Medicinal plants have historically proven their value as a source of molecules with therapeutic potential, and nowadays still represent an important pool for the identification of novel drug leads. In the past decades, pharmaceutical industry focused mainly on libraries of synthetic compounds as drug discovery source. They are comparably easy to produce and resupply, and demonstrate good compatibility with established high throughput screening (HTS) platforms. However, at the same time there has been a declining trend in the number of new drugs reaching the market, raising renewed scientific interest in drug discovery from natural sources, despite of its known challenges. In this survey, a brief outline of historical development is provided together with a comprehensive overview of used approaches and recent developments relevant to plant-derived natural product drug discovery. Associated challenges and major strengths of natural product-based drug discovery are critically discussed. A snapshot of the advanced plant-derived natural products that are currently in actively recruiting clinical trials is also presented. Importantly, the transition of a natural compound from a “screening hit” through a “drug lead” to a “marketed drug” is associated with increasingly challenging demands for compound amount, which often cannot be met by re-isolation from the respective plant sources. In this regard, existing alternatives for resupply are also discussed, including different biotechnology approaches and total organic synthesis.While the intrinsic complexity of natural product-based drug discovery necessitates highly integrated interdisciplinary approaches, the reviewed scientific developments, recent technological advances, and research trends clearly indicate that natural products will be among the most important sources of new drugs also in the future.

N ature has evolved highly efficient systems in the form of cascade reactions, which assemble the metabolic networks that support life (growth and survival). The basic principle of cascade reactions is also frequently used in biocatalysis, using enzymes in isolation, as well as in combination with chemocatalysts (see Fig. 1 and Box 1 for definition of terms) [1][2][3][4][5][6][7] . As there is no need for purification and isolation of intermediates, operating time, production costs and waste are reduced, and concomitantly, overall yields are improved. In addition, the problem of unstable or difficult to handle intermediates can be overcome, and reactivity as well as selectivity can be enhanced by avoiding unfavourable reaction equilibria through the cooperative effects of multiple catalysts 8 . Starting in the 1980s, the early examples of the combination of chemo-and biocatalysts were reported by the van Bekkum group, who pioneered the development of a process to make the sugar substitute d-mannitol from readily available d-glucose through the combination of a heterogeneous metal-catalysed hydrogenation and an enzyme-catalysed isomerization 9 . The first broadly applied technology for the combination of enzyme and metalcatalysts, which was the research subject of many academic groups as well as industry, emerged in the 1990s from the Williams group 10,11 and aimed to achieve higher yields than classical kinetic resolution of racemates, thus overcoming the limitation of a maximum yield of 50% in the latter case. A prominent example of work that developed this theme is the combination of lipase-catalysed kinetic resolution via acylation of secondary alcohols with Pd-or Rh-catalysed racemization via reversible transfer hydrogenation to achieve a dynamic kinetic resolution (DKR) [12][13][14] . This example was facilitated in part because lipases are active and stable in organic solvents. Later, for instance, Turner's group combined a monoamine oxidase-catalysed imine formation with a chemical reduction 15 to achieve the 100% theoretical yield through a deracemization process. In addition to the combination of metalcatalysis with enzymes, organocatalysis, electrochemistry and light-induced reaction couples have since then been studied extensively, going far beyond the scope of a DKR.The challenges to combining chemo-and biocatalysis in cascades (see Box 1 for definitions and Table 1) can be daunting, not least the requirement for the chemical step to occur in the presence of water, the preferred solvent for enzymes 3 . This Review therefore highlights recent examples of the combination of chemo-and biocatalysts in aqueous multistep syntheses, and looks at how to overcome limitations by, for example, design of appropriate reaction conditions, protein engineering and advanced reactor concepts. Furthermore, trends such as the integration of transition metalcatalysis into microorganisms and the introduction of novel chemistry into engineered enzymes are discussed, and a critical assessment of the impact of this research ...

Nature's smartness and efficient assembling cascade type reactions inspired biologists and chemists all around the world. Tremendous effort has been directed towards the understanding and mimicking of such networks. In recent years considerable progress has been made in developing multistep one-pot reactions combining either advantage of chemo-, regio-, and stereoselectivity of biocatalysts or promiscuity and productivity of chemocatalysts. In this context several concepts, inspired by different disciplines (biocatalysis, metabolic engineering, synthetic chemistry, and material science), have been evolved. This review will focus on major contributions in the field of cascade reactions over the last three years.

Baeyer–Villiger monooxygenases (BVMOs) represent a specific class of monooxygenases that are capable of catalyzing a variety of oxidation reactions, including Baeyer–Villiger oxidations. The recently elucidated BVMO crystal structures have provided a more detailed insight into the complex mechanism of these flavin‐containing enzymes. Biocatalytic studies on a number of newly discovered BVMOs have shown that they are very potent oxidative biocatalysts. In addition to catalyzing the regio‐ and enantioselective Baeyer–Villiger oxidations of a wide range of carbonylic compounds, epoxidations, and enantioselective sulfoxidations have also been shown to be part of their catalytic repertoire. This review provides an overview on the recent developments in BVMO‐mediated biocatalytic processes, identification of the catalytic role of these enzymes in metabolic routes and prodrug activation, as well as the efforts in developing effective biocatalytic methodologies to apply BVMOs for the synthesis of high added value compounds.

HAL is a multidisciplinary open access archive for the deposit and dissemination of scientific research documents, whether they are published or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers. L'archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

BACKGROUND AND PURPOSEGABAA receptors are the major inhibitory neurotransmitter receptors in the mammalian brain and the target of many clinically important drugs interacting with different binding sites. Recently, we demonstrated that CGS 9895 (2-(4-methoxyphenyl)-2H-pyrazolo [4,3-c]quinolin-3(5H)-one) acts as a null modulator (antagonist) at the high affinity benzodiazepine binding site, but in addition elicits a strong enhancement of GABA-induced currents via a novel drug binding site at the extracellular a+b-interface. Here, we investigated 32 structural analogues of CGS 9895 for their ability to mediate their effects via the a1+b3-interface of GABAA receptors. EXPERIMENTAL APPROACHGABAA receptors were expressed in Xenopus laevis oocytes and investigated by the two-electrode voltage clamp method. KEY RESULTSWe not only identified compounds with higher efficacy/potency than CGS 9895 for stimulating GABA-induced currents via the a1+b3-binding site, but also discovered compounds acting as null modulators at this site. Most of the compounds also acted as null modulators via the benzodiazepine binding site of GABAA receptors. But some of the positive allosteric modulators or null modulators exclusively exerted their action via the a+b-binding site. CONCLUSION AND IMPLICATIONSPyrazoloquinolinones and pyrazolopyridinones represent the first prototype of drug candidates mediating benzodiazepine like modulatory effects via the a+b-interface of GABAA receptors. The discovery of null modulators acting as inhibitors of the plus modulators provides a highly useful tool for the discovery of additional classes of compounds that can modulate GABAA receptors via this site, which may lead to novel therapeutic principles. LINKED ARTICLEThis article is accompanied by Varagic et al.,

Halogen Dance (HD) reactions are a useful tool for synthetic chemists as they enable access to positions in aromatic and heteroaromatic systems for subsequent functionalization which are often difficult to address by other methods, hence, allowing entry to versatile scaffolds. While the method can be extremely useful, this transformation is often neglected upon designing synthetic sequences. This may be largely attributed to the lack of comprehensive reference works covering the general principles and outlining the versatility and limitations of the technique. The following review tries to present HD reactions in a clear and concise manner in order to convince more chemists of its advantages. It covers the field of HD reactions from their first observation in 1951 until the present. The important contributions leading to the elucidation of the mechanism are briefly outlined followed by a detailed mechanistic section and a discussion of factors which influence HD reactions. Finally, an overview of HD reactions on various carbocyclic and heterocyclic ring systems and its applications in the synthesis of complex compounds is given.

Recent progress in the field of transition‐metal‐catalyzed cross‐coupling reactions on various azole systems is summarized. Most important C–C‐ and C–X‐bond formation methodologies (Negishi, Suzuki–Miyaura, Stille, Kumada–Corriu–Tamao, Hiyama, Sonogashira, Heck, C–H activation) are reviewed and discussed for the imidazole, oxazole, thiazole, pyrazole, isoxazole, and isothiazole system, as well as for azoles with more than two heteroatoms. This review covers the literature that appeared in the past ten years up to the end of 2005 with corresponding azoles used either as metal organyl or halide (including triflates and some other less frequently applied leaving groups); literature describing azole structures only as ligands was not included.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)