The first enantioselective synthesis of the natural pesticide, rotenone

Georgiou, Kathy Hadje
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The 2-isopropenyl-2,3-dihydrobenzofuran moiety is found in many naturally occurring compounds including rotenone, a complex pentacyclic molecule isolated from several leguminous plants of the Derris and Lonchocarpus species. Interest in rotenone stems from the fact that it possesses significant pesticidal and piscicidal properties which have been employed for centuries. Furthermore, as it has three stereogenic centres, rotenone poses an interesting and challenging synthetic target for organic chemists. Although various syntheses of this natural compound have been reported, none of these were stereoselective. The first stereoselective total synthesis of rotenone is described in this dissertation. Initially, a model study was conducted in which the simplest of the natural rotenoids, munduserone, was synthesised. The key step in this transformation involves the use of a platinum catalysed 6-endo-hydroarylation reaction of an alkynone intermediate, thus affording munduserone in 6 steps and an overall yield of 23%. We then attended to the synthesis of the more complex rotenoid, rotenone. Rotenone was synthesised by the initial assembly of a chiral (-)-(R)-2-isopropenyl-2,3-dihydrobenzofuran-4-ol moiety, asymmetrically accessible using a stereoselective Pd π-allyl mediated cyclisation of (E)-4-(2,6-dihydroxyphenyl)-2-methylbut-2-enyl methyl carbonate. Having constructed the dihydrobenzofuran in an enantiomeric excess of 94.8%, the chromene part of rotenone could then be synthesised. To this end, the LDA mediated coupling reaction of the formylated dihydrobenzofuran and 1,2-dimethoxy-4-(prop-2-ynyloxy)benzene, gave a secondary alcohol which was subsequently oxidised to the corresponding alkynone, (-)-(R)-(6,7-dimethoxy-2H-chromen-4-yl)(4-methoxy-2-isopropenyl-2,3-dihydrobenzofuran-5-yl)methanone. A 6-endo-hydroarylation reaction was employed as a mild strategy to construct the chromene moiety, (-)-(R)-(6,7-dimethoxy-2H-chromen-4-yl)(4-methoxy-2-isopropenyl-2,3-dihydrobenzofuran-5-yl)methanone. Finally, a deprotection and a base-catalysed intramolecular oxo-Michael addition concluded the first stereoselective synthesis of rotenone in 17 steps and an overall yield of 0.02%
MSc., Faculty of Science, University of the Witwatersrand, 2011
Organic compounds (Synthesis) , Enantiomers (synthesis) , Natural pesticides