References and Notes
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Friedländer reaction. For examples, see:
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<A NAME="RU06707ST-6">6</A>
When this reaction was carried out at reflux in toluene, the major product was the
1,4-dihydropyridines. The investiga-tion regarding the synthesis of 1,4-dihydropyridines
will be reported elsewhere in due course.
<A NAME="RU06707ST-7">7</A> We performed this reaction using TfOH (10 mol%) at reflux in EtOH, and obtained
the quinoline 4aa in 50% yield.
<A NAME="RU06707ST-8">8</A>
General Procedure: To the EtOH (5 mL) solution of 1a (53.9 mg, 0.5 mmol), 2a (55 µL, 0.54 mmol) and Lewis acid (0.05 mmol, 10 mol%) was added 3 (61 µL, 0.6 mmol) using a microsyringe and then the mixture was refluxed for 24 h.
The reaction was quenched with sat. aq NaHCO3 and the mixture was extracted with EtOAc. The combined organic layers were dried
over MgSO4. The organic layer was filtered and concentrated under reduced pressure. The yield
of 4aa was determined by GC using biphenyl as the internal standard. The quinoline 4aa was purified by preparative TLC (SiO2; hexane-EtOAc, 7:1) and/or recycling preparative HPLC (GPC column, CHCl3 as an eluent) and was fully characterized.
Ethyl 6-Methyl-2-phenylquinoline-3-carboxylate (4aa): 1H NMR (300 MHz, CDCl3): δ = 1.06 (t, J = 6.9 Hz, 3 H), 2.55 (s, 3 H), 4.17 (q, J = 6.9 Hz, 2 H), 7.44 (m, 3 H), 7.62 (m, 4 H), 8.07 (d, J = 9.0 Hz, 1 H), 8.55 (s, 1 H). 13C NMR (CDCl3): δ = 13.6, 21.5, 61.4, 125.4, 125.8, 126.8, 128.1, 128.3, 128.4, 129.1, 133.8, 137.2,
138.3, 140.8, 146.9, 157.2, 168.1.
<A NAME="RU06707ST-9">9</A>
The reaction with imine 5, which was prepared from 1a and 2a, using Sc(OTf)3 yielded the quinoline 4aa in 60% yield.
<A NAME="RU06707ST-10">10</A>
CCDC 650434 contains the supplementary crystallographic data for compound 4ha. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif,
or by emailing data_request@ccdc.cam.ac.uk, or by contacting The Cambridge Crystallographic
Data Centre, 12, Union Road, Cambridge CB2 1EZ, UK; fax: +44(1223)336033.
<A NAME="RU06707ST-11">11</A>
Typical Procedure (Scheme 2): To the EtOH (7 mL) solution of 1a (107.2 mg, 1.0 mmol), 2d (110 µL, 1.1 mmol), which was distilled from P2O5 before use, and Sc(OTf)3 (49.2 mg, 0.10 mmol) was added 3 (122 µL, 1.2 mmol) using a microsyringe and then the mixture was refluxed for 24
h. The reaction was quenched with sat. aq NaHCO3 and the mixture was extracted with EtOAc. The combined organic layers were dried
over MgSO4. This organic layer was filtered and concentrated under reduced pressure. The residue
was purified by short column chromatography (hexane-EtOAc, 6:1) and recycling preparative
HPLC (GPC column, CHCl3 as an eluent) to give the pure 4ad in 34% yield.
Diethyl 6-Methylquinoline-2,3-dicarboxylate (4ad): 1H NMR (300 MHz, CDCl3): δ = 1.45 (m, 6 H), 2.57 (s, 3 H), 4.43 (q, J = 7.0 Hz, 2 H), 4.52 (q, J = 7.0 Hz, 2 H), 7.67 (m, 2 H), 8.09 (d, J = 5.8 Hz, 1 H), 8.67 (s, 1 H). 13C NMR (CDCl3): δ = 14.1, 21.6, 61.4, 62.2, 122.5, 127.1, 127.3, 129.4, 134.6, 138.7, 138.9, 146.6,
147.0, 165.3, 166.9.
<A NAME="RU06707ST-12">12</A> The CuOTf-catalyzed 1,4-addition of EtOH to ethyl propiolate has been reported.
See:
Bertz SH.
Dabbagh G.
Cotte P.
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1982,
47:
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<A NAME="RU06707ST-13A">13a</A>
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Shindo T.
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Takaki K.
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Synthesis
1995,
801
<A NAME="RU06707ST-13B">13b</A>
Kobayashi S.
Ishitani H.
Nagayama S.
Synthesis
1995,
1195
<A NAME="RU06707ST-14">14</A>
In this reaction, ethyl 3,3-diethoxypropionate was obtained predominantly.