Merrifield syntheses support

Such biosyntheses were models for the Merrifield-synthesis [8] (Fig. 3), which culminated in the development of fully automated peptide synthesizers [9]. In a repeated reaction cycle a N-terminal protected amino acid, which is attached with its C-terminal end to an insoluble solid support, is deprotected, activated and lengthened by a second protected amino acid unit. The deprotect -ing and coupling steps can be repeated until the entire peptide is assembled. 

NHCH2COH, which in oxytocin has been modified so that it appears as —NHCH2CNH2. Therefore, attach glycine to the solid support in the first step of the Merrifield synthesis. The carboxyl group can be modified to the required amide after all the amino acid residues have been added and the completed peptide is removed from the solid support. 

Figure 3-2 Merrifield synthesis on a polymer bead support. The growing peptide chain is attached to a polymer support, usually in the form of small beads. The next amino acid (bearing R2) IS attached, and its protecting group (BOO is removed with acid ase treatment. flOC. butyl-oxycarbonyl, DCC, dicyclohexyl-carbodiimide.

Several heterogeneous catalysts have been shown to effect related multicomponent couplings. These include cross-linked polymeric ionic liquid material-supported copper (Cu-CPSIL), silica-dispersed CuO (CuO/Si02), and imidazolium-loaded Merrifield resin-supported copper (Cu-PSIL), all of which can be used in water at room temperature to arrive at 1,4-disubstituted-1,2,3-triazoles from alkyl halides, NaNs, and terminal alkynes. Each can be filtered and reused several times with minimal loss of efficacy. Multistep flow synthesis, specifically including generation of underused vinyl azides and their subsequent click conversions to vinyl triazoles, has also been reported. ... 

Contrary to the principles of the ribosomal protein biosynthesis, where the carboxylic functions of amino acids are reactively bound to a polymer — the transfer ribonucleic acid (t-RNA) — which itself is orientated specifically on the ribosomal messenger ribonucleic acid (m-RNA) (Fig. 1 for details see [36]), the basic idea of the Merrifield synthesis depends upon a nonreactive covalent fixation of the C-terminal amino acid of the target peptide on a solid support (Fig. 2). On this insoluble but swollen polymer, the pep-... 

Frank and Hagenmaier [70] in a detailed study investigated the influence of various chloromethylation procedures and varying degrees of cross-linkage of polystyrene on the course of the Merrifield synthesis of a model pentapeptide, performed on the supports. 

It should be emphasized that the spacer bearing support modified in this way is indeed uniformly functionalized. The tertiary glycolic group was found entirely inert to the reaction conditions of the Merrifield synthesis, if low concentrations of trifluoroacetic acid (< 10%, in dichloromethane) are used in the deprotection procedures [81]. The subsequent elimination of water from the remaining tertiary alcoholic function of the glycolic handle which leads to the activation of the C-termini of peptides synthetized on this support, will be described in Sect. 3.5.3.1. 

For reasons explained on p. 20 a modified N,N-dimethyl-polyacrylamide support was introduced [83] into the system of the Merrifield synthesis. This polar polymer is prepared by a ternary copolymerization of N,N-dimethyl-acrylamide, N,N-bisacryloylethylenedi-amine and N-tert.butoxycarbonyl-j3-alanyl-N -acryloylhexamethylenediamine (Fig. 27). 

The polymer support, loaded with a well-known amount of the first C-terminal amino acid, has to be preswollen and washed with an appropriate solvent, e.g., dichloromethane, before the repetitive procedure of the Merrifield synthesis is initiated with the cleavage of the temporary N-terminal protecting group to liberate the first amino function. Deprotect-ing reagents and fission products are washed out with an inert solvent. The completion of these steps has to be monitored by a suitable procedure [92,95] to prevent undesired side reactions. Assuming that the temporary N-terminal protecting group was cleaved by acid. 

To summarize, the masking functions in a Merrifield synthesis should be selected in such a gradation of lability that the often repeated N-terminal deprotections endanger neither the C-terminal link to the support nor the masked side functions of the growing peptide. The type of anchoring used should allow us to liberate the peptide in an entirely protected form from its support, to allow further use of the sequence in fragment condensation reactions. 

Series of reagents are in use to deprotect specifically peptide side functions following a Merrifield synthesis. Since this aspect, however, is not a problem in connection with the peptide synthesis on polymer phase, the discussion is omitted here. Questions relating to cleavage of protected peptides from the polymer support are discussed in Sect. 3.5. 

The method of liberation of the end product from its support depends, first, on the type of C-terminal anchor bond to the polymer and, second, on the question of whether the synthetic material has to be released fully protected, partially masked but with free carboxylic terminus, or entirely deprotected. These aspects are part of the strategic concept — as mentioned in the beginning — which have to be carefully considered before the Merrifield synthesis is initiated. The conditions of deprotection of the mobile temporary N-terminal protecting group and those to cleave masking groups from side functions of the desired peptide are interrelated with the detachment conditions projected to liberate the end product from its support. 

This book was written in the context of the daily confrontation with problems in the utilization of polymeric supports for the synthesis of peptides. Therefore, views and experiences which usually are not mentioned in scientific journals are collected in these pages. The author has deliberately discussed in detail the possible influence of the polymer phase on the varying reaction conditions in the Merrifield synthesis this aspect is neglected in most publications dealing with peptide synthesis. However, in view of the growing body of information on the chemistry of polymer-supported peptide syntheses, the international readership should regard the author s arguments as open to discussion. 

I am very much indebted to all of my colleagues with whom I have had the opportunity to cooperate in studying the potential of the Merrifield synthesis. Above all I like to express my gratitude to my teacher. Professor Dr. Theodor Wieland, Heidelberg, for his boundless encouragement and support in my efforts in the field of peptide synthesis, particularly in its polymer phase bound version. Last but not at least I wish to thank Miss Hildegard Leyden. With infinite patience and great accuracy she typed the manuscript in addition to her daily duties. 

Just as the Merrifield synthesis in Chapter 27 (Section 27.4.2) is used to prepare polyamides by using a polymer bead to anchor the growing polyamide chain, there is a method for making nucleic acids using a pol3rmer support. Of the several polymer supports used, styrene or copolymers of styrene are used most of the time. If polystyrene (see Chapter 10, Section 10.8.3) is used as a support, a chemical reaction is required to attach the first nucleotide. This is accomplished by preparing a polystyrene derivative in which the benzene rings are functionalized. 

Draw the structure of the protected amino acid that must be anchored to the solid support in order to use a Merrifield synthesis to prepare leucine enkephalin. 

The synthesis of a novel Merrifield resin-supported dipeptide Pro-Ala-O-P, derived from proline and alanine, was reported by Wang and Yan with the aim of being used as an organocatalyst in asymmetric aldol reactions of ketones with aldehydes. " Indeed, this supported dipeptide was found to be an efficient catalyst to promote the asymmetric aldol reaction under neat conditions between aromatic aldehydes and cyclic ketones, generating the corresponding aldol products with moderate to high yields and diastereoselectivities of up to 80% de combined with good enantioselectivities of up to 95% ee, as shown in Scheme 2.18. Moreover, this catalyst could be used for seven times with only a minor decrease in product yields, but maintained stereoselectivities. 

This type of polymer support seems to be an interesting alternative to conventional polystyrene because the simple procedures for the experimental work-up of the Merrifield synthesis can be applied and simultaneously quasi-homogeneous reaction conditions are attained by reason of the hydrophilic polyfoxyethylene) spacer group. 

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