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The systematic modification of natural products through diverted total synthesis is a powerful concept for the systematic modification of natural products with the aim of studying mechanistic aspects of their biological activity. This concept offers far-reaching opportunities for discovery at the interface of biology and chemistry. It is underpinned by the power of chemical synthesis, which manifests itself in the ability to modify structure at will. Its implementation, when combined with innovative design, enables the preparation of unique mechanistic probes that can be decisive in differentiating and validating biological hypotheses at the molecular level. This Review assembles a collection of classic and current cases that illustrate and underscore the scientific possibilities for practitioners of chemical synthesis.

Enolonium species/iodo(III)enolates of carbonyl compounds have been suggested to be intermediates in a wide variety of hypervalent iodine induced chemical transformations of ketones, including α-C-O, α-C-N, α-C-C, and α-carbon-halide bond formation, but they have never been characterized. We report that these elusive umpoled enolates may be made as discrete species that are stable for several minutes at -78 °C, and report the first spectroscopic identification of such species. It is shown that enolonium species are direct intermediates in C-O, C-N, C-Cl, and C-C bond forming reactions. Our results open up chemical space for designing a variety of new transformations. We showcase the ability of enolonium species to react with prenyl, crotyl, cinnamyl, and allyl silanes with absolute regioselectivity in up to 92 % yield.

Systemic fungal infections are an increasingly prevalent health problem, especially among immunocompromised patients. Antifungal drug development lags far behind in comparison to other types of antimicrobial drugs. Current commercially available antifungals are limited by their insufficient potency, side effects, drug-drug interactions, developing drug-resistance, and narrow formulation options. Here, we report the preparation and evaluation of two novel PEG amide conjugates of amphotericin B (AMB (1)): AB1 (4) and AM2 (5). These compounds are nonlabile, they are prepared in only two and three synthetic steps, respectively, and they show antifungal activity against a wide range of clinical fungal isolates. Their toxicity is significantly lower, and their water solubility is up to 5000-fold higher than that of AMB (1). In vivo efficacy studies in a mouse model of systemic candidiasis showed that AM2 (5) successfully cured all the mice at concentrations above 3.5 mg/kg body weight. In conclusion, these properties make AB1 (4) and AM2 (5) promising candidates for clinical use.

Identifying the mechanism of a catalytic reaction is paramount for designing new and improved catalysts. Several alternative catalytic cycles for the copper/2,2,6,6-tetramethylpiperidine-N-oxyl (TEMPO)-catalyzed aerobic oxidation of alcohols to the corresponding aldehydes or ketones were examined using DFT at the SMD(CH CN)-RIJCOSX-DSD-PBEB95/def2-TZVP//DF-PBE /def2-SVP level of theory. A catalytic cycle in which TEMPO remains coordinated to copper throughout was identified as the most likely mechanism. There are three components to the catalytic cycle: 1) hydrogen transfer from the alkoxyl ligand to coordinated TEMPO, 2) oxygen activation with formation of a peroxo complex, and 3) alcohol activation with transfer of the OH proton to the peroxo ligand. The oxidation takes place via a six-membered intramolecular hydrogen-transfer transition state. Importantly, this is not the rate-determining step, which instead involves oxygen activation and/or the initial alcohol activation.

Reactions of antimalarial beta-sulfonyl endoperoxides 9 and 10, which, like yingzhaosu A (2), derive from the 2,3-dioxabicyclo[3.3.1]nonane system 3, with iron(II) salts were studied. Product analysis of the iron(II)-induced degradations provided evidence for the intermediacy of carbon-centered cyclohexyl radicals 20 and 31 and their possible oxidation to the corresponding carbocations 21 and 32. It is conceivable that the antimalarial activity of beta-sulfonyl endoperoxides of type 5 may derive from alkylation of vital intraparasitic biomolecules by free radicals and/or carbocations, generated within the malaria parasite through a similar iron(II)-induced degradation process.

The syntheses and in vitro antimalarial screening of 50 bridged, bicyclic endoperoxides of types 9-13 are reported. In contrast to antimalarial trioxanes of the artemisinin family, but like yingzhaosu A and arteflene, the peroxide function of compounds 9-13 is contained in a 2,3-dioxabicyclo[3.3.1]nonane system 6. Peroxides 9 and 10 (R(1) = OH) are readily available through a multicomponent, sequential, free-radical reaction involving thiol-monoterpenes co-oxygenation (a TOCO reaction). beta-Sulfenyl peroxides 9 and 10 (R(1) = OH) are converted into beta-sulfinyl and beta-sulfonyl peroxides of types 11-13 by controlled S-oxidation and manipulation of the tert-hydroxyl group through acylation, alkylation, or dehydration followed by selective hydrogenation. Ten enantiopure beta-sulfonyl peroxides of types 12 and 13 exhibit in vitro antimalarial activity comparable to that of artemisinin (IC(50) = 6-24 nM against Plasmodium falciparum NF54). In vivo testing of a few selected peroxides against Plasmodium berghei N indicates that the antimalarial efficacies of beta-sulfonyl peroxides 39a, 46a, 46b, and 50a are comparable to those of some of the best antimalarial drugs and are higher than artemisinin against chloroquine-resistant Plasmodium yoelii ssp. NS. In view of the nontoxicity of beta-sulfonyl peroxides 39a, 46a, and 46b in mice, at high dosing, these compounds are regarded as promising antimalarial drug candidates.

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