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Synlett 2008(17): 2613-2616  
DOI: 10.1055/s-0028-1083515
LETTER
© Georg Thieme Verlag Stuttgart ˙ New York

Ethyl Vinyl Ether as a Synthetic Equivalent of Acetylene in a DABCO-Catalyzed Microwave-Assisted Diels-Alder-Elimination Reaction Sequence Starting from 2H-Pyran-2-ones

Krištof Kranjc*, Marijan Kočevar
Faculty of Chemistry and Chemical Technology, University of Ljubljana, Aškerčeva 5, 1000 Ljubljana, Slovenia
Fax: +386(1)2419220; e-Mail: kristof.kranjc@fkkt.uni-lj.si;
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Publication History

Received 23 June 2008
Publication Date:
01 October 2008 (online)

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Abstract

We present a study of the Diels-Alder reaction between various electron-deficient 2H-pyran-2-ones and ethyl vinyl ether. This microwave-accelerated sequence of a cycloaddition followed by a retro-Diels-Alder reaction (the elimination of CO2) and a second elimination step of EtOH yields substituted aniline derivatives. The reaction sequence is greatly accelerated by the application of DABCO as a suitable base.

Key words

Diels-Alder reactions - arenes - catalysis with bases - anilines - microwave reactions

    References and Notes

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10

General Procedure
A mixture of the starting 2H-pyran-2-one 1 (1 mmol), ethyl vinyl ether (2, 721.1 mg, 10 mmol), and DABCO (11.2 mg, 0.1 mol) in MeCN (2 mL) was irradiated in the focused microwave equipment (CEM Discover) for the time specified (Table  [¹] ). The final temperature was set to 120 ˚C, the power to 120 W, and the ramp time to 5 min. Thereafter, the reaction mixture was cooled, the volatile components were removed in vacuo, the remaining solid was treated with mixture of EtOH and H2O (10:1), and cooled. The precipitated product 5 was filtered off and washed with EtOH-H2O (10:1).
Selected Data of the ProductsMethyl 5-(Benzoylamino)-2-[(methoxycarbonyl)-methyl]benzoate (5a) Mp 169-171 ˚C (MeOH). IR (KBr): 3339, 1737, 1720, 1653, 1590, 1526, 1435, 1419 cm-¹. ¹H NMR (300 MHz, CDCl3): δ = 3.71 (s, 3 H, Me), 3.83 (s, 3 H, Me), 3.98 (s, 2 H, CH2), 7.21 (d, 1 H, J = 8.4 Hz, 3-H), 7.52 (m, 3 H, Ph), 7.86 (m, 3 H, Ph, 4-H), 8.10 (br s, 1 H, NH), 8.18 (d, 1 H, J = 2.1 Hz, 6-H). ¹³C NMR (75.5 MHz, DMSO-d 6): δ = 39.1, 51.4, 51.9, 122.0, 123.8, 127.6, 128.4, 129.4, 130.9, 131.7, 132.8, 134.5, 138.3, 165.6, 166.7, 171.5. MS (EI): m/z (%) = 327 (5) [M+], 105 (100). Anal. Calcd for C18H17NO5: C, 66.05; H, 5.23; N, 4.28. Found: C, 66.32; H, 5.17; N, 4.26.
Ethyl 5-(Benzoylamino)-2-[(ethoxycarbonyl)-methyl]benzoate (5b)
Mp 170.5-171.5 ˚C (MeOH). IR (KBr): 3298, 1736, 1714, 1651, 1588, 1523, 1420 cm-¹. ¹H NMR (300 MHz, CDCl3): δ = 1.25 (t, 3 H, J = 7.1 Hz, CH2CH 3), 1.37 (t, 3 H, J = 7.1 Hz, CH2CH 3), 3.99 (s, 2 H, CH2), 4.15 (q, 2 H, J = 7.1 Hz, CH 2CH3), 4.32 (q, 2 H, J = 7.1 Hz, CH 2 CH3), 7.24 (d, 1 H, J = 8.9 Hz, 3-H), 7.53 (m, 3 H, Ph), 7.88 (m, 2 H, Ph), 7.95 (dd, 1 H, J 1 = 8.9 Hz, J 2 = 2.4 Hz, 4-H), 7.98 (br s, 1 H, NH), 8.10 (d, 1 H, J = 2.4 Hz, 6-H).
Methyl 5-(Benzoylamino)-2-methylbenzoate (5c)
Mp 126-127 ˚C (EtOH-H2O). IR (KBr): 3270, 1732, 1720, 1649, 1582, 1528, 1498, 1428 cm-¹. ¹H NMR (300 MHz, CDCl3): δ = 2.57 (s, 3 H, Me), 3.86 (s, 3 H, Me), 7.22 (d,
1 H, J = 8.3 Hz, 3-H), 7.50 (m, 3 H, Ph), 7.81 (dd, 1 H, J 1 = 2.2 Hz, J 2 = 8.3 Hz, 4-H), 7.86 (m, 2 H, Ph), 8.02 (br s, 1 H, NH), 8.08 (d, 1 H, J = 2.2 Hz, 6-H). ¹³C NMR (75.5 MHz, CDCl3): δ = 21.1, 51.9, 122.3, 124.0, 127.0, 128.7, 129.8, 131.9, 132.3, 134.6, 135.7, 136.3, 165.8, 167.5. MS (EI): m/z (%) = 269 (30) [M+], 105 (100). Anal. Calcd for C16H15NO3: C, 71.36; H, 5.61; N, 5.20. Found: C, 71.30; H, 5.68; N, 5.22.
Ethyl 5-(Benzoylamino)-2-methylbenzoate (5d) Mp 119-120.5 ˚C (EtOH-H2O). IR (KBr): 3296, 1726, 1648, 1583, 1530, 1448, 1404 cm-¹. ¹H NMR (300 MHz, CDCl3): δ = 1.38 (t, 3 H, J = 7.1 Hz, CH2CH 3), 2.57 (s, 3 H, Me), 4.35 (q, 2 H, J = 7.1 Hz, CH 2CH3), 7.23 (d, 1 H, J = 
8.1 Hz, 3-H), 7.50 (m, 3 H, Ph), 7.87 (m, 3 H, Ph, 4-H), 7.96 (br s, 1 H, NH), 8.02 (d, 1 H, J = 2.4 Hz, 6-H).
N -(3-Acetyl-4-methylphenyl)benzamide (5e) ¹¹
Mp 131-132 ˚C (EtOH-H2O); lit.¹¹ mp 137-138 ˚C (EtOH). IR (KBr): 3410, 1861, 1788, 1699, 1653, 1533, 1492 cm-¹. ¹H NMR (300 MHz, CDCl3): δ = 2.49 (s, 3 H, Me), 2.57 (s, 3 H, Me), 7.21 (d, 1 H, J = 8.4 Hz, 5-H), 7.53 (m, 4 H, Ph,
6-H), 7.87 (m, 2 H, Ph), 8.05 (br s, 1 H, NH), 8.13 (d, 1 H, J = 2.4 Hz, 2-H).
N -(3-Benzoyl-4-methylphenyl)benzamide (5f)
Mp <35 ˚C (EtOH-H2O). IR (neat): 1671, 1597, 1522, 1501 cm-¹. ¹H NMR (300 MHz, CDCl3): δ = 2.28 (s, 3 H, Me), 7.28 (d, 1 H, J = 8.4 Hz, 5-H), 7.52 (m, 7 H, 2 × Ph, 2-H), 7.77 (dd, 1 H, J 1 = 2.4 Hz, J 2 = 8.4 Hz, 6-H), 7.83 (m, 4 H, Ph), 7.94 (br s, 1 H, NH).
N -(3-Acetyl-4-methylphenyl)acetamide (5g) ¹¹,¹² Mp 86-88 ˚C (EtOH-H2O); lit.¹¹ mp 94-95 ˚C (EtOH); lit.¹² mp 124 ˚C (EtOH). IR (KBr): 3331, 1686, 1668, 1585, 1536, 1497, 1453 cm-¹. ¹H NMR (300 MHz, CDCl3): δ = 2.19 (s, 3 H, Me), 2.47 (s, 3 H, Me), 2.57 (s, 3 H, Me), 7.16 (br s,
1 H, NH), 7.18 (d, 1 H, J = 8.2 Hz, 5-H), 7.40 (dd, 1 H, J 1 = 2.0 Hz, J 2 = 8.2 Hz, 6-H), 7.97 (d, 1 H, J = 2.0 Hz, 2-H).