Studies towards the total synthesis of insect antifeedant clerodanes

An approach towards the total synthesis of insect antifeedant clerodanes is described in this thesis.In chapter 1 the structure, nomenclature, occurrence, biosynthesis and physiological activities of the clerodanes are described. A number of structure/activity studies, concerning the insect antifeedant properties of these compounds, is summarized in a tabular form. Such studies have led to the conclusion that for evoking strong antifeedant activity both the furofuran and the decalin part of the molecule are required (de Groot/ Schoonhoven and Ley/Blaney). A literature survey of the most relevant synthetic studies towards the clerodanes is presented in chapter 2.A retrosynthetic study of the targetmolecule dihydroclerodin 26n is described in chapter 3. This approach offers some flexibility with respect to the decalin part and the furofuran part of the molecule. According to this synthetic plan the enone 48 has to be converted into the y-dioxolanyi-α,β-unsaturated ketone 130 . The problems connected with this transformation and the solution for these problems are described in chapter 4.New methods for the stereoselective synthesis of the furofuran unit had to be developed. Such methods must be applicable on the planned intermediates 127 and/or 128 and therefore had to start from an aldehyde or an oxirane function and had to be compatible with the functional groups present in other parts of these molecules, so strongly oxidizing or electrophilic reactions had to be avoided. Moreover reactions have to be carried out on a neopentylic carbon atom and therefore pivalaldehyde and tert-butyloxirane were chosen as the principal model compounds. In chapter 5 stereoselective perhydrofurofuran syntheses, based on the reaction of lithiated sulfones with carbonyl compounds, are described. In chapter 6 stereoselective furofuran syntheses, based on the reaction of lithiated nitriles with oxiranes, are described.Having appropiate methods for the introduction of a furofuran unit in hand, the aldehyde 128 was considered to be a very promising clerodane precursor. So the enone 130 was converted into the cyclic ether 129 via two stereoselective reductions. Additional prove for the correct stereochemistry of this molecule was obtained by an X- ray crystal structure determination. The subsequent cleavage of the cyclic ether structure determination. The subsequent cleavage of the cyclic ether proceeded in a remarkable chemo- and regioselective manner to give a bromide diacetate, which was further transformed into the aldehyde 128 . The latter transformation was not studied in full detail owing to the limited availability of material. Nevertheless a small amount of this aldehyde 128 was obtained. Its transformation into dihydroclerodin 26n will need further investigations.