womack,* John G. Angeles, Vincent E. Fanelli, and
Christie A. Heyer
Firmenich, Corporate R&D DiVision, P.O. Box 5880,
Princeton, New Jersey 08543
gary.womack@firmenich.com
ReceiVed May 23, 2007
Recently, Yonezawa et al.8 and we9 have reported that the
acetals of 2-alkylcinnamaldehydes will cyclize in the pres-
ence of acid catalysts to yield 1-alkoxy-2-alkyl-1H-indenes
(2). Yonezawa reported the isolation of the indenyl ethers
in moderate yields after treating (E)-2-alkylcinnamaldehydes
(1) with 4 equiv of trimethyl orthoformate and 1 equiv of
BF3‚OEt2 for 3 h at 25 °C (Scheme 1). The reaction also
occurred when starting with the dimethyl acetal of 1a, and a
mechanism was proposed in which intramolecular electrophilic
substitution proceeds through an allylic oxocarbenium ion 4.
We have found that this cyclization can be accomplished with
catalytic quantities of FeCl3 and that the resulting indenyl ethers
can be converted in high yields into 2-alkylindanones. In
addition to achieving a catalytic, intramolecular Friedel-Crafts
reaction leading to indanones, the process uses the more
environmentally benign reaction conditions of FeCl3 as the
catalyst and methyl acetate as the solvent.
The preparation of indanones by the intramolecular acylation
of 3-arylpropanoic acids or halides requires the use of non-
catalytic acid promoters. In the presence of 5-10 mol %
FeCl3, in situ generated dimethyl acetals of (E)-2-alkylcin-
namaldehydes cyclize to 1-methoxy-2-alkyl-1H-indenes in
good-to-high yields. The 1-methoxyindenes were converted
in high yield into 2-alkylindanones by treatment with
triethylamine, to effect isomerization to the isomeric enol
ethers, followed by acid-catalyzed hydrolysis. Thus, a
catalytic, intramolecular Friedel-Crafts reaction leading to
2-alkylindanones from 2-alkylcinnamaldehydes was devel-
oped.
Table 1 presents selected optimization experiments using 1a
as the substrate. At 40 mol % FeCl3 in dichloromethane (DCM),
1a was consumed within 1 h at rt in the presence of 1.1 equiv
Intramolecular Friedel-Crafts acylation of 3-arylpropanoic
acids1 and 3-arylpropanoyl halides,2 or intramolecular alkylation
using 1-aryl-2-propen-1-ones,3 are well-established methods for
(4) For reviews on intramolecular acylation yielding 1-indanones, see
(a) Heaney, H. In ComprehensiVe Organic Synthesis; Trost, B. M., Fleming,
I., Eds.; Pergamon Press: Oxford, UK, 1991; Vol. 2, pp 753-768. (b)
Larock, R. ComprehensiVe Organic Transformations, 2nd ed.; Wiley-
VCH: New York, 1999; pp 1422-1433. (c) Popp, F. D.; McEwen, W. E.
Chem. ReV. 1958, 58, 321-401. (d) Johnson, W. S. Org. React. 1944, 2,
114-177.
(5) (a) Gevorgyan, V.; Quan, L. G.; Yamamoto, Y. Tetrahedron Lett.
1999, 40, 4089-4092. (b) Larock, R. C.; Pletnev, A. A. Tetrahedron Lett.
2002, 43, 2133-2136. (c) Larock, R. C.; Gagnier, S. V. J. Am. Chem. Soc.
2003, 125, 4804-4807. (d) Hayashi, T.; Shintani, R. Org. Lett. 2005, 7,
2071-2073. (e) Shintani, R.; Okamoto, K.; Hayashi, T. J. Am. Chem. Soc.,
2005, 127, 2872-2873. (f) Yamabe, H.; Mizuno, A.; Kusama, H.; Iwasawa,
N. J. Am. Chem. Soc., 2005, 127, 3248-3249.
(6) Cui, D.-M.; Zhang, C.; Kawamura, M.; Shimada, S. Tetrahedron Lett.
2004, 45, 1741-1745.
(7) Fillion, E.; Fishlock, D.; Wilsily, A.; Goll, J. M. J. Org. Chem. 2005,
70, 1316-1327.
(8) (a) Jobashi, T.; Kawai, A.; Kawai, S.; Maeyama, K.; Oike, H.;
Yoshida, Y.; Yonezawa, N. Tetrahedron 2006, 62, 5717-5724. (b) Jobashi,
T.; Maeyama, K.; Noguchi, K.; Yoshida, Y.; Yonezawa, N. Bull. Chem.
Soc. Jpn. 2006, 79, 627-633.
(9) Womack, G. B.; Snowden, R. L.; Mosimann, H.; Birkbeck, A. A.
Process for Producing Indenol Esters or Ethers. WO Pat. Appl. WO 2005/
113473 A2, December 1, 2005.
(1) (a) Ho, T.-L.; Lee, K.-Y.; Chen, C.-K. J. Org. Chem. 1997, 62, 3365-
3369. (b) Budhhram, R. S.; Palaniswamy, V. A.; Eisenbraun, E. J. J. Org.
Chem. 1986, 51, 1402-1406. (c) Premasagar, V.; Palaniswamy, V. A.;
Eisenbraun, E. J. J. Org. Chem. 1981, 46, 2974-2976. (d) Koo, J. J. Am.
Chem. Soc. 1953, 75, 1891-1895. (e) Rendy, R.; Zhang, Y.; McElrea, A.;
Gomez, A.; Klumpp, D. A. J. Org. Chem. 2004, 69, 2340-2347.
(2) (a) House, H. O.; Hudson, C. B. J. Org. Chem. 1970, 647-651. (b)
Hulin, B.; Koreeda, M. J. Org. Chem. 1984, 207-209. (c) Yamato, T.;
Hideshima, C.; Surya Prakash, G. K.; Olah, G. A. J. Org. Chem. 1991, 56,
3955-3957. (d) Hanessian, S.; Ma, J. Tetrahedron Lett. 2001, 42, 8785-
8788. (e) Buckley, T. F., III.; Rapoport, H. J. Am. Chem. Soc. 1980, 102,
3056-3062.
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K. Y.; Walspurger, S.; Sommer, J. Tetrahedron Lett. 2005, 46, 8391-8394.
10.1021/jo070952o CCC: $37.00 © 2007 American Chemical Society
Published on Web 08/10/2007
7046
J. Org. Chem. 2007, 72, 7046-7049

TABLE 2. Preparation of 2b-e and 5ba
SCHEME 1. Reported Preparation of
1-Alkoxy-2-alkylindenes8a
entry
R
enal (E/Z)
product
yield (%)
1
2
3
Et
Pr
Bu
Pentyl
Bu
1b (96:4)
1c (96:4)
1d (95:5)
1e (92:8)
1d
2b
2c
2d
2e
5b
93
92
95
90
96
4
5b
a 1.1 equiv (MeO)3CH, 0.05 equiv FeCl3, refluxing MeOAc, 3-4 h. b 1.1
equiv (EtO)3CH, 0.05 equiv FeCl3, refluxing DCM, 4 h.
the liberation of alcohols as cyclization proceeds. A possible
explanation for the yield difference is a slower rate of cyclization
of the diethyl acetal, which allows for increased degradative
side reactions prior to indene formation. Attempts to prepare
5a with 10 mol % catalyst in refluxing DCM failed to achieve
complete consumption of 1a and necessitated the use of 15 mol
% catalyst which afforded 5a in 40% yield (entry 8). The yield
was improved to 69% using EtOAc as the solvent (entry 9) but
was still lower than the 89% yield obtained for 2a using similar
reaction conditions (entry 5).
TABLE 1. FeCl3-Catalyzed Formation of 2a and 5a from 1a
The cyclization of other 2-alkylcinnamaldehydes (1b-e) was
more efficient than that of 1a since 5 mol % catalyst was
sufficient to completely consume the enals within 4 h and
resulted in high isolated yields (90-96%) of the 1-alkoxy-2-
alkylindenes (Table 2). In the case of 2-butylcinnamaldehyde
(1d), a further reduction in catalyst level to 1 mol % resulted
in incomplete reaction after 8 h at reflux (ratio 2d/1d ) 68:32,
by GC). In contrast to the results obtained with 1a, 1d cyclized
in high yield with either trimethyl or triethyl orthoformate,
resulting in 95 and 96% yields, respectively (entries 3 and 5).
The higher cyclization efficiency of 1b-e probably results from
the larger 2-alkyl groups sterically preventing competitive
decomposition reactions that would lead to lower yields and
catalyst deactivation.
mol % FeCl3, solvent,
entry
R
temp, time (h)
yield (%)a
1
2
3
4
5
6
7
8
9
Me
Me
Me
Me
Me
Me
Et
40, DCM, rt, 1
40, DCM, rt, 24
20, DCM, rt, 24
10, DCM, reflux, 4
10, MeOAc, reflux, 3
5, MeOAc, reflux, 8
40, DCM, rt, 1
15, DCM, reflux, 7
15, EtOAc, 70 °C, 7
78
77
72
79
89
85b
45
40
69
Et
Et
a Isolated by bulb-to-bulb distillation of reaction mixture. b Water
added prior to work up to hydrolyze remaining acetal, 2a/1a ) 97:3 by 1H
NMR.
of trimethyl orthoformate yielding 2a in 78% (entry 1). Reducing
the catalyst level to 20 mol % resulted in a slower reaction, but
complete consumption of 1a was achieved within 24 h (72%
yield, entry 3). Reducing the catalyst level to 10 mol %
necessitated heating the reaction mixture at reflux to consume
all of 1a and afforded 2a in 79% yield after 4 h (entry 4). Using
these conditions with MeOAc as the solvent resulted in an
improved yield of 89% (entry 5). With MeOAc as solvent and
a catalyst level of 5 mol %, not quite all of 1a was consumed
after 8 h at reflux, but still, 2a (containing 3 mol % 1a) was
obtained in 85% yield after bulb-to-bulb distillation of the
reaction mixture (entry 6).
Treating 1a with triethyl orthoformate using the conditions
of Table 1, entry 1 produced 1-ethoxy-2-methylindene 5a in
45% yield (Table 1, entry 7), much lower than the 78% yield
obtained for methoxy analog 2a (entry 1). The reason for this
yield difference does not appear to be due to increased sensitivity
of 5a to acid-catalyzed decomposition, since both 2a and 5a
decomposed to similar degrees, 87 and 84% losses, respectively,
after 6 h at rt when treated with 40 mol % FeCl3 in the absence
of trialkyl orthoformate (DCM, GC with dodecane internal
standard). Stability of the alkoxyindenes under the preparative
reaction conditions is probably due to catalyst deactivation with
Cyclization also was achieved with either alkyl or methoxy
group substituents on the aromatic ring (Table 3). As in the
case of the simple 2-alkylcinnamaldehydes, when the 2-alkyl
group was varied from Me to Et, higher yields of the indenyl
ethers were isolated and a lower catalyst level (5 vs 10 mol %)
could be employed. This was especially evident for the para-
methoxy substituted enals 12 and 13, in which only trace
amounts of indenyl ether 6c were detected (GC/MS) starting
from 12 while the 2-ethyl analog 13 resulted in an 82% yield
of 7c (entries 5 and 6). Attempted bulb-to-bulb distillation of
the 6c reaction mixture did not yield any distillate from the
black polymeric residue. Both para and ortho EDGs in enals
8-17 are expected to help stabilize intermediate carbenium ions
3 and 4. Despite this, the yields of 2-methylindenyl ethers 6
were lower than the 89% yield for 2a. Thus, inductive effects
that help stabilize 3 and 4 also promote reactions leading to
degradation products unless sterically restricted by the substitu-
ent at C(2).
Indene formation failed to occur using (E)-cinnamaldehyde
as the substrate (conditions of Table 1, entry 5). GC/MS analysis
indicated formation of the dimethyl acetal of benzaldehyde and
higher molecular weight products but no alkoxyindene. Cy-
clization also failed to occur for enals 1 in the absence of trialkyl
J. Org. Chem, Vol. 72, No. 18, 2007 7047

TABLE 3. Indenyl Ethers with Substituents on the Aromatic
Ringa
SCHEME 2. Preparation of 2-Alkylindanones
Examples include indenes with alkyl, alkoxy, acetoxy, and
hydroxy subsituents. With a hydroxy substituent, the resulting
enol yields the corresponding indanone.13 For indenyl ethers 2,
6, and 7, such an isomerization would yield the enol ethers 18
that could then be hydrolyzed to the 2-alkylindanones 19. We
tested this possibility by treating 2d with 2 equiv of triethylamine
at room temperature. After 3 days, 2d was consumed and 18a
was isolated by bulb-to-bulb distillation in 98% yield. Acid-
catalyzed hydrolysis of the enol ether yielded 2-butylindanone
(19a) in 95% yield (93% yield from 2d). In a like manner, other
2-alkylindanones also were isolated in high overall yields from
alkoxyindenes (Scheme 2).
In conclusion, we have shown that (E)-2-alkylcinnamalde-
hydes can be cyclized to 1-alkoxy-2-alkylindenes in the presence
of trialkyl orthoformate (1.1 equiv) and catalytic quantities of
FeCl3 (5-10 mol %). Activation of the aryl group by means of
EDGs is not necessary for the intramolecular Friedel-Crafts
reaction to proceed. The resulting 1-alkoxyindenes can be
converted to 2-alkylindanones in high yield after a base-
promoted isomerization to enol ethers 18 followed by acid-
catalyzed hydrolysis. 2-Alkylindanone synthesis is achieved
starting from 2-alkylcinnamaldehydes, readily available from
aldol condensation of enolizable aldehydes with arylaldehydes.14
In contrast, a more traditional preparation of such indanones
from 2-alkylcinnamaldehydes would require oxidation and
hydrogenation of the enal to the 3-arylpropanoic acids followed
by intramolecular acylation using excess amounts of Brønsted
acids.
a Reaction conditions: 1.1 equiv (MeO)3CH, 10 mol % FeCl3 (R ) Me)
or 5 mol % FeCl3 (R ) Et), refluxing MeOAc, 3-4 h.
orthoformate. There was no measurable loss of 1d after 8 h
when treated with 5 mol % FeCl3 in refluxing MeOAc (GC,
dodecane as internal standard). These observations are consistent
with those reported by Yonezawa et al.8a and show that a 2-alkyl
group in the enal and acetal formation are both necessary for
cyclization to occur. Acid-catalyzed cyclization to indenols has
been reported for a few 3-aryl-2-alkyl-2-propenals with electron-
rich aryl groups,10 but there are no reports of the direct formation
of indenols from 1.11
The migration of the indene double bond is known to occur
in the presence of basic catalysts.12 For indenes bearing a
substituent on C(1), an equilibrium mixture is established
favoring the product with the more substituted double bond.
Experimental Section
General Procedure for the Preparation of Indenyl Ethers.
2-Alkylcinnamaldehyde (100 mmol), trimethyl orthoformate (110
mmol), and FeCl3 (5 mmol, 10 mmol for 2-methylcinnamaldehydes)
were mixed in refluxing MeOAc (100-200 mL) until all enal was
consumed (typically 2-6 h). The reaction mixture was diluted with
diethyl ether (200 mL) and washed with water (3 × 100 mL). The
organic phase was dried (MgSO4), filtered, and concentrated.
Indenyl ethers 2, 6, and 7 were isolated as pale-yellow oils (6d
crystalline solid) by bulb-to-bulb distillation. 6f was isolated by
column chromatography on silica gel followed by bulb-to-bulb
distillation.
(10) (a) Gupton, J. T.; Clough, S. C.; Miller, R. B.; Lukens, J. R.; Henry,
C. A.; Kanters, R. P. F.; Sikorski, J. A. Tetrahedron 2003, 59, 207-215.
(b) Singh, P. P.; Reddy, P. B.; Sawant, S. D.; Koul, S.; Taneja, S. C.; Kumar,
H. M. S. Tetrahedron Lett. 2006, 47, 7241-7243.
(11) For a report on the treatment of cinnamaldehyde with TFSA, see:
Ohwada, T.; Yamagata, N.; Shudo, K. J. Am. Chem. Soc. 1991, 113, 1364-
1373.
(12) (a) Friedrich, E. C.; Taggart, D. B. J. Org. Chem. 1975, 40, 720-
723. (b) Almy, J.; Hoffman, D. H.; Chu, K. C.; Cram, D. J. J. Am. Chem.
Soc. 1973, 95, 1185-1190.
1-Methoxy-2-butyl-1H-indene (2d). A mixture of 1d (18.8 g,
100 mmol), trimethyl orthoformate (11.7 g, 110 mmol) and FeCl3
(0.81 g, 5 mmol) in 200 mL of MeOAc was heated at reflux for
2.5 h. After work up, bulb-to-bulb distillation (bp 85-90 °C, 0.019
(13) (a) Clark, W. M.; Tickner-Eldridge, A. M.; Huang, G. K.; Pridgen,
L. N.; Olsen, M. A.; Mills, R. J.; Lantos, I.; Baine, N. H. J. Am. Chem.
Soc. 1998, 120, 4550-4551. (b) Gevorgyan, V.; Quan, L. G.; Yamamoto,
Y. Tetrahedron Lett. 1999, 40, 4089-4092.
(14) (a) Nongkhlaw, R. L.; Nongrum, R.; Myrboh, B. J. Chem. Soc.,
Perkin Trans. 2001, 1,1300-1303. (b) Bogert, M. T.; Powell, G. Am. Perf.
Ess. Oil ReV. 1930, 25, 617-620.
7048 J. Org. Chem., Vol. 72, No. 18, 2007

mmHg) afforded 19.2 g (95 mmol, 95% yield) of 2d as a pale-
yellow oil: 1H NMR (400 MHz, CDCl3): δ 0.95 (t, J ) 7.2 Hz,
3H), 1.36-1.46 (m, 2H), 1.51-1.70 (m, 2H), 2.29-2.43 (m, 2H),
3.02 (s, 3H), 4.93 (s, 1H), 6.43 (s, 1H), 7.11 (t, J ) 7.2 Hz, 1H),
7.14 (d, J ) 7.2 Hz, 1H), 7.22 (t, J ) 7.2 Hz, 1H), 7.41 (d, J ) 7.2
Hz, 1H); 13C NMR (100 MHz, CDCl3): δ 14.0 (q), 22.7 (t), 28.1
(t), 30.4 (t), 51.8 (q), 83.8 (d), 120.3 (d), 123.8 (d), 124.6 (d), 127.4
(d), 128.4 (d), 141.8 (s), 143.8 (s), 150.7 (s); IR (film): 2958, 2931,
1618, 1464, 1321, 1202, 1106, 1079, 948, 898 cm-1; MS (m/z):
202 (M+, 17), 159 (100), 145 (18), 141 (5), 129 (19), 128 (21),
115 (15), 102 (4), 91 (5); HRMS calcd for C14H18O: 202.1358,
found: 202.1356.
General Procedure for the Preparation of 2-Alkylindanones
19. In a typical procedure, 3 g of indenyl ether 2 or 6 were mixed
with 2 equiv of triethylamine and 25 mL of EtOAc. The mixture
was monitored by gas chromatography until the ratio of the enol
ether to starting material was greater than 99:1 (3-5 days). The
mixture was concentrated to remove solvent and subjected to bulb-
to-bulb distillation to first remove triethylamine (50-60 °C oven
temperature, 2.2 mmHg) and then enol ether 18. 18 was mixed
with THF (20 mL) and 0.1 N HCl (40 mL), and the mixture was
heated (75 °C) for 4 h. The mixture was diluted with ethyl ether
and the aqueous phase separated. The organic phase was washed
with water, dried (MgSO4), filtered, and concentrated. Bulb-to-bulb
distillation yielded the 2-alkylindanones 19.
2-Butylindanone (19a).15 From 25.3 g (0.125 m) of 2d, 24.8 g
(0.123 m, 98% yield) of 18a were isolated by bulb-to-bulb
distillation (bp 84-94 °C, 0.02 mmHg). 3 g (14.9 mmol) of 18a
were hydrolyzed to yield 2.65 g (14.1 mmol, 95% yield) of 19a by
bulb-to-bulb distillation (bp 120 °C, 0.02 mmHg) as a pale-yellow
oil: 1H NMR (400 MHz, CDCl3): δ 0.92 (t, J ) 7.2 Hz, 3H),
1.30-1.50 (m, 5H), 1.90-2.00 (m, 1H), 2.61-2.67 (m, 1H), 2.81
(dd, J ) 17.1, 3.8 Hz, 1H), 3.31 (dd, J ) 17.1, 7.9 Hz, 1H), 7.34
(t, J ) 7.6 Hz, 1H), 7.44 (d, J ) 7.6 Hz, 1H), 7.57 (t, J ) 7.6 Hz,
1H), 7.74 (d, J ) 7.6 Hz, 1H); 13C NMR (100 MHz, CDCl3): δ
14.0 (q), 22.7 (t), 29.6 (t), 31.2 (t), 32.9 (t), 47.4 (d), 123.8 (d),
126.6 (d), 127.3 (d), 134.6 (d), 136.9 (s), 153.8 (s), 209.1 (s); IR
(film): 2958, 2931, 2858, 1713, 1610, 1465, 1287 cm-1; MS
(m/z): 188 (2), 145 (13), 133 (10), 132 (100), 131 (16), 115 (11);
HRMS calcd for C13H16O: 188.1201, found: 188.1209.
Supporting Information Available: Experimental procedures,
spectral data, and 1H NMR and 13C NMR spectra of products 2, 5,
6, 7, 18a, and 19 are given. This material is available free of charge
via the Internet at http://pubs.acs.org.
JO070952O
(15) Ichikawa, J.; Jyono, H.; Kudo, T. Fujiwara, M.; Yokota, M. Synthesis
2005, 39-46.
J. Org. Chem, Vol. 72, No. 18, 2007 7049