Introduction

Steroids are important pharmacologically active scaffolds [1]. The relevance of microbiological transformations for the production of steroidal drugs and hormones was first recognized when the 11a-hydroxylation of progesterone by Rhizopus ar-rhizus and R. nigricans was developed by Murray and Petterson [2]; this resulted in a tremendous improvement of the synthesis of corticoids. Starting from deoxy-cholic acid 31 steps had been needed for chemical synthesis of cortisone, whereas the microbiological procedure starting from diosgenin shortened the synthesis to 13 steps [3]. Since then, microbiological transformations have found broad application in the synthesis of pharmaceuticals and natural products [4].

In microbiological chemistry whole cells are used instead of isolated enzymes. This approach frequently proves to be beneficial, because isolation and purification of enzymes [5] are tedious and often uneconomical procedures which can also result in significant loss of enzyme activity. The main applications of microbiological reactions, which can be performed by use of growing, resting [6], or immobilized cells [7], include hydroxylation of non-activated C-H bonds [8], oxidation reactions [9], dehydrogenation of saturated to a,S-unsaturated ketones [10], reduction of keto groups [11], and partial degradation of complex molecules [12] (two such examples are discussed in Chapters 6.3 and 6.4). In these examples, the oxidizing or reducing enzyme is regenerated by other enzymes in the cell. Although isolated enzymes are often used for hydrolyzing reactions, because no coenzyme needs to be regenerated [5, 13], it is also possible to hydrolyze esters selectively using whole cells [14]. Similar to enzymatic reactions, microbiological transformations can be rather substrate-specific, thus limiting the flexibility of the approach. Similar to chemical catalytic reactions, moreover, microbiological transformations often require optimization of the reaction conditions before high yields and high enantio-meric purities can be obtained. Once a microorganism performing the desired transformation is found, there are several ways of improving the yield of a reaction. For example, addition of water-soluble organic solvents [15], cyclodextrins, surfac tants, or organic carbon or nitrogen sources [16], and optimization of pH [17] and incubation conditions often lead to higher yields. Transformations can also be improved by mutation and selection [16].

Combinatorial chemistry [18], on the other hand, is a well established means of synthesizing relatively large numbers of analogs relatively quickly (see also Chapters 2.3, 3.1 or 5.4 for examples). Use of laboratory automation [19], in conjunction with the advanced chemoinformatics systems needed to handle the enormous amounts of related data [20], enables full exploitation of this principle. These technologies have recently been widely applied to drug discovery programs [21] and to chemical development and material sciences [22]. In drug discovery early chemistry-driven approaches typically directed at large numbers of crude products have been widely supplemented or replaced by more focussed, structure-based and property-biased strategies [23]. In these approaches only a subset of a virtual library most likely to show the desired properties is synthesized, preferably selected by means of virtual screening, pharmacokinetic property calculations, and medicinal chemistry know-how, thus leading to high-quality compounds.

Combination of microbiological chemistry, often yielding scaffolds not easily obtained by purely chemical means, and combinatorial chemistry, enabling rapid and efficient synthesis of analogs, provides a valuable tool for generation of novel test compounds. As an example [24] we describe here the application of our lipoic acid-derived thioketal linker [25] to the solid-phase synthesis of A4-3-keto steroidal ureas from ¿-sitosterol.

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