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Synlett 2004(12): 2209-2211  
DOI: 10.1055/s-2004-831307
LETTER
© Georg Thieme Verlag Stuttgart · New York

Protecting Group Effects on the Efficiency of the Ruthenium-Catalyzed Alder-Ene Reaction

Joseph P. Hartley*, Stephen G. Pyne*
Department of Chemistry, University of Wollongong, Wollongong, New South Wales, 2522, Australia
Fax: +61(2)42214287; e-Mail: jhartley@uow.edu.au; e-Mail: spyne@uow.edu.au;
Further Information

Publication History

Received 14 July 2004
Publication Date:
26 August 2004 (online)

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Abstract

The efficiency of the ruthenium-catalyzed Alder-ene reaction of hydroxy alkenes depends heavily on the nature of the O-protecting groups employed, as well as the length of the carbon spacer between the hydroxy and alkene group.

Key words

Alder-ene reaction - ruthenium catalysis - O-protecting groups

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2

(1 Z ,4 E )-( R )-1-Trimethylsilyl-2-(2′-methyl-3′- tert -butyldiphenylsilyloxy)-6-acetoxy-1,4-hexadiene. CpRu(MeCN)3PF6 (39 mg, 0.090 mmol) under Ar was treated with an acetone solution (2 mL) of 1-acetoxy-3-butene (170 mg, 1.5 mmol) and (R)-1-(trimethylsilyl)-4-methyl-5-(tert-butyldiphenylsilyloxy)-1-pentyne (123 mg, 0.300 mmol). The yellow solution was stirred at r.t. for 20 h. The reaction mixture was passed through a short plug of silica and concentrated in vaccuo. The residue was purified by column chromatography (increasing polarity from 1% to 10% EtOAc in petroleum spirit), which gave the title compound (134 mg, 0.256 mmol, 85% yield) as a colorless oil. Rf (11% EtOAc in petroleum spirit): 0.67. [α]D 25 +1.1 (c 6.7, CH2Cl2). MS (ES+): m/z (%) = 463.28 (100) [M - OAc], 540.32 (23) [M + NH4 +], 545.29 (33) [M + Na+], 561.26 (12) [M + K+], 655.20 (33) [M + Cs+]. HRMS (ES+): m/z [M + Na+] calcd for C31H46O3NaSi2: 545.2883; found: 545.2904. 1H NMR (300 MHz, CDCl3): δ = 0.06 [9 H, s, (CH3)3Si], 0.86 (3 H, d, J = 6.0 Hz, CH3CH), 1.06 [9 H, s, (CH3)3CSi], 1.82-2.02 (2 H, m, CH3CH and CHCH2C=), 2.05 [3 H, s, CH3C(O)O], 2.27 (1 H, dd, J = 12.9, 4.8 Hz, CH3CH), 2.74 (2 H, d, J = 6.6 Hz, =CHCH2CH=), 3.47 (2 H, d, J = 6.0 Hz, CH2OSi), 4.52 (2 H, d, J = 6.3 Hz, CH2OAc), 5.23 (1 H, s, TMSCH), 5.54 (1 H, dt, J = 15.3, 6.3 Hz, AcOCH2CH=CH), 5.73 (1 H, dt, J = 15.6, 6.9 Hz, AcOCH2CH=CH), 7.36-7.44 (6 H, m, SiPh), 7.62-7.68 (4 H, m, SiPh). 13C NMR (75 MHz, CDCl3): δ = 0.8 (CH3), 16.4 (CH3), 19.5 (C), 21.2 (CH), 27.2 (CH3), 34.2 (CH3), 39.6 (CH2), 41.8 (CH2), 65.3 (CH2), 69.5 (CH2), 125.8 (CH), 126.8 (CH), 127.8 (CH), 129.8 (CH), 134.1 (C), 134.3 (CH), 135.9 (CH), 155.3 (C), 171.0 (C=O).

4

Whilst there are examples of other processes occurring when β-hydrogen elimination is inhibited,5 under the mild conditions employed only starting materials are recovered.