Natural analogues sources

The yellow analogues, the betaxanthins, are composed of betalamic acid with amino acids or amines, respectively, amounting to 26 structures known to occur naturally. " Structures unambiguously assigned by NMR spectroscopy usually carry trivial names derived from the plant material from which they have been first isolated. The substitution patterns of betalyanins and betaxanthins hitherto reported together with their particular plant sources are listed in Table 4.4.1 and 4.4.2, respectively. 

The second source of partitioning data is experimental equilibration of crystals and liquids followed by microbeam analysis of quenched run products. Starting materials can be natural rocks, or synthetic analogues. In either case it is customary to dope the starting material with the U-series element(s) of interest, in order to enhance analytical precision. Of course, doping levels should not be so high as to trigger trace phase saturation (e g.. 

Figure 7.23 shows the prototype diazobenzo[Z ]fluorene-based natural products kinamycin A and prekinamycin. The kinamycin A-D family were first isolated from Streptomyces murayamaensis, but the structures were incorrectly characterized as having a cyanobenzo[Z ]carbazole ring. Since the initial discovery of the kinamycins, many new analogues have been discovered from natural sources.88-92... 

In 1972, Chinese researchers isolated, by extraction at low temperature from a plant, a crystalline compound that they named qinghaosu [the name artemisinin (la) is preferred by Chemical Abstracts, RN 63968-64-9]. The plant source of artemisinin is a herb, Artemisia annua (Sweet wormwood), and the fact that artemisinin is a stable, easily crystallizable compound renders the extraction and purification processes reasonably straightforward. The key pharmacophore of this natural product is the 1,2,4-trioxane unit (2) and, in particular, the endoperoxide bridge. Reduction of the peroxide bridge to an ether provides an analogue, deoxyartemisinin 3, that is devoid of antimalarial activity. ... 

Recently, analogues of nucleosides [60], natural products Huperzine-A [61] and Hydroartemisinin [62], and inhibitors of metallo-/ -lactamases have been synthesised [63]. With acylsilane electrophiles, the initial adducts undergo Brook rearrangement which is interrupted by -Si bond fission with loss of fluoride anion (Eq. 16), leading to the formation of extremely useful difluoro-enol silanes [64]. Of the various fluoride sources employed, the tetrabutylam-monium triphenyldifluorostannate described by Gingras appears to be particularly effective. The numerous other methods for trifluoromethylation formed the subject of an exhaustive review [65]. More recently, the Olah group described a chlorodifluoromethyl trimethylsilane which is expected to have a rich chemistry [66]. 

This transformation is in contrast to the corresponding more sterically crowded [Mo N (Bu )Ar 3] which does not exhibit this type of behaviour. Initially it was assumed that the cyclometallated structure would inhibit the type of reactions (e.g. with N2 or N2O) shown by the t-Bu substituted tris-amido complex. Investigation of the chemistry, however, showed that this is not the case and the imine-hydrido product behaves as an effective source of [Mo N(Pr )Ar 3] which, because of its less sterically encumbered nature, exhibits reactivity not observed for its more hindered [Mo N(Bu )Ar 3] counterpart. A simple example is provided by complex formation with benzophenone to give [Mo N(Pr )Ar 3(r 2-OCPh2)], whereas the Bu analogue did not react with this reagent. ... 

As you might suspect a great number of drugs and poisons can be isolated from natural sources. These were the materials of ancient folk medicine and witch doctors, of healing and war. With the advent of modern chemistry the structural nature of each compound could be analyzed atom by atom and similar structures (analogues) could be constructed and tested for their activity. 

---