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Typical Procedure
for the Asymmetric Nazarov Cyclization of 1a
A solution
of Fe(ClO4)˙6H2O (18.1 mg, 0.050
mmol) and (S,S)-tb-pybox (16.5 mg, 0.050 mmol) in anhyd
CH2Cl2 (0.50 mL) was stirred at r.t. for 3
h, then 2-benzyliden-3-oxo-4,5-diphenylpent-4-enoic acid ethyl ester
(1a, 38.2 mg 0.10 mmol) in anhyd CH2Cl2 (0.50
mL) and hexane (0.50 mL) was added slowly, then stirred at 60 ˚C
for 15 h. The reaction mixture was quenched with H2O,
and extracted with EtOAc. The combined organic layers was washed
with brine, dried over MgSO4, and evaporated. The residue
was chromatographed on silica gel (hexane-Et2O = 7:3)
to give 26.4 mg (69%) of 2a.
Analytical
Data
[α]D
²4 +289
(c 1.0, CHCl3). The enantiomeric
purity was determined to be 83% ee by chiral HPLC analysis
with a Daicel CHIRALPAK AD-H [hexane-2-PrOH (95:5),
flow: 0.5 mL/min, 35 ˚C, t
major = 23
min, t
minor = 33
min]. The absolute configuration of the major enantiomer
is (1R,5S)
by the comparison of the reported data. ¹H NMR
(400 MHz, CDCl3): δ = 1.31 (t, J = 6.8 Hz,
3 H), 3.64 (m, 1 H), 4.26 (m, 2 H), 5.02 (d, J = 2.8
Hz, 1 H), 7.10-7.34 (m, 15 H). ¹³C NMR
(125 MHz, CDCl3): δ = 14.2, 50.9, 61.9,
62.5, 127.2, 127.7, 128.1, 128.2, 128.3, 128.9, 128.9, 129.6, 129.7, 131.2,
134.0, 138.7, 140.5, 168.3, 170.0, 199.4.
<A NAME="RU01310ST-8">8</A>
We also examined the reaction using
other chiral pybox-type ligands, such as (S,S)-phe-pybox and (S,S)-inda-pybox,
but enantioselectivities were 2% ee (92% yield)
and 33% ee (22% yield), respectively.
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Surprisingly, the reaction of 1a with Fe(OTf)2 in toluene
did not proceed.
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Cui H.-F.
Dong K.-Y.
Zhang G.-W.
Wang L.
Ma J.-A.
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2007,
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The relative stereochemistry of 3 was estimated by comparison with the
product from the reaction of 1 by Cu(OTf)2,
see ref. 10a.
