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Synthesis 2005(19): 3293-3296  
DOI: 10.1055/s-2005-918459
PAPER
© Georg Thieme Verlag Stuttgart · New York

Total Synthesis of Japanese Hop Ether Using an Efficient Intramolecular Pauson-Khand Reaction

John J. Caldwell, Iain D. Cameron, Steven D. R. Christie, Alastair M. Hay, Craig Johnstone, William J. Kerr*, Anthony Murray
WestCHEM, Department of Pure and Applied Chemistry, University of Strathclyde, 295 Cathedral Street, Glasgow, Scotland G1 1XL, UK
Fax: +44(141)5484246; e-Mail: w.kerr@strath.ac.uk;
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Publication History

Received 11 September 2005
Publication Date:
14 November 2005 (online)

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Abstract

The naturally occurring monoterpene Japanese Hop Ether has been synthesised in 14 steps in an overall yield of 29%. The key step of the synthesis, an intramolecular Pauson-Khand reaction, has been shown to proceed in good to excellent yield under mild N-oxide promotion conditions and with complete retention of alkene stereochemistry (for both cis- and trans-alkenes) in the product cyclopentenone.

Key words

amine N-oxides - cyclopentenones - natural products - Pauson-Khand cyclizations - stereoselectivity

13

The identity of each cyclopentenone product was initially elucidated by 1H NMR. In particular, the non-olefinic proton α to the carbonyl in product 19 (from E-enyne 18), would be in an endo-position and, hence, would be more shielded; this proton appears as a multiplet at δ = 2.42-2.47 ppm. The equivalent non-olefinic proton α to the carbonyl in product 6 (from Z-enyne 7), would be in an exo-position and, hence, would be more deshielded; this proton appears as a multiplet at δ = 2.90-2.97 ppm.