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Several of the enzymes related to the folate cycle are well-known for their role as clinically validated antimalarial targets. Nevertheless for serine hydroxymethyltransferase (SHMT), one of the key enzymes of this cycle, efficient inhibitors have not been described so far. On the basis of plant SHMT inhibitors from an herbicide optimization program, highly potent inhibitors of Plasmodium falciparum (Pf) and Plasmodium vivax (Pv) SHMT with a pyrazolopyran core structure were identified. Cocrystal structures of potent inhibitors with PvSHMT were solved at 2.6 Å resolution. These ligands showed activity (IC50/EC50 values) in the nanomolar range against purified PfSHMT, blood-stage Pf, and liver-stage P. berghei (Pb) cells and a high selectivity when assayed against mammalian cell lines. Pharmacokinetic limitations are the most plausible explanation for lack of significant activity of the inhibitors in the in vivo Pb mouse malaria model.

Cyclooctatetraene—via its 1α,2α,5α,6α-diepoxy-3β,4β-diol (4)—is the basis for the construction of specifically polyfunctionalized C8 building blocks. Whilst with monofunctional nucleophiles only monosubstitution and with 1,1-dinucleophiles neatly intramolecular cyclization to aza(oxa,thia)bicyclo[4.2.1]nonenes occurred, the desired substitution pattern became, in principle, accessible with 1,2-disubstituted hydrazines. The usefulness of the 7-hetero[4.2.1]bicycles for the preparation of meso-persubstituted azepanes and oxepanes is exemplified.

In specifically designed diazene/diazene(oxide) substrates, in spite of seemingly favorable geometrical prerequisites (1, d = 2.85 A, ω = 170°), no [π2+π2] photocycloaddition, particularly no photometathesis, is observed; diazenoxide → oxadiaziridine cyclizations are the alternative photoreactions.

We describe in full the first synthesis of the potent insect antifeedant azadirachtin through a highly convergent approach. An O-alkylation reaction is used to unite decalin ketone and propargylic mesylate fragments, after which a Claisen rearrangement constructs the central C8-C14 bond in a stereoselective fashion. The allene which results from this sequence then enables a second critical carbon-carbon bond forming event whereby the [3.2.1] bicyclic system, present in the natural product, is generated via a 5-exo-radical cyclisation process. Finally, using knowledge gained through our early studies into the reactivity of the natural product, a series of carefully designed steps completes the synthesis of this challenging molecule.

The first total synthesis of three naturally occurring cyclophane derivatives belonging to the turriane family of natural products is described. Their sterically hindered biaryl entity is formed by reaction of the Grignard reagent derived from aryl bromide 10 with the oxazoline derivative 18, and the macrocyclic tether of the targets is efficiently forged by ring closing metathesis. While conventional RCM catalyzed by the ruthenium-carbene complexes 33 or 34 invariably leads to the formation of mixtures of both stereoisomers with the undesirable (E)-alkene prevailing, ring closing alkyne metathesis (RCAM) followed by Lindlar reduction of the resulting cycloalkynes 37 and 38 opens a convenient and stereoselective entry into this class of compounds. RCAM can either be accomplished by using the tungsten alkylidyne complex [(tBuO)3 [triple bond] WCCMe3] or by means of a catalyst formed in situ from [Mo(CO)6] and para-trifluoromethylphenol. The latter method is significantly accelerated when carried out under microwave heating. Furthermore, the judicious choice of the protecting groups for the phenolic -OH functions turned out to be crucial. PMB-ethers were found to be compatible with the diverse reaction conditions en route to 3-5; their cleavage, however, had to be carried out under carefully optimized conditions to minimize competing O-C PMB migration. Turrianes 3-5 are shown to be potent DNA cleaving agents under oxidative conditions when administered in the presence of copper ions.

PtCl(2) constitutes an efficient and practical catalyst for a set of different atom economical rearrangement reactions of enynes. This includes (i) a formal enyne metathesis reaction delivering 1,3-dienes, (ii) the formation of polycyclic vinylcyclopropane derivatives, and (iii) an unprecedented O--C allyl shift reaction if unsaturated ethers are employed. Although these transformations produce significantly different structural motifs, they share a common mechanism comprising a cationic manifold triggered by the pi-complexation of Pt(II) onto the alkyne unit of the substrates. Strong experimental support for the proposed mechanism comes from deuterium-labeling studies, a careful analysis of the product distribution pattern, and the fact that in some cases PtCl(2) can be replaced by simple Lewis or Brønsted acids as the catalysts.

Exchange of one PCy3 unit of the classical Grubbs catalyst 1 by N-heterocyclic carbene (NHC) ligands leads to "second-generation" metathesis catalysts of superior reactivity and increased stability. Several complexes of this type have been prepared and fully characterized, six of them by X-ray crystallography. These include the unique chelate complexes 13 and 14 in which the NHC- and the Ru-CR entities are tethered to form a metallacycle. A particularly favorable design feature is that the reactivity of such catalysts can be easily adjusted by changing the electronic and steric properties of the NHC ligands. The catalytic activity also strongly depends on the solvent used; NMR investigations provide a tentative explanation of this effect. Applications of the "second-generation" catalysts to ring closing alkene metathesis and intramolecular enyne cycloisomerization reactions provide insights into their catalytic performance. From these comparative studies it is deduced that no single catalyst is optimal for different types of applications. The search for the most reactive catalyst for a specific transformation is facilitated by IR thermography allowing a rapid and semi-quantitative ranking among a given set of catalysts.

The first total synthesis of three naturally occurring cyclophane derivatives belonging to the turriane family of natural products is described. Their sterically hindered biaryl entity is formed by reaction of the Grignard reagent derived from aryl bromide 10 with the oxazoline derivative 18, and the macrocyclic tether of the targets is efficiently forged by ring closing metathesis. While conventional RCM catalyzed by the ruthenium-carbene complexes 33 or 34 invariably leads to the formation of mixtures of both stereoisomers with the undesirable (E)-alkene prevailing, ring closing alkyne metathesis (RCAM) followed by Lindlar reduction of the resulting cycloalkynes 37 and 38 opens a convenient and stereoselective entry into this class of compounds. RCAM can either be accomplished by using the tungsten alkylidyne complex [(tBuO)3 [triple bond] WCCMe3] or by means of a catalyst formed in situ from [Mo(CO)6] and para-trifluoromethylphenol. The latter method is significantly accelerated when carried out under microwave heating. Furthermore, the judicious choice of the protecting groups for the phenolic -OH functions turned out to be crucial. PMB-ethers were found to be compatible with the diverse reaction conditions en route to 3-5; their cleavage, however, had to be carried out under carefully optimized conditions to minimize competing O-C PMB migration. Turrianes 3-5 are shown to be potent DNA cleaving agents under oxidative conditions when administered in the presence of copper ions.