Syntheses of Allelochemicals for Insect Control
Abstract: This thesis describes the synthetic preparation of somecompounds, which can serve as chemical signals for use in thedevelopment of control methods for pest insects. The compoundssynthesised are of the isoprenoid type and of two kinds:carvone derivatives and germacranes. The derivatives of carvoneare based on modifications of this compound, by reactions ofeither its endocyclic or its exocyclic double bond. One type ofmodifications was accomplished by chemoselective additions ofthiophenol. The latter ones imply additions to the exocyclicdouble bond and seem to constitute general, previously rarelystudied reactions.In other modifications of its exocyclic side chain, carvoneafforded some sesqui- and diterpeniod natural products. Thefollowing compounds were synthesised in an enantioselectiveway: (-)-epi-delobanone, (-)-delobanone,(-)-7-hydroxy-3,10-prenylbisaboladien- 2-one (an insecticidalconstituent of Croton linearis) as well as its diastereomer andsome other compounds with similar structures. All of thesecompounds weretested for their antifeedant/feeding deterrentcapability against gnawing of the pine weevil, Hylobiusabietis.The germacranes prepared by means of enantioselective totalsyntheses are: (?)- 1(10),5-germacradien-4-ol and(?)-germacrene D. The former is a constituent of thedefence secretion (an allomone) from the larvae of the pinesawfly, and the needles of Scots pine. (?)-Germacrene D isa ubiquitous compound in nature. For example, it occurs in thepeels of apples and acts as one component of a lure (akairomone) to the apples, which attracts the codling moth,Cydia pomonella.The main problem in the total syntheses of the germacraneswas the formation of the unsaturated monocyclic 10-memberedring. This was achieved by intramolecular alkylation with asuitably functionalised/protected cyanohydrin derivative,which, after further elaboration, afforded a monocyclic10-membered enone, that was used in the syntheses of the twogermacranes mentioned above. In the initial steps in thesynthetic sequence the stereochemistry was established byalkylation of an amide enolate attached to a chiral auxiliary.This approach could most likely also readily furnish the(+)-enantiomers of these germacrenes (of the germacraneterpenoid class) using the opposite enantiomer of the chiralauxiliary in the initial steps.Keywords: isoprenoids, natural product synthesis,allelochemicals, kairomones, allomones, bisabolane terpenoids,Hylobius abietis, germacrane terpenoids, Neodiprion sertifer,stereoselective synthesis.
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