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Email Discussion: Paper 31 (forwarded for Dr. Yannick LANDAIS) Dr J M Goodman
Email Discussion: Re: Paper 31 (forwarded for Dr. Yannick LANDAIS) Pascal GOSSELIN

Diastereoselectivesynthesis Of (-)-Karahana Ether By Diels-Alder Reaction Of Enantiopure(+)-(R)-4-Methyl-2-P-Tolylsulfinyl-1,3-Pentadiene.

C. Maignan, E. Bonfand and P. Gosselin*

Laboratoire de Synthèse Organique, URA CNRS 482,Faculté des Sciences, Université
du Maine, 72017 Le Mans CEDEX,France.

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Diene 1 was prepared bydirect condensation of lithium (+)-(R)-p-tolylvinylsulfoxide 2 on α-phenylselanylisobutyraldehyde3 and subsequent Krief-Reich anti-elimination on the resultingsulfinyl-β-hydroxyselenide 4.

Diene 1 is apotentially useful precursor of natural terpenoids. As anillustration, the total stereoselective synthesis of (-)-(1S,5R)-Karahanaether 10 was realized by mean of an asymmetric Diels-Alder reaction as thekey-step:

The main hydrolyzed cycloadduct 5 resulted probably from an endo approach of maleic anhydride bythe less sterically hindered side of diene 1 (opposite to the p-tolyl group) which is also the most nucleophilic one.This indicates a s-trans conformation of the S=O and C=Cbonds in compound 1, at least in the transition state (as previously reported in the case ofanother 2-sulfinyldiene :Tetrahedron : Asymmetry 5 (5) 781-4 1994 ).

Enantiopure diol 7 wasobtained via a two steps reduction. This process allowedthe complete separation of intermediate lactones 6a +6b from their exo-diastereomers.

Heterocyclization to ether 8 was realized by an intramolecular conjugate addition of the solepseudo-axial heteroanion. Further hydrogenolysis and deshydratation of8 gave natural (-)-(1S,5R)-Karahana ether 10 (33% yieldfrom 1).

This synthesis utilizes both main properties of the sulfinylgroup :

* good chiral induction agent in cycloaddition

* efficient activating group in Michaël addition.

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