Solid phase technique

Memfield s concept of a solid phase method for peptide synthesis and his devel opment of methods for carrying it out set the stage for an entirely new way to do chem ical reactions Solid phase synthesis has been extended to include numerous other classes of compounds and has helped spawn a whole new field called combinatorial chemistry Combinatorial synthesis allows a chemist using solid phase techniques to prepare hun dreds of related compounds (called libraries) at a time It is one of the most active areas of organic synthesis especially m the pharmaceutical industry... 

The details of the solid-phase technique have been improved substantially over the years, but the fundamental idea remains the same. The most commonly used resins at present are either the Wang resin or the PAM (phenyl-acetamidomethyl) resin, and the most commonly used N-protecting group is the fluorenylmethyloxycarbonyl, or Fmoc group, rather than Boc. 

S.P. solid-phase technique L.P. liquid-phase technique OPTcp pentachlorophenyl ester TEEP tetraethyl pyrophosphit OTCp trichlorophenyl ester ONp p-nitrophenyl ester ONSu N-hydroxysuccinimido ester OPFp pentafluorophenyl ester OQu 8-hydroxyquinyl ester OPy 3-hydroxy-pyridyl ester ODnp 2,4-dinitrophenyl ester DCC dicyclohexylcarbodiimide HOBn 3-hydroxy-4-oxo-3,4-dihydro-l,2,3-benzotriazin Opi JV-hy-droxypiperidine EEDQ 2-ethoxy-l-ethoxycarbonyl-l,2-dihydroquinoline Tos p-toluenesulfonyl PTC propanetricarboxylic acid OBu tm-butyl ester Nva norvaline Aha aminohexanoic acid Om ornithine... 

Difficulties due to side reactions (cyclization) and a broad molecular weight distribution accompanying the polycondensation of active esters led to the application of methods wherein the polymers are built up stepwise. In 1968, Sakakibara et al.31) introduced the solid-phase technique using Merrifield s resin. By stepwise addition of tert-pentyloxycar-bonyl tripeptides, they have synthesized (Pro-Pro-Gly)n with n = 5, 10, 15 and 20. 

In the case of (Ala-Gly-Pro)n with n = 5-15, the tripeptide chains were synthesized by the liquid-solid phase technique. As mentioned above, the coupling of longer preformed peptide chains was difficult and the yield of the trimer was low. Therefore, a liquid-solid phase technique was applied in which a trimer was grown in a stepwise manner, beginning from a trifunctional crosslinked base A. 

In addition to the insoluble polymers described above, soluble polymers, such as non-cross-linked PS and PEG have proven useful for synthetic applications. However, since synthesis on soluble supports is more difficult to automate, these polymers are not used as extensively as insoluble beads. Soluble polymers offer most of the advantages of both homogeneous-phase chemistry (lack of diffusion phenomena and easy monitoring) and solid-phase techniques (use of excess reagents and ease of isolation and purification of products). Separation of the functionalized matrix is achieved by either precipitation (solvent or heat), membrane filtration, or size-exclusion chromatography [98,99]. 

PEG polymers are widely used as water soluble supports [99]. Although these polymers suffer from easy loss of PEG oligomers, they are frequently used for the preparation of small organic molecules [100-105] and biopolymers [106,107]. The main benefit of PEG supports is their solubility in water as well as most organic solvents. Also, as opposed to most solid-phase techniques, PEG polymers allow for easy on-bead NMR monitoring. Soluble PEG supports have been used frequently in synthetic microwave chemistry protocols [108-122]. 

Although the solid-phase technique was first developed for the synthesis of peptide chains and has seen considerable use for this purpose, it has also been used to synthesize chains of polysaccharides and polynucleotides in the latter case, solid-phase synthesis has almost completely replaced synthesis in solution. The technique has been applied less often to reactions in which only two molecules are brought together (nonrepetitive syntheses), but many examples have been reported. 

Solution and solid phase techniques have each been used... 

Peptide synthesis was amenable to solid-phase techniques since the process was repetitive. The C-terminal amino acid is attached to polymeric surface and the peptide chain is assembled via a two-step process coupling of the incoming amino acid that has the alpha-amino group protected... 

The use of solid-phase techniques has especially increased in the last decades in the case of fungal-mediated bioprocesses, since they reproduce the natural habitat and growth conditions for filamentous fungi. Moreover, solid-phase bioreactors... 

The solid-phase technique of split and mix synthesis relies on the efficiency of mixture-based synthesis to provide very large libraries (millions) of discrete compounds (Figure 4).[161 In this approach, each resin bead is treated with a single building block for each synthesis step. Thus any single resin bead possesses identical copies of one library member, but the identity of the library member on any bead is lost due to the mix step of the process. Elegant strategies have been developed to chemically encode the syn-... 

In general, solid-phase synthesis, rather than solution-phase synthesis, can be the preferred method for the generation of combinatorial libraries because of the greater abihty to automate a solid-phase protocol, primarily due to the use of excess reagents in solution to effect cleaner reactions and to the ease of workup by simple filtration. The solid-phase method of peptide synthesis has had many notable successes. However, the preparation of peptides containing more than 20 amino acids in length using the solid-phase technique often causes major problems in that very extensive purification of the final product is needed. 

Some other aspects of nitrile oxide chemistry that will grow in importance in the future involve the use of high pressure (> 10 kbar) to enforce hitherto slow or not feasible cycloadditions. Of particular relevance for nitrile oxides will be to find substantial improvements for practical, cost-efficient, large-scale reactions (351). Solid-phase techniques, adaptation for combinatorial chemistry, or improved versions for dehydrating nitroalkanes will all play an increasingly important role. 

One other synthetic approach that does not utilize adjacent functional groups in the precursor has been reported. Pyrrole 234 has been constructed in such a way that a cyclization to 235 is the preferred course of the reaction (Equation 83). This represents the lone approach envisioned in this series <2003JOC6984>. A related synthesis using solid-phase techniques has been described <2005JC0977>. 

In preparing these various libraries, extensive use is made of solid phase synthetic methods. These methods are all derived from the solid phase peptide synthesis (SPPS) method developed by Merrifield in 1963. When performing a large number of syntheses, it is preferable to perform the synthetic steps on a solid bead rather than completing the entire synthesis in the solution phase. The solid-phase technique makes byproduct removal and final compound purification easier. The organic chemistry literature contains a wealth of different types of solid-phase supports and novel linkers for attaching the synthetic substrate to the bead. 

FIGURE 10.2 An outline of dipeptide synthesis by the solid phase technique... 

Phenylacetylene chemistry allows construction of shape-persistent molecular architectures through structural control and monomer diversity.27 Combination of this method with solid-phase techniques enhances the rate at which compounds can be made, especially because automation is possible. Oligomer libraries may also be accessible using this type of chemistry. 

Despite the obstacles discussed above, solid-phase syntheses of oligomers do have two significant advantages over solution techniques. First, purifications in solid-phase techniques are easier and faster than in solution and are more likely to be amenable to automation. Second, hetero-oli-gomerizations are relatively easy to arrange if one component is anchored to a solid phase. Overall, the advantages of solid-phase syntheses of oligomers, as well as other products, can be appreciable for reproducible, reliable, and easily monitored reaction schemes. 

Solid-phase techniques can be applied to the N-activated esters.16 In the activated ester method, 4-nitrophenyl, 12 21-27 2,4-dinitrophenyl, 614 28 2,4,5-trichlorophenyl, 29 and penta-fluorophenyl 8 esters can be used, the amide bonds being formed with aminolytic release of the corresponding phenol (Scheme 4). For example, the use of pentafluorophenyl esters in the synthesis of pure azapeptides (named azatides for the first time) proved to be advantageous. 8 ... 

The solid-phase technique has been applied in this field with the amino component linked to a polymer/7,9 To meet the requirements of combinatorial chemistry the coupling conditions of resin-bound Asp with carbazic acid chloride of Fmoc-AzSar-OH and Fmoc-AzAla-OH were individually optimized. Remarkably, the carbazic acid chloride of Fmoc-AzSar-OH proved to be much more reactive than the activated Fmoc-AzAla-OH/9 ... 

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