A New Technology Solid Phase Peptide Synthesis

Once an amino acid is linked to an insoluble polymer, it can be converted to a dipeptide derivative that remains attached to the support while excess starting materials and the by-products of acylation can be removed simply by washing the peptidyl polymer with appropriately selected solvents. Partial deprotection is then carried out in a similar fashion and the dipeptide transformed into a tripeptide while the link between peptide and resin remains intact. In this way the chain can be lengthened until complete, without a single concentration or precipitation or transfer from one container into another. Only addition of reactants and repeated washings are necessary. At the conclusion of the chain building process the bond between peptide and polymer is cleaved and the peptide purified. 

This ingenious concept was developed simultaneously and independently by R.L. Letsinger and M.J. Kornet at Northwestern University in Evanston, Illinois, 

The anchoring ester bond, that should resist the various operations of the chain-building process, is cleaved at the completion of the procedure by alkaline hydrolysis or better by acidolysis or, when a peptide amide is desired, by ammonolysis. 

In connection with different kinds of anchoring groups we should recall that numerous biologically active peptides have a carboxamide rather than a free carboxyl at their C-termini. As mentioned earlier, such peptide amides can be obtained by ammonolysis of the ester bond between peptide and polymer. A second possibility, to wit, acidolysis is equally useful and often practiced. In 1970 Pieta and Marshall constructed a new polymeric support in which the acid labile N—C bond in benzhydrylamine serves as the point of cleavage (Fig. 7). Of course, the bond s sensitivity to acids can be further increased by electron-releasing substituents such as methoxy groups. 

Peptide synthesis is quite regularly accompanied by undesired side reactions, for instance by the alkylation of side chains during deprotection. In addition to such general problems for which at least partial remedies have been found, also a special problem is encountered in solid phase syntheses the formation of deletion peptides and truncated sequences . Both incomplete acylation and incomplete deprotection result in peptide chains from which one or more amino acid residues are missing. A second kind of deletion, the loss of the C-terminal dipeptide sequence, is caused by diketopiperazine formation followed by acylation of the resulting hydroxymethyl polymer (Fig. 8). Premature chain termination (truncation) takes place if acetic acid or trifluoroacetic acid are not completely removed after deprotection and then co-activated in the following coupling step. 

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The origins of combinatorial chemistry can be traced back to solid phase peptide synthesis in the 1960s, but as applied to small druglike molecules, the technology started to become widely applied during the latter part of the 1990s. The use of combinatorial chemistry in drug discovery ushered in a new chapter in chemical synthesis within the pharmaceutical laboratory. For the... 

The automated solid-phase synthesis of oligosaccharides was first reported in 2001 [38], A peptide synthesizer was modified to enable solid-phase oligosaccharide synthesis. The synthesizer was equipped with a cooling system for low-temperature glycosylations. As in the case with other automated platforms, a combination of new technologies and synthetic methods were developed, and distinctive breakthroughs were achieved. Many state-of-the-art synthetic and analytical methods were adopted and adjusted over the years. The first decade of automated solid-phase oligosaccharide synthesis relied on the development of new methods to access complex structures (Fig. 7.3). 

More recently, several papers reporting the solid phase synthesis of peptide aldehydes have been published. Dinh and Amstrong (61) outline the use of Weinreb-type amides on solid support for the synthesis of ketones and aldehydes Ede and Bray (62) report a new linker based on the oxazolidine moiety and use this method with Multipin technology Galeotti et oL (63) apply their thiazolidinyl linker to a solid support. The great interest in these aldehydic compounds has also been demonstrated by the fact that the Weinreb amide resin is now commercially available from Bachem and Novabiochem. 

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