The portioning-mixing (PM) procedure is one of the earliest combinatorial synthetic methods. It was introduced 10 years ago, in 1988 [1]-[3]. This method exemplified the first time that it is possible to prepare with high confidence a previously unimaginable large number of compounds. It taught a large number of chemists to think combinatorially. The tenth anniversary of the method offers an occasion to summarize its main advantages, the areas of application and new developments.

The general approach of using a bicyclic template to produce inhibitors of the protease superfamily of enzymes has been investigated. The Diels-Alder cycloaddition reaction on solid support has been found to be highly efficient for the synthesis of libraries of compounds that mimic the b-strand secondary structure of proteins. Several potent and selective inhibitors of proteases have been discovered

1,2,3-thiadiazoles were synthesized in parallel using a polymer sulfonyl hydrazide resin (PS-TsNHNH2) and employing a "catch and release" synthesis strategy. Resin capture of ketones synthesized from Weinreb amides and Grignard reagents afforded resin-bound sulfonylhydrazones. Cyclizative cleavage of supportbound sulfonylhydrazones with thionyl chloride afforded 1,2,3-thiadiazoles. Excess thionyl chloride was neutralized using liquid-liquid extraction cartridges.

A focused library of tertiary amines was synthesized by reacting alcohols with polystyrene sulfonyl chloride resin to give polystyrene sulfonates, which were then reacted with secondary amines to give tertiary amines.

In the drug discovery process, chemists are charged with the task of synthesising compounds with the ultimate aim of producing a clinical candidate. With the aid of combinatorial chemistry, it is hoped that the number and diversity of compounds available for screening will increase, the time taken to optimise a lead will decrease and ultimately the time from the initial high-throughput screen to selection of a clinical candidate will be minimised. In our combinatorial chemistry effort, all compounds are prepared in a single compound per well format and are tested in solution. The advantages of this approach are that it is compatible with all screening methods, and no deconvolution is necessary once an active compound is identified. Before submission to either therapeutic biology or high throughput screening (HTS), each compound is characterised by HPLC for purity and MS for identity. Finally, the mass of each compound is obtained. This not only allows for the calculation of a yield, but also helps further downstream in that quantitative HTS data can now be obtained.

After the novel and unusual structure of the cephalostatins (scheme 1) had been elucidated 1 and the limited availability of these powerful cytotoxins had been realized2, synthetic efforts aiming at compounds of this type became known from various places in the world.

Nature's pathways have been a strong motivation for synthetic chemists. Trying to imitate what has been shown or what is supposed to be a biosynthetic transformation always pursues several objectives: One evident goal is to obtain the natural product via the elegant way used in the natural process. The second not less important goal is to obtain insight into the mechanistic details of the biosynthetic pathway. Finally during the synthetic effort compounds are obtained which are potentially interesting as inhibitors of the natural process. Porphobilinogen, the second dedicated intermediate in the biosynthesis of ´pigments of lifeª was synthesised following the mechanistic rationale proposed 30 years ago by Shemin for the biosynthesis. Substances obtained during this synthetic effort and inhibitors especially synthesised for our studies were tested in order to evaluate the different mechanistic proposals. This biomimetic approach allowed to guide at the same time the synthetic as well as the biosynthetic studies.

Intermediates of the catalytic cycles of chloroperoxidase and cytochrome P450cam are identified and characterized by means of synthetic enzyme models.

A group of 26 new 6-halogeno- or 6-alkylsulfanyl-2-alkylsulfanylpyridine-4-carboxamides and corresponding carbothioamides was synthesised. Some of the amides and all thioamides were tested for their antimycobacterial activity against atypical mycobacterial strains. Promising photosynthesis-inhibiting activity was also found for some of the amides.

The tripeptide sequence Arg-Gly-Asp (RGD) has been shown to inhibit the adhesive and aggregatory functions of platelets by binding to platelet receptor GP IIb/IIIa. Fibrinogen binding to GP IIb/IIIa represents the final common event that leads to platelet aggregation regardless of platelet activation and, in certain circumstances, is the primary cause of a variety of human cerebral and cardiovascular diseases[1]. The knowledge of the preferred conformation of the RGD sequence together with structure-activity relationship (SAR) analyses should lead to a better understanding of the characteristics important for high affinity binding to GP IIb/IIIa.