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Synlett 2002(3): 0477-0479
DOI: 10.1055/s-2002-20470
LETTER
© Georg Thieme Verlag Stuttgart · New York

Self-Activation and 1,8-Stereoinduction in a Boronate-substituted Dienophile

Jason W. J. Kennedy, Dennis G. Hall*
Department of Chemistry, University of Alberta, Edmonton, Alberta, Canada T6G 2G2
Fax: +1(780)4928231; e-Mail: dennis.hall@ualberta.ca;
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Publication History

Received 23 October 2001
Publication Date:
05 February 2007 (online)

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Abstract

The [4+2] cycloaddition of ortho-boronoanilide dienophile 4 with cyclopentadiene was found to proceed faster than both its para isomer 8 and the unsubstituted derivative 6, thereby confirming that self-activation by internal coordination is operative in the case of 4. Chiral boronic esters derivatives 9 and 10 provided a small level of remote 1,8-stereoinduction. These results show that dialkoxyboronic esters can operate as weak, internal Lewis acids and activate carbonyl-containing functionalities in cycloaddition reactions.

Key words

Lewis acids - Diels-Alder reactions - substituent effects - chiral auxiliaries - stereoselective synthesis

    References

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    All compounds reported herein were obtained in a high state of purity and exhibited spectral data (NMR, MS) in accord with proposed structures. Characterization data for dienophile substrates 4, 6, 8: Pinacol ortho-boronoanilide 4: white solid, mp: 168-170 °C; 1H NMR (Acetone-d 6 , 400 MHz): δ = 10.34 (br s, 1 H), 8.06 (m, 1 H), 7.67 (d, J = 8 Hz, 1 H), 7.37 (m, 1 H), 7.10 (apparent t, J = 8 Hz, 1 H), 6.38 (m, 2 H), 5.82 (dd, J = 7 Hz, 5 Hz, 1 H), 1.38 (s, 12 H); 13C NMR (Acetone-d 6 , 100 MHz): δ = 164.1, 143.6, 136.0, 135.9, 132.2, 131.8, 131.7, 127.9, 127.8, 124.6, 124.5, 119.0, 84.1, 84.0, 25.6, 25.5; 13C NMR (Benzene-d 6 , 100 MHz): δ = 163.2, 145.3, 136.4, 133.1, 132.8, 125.8, 123.4, 119.6, 83.9, 24.8. A very slight peak duplication effect, ca. 0.05 ppm, was observed for arene carbons in acetone. This may be due to a slow competing effect from this coordinating solvent. 11B NMR (Benzene-d 6 , 64 MHz): δ = 30.1; IR (CH2Cl2 cast, cm-1): 3316 (m, N-H), 3058 (w, Csp 2-H), 2970 (s, Csp 3-H), 1646 (s, C=O), 731 (s, o-sub Ar); HRMS (EI): m/z calcd for C15H20NO3 11B: 273.15363; found: 273.15376. Acrylanilide 6: [5b] 1H NMR (CDCl3, 300 MHz): δ = 7.58 (m, 2 H), 7.31 (m, 2 H), 7.18 (br s, 1 H), 7.10 (m, 1 H), 6.43 (d, J = 17 Hz, 1 H), 6.25 (dd, J = 17 Hz, 10.2 Hz, 1 H), 5.78 (d, J = 10 Hz, 1 H). 13C (CDCl3, 75 MHz): δ = 164.0, 140.2, 132.8, 129.5, 126.9, 124.4, 120.3. Pinacol para-boronoanilide 8: 1H NMR (Acetone-d 6 , 300 MHz): δ = 7.75 (AB, J = 9 Hz, 2 H), 7.68 (AB, J = 9 Hz, 2 H), 6.46 (dd, J = 17 Hz, 10 Hz, 1 H), 6.34 (dd, J = 16 Hz, 3 Hz, 1 H), 5.70 (dd, J = 10 Hz, 3 Hz, 1 H), 1.31 (s, 12 H); 13C NMR (Acetone-d 6 , 100 MHz): δ = 164.1, 142.8, 136.3, 136.2 (broad weak signal, C-B), 132.7, 127.4, 119.2, 84.3, 25.2; 11B NMR (Benzene-d 6 , 64 MHz): δ = 30.8; IR (CH2Cl2 cast, cm-1): 3303 (m, N-H), 3102 (w, Csp 2-H), 2978 (m, Csp 3-H), 1667 (s, C=O), 1635 (s, C=C alkyl), 1593 (s, C=C aromatic), 1361 (vs, B-O), 860 (m, p-sub Ar); HRMS (EI): m/z calcd for C15H20NO3 11B: 273.15363, found: 273.15381.
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7

X-ray crystallographic data have been deposited to the Cambridge Crystallographic Data Centre (file number: CCDC 166880).