ran derivatives under mild conditions. Therefore, we attempted
to examine the reaction of MCPs 1 with o-quinonemethide
analogues, generated from salicylaldehydes and CH(OEt)3 in
the presence of a Lewis acid. In this Letter, we wish to present
a novel Sc(OTf)3- or BF3·Et2O-catalyzed cycloaddition of MCPs
with o-quinonemethide analogues to produce the corresponding
chromene derivatives in moderate to good yields under mild
conditions along with the further transformation to indene
derivatives at high temperature.
Initial examinations using diphenylmethylenecyclopropane
2a (1.0 equiv) as the substrate to react with salicylaldehyde 1a
(2.5 equiv) and triethoxymethane (CH(OEt)3, 3.0 equiv) in the
presence of a Lewis acid (0.3 equiv) in various solvents were
aimed at determining the optimal conditions, and the results of
these experiments are summarized in Table 1. The reaction
Bi(OTf)2Cl, Fe(OTf)2·2CH3CN, and BF3·Et2O were not as
effective as Sc(OTf)3, affording 3a in 41-75% yields (Table
1, entries 3-7). The examination of solvent effects revealed
that in tetrahydrofuran (THF), Et2O, and toluene, no reaction
occurred, and in dichloromethane or acetonitrile, 3a was
produced in 66% or 28% yield, respectively, under the standard
conditions (Table 1, entries 9-11 and 8, 12). Therefore, the
best conditions are to carry out the reaction in DCE at room
temperature using 1a (1.0 equiv), 2a (2.5 equiv), and CH(OEt)3
(3.0 equiv) in the presence of Sc(OTf)3 (0.3 equiv).
The structure of 3a was determined by NMR spectroscopic
data and mass and HRMS analyses (see Supporting Informa-
tion). Furthermore, the X-ray crystal structure of 3a was
determined and is presented with its CIF data in Supporting
Information.6
With these optimal conditions in hand, we next carried
out this reaction using a variety of starting materials 1 and
methylenecyclopropanes 2 as shown in Table 2 to examine
Table 1. Optimization of the Reaction Conditions
Table 2. Scope and Limitations of This Cycloaddition Reaction
entrya
Lewis acid
Yb(OTf)3
Sc(OTf)3
Yb(NTf2)3
Zr(OTf)4
Bi(OTf)2Cl
Fe(OTf)2·2CH3CN
BF3·Et2O
Yb(OTf)3
Sc(OTf)3
Sc(OTf)3
Sc(OTf)3
Sc(OTf)3
solvent
3, yield (%)b
1
2
3
4
5
6
7
8
DCE
DCE
DCE
DCE
DCE
DCE
DCE
CH2Cl2
THF
Et2O
71
78
41
47
38
53
75
66
NR
NR
NR
28
9
10
11
12
toluene
CH3CN
a Reaction conditions: 1a (0.25 mmol), CH(OEt)3 (0.3 mmol), Sc(OTf)3
(0.03 mmol), and DCE (2.0 mL) were used, the reactions were carried out
at rt for 20 min, and then MCP 2a (0.1 mmol) was added. The reaction
mixtures were stirred at rt for 24 h. b Isolated yields.
a Reaction conditions: 1 (0.25 mmol), CH(OEt)3 (0.3 mmol), Sc(OTf)3
(0.03 mmol), and DCE (2.0 mL) were used, the reactions were carried out
at rt for 20 min, then MCPs 2 (0.1 mmol) were added, and the reaction
mixtures were stirred for 24 h. b Isolated yields. c The ratio of isomeric
procedure is that after the reaction mixtures of salicylaldehyde
1a, CH(OEt)3, and Lewis acid were stirred in 1,2-dichloroethane
(DCE) at room temperature (20 °C) for 20 min, then MCP 2a
was added, and the resulting mixtures were further stirred for
24 h. After the usual workup, the residue was subjected to the
silica gel column chromatography to give the product. It was
found that using Yb(OTf)3 as the catalyst afforded the cycload-
dition compound 3a in 71% yield (Table 1, entry 1). Using
Sc(OTf)3 instead of Yb(OTf)3 produced 3a in 78% yield (Table
1, entry 2). Other Lewis acids such as Yb(NTf2)3, Zr(OTf)4,
1
mixtures was determined by H NMR spectroscopic data.
the scope and limitations of this cycloaddition. As for
salicylaldehydes 1b-1f having a variety of substituents on
the benzene rings, the reactions with MCP 2a proceeded
smoothly to afford the corresponding cycloadducts 3b-3f
in 61-80% yields, indicating that the substituents on the
aromatic ring of 1 did not have significant influence on the
(5) Selected recent contributions from our group: (a) Shi, M.; Liu, L. P.;
Tang, J. J. Am. Chem. Soc. 2006, 128, 7430–7431. (b) Shao, L.-X.; Xu,
B.; Huang, J.-W.; Shi, M. Chem.sEur. J. 2006, 12, 510–517. (c) Shi, M.;
Xu, B.; Huang, J.-W. Org. Lett. 2004, 6, 1175–1178. (d) Tian, G.-Q.; Shi,
M. Org. Lett. 2007, 9, 4917–4920. (e) Shao, L.-X.; Li, Y.-X.; Shi, M.
Chem.sEur. J. 2007, 13, 862–869. (f) Yao, L. F.; Shi, M. Org. Lett. 2007,
9, 5187–5190. (g) Jiang, M.; Shi, M. Organometallics 2009, 28, 5600–
5602. (h) Jiang, M.; Shi, M. Tetrahedron 2009, 65, 5222–5227. (i) Jiang,
M.; Shi, M. J. Org. Chem. 2009, 74, 2516–2520.
(6) The crystal data of 3a have been deposited in CCDC with number
760083. Empirical formula: C25H24O2. Formula weight: 356.44. Crystal
color, habit: colorless, prismatic. Crystal dimensions: 0.402 × 0.307 × 0.231
mm3. Crystal system: monoclinic. Lattice type: primitive. Lattice parameters:
a ) 9.4459(9) Å, b ) 11.0752(10) Å, c ) 18.8988(18) Å, R ) 90°, ꢀ )
100.572(2)°, γ ) 90°, V ) 19435(3)Å3. Space group: P2(1)/n; Z ) 4; Dcalc
) 1.218 g/cm3; F000 ) 760. Diffractometer: Rigaku AFC7R. Residuals: R,
Rw 0.0479, 0.1174.
Org. Lett., Vol. 12, No. 11, 2010
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