Supports in solid-phase synthesis

Rasoul, F., Ercole, F., Pham, Y., et al. (2000) Grafted supports in solid-phase synthesis. Biopolymers (Pept. Sci.) 55, 207-216. 

Structural analysis of linear polymers molecularly dissolved in a suitable solvent using and solution phase NMR spectroscopy is long established [87-89]. Not surprisingly therefore when a linear soluble polymer is used as a support in solid phase synthesis and solution phase NMR spectroscopy can be a powerful tool in following the chemical synthesis on the support [90]. Figure 15.3.58, for example, shows a series of H NMR spectra of dissolved linear polymer samples taken at various stages in the solid phase synthesis of oligoethers on soluble polystyrene [91]. The various chemical steps Fig. 15.3.59 are clearly demonstrated. 

Polyamide support in solid-phase synthesis 16, 424s31 ... 

However, other polymer composite materials also popular in solid-phase synthesis, such as polyethylene or polypropylene tea bags , lanterns, crowns, or plugs, are generally less suitable for high-temperature reactions (>160 °C). Therefore, micro-wave irradiation is typically not a very suitable tool to speed up reactions that utilize these materials as either a solid support or as containment for the solid support. 

Bis(indolyl)nitroethanes are obtained readily in 7-10 min in high yields (70-86%) on fine TLC-grade silica gel (5-40 pm) by Michael reaction of 3-(2 -nitrovinyl) indole with indoles. The same reaction reported requires 8-14 h for completion at room temperature [77]. Several functionalized resins have been prepared from Merrifield resin via a MW-assisted procedure that utilized mixed solvent system to facilitate the swelling of resins and coupling with microwaves [78], These resins can function as solid supports or polymeric scavengers in solid phase synthesis. 

Scheme 4.8 Use of trichloracetimidate donors of polystyrene supports in solid-phase oligosaccharide synthesis.

Scheme 4.12 Application of CPG support in solid-phase oligosaccharide synthesis using glycosyl trichloroacetimidates.

A van Vliet, RH Smulders, BH Rietman, GI Tesser. Protected peptide intermediates using a trityl linker on a solid support, in R Epton, ed. Innovations and Perspectives in Solid Phase Synthesis. Proceedings of the 2nd Symposium. Intercept, Andover,... 

Once it is part of a cyclic dipeptide, the prolyl residue becomes susceptible to enantiomerization by base (see Section 7.22). The implication of the tendency of dipeptide esters to form piperazine-2,5-diones is that their amino groups cannot be left unprotonated for any length of time. The problem arises during neutralization after acidolysis of a Boc-dipeptide ester and after removal of an Fmoc group from an Fmoc-dipeptide ester by piperidine or other secondary amine. The problem is so severe with proline that a synthesis involving deprotection of Fmoc-Lys(Z)-Pro-OBzl produced only the cyclic dipeptide and no linear tripeptide. The problem surfaces in solid-phase synthesis after incorporation of the second residue of a chain that is bound to the support by a benzyl-ester type linkage. There is also the added difficulty that hydroxymethyl groups are liberated, and they can be the source of other side reactions. 

This chapter will focus exclusively on cross-linked vinyl polymer supports either in a spherical bead or resin form, or in some other macroscopic format. These essentially insoluble materials lead to considerably simplified reaction work-up and product isolation procedures when used e.g. in solid phase synthesis or as catalyst or... 

Infrared and Raman spectroscopy are nondestructive, quick and convenient techniques for monitoring the course of solid-phase reactions, and have therefore been widely used for the characterization of polymer supports and supported species [156-160]. In fact, the application of infrared spectroscopy in solid-phase synthesis has received much attention and has been the subject of several recent reviews [127, 128, 161-164]. Reactions involving either the appearance or disappearance of an IR-active functional group can be easily monitored using any of the IR techniques described in this section. Some beads are typically removed from the reaction mixture, then they are quickly washed and dried prior to IR analysis. Traditionally, polymer supports are diluted and ground with KBr, then conventional FT-IR analysis of the KBr disk is carried out Although this is a commonly used... 

Note 3 Use of the term resin to describe the polymer beads used in solid-phase synthesis and as polymer supports, catalysts, reagents, and scavengers is also discouraged. 

The ready alkylation of heterocyclic thiols lends this link to applications in solid-phase synthesis. Although much more work has been done in other heterocyclic systems, a prototype solid-phase synthesis has been described in which the pteridine is built from a 2- or 4-alkylthiopyrimidine attached to a cross-linked polystyrene support <20030BG1909, 2003TL1267> Oxidative cleavage was preferred using DMDO to avoid unwanted by-products... 

An important criteria in solid-phase synthesis is product purities immediately after cleavage from the support. Ideally, the target compound should be cleaved into a solvent-reagent system that can be easily removed, usually by evaporation. Solvents/cleavage reagents that are difficult to remove may compromise subsequent biological screening of the libraries. Consequently,... 

In this section the use of polystyrene and copolymers of styrene with various cross-linking agents as supports for solid-phase organic synthesis is discussed. Copolymers of styrene with divinylbenzene are the most common supports for solid-phase synthesis. Depending on the kind of additives used during the polymerization and on the styrene/divinylbenzene ratio, various different types of polystyrene can be prepared. However, non-cross-linked polystyrene has also been used as a support for organic synthesis [10,16-22], Linear, non-cross-linked polystyrene is soluble in organic solvents such as toluene, pyridine, ethyl acetate, THF, chloroform, or DCM, even at low temperatures, but can be selectively precipitated by the addition of methanol or water. 

Cross-linked polystyrene can be functionalized in many ways [38 41]. Those functionalized resins that are frequently used as supports for solid-phase synthesis are commercially available, and their preparation will be mentioned only in brief here. 

Meldal and coworkers developed polyacrylamides cross-linked with poly(ethylene glycol), referred to as PEGA, as supports for solid-phase synthesis and on-bead enzymatic assays [181-183]. Functionalization of the polymer was performed in a similar fashion as in the case of other polyacrylamides, i.e. either by copolymerization with N-acryloylsarcosine ethyl ester followed by aminolysis with ethylenediamine, or by copolymerization with an amino group containing monomer. The monomers used for a high-capacity (0.4-0.8 mmol/g [182]) and a low-capacity (0.2-0.4 mmol/g [181]) PEGA support are sketched in Figure 2.7. 

Linkers are molecules which keep the intermediates in solid-phase synthesis bound to the support. Linkers should enable the easy attachment of the starting material to the support, be stable under a broad variety of reaction conditions, and yet enable selective cleavage at the end of a synthesis without damage to the product. Several types of linker have been developed, which meet these conflicting requirements to different extents [1]. Newer developments include linkers containing fluorine to facilitate the monitoring of solid-phase chemistry by NMR (see Section 1.3.5), and enantio-merically pure linkers that enable the synthesis on solid phase of enantiomerically enriched products [2],... 

Acid-labile linkers are the oldest and still the most commonly used linkers for carboxylic acids. Most are based on the acidolysis of benzylic C-O bonds. Benzyl esters cleavable under acidic conditions were the first type of linker to be investigated in detail. The reason for this was probably the initial choice of polystyrene as an insoluble support for solid-phase synthesis [13]. Polystyrene-derived benzyl esters were initially prepared by the treatment of partially chloromethylated polystyrene with salts of carboxylic acids (Figure 3.3). 

A flexible means of access to functionalized supports for solid-phase synthesis is based on metallated, cross-linked polystyrene, which reacts smoothly with a wide range of electrophiles. Cross-linked polystyrene can be lithiated directly by treatment with n-butyllithium and TMEDA in cyclohexane at 60-70 °C [1-3] to yield a product containing mainly meta- and para-Iithiated phenyl groups [4], Metallation of noncross-linked polystyrene with potassium ferf-amylate/3-(lithiomethyl)heptane has also been reported [5], The latter type of base can, unlike butyllithium/TMEDA [6], also lead to benzylic metallation [7]. The C-Iithiation of more acidic arenes or heteroar-enes, such as imidazoles [8], thiophenes [9], and furans [9], has also been performed on insoluble supports (Figure 4.1). These reactions proceed, like those in solution, with high regioselectivity. 

Carbamates are by far the most common type of amine protection used in solid-phase synthesis. Various types of carbamate have been developed that can be cleaved under mild reaction conditions on solid phase. Less well developed, however, are techniques that enable the protection of support-bound amines as carbamates. Protection of amino acids as carbamates (Boc or Fmoc) is usually performed in solution using aqueous base (Schotten-Baumann conditions). These conditions enable the selective protection of amines without simultaneous formation of imides or acylation of hydroxyl groups. Unfortunately, however, Schotten-Baumann conditions are not compatible with insoluble, hydrophobic supports. Other bases and solvents have to be used in order to prepare carbamates on, for example, cross-linked polystyrene, and more side reactions are generally observed than in aqueous solution. 

The acylation of amines on insoluble supports is one of the most thoroughly investigated reactions in solid-phase synthesis. The continuous optimization of peptide bond formation in recent decades has led to protocols that enable racemization-free, quantitative acylations of support-bound peptides with protected amino acids. In recent years, the range of amides available by solid-phase synthesis has expanded significantly to include, for example, N-alkylated peptides and anilides. New strategies for the preparation of amides, such as C-carbamoylations and the Ugi reaction, have also been successfully realized on insoluble supports. 

Trichloroacetimidates are the only type of imino ethers to have found some application in solid-phase synthesis. Trichloroacetimidates can readily be prepared from support-bound alcohols by treatment with trichloroacetonitrile and a base (Entry 6, Table 13.18). Because trichloroacetimidates are good alkylating agents, this reaction offers a convenient alternative for converting support-bound aliphatic alcohols into alkylating agents. Trichloroacetimidates prepared from Wang resin or from hydroxymethyl polystyrene are quite stable and can be stored for several months without decomposition [253],... 

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