Preparation of cyclopenta-fused N-, O-, and S-heterocycles by Lewis acid catalyzed Nazarov reaction

Article abstract of DOI:10.1055/s-2007-965965

The products of carbonylative coupling between lactam-, lactone- and thiolactone-derived vinyl triflates and alkenylboronic acids are suitable substrates for the Lewis acid catalyzed Nazarov reaction. The most efficient Lewis acids for the Nazarov reactio

Full text of DOI:10.1055/s-2007-965965

PRACTICAL SYNTHETIC PROCEDURES
1733
Preparation of Cyclopenta-Fused N-, O-, and S-Heterocycles by Lewis Acid
Catalyzed Nazarov Reaction
Preparationof
C
a
yclopenta-
u
Fused
N
-,
O
r
-,
a
ndS-
a
H
eterocycles Bartali, Paolo Larini, Antonio Guarna, Ernesto G. Occhiato*
Department of Organic Chemistry ‘U. Schiff’ and Laboratory of Design,
Synthesis and Study of Biologically Active Heterocycles (HeteroBioLab), Università di Firenze,
Via della Lastruccia 13, 50019 Sesto Fiorentino, Italy
Fax +39(055)4573531; E-mail: ernesto.occhiato@unifi.it
Received 7 February 2007
PSP
No86
Abstract: The products of carbonylative coupling between lactam-, lactone- and thiolactone-derived vinyl triflates and alkenylboronic ac-
ids are suitable substrates for the Lewis acid catalyzed Nazarov reaction. The most efficient Lewis acids for the Nazarov reaction are scan-
dium(III) and indium(III) triflates (3–15 mol%) in 1,2-dichloroethane, which provide the Nazarov products in moderate to excellent yield
(53–86%). The electrocyclization rate depends also on the heteroatom (N, O, S) and the N-protecting group. On the whole, the entire pro-
cedure is an expeditious synthesis of hexahydro[1]pyrindenes, -cyclopenta[b]pyrans, and -cyclopenta[b]thiopyrans of noteworthy interest
as they form the structural core of several natural molecules.
Key words: carbonylative couplings, electrocyclic reactions, Nazarov reaction, Lewis acids, bicyclic compounds
1. KHMDS, PhNTf2, THF, –78 °C
2. Alkenylboronic acid, 5% Pd(OAc)2,
Ph3P or dppf, CsF, CO (1 atm), THF, r.t.
R2
3–15% Sc(OTf)3
or In(OTf)3, DCE, 0–55 °C
R2
R3
R1
R3
R1
R1
50–86% overall
53–86%
X
O
X
X
O
O
X = N-EWG
X = O
X = S
1
2
3
X = N-EWG
X = O
X = S
X = N-EWG
X = O
X = S
4
5
6
7
8
9
Scheme 1
Introduction
structural core of several natural compounds.3 The proce-
dure encompasses the transformation of lactams, lactones,
The acid-catalyzed cyclization of a doubly a,b-unsaturat- and thiolactones 1–3 into the corresponding enol triflates
ed ketone is referred to as the Nazarov reaction and it is followed by carbonylative coupling with vinylboronic
currently one of the most versatile and powerful methods acids and, eventually, the Lewis acid catalyzed Nazarov
for the synthesis of five-membered carbocycles.1 The reaction of coupling products 4–6.
abundance of five-membered carbocycles among natural
products has inspired much research around the Nazarov
reaction, so that the limitations of the original procedure
such as the harsh reaction conditions (strong acids and
Scope and Limitation
high temperature) and the poor regioisomeric control have The carbonylative coupling of the triflates4 with substitut-
been fully surmounted.2
ed vinylboronic acids (2.5 equiv) is carried out in the pres-
ence of palladium(II) acetate (5 mol%) and using
triphenylphosphine (10 mol%) as a ligand (Scheme 1) as
already reported by us for a N-heterocyclic triflate.5 Un-
der these conditions the reaction occurs at room tempera-
ture and under one atmosphere of carbon monoxide. With
the O-heterocyclic triflates6 the yield of the carbonylated
products 5 can be increased by using 1,1¢-bis(diphe-
nylphosphino)ferrocene (dppf, 6.25 mol%) as a ligand,
whereas with S-containing triflates7 using cesium acetate
as a base, instead of cesium fluoride (3 equiv), provides
higher yields of the carbonylated products 6.
In several instances, Nazarov reactions have been carried
out on substrates in which one of the double bonds in-
volved in the electrocyclization is present in a carbocycle,
thus leading to the formation of hexahydroindenes and
other cyclopenta-fused carbocycles;1 only a few examples
of the Nazarov reaction involving nonaromatic heterocy-
clic compounds are known. The procedure summarized in
Scheme 1 entails a short sequence which allows an expe-
ditious synthesis of cyclopenta-fused N-, O-, and S-het-
erocycles that are of noteworthy interest as they form the
A series of Lewis acids was tested in order to optimize the
conditions for the Nazarov cyclization of dienones 4–6.
Metal triflates, in particular scandium(III) triflate and in-
SYNTHESIS 2007, No. 11, pp 1733–1737
Advanced online publication: 28.02.2007
DOI: 10.1055/s-2007-965965; Art ID: Z03207SS
© Georg Thieme Verlag Stuttgart · New York
x
x.
x
x
.
2
0
0
7

1734
L. Bartali et al.
PRACTICAL SYNTHETIC PROCEDURES
dium(III) triflate were the most efficient in catalyzing the acidic among the catalysts tested as the two cations Sc3+
electrocyclization process [Zn(OTf)2, Cu(OTf)2, and In3+ possess the highest charge-to-radius ratio.8 A cor-
Yb(OTf)3, and Y(OTf)3 were all inferior catalysts]. Scan- relation between the efficiency of the electrocyclization
dium(III) and indium(III) triflates are the most Lewis and the Lewis acidity of the metal triflates thus exists.
Table 1 Nazarov Products Obtained by Treatment of the Corresponding Dienones 4–6 with a Lewis Acid
Entry
Lewis acid (mol%)
Temp (°C)
Time (h)
Product
Yielda (%)
86
n-Pr
1
Sc(OTf)3 (3)
55
3
7a
N
O
CO2Me
2
3
In(OTf)3 (15)
Sc(OTf)3 (3)
35
55
12
7a
7b
85
83
Ph
2.5
N
O
CO2Me
4
5
In(OTf)3 (10)
Sc(OTf)3 (10)
20
22
4
7b
7c
81
64
Ph
20
N
O
CO2Me
Ph
6
7
8
Sc(OTf)3 (3)
Sc(OTf)3 (3)
Sc(OTf)3 (10)
55
0
6.5
4
7d
55
65
63
N
O
Ts
n-Pr
8ab
8bc
O
O
O
Ph
0
1
O
9
In(OTf)3 (3)
Sc(OTf)3 (3)
20
20
1
8b
64
53
8cd
Ph
10
1.5
O
O
n-Pr
11
Sc(OTf)3 (3)
20
1
9a
67
S
O
Ph
12
13
Sc(OTf)3 (3)
In(OTf)3 (3)
20
20
1
1
9b
9b
73
72
S
O
a Yield of isolated product.
b Ratio trans/cis 9:1.
c Ratio trans/cis 20:1.
d Ratio trans/cis 1.2:1.
Synthesis 2007, No. 11, 1733–1737 © Thieme Stuttgart · New York

PRACTICAL SYNTHETIC PROCEDURES
Preparation of Cyclopenta-Fused N-, O-, and S-Heterocycles
1735
R
4
H
R
4a
5
H
Me
O
a
H
+
OLA
H
LAO
Me
O
+
2
Me
O
O
R
5a,b
8a,b
pseudo-axial bond formation
(favored)
trans product
b
R
H
H
O
LAO
+
H
R
Me
Me
O
H
O
8a,b
pseudo-equatorial bond formation
(disfavored)
cis product
Scheme 2 Torquoselectivity in the Nazarov reaction of dienones 5
With N-methoxycarbonyl-protected dienones 4a–c, all re- in good yield just after one hour at room temperature.
actions were successfully carried out at either room tem- When the reaction was carried out on 6a (R2 = Pr, R3 = H)
perature or by heating at 35–55 °C in the presence of 3–15 at 55 °C, we observed a rapid darkening of the solution
mol% of the catalyst (Table 1, entries 1–5) providing the and, by TLC, degradation of the substrate and/or the Naz-
Nazarov products 7a–c in good yields. The N-tosyl pro- arov product into several byproducts. By lowering the re-
tecting group had a slightly detrimental effect on the reac- action temperature and using the same low amount of
tion rate (entry 6), as an acceptable yield (55%) of the scandium triflate (3 mol%) the reaction was still rapid, as
Nazarov product 7d was obtained only after heating for the conversion of 6a into 9a was complete after one hour
6.5 hours in 1,2-dichloroethane at 55 °C in the presence of (entry 11), without any appreciable degradation (67%
scandium(III) triflate (3 mol%); at room temperature with yield after chromatography). Treatment of dienone 6b
0.1 equivalent of the Lewis acid the reaction was very (R2 = Ph, R3 = H) with both Lewis acids (entries 12 and
slow and provided the Nazarov product in 26% yield after 13) provided good yields of the Nazarov product 9b after
24 hours.
one hour at room temperature.
For the Nazarov reaction of 2-alkoxy-1,4-dien-3-ones,9 The (high) relative rates of cyclization of heteroatom-con-
we chose 6-methyl-substituted compounds 5 (X = O, taining dienones 4–6 can be explained on the basis of the
R1 = 6-Me) in order to address, besides the reactivity, the role of the heteroatom in stabilizing the positive charge
problem of torquoselectivity in the Lewis acid catalyzed that develops at the a-position in the transition state of the
cyclization, an issue that has been examined by us in the process (Scheme 2).10a,b,11 Also, bidentate binding of the
case of Brønsted acid catalyzed Nazarov reaction,10 which Lewis acid to the substrate (C=O and X) could help the
could be of crucial importance in synthetic applications of two vinyl moieties in adopting the proper s-cis/s-trans ori-
the methodology. Cyclization of dienone 5a (R2 = Pr, entation for the cyclization.9
R3 = H) in the presence of 3 mol% of scandium triflate at
0 °C for four hours provided the Nazarov product 8a (en-
As for the torquoselectivity in the cyclization of 5a,b, it is
interesting to notice that the Lewis acid has no effect in
try 7) in 65% yield after chromatography. Cyclization of
varying the sense of conrotation with respect to the min-
7b (R2 = Ph, R3 = H) was rapid with 10 mol% of scandi-
eral acid catalyzed process,10a as the 2,5-trans-products
um triflate at 0 °C (entry 8) as well as in the presence of 3
8a,b are obtained as the major diastereomers. The electro-
cyclization of the pentadienyl cations deriving from 5a,b
(Scheme 2) occurs through a transition state in which the
mol% of indium(III) triflate (entry 9) at 20 °C. Both com-
pounds 8a and 8b were obtained as a mixture of diaste-
reomers in 9:1 and 20:1 ratio, respectively. The major
isomers were assigned the 2,5-trans relative stereochem-
methyl group is equatorially placed. On the basis of previ-
ous DFT studies,11 the formation of the new bond along
istry by NOE studies, which is in accordance with the re-
the axial direction (path a) should occur through a more
sults obtained in the mineral acid catalyzed Nazarov
stable transition state and allow for the maximum orbital
reaction of analogous substrates.10a The cyclization of 7c
overlap in the formation of the new C4a–C5 bond.
(R2 = H, R3 = Ph) was carried out at room temperature in
Finally, irrespective of the sense of conrotation, the for-
the presence of 3 mol% of scandium triflate (entry 10). In
mation of the thermodynamic 1.2:1 mixture 8c is due to
this case a 1.2:1 mixture of diastereomers 8c (not separat-
equilibration at C6 after cyclization and loss of H4a.
ed) was isolated by chromatography in 53% yield. TLC
monitoring showed that some degradation of the starting
material and/or the Nazarov product had occurred.
The typical procedures for the three classes of dienones depicted in
Scheme 1 are described. For the carbonylative coupling procedure
and characterization of 7a and 7b see ref. 5. Analytical data for all
new compounds are provided in detail.
The cyclization of thiopyranyl derivatives 6 was also rap-
id, as the Nazarov products (entries 11–13) were obtained
Synthesis 2007, No. 11, 1733–1737 © Thieme Stuttgart · New York

1736
L. Bartali et al.
PRACTICAL SYNTHETIC PROCEDURES
Methyl 7-Oxo-5-propyl-2,3,4,5,6,7-hexahydro-1H-[1]pyrind-
ine-1-carboxylate (7a);5 Typical Procedure
(2R*,5R*)-2-Methyl-5-phenyl-3,4,5,6-tetrahydrocyclopen-
ta[b]pyran-7(2H)-one (8b)
Rf = 0.18 (EtOAc–hexane, 1:5, 0.5% Et3N).
To a soln of 4a (72 mg, 0.3 mmol) in anhyd DCE (1.5 mL) was add-
ed Sc(OTf)3 (4.5 mg, 0.009 mmol) and the resulting soln was stirred
at 55 °C for 3 h under an N2 atmosphere. The dark soln was diluted
with Et2O (8 mL) and washed with 5% aq NaHCO3 (30 mL). The
aqueous layer was extracted with Et2O (2 × 25 mL) and the com-
bined organic layers were dried (Na2SO4). Chromatography (silica
gel 60, EtOAc–petroleum ether, 1:2) provided 7a (62 mg, 86%) as
a colorless oil.
1H NMR (400 MHz, CDCl3): d = 1.41 (d, J = 6.2 Hz, 3 H), 1.57–
1.78 (m, 1 H), 1.84–2.00 (m, 1 H), 2.07–2.19 (m, 2 H), 2.33 (dd,
J = 18.7, 1.5 Hz, 1 H), 2.91 (dd, J = 18.7, 6.6 Hz, 1 H), 3.81 (dd,
J = 6.6, 1.5 Hz, 1 H), 4.03–4.17 (m, 1 H), 7.15–7.18 (m, 2 H), 7.20–
7.40 (m, 3 H).
13C NMR (CDCl3): d = 20.6 (q), 22.6 (t), 28.4 (t), 43.2 (t), 43.7 (d),
73.5 (d), 126.9 (d, 2 C), 127.0 (d), 128.8 (d, 2 C), 141.6 (s), 146.9
(s), 151.5 (s), 199.8 (s).
Methyl 7-Oxo-6-phenyl-2,3,4,5,6,7-hexahydro-1H-[1]pyrind-
ine-1-carboxylate (7c)
Rf = 0.31 (EtOAc–petroleum ether, 1:1).
MS (EI, 70 eV): m/z (%) = 228 (M+, 100), 200 (94), 129 (58), 115
(55), 91 (22).
1H NMR (400 MHz, CDCl3): d = 1.96–2.02 (m, 2 H), 2.46–2.50 (m,
2 H), 2.60 (d, J = 18.2 Hz, 1 H), 3.07 (dd, J = 18.2, 7.2 Hz, 1 H),
3.65–3.73 (m, 2 H + 1 H), 3.74 (s, 3 H), 7.19–7.36 (m, 5 H).
Anal. Calcd for C15H16O2 (228.29): C, 78.92; H, 7.06. Found: C,
78.63; H, 7.19.
13C NMR (CDCl3): d = 22.4 (t), 26.5 (t), 37.6 (t), 44.3 (t), 50.7 (d),
53.2 (q), 126.8 (d), 127.6 (d, 2 C), 128.6 (d, 2 C), 137.7 (s), 139.4
(s), 154.4 (s), 156.0 (s), 198.6 (s).
2-Methyl-6-phenyl-3,4,5,6-tetrahydrocyclopenta[b]pyran-
7(2H)-one (8c)
Major diastereomer
MS (EI, 70 eV): m/z (%) = 271 (M+, 100), 243 (12), 226 (12), 212
Rf = 0.15 (EtOAc–hexane, 1:5, 0.5% Et3N).
1H NMR (400 MHz, CDCl3): d = 1.43 (d, J = 6.2 Hz, 3 H), 1.64–
1.87 (m, 1 H), 1.94–2.10 (m, 1 H), 2.38–2.51 (m, 3 H), 2.96 (dd,
J = 17.6, 6.9 Hz, 1 H), 3.59 (d, J = 6.9 Hz, 1 H), 4.06–4.22 (m, 1 H),
7.10–7.38 (m, 5 H).
(23), 77 (18).
Anal. Calcd for C16H17NO3 (271.31): C, 70.83; H, 6.32; N, 5.16.
Found: C, 70.49; H, 6.14; N, 5.27.
13C NMR (CDCl3): d = 20.6 (q), 24.3 (t), 28.6 (t), 35.9 (t), 49.7 (d),
73.5 (d), 126.8 (d, 2 C), 127.5 (d), 128.6 (d, 2 C), 139.5 (s), 144.4
(s), 150.5 (s), 199.8 (s).
5-Phenyl-1-tosyl-1,2,3,4,5,6-hexahydro-7H-[1]pyrindin-7-one
(7d)
Rf = 0.41 (CH2Cl2–MeOH, 50:1).
1H NMR (400 MHz, CDCl3): d = 1.90–1.96 (m, 2 H), 2.08–2.16 (m,
2 H), 2.43 (s, 3 H), 2.44 (dd, J = 18.3, 2.2 Hz, 2 H), 2.98 (dd,
J = 18.3, 7.0 Hz, 1 H), 3.40–3.49 (m, 2 H), 3.87 (d, J = 7.0 Hz, 1 H),
7.11 (d, J = 8.4 Hz, 2 H), 7.25–7.35 (m, 5 H), 8.04 (d, J = 8.4 Hz, 2
H).
MS (EI, 70 eV): m/z (%) = 228 (M+, 100), 199 (16), 129 (35), 91
(22).
5-Propyl-3,4,5,6-tetrahydrocyclopenta[b]thiopyran-7(2H)-one
(9a)
13C NMR (CDCl3): d = 21.6 (q), 21.7 (t), 24.1 (t), 43.7 (t), 45.4 (d),
46.2 (t), 127.2 (d, 2 C), 127.3 (d), 127.8 (d, 2 C), 129.1 (d, 2 C),
129.4 (d, 2 C), 138.0 (s), 141.3 (s), 143.6 (s), 161.2 (s), 199.8 (s).
MS (EI, 70 eV): m/z (%) = 367 (M+, 100), 261 (37), 212 (76), 184
(94), 91 (100).
To a soln of 6a (52 mg, 0.27 mmol) in anhyd DCE (2 mL) was add-
ed Sc(OTf)3 (3.9 mg, 0.008 mmol) and the resulting soln was stirred
at r.t. for 1 h under an N2 atmosphere. Usual workup and chroma-
tography (silica gel 60, EtOAc–petroleum ether, 1:8, 0.5% Et3N,)
provided 9a (35 mg, 67%) as a colorless oil.
Rf = 0.28 (EtOAc–petroleum ether, 1:8, 0.5% Et3N).
Anal. Calcd for C21H21NO3S (367.46): C, 68.64; H, 5.76; N, 3.81.
Found: C, 68.33; H, 5.49; N, 3.44.
1H NMR (400 MHz, CDCl3): d = 0.94 (t, J = 7.0 Hz, 3 H), 1.18–
1.40 (m, 3 H), 1.64–1.68 (m, 1 H), 1.96–2.07 (m, 1 H), 2.10 (dd,
J = 18.7, 2.9 Hz, 1 H), 2.11–2.20 (m, 1 H), 2.33 (dt, J = 18.7, 5.6
Hz, 1 H), 2.48–2.56 (m, 1 H), 2.57 (dd, J = 18.7, 6.4 Hz, 1 H), 2.73–
2.80 (m, 1 H), 2.82–2.92 (m, 2 H).
13C NMR (CDCl3): d = 14.3 (q), 20.3 (t), 22.4 (t), 25.6 (t), 26.0 (t),
35.1 (d), 40.2 (t), 43.5 (t), 132.8 (s), 166.8 (s), 200.8 (s).
2-Methyl-5-propyl-3,4,5,6-tetrahydrocyclopenta[b]pyran-
7(2H)-one (8a)
To a soln of 5a (78 mg, 0.4 mmol) in anhyd DCE (3 mL) was added
Sc(OTf)3 (6 mg, 0.012 mmol) and the resulting soln was stirred at
0 °C for 4 h under an N2 atmosphere. Usual workup and chromatog-
raphy (silica gel 60, EtOAc–hexane, 1:4, 1% Et3N) provided 5b (51
mg, 65%) as a pale yellow oil (9:1 diastereomeric mixture).
MS (EI, 70 eV): m/z (%) = 196 (M+, 47), 153 (100), 125 (7).
Anal. Calcd for C11H16OS (196.31): C, 67.30; H, 8.22. Found: C,
67.46; H, 8.04.
Major diastereomer
Rf = 0.24 (EtOAc–hexane, 1:4, 1% Et3N).
1H NMR (400 MHz, CDCl3): d = 0.93 (t, J = 7.3 Hz, 3 H), 1.08–
1.26 (m, 1 H), 1.09–1.40 (m, 2 H), 1.37 (d, J = 6.4 Hz, 3 H), 1.56–
1.73 (m, 2 H), 1.91–1.97 (m, 1 H), 2.02 (d, J = 18.4 Hz, 1 H), 2.16–
2.22 (m, 1 H), 2.39–2.48 (m, 1 H), 2.51 (dd, J = 18.4, 6.1 Hz, 1 H),
2.57–2.65 (m, 1 H), 3.95–4.03 (m, 1 H).
13C NMR (CDCl3): d = 14.1 (q), 20.2 (t), 20.6 (q), 22.8 (t), 28.4 (t),
35.7 (t), 37.4 (d), 39.4 (t), 73.2 (d), 148.3 (s), 150.9 (s), 200.2 (s).
5-Phenyl-3,4,5,6-tetrahydrocyclopenta[b]thiopyran-7(2H)-one
(9b)
Mp 108–135 °C (dec); Rf = 0.24 (EtOAc–petroleum ether, 1:6,
0.5% Et3N).
1H NMR (400 MHz, CDCl3): d = 1.96–2.07 (m, 2 H), 2.07–2.27 (m,
2 H), 2.42 (dd, J = 18.7, 2.0 Hz, 1 H), 2.86–2.91 (m, 2 H), 2.97 (dd,
J = 18.7, 6.7 Hz, 1 H), 3.91 (d, J = 6.7 Hz, 1 H), 7.10–7.13 (m, 2 H),
7.23–7.35 (m, 3 H).
13C NMR (CDCl3): d = 22.3 (t), 25.6 (t), 26.0 (t), 44.0 (t), 49.6 (d),
127.0 (d, 3 C), 128.9 (d, 2 C), 134.0 (s), 141.3 (s), 165.3 (s), 204.0
(s).
MS (EI, 70eV): m/z (%) = 194 (33), 151 (100), 123 (28), 55 (30).
Anal. Calcd for C12H18O2 (194.27): C, 74.19; H, 9.34. Found: C,
70.33; H, 9.01.
Synthesis 2007, No. 11, 1733–1737 © Thieme Stuttgart · New York

PRACTICAL SYNTHETIC PROCEDURES
Preparation of Cyclopenta-Fused N-, O-, and S-Heterocycles
1737
MS (EI, 70 eV): m/z (%) = 230 (M+, 100), 202 (47), 141 (46), 115
Prod. 2001, 64, 42. (f) Trost, B. M.; Chung, C. K.;
(55), 77 (55).
Pinkerton, A. B. Angew. Chem. Int. Ed. 2004, 43, 4327.
(g) Omura, S.; Tanaka, H.; Awaya, J.; Narimatsu, Y.;
Konda, Y.; Hata, T. Agric. Biol. Chem. 1974, 38, 899.
(h) Gurevich, A. I.; Kolosov, M. N.; Korobko, V. G.;
Onoprienko, V. V. Tetrahedron Lett. 1968, 9, 2209.
(i) Chen, C.-W.; Lu, G.; Ho, C.-T. J. Agric. Food Chem.
1997, 45, 2996.
Anal. Calcd for C14H14OS (230.33): C, 73.01; H, 6.13. Found: C,
72.88; H, 6.23.
Acknowledgment
We thank MIUR and University of Florence for financial support,
and Cassa di Risparmio di Firenze for granting a 400 MHz NMR
spectrometer.
(4) For a review on lactam-derived enol triflates, see: Occhiato,
E. G. Mini-Rev. Org. Chem. 2004, 1, 149.
(5) Larini, P.; Guarna, A.; Occhiato, E. G. Org. Lett. 2006, 8,
781.
(6) (a) Milne, J. E.; Jarowicki, K.; Kocienski, P. J. Synlett 2002,
607. (b) Milne, J. E.; Kocienski, P. J. Synthesis 2003, 584.
(7) Enol triflates from thiolactones can be prepared as reported
in ref. 5 for N-methoxycarbonyl-protected d-valerolactam
1a.
(8) Kobayashi, S.; Ogawa, C. Chem. Eur. J. 2006, 12, 5984.
(9) The Lewis acid catalyzed Nazarov reaction of 2-alkoxy-1,4-
dien-3-ones has been reported: (a) Liang, G.; Gradl, S. N.;
Trauner, D. Org. Lett. 2003, 5, 4931. (b) He, W.; Sun, X.;
Frontier, A. J. J. Am. Chem. Soc. 2003, 125, 14278.
(10) (a) Prandi, C.; Ferrali, A.; Guarna, A.; Venturello, P.;
Occhiato, E. G. J. Org. Chem. 2004, 69, 7705.
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Synthesis 2007, No. 11, 1733–1737 © Thieme Stuttgart · New York