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Synlett 2004(2): 0239-0242  
DOI: 10.1055/s-2003-44974
LETTER
© Georg Thieme Verlag Stuttgart · New York

Zn(ClO4)2·6H2O as a Powerful Catalyst for the Conversion of β-Ketoesters into β-Enamino Esters

Giuseppe Bartoli*a, Marcella Boscoa, Manuela Locatellia, Enrico Marcantonib, Paolo Melchiorrea, Letizia Sambri*a
a Dipartimento di Chimica Organica ‘A. Mangini’, v.le Risorgimento 4, 40136 Bologna, Italy
Fax: +39(051)2093654; e-Mail: giuseppe.bartoli@unibo.it;
b Dipartimento Scienze Chimiche, Università di Camerino, via S.Agostino 1, 62032 Camerino, (Macerata), Italy
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Publication History

Received 8 October 2003
Publication Date:
04 December 2003 (online)

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Abstract

Zn(ClO4)2·6H2O proved to be a very powerful catalyst for the condensation of primary and secondary amines with β-ketoesters to give N-substituted β-enaminoesters.

Key words

amino acid derivatives - zinc perchlorate - condensation - amines - keto esters

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These conditions are sufficient to reactivate the catalyst, without any decomposition process. Zn(ClO4)2·6H2O in fact decomposes under vacuum at temperatures higher than 140 °C.

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Representative Experimental Procedure: Synthesis of tert-butyl-3-anilino-2-butenoate (1a). To a round-bottom flask Zn(ClO4)2·6H2O (24 mg, 0.063 mmol), MgSO4 (46 mg, 0.38 mmol), t-butyl acetoacetate (0.21 mL, 1.26 mmol), CH2Cl2 (0.5 mL) and aniline (0.17 mL, 1.90 mmol) were added. The reaction mixture was stirred at r.t. for 21 h. After addition of 5 mL of CH2Cl2, the catalyst was filtered off and the solution was concentrated at reduced pressure. The crude product was purified by filtration on a short silica gel column pre-treated with Et3N. The filtered catalyst was reactivated by heating in a oven at 60 °C overnight and reused. Compounds 5a, 5b and 5e are commercial products. 1f, [21] 1h, [21] 1o, [22] 5c [23] and 5d [19] are known compounds. Spectroscopic data for selected examples follow.
t -Butyl ( Z )-3-anilino-2-butenoate ( 1a): 1H NMR (300 MHz, CDCl3): δ = 1.50 (s, 9 H, 3 × CH3), 1.97 (s, 3 H, CH3), 4.60 (bs, 1 H, CH), 7.05-7.20 (m, 3 H, Ph), 7.25-7.35 (m, 2 H, Ph), 10.40 (bs, 1 H, NH). 13C NMR (75 MHz, CDCl3):
δ = 20.2 (CH3), 28.6 (CH3), 78.5 (C), 87.8 (CH), 124.2 (CH), 124.6 (CH), 128.9 (CH), 139.5 (C), 158.0 (C), 170.3 (C). IR (nujol): νNH = 3272 cm-1. MS (EI): m/z (%) = 233(6) [M+], 17 7(16), 118 (41), 77 (30), 59 (100).
Ethyl ( Z )-2-(butylamino)-1-cyclopentene-1-carboxylate ( 1g): 1H NMR (300 MHz, CDCl3): δ = 0.93 (t, 3 H, CH3, J HH = 7.2 Hz), 1.27 (t, 3 H, CH3, J HH = 7.2 Hz), 1.35-1.40 (m, 2 H, CH2), 1.45-1.60 (m, 2 H, CH2), 1.80-1.90 (m, 2 H, CH2), 2.45-2.60 (m, 4 H, 2 × CH2), 3.15-3.25 (m, 2 H, CH2), 4.13 (q, 2 H, CH2, J HH = 7.2 Hz), 7.40 (bs, 1 H, NH). 13C NMR (75 MHz, CDCl3) δ = 13.6 (CH3), 14.7 (CH3), 19.8 (CH2), 20.8 (CH2), 28.9 (CH2), 31.9 (CH2), 32.9 (CH2), 44.2 (CH2), 58.2 (CH2), 91.9 (C), 164.8 (C), 168.4 (C). IR (neat): νNH = 3320 cm-1. MS (EI): m/z (%) = 211 (43) [M+], 182 (9), 166 (28), 122 (100).