Solution-phase synthesis chemistry assessment

The tedious purification of intermediates in classical solution synthesis produces pure compounds, which are then carried forward to the subsequent steps of the synthetic scheme. When a side product of the SP reaction remains attached to the resin, it becomes impossible to separate it from the desired intermediate/target molecule, thus irreversibly affecting the quality of the synthesis and the purity of the final product. This and other factors, which will be discussed in the following chapters, are considered during the so-called chemistry assessment phase of the synthesis in which the... 

The next phase for SP is the validation of the selected route in solution. It is useful to check the outcome of the reaction on several compounds to uncover potential weaknesses. This step is obviously redundant in the solution phase, because it corresponds to the following chemical assessment step, even though the synthesis of a few more fully characterized discretes may be desirable (Fig. 8.7, top). Typically, the assessment of the chemistry in solution takes several weeks to check a few representative monomers with different reactivities in order to discard unsuitable monomer... 

The optimized reaction conditions to make the same library in solution or on SP differ significantly in most examples because of the influence exerted by the heterogeneous support (see Chapter 1), which may lead to different purities and yields of the final compounds. When the chemistry assessment, either in solution or on solid phase, does not give satisfactory results after having exploited all the reasonable experimental conditions, the other phase should at least be considered before abandoning the library synthesis. 

The synthetic scheme used to prepare the library is shown in Fig. 8.10. The reaction steps, amide coupling, ozonization, reductive amination, and catalytic reduction, are trivial for carbohydrate substrates, and the authors decided that assessment of the chemistry for the library synthesis would not have been necessary. The availability of 8.19 in multigram quantities reduced the significance of potentially low-yielding steps. The rehearsal of the monomers was also avoided because of the small size of the two monomer sets Mi (four Fmoc-protected a-amino acids, Fig. 8.10) and M2 (six amines. Fig. 8.10), which were inspired by a model for the interaction between paromomycin and RNA (55). Finally, such a small array could be considered as a model library for a much larger solution-phase library of potential RNA binding molecules. 

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