Cleaved from the Solid Support

As already mentioned, components of libraries produced by the original split-mix method are discrete compounds. If the libraries are cleaved from the solid support, however, mixtures are formed. The products of the liquid phase synthesis are always mixtures. At the beginnings, finding an active component in a mixture of thousands, or millions of structurally related compounds seemed to be a task like finding a needle in a haystack. Later on, however, reliable methods have been developed to solve this problem. All these methods are based on preparation and screening of properly designed partial libraries. 

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DNA synthesizers operate on a principle similar to that of the Merrifield solid-phase peptide synthesizer (Section 26.8). In essence, a protected nucleotide is covalently bonded to a solid support, and one nucleotide at a time is added to the growing chain by the use of a coupling reagent. After the final nucleotide has been added, all the protecting groups are removed and the synthetic DNA is cleaved from the solid support. Five steps are needed ... 

The solid-phase synthesis of dendritic polyamides was explored by Frechet et al. [49]. Inspired by the technique used by Merrifield for peptide synthesis, the same strategy was used to build hyperbranched polyamides onto a polymeric support. The idea was to ensure the preservation of the focal point and to ease the purification between successive steps. The resulting polymers were cleaved from the solid support, allowing ordinary polymer characterization. The reaction was found to be extremely sluggish beyond the fourth generation. 

A hexaglycine spacer was attached to the solid support to give a substitution of 0.2 mmol g of dry silica and the excess amino groups were then capped using acetic anhydride. In the next step a selectively cleavable, a-chymotrypsin sensitive, phenylalanine ester (2) was implemented for the release of the products from the solid support under mild conditions. Subsequently it was transformed to (3) followed by reactions with glycosyl transferases to yield (4). Finally, the desired glycopeptide was cleaved from the solid support in high yield by treatment of (4) with a-chymotrypsin (Scheme 10.1). 

The PNA chain was linked to the peptide spacer glutamic acid-(y-tert-butyl ester)-(fi-aminohexanoic acid)-(fi-aminohexanoic acid) (Glu [OtBuj-fiAhx-fiAhx) via an enzymatically cleavable Glu-Lys handle. The Glu [OtBuj-fiAhx-fiAhx spacer was coupled to the amino-functionalized membrane by standard Fmoc-Chemistry. Then the membranes were mounted in an ASP 222 Automated SPOT Robot and a grid of the desired format was dispensed at each position. The free amino groups outside the spotted areas were capped and further chain elongation was performed with Fmoc-protected PNA monomers to synthesize the desired PNA oligomers (18). After completion of the synthesis, the PNA oligomers were cleaved from the solid support by incubation with bovine trypsin solution in ammonium bicarbonate at 37 °C for 3 h. 

The linker 37 with the first amino acid attached, compound 38, can be applied to stepwise solid-phase peptide synthesis. At the end of the synthesis, when the desired peptide sequence is completed, the thioacid-modified peptide fragment is cleaved from the solid support by HF and further S-alkylated with a N-bromoacetylated peptide to form an endothioester bond. The cleaved thioacid can also be reacted with an alkyl bromide to form the corresponding thioester. 

When the peptide is finished it can be cleaved from the solid support (resin) using TFA. This acid-based cleavage process also results in deprotection of the amino acid side chains. 

Once the final compound has been cleaved from the solid support, the contents of the blocks are drained into a standard 96-well microtiter plate. The solvent is removed in a vacuum oven and appropriate daughter plates (master chemistry plate, master biology plate and analytical plate) are prepared using a 96-channel pipetting robot. The average production capacity of such a work station is between 1000 and 2000 single spatially dispersed compounds per day. 

Numerous linkers have been developed with the aim of immobilizing substrates on a solid support. Commercially available (+)-a-lipoic acid has been employed as a novel, chemically stable linker for the immobilization of ketones. The utility of this thioacetal-based linker in solid-phase synthesis has been demonstrated by the synthesis of several 4-acetylbiphenyls by means of the Suzuki reaction. The products were readily cleaved from the solid support by treatment with [bis(trifluoroacetoxy)iodo]benzene [PhI(OCOCF3)2] [107]. 

A method in which the C-terminal amino acid is attached to a solid support (polystyrene beads) and the peptide is synthesized in the C —> N direction by successive coupling of protected amino acids. When the peptide is complete, it is cleaved from the solid support, (p. 1185)... 

More recently Kunz and co-workers described a synthesis of 2,6-dideoxy-2,6-diaminopyra-noside libraries [73]. The scaffold 120 allows for selective deprotection of four positions in a manner that is independent of the sequence (O Scheme 11). Different functional groups have been incorporated at each position to give 121-4, which can then be cleaved from the solid support. For example, the 2-A-allyloxycarbonyl protecting group of 120 can be selec-... 

Release of resin-bound peptides. Bound peptides are quantitatively cleaved from the solid support on exposure to ammonia vapor. The method is general. 

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