Resin cleavage photochemical

Various authors have studied the ageing of triterpenoid resins to understand and possibly slow their deterioration [3, 4, 12, 13, 17 36]. The main degradation pathway is autoxida-tion, an oxidative radical chain reaction [37, 38] after formation of radicals, oxygen from the air is inserted, leading to peroxides. The peroxides can be homolytically cleaved, resulting in new radicals that continue the chain reaction. The cleavage of peroxide bonds can be induced thermally or photochemically. 

Solution-phase chemistry is simpler than solid-phase. Solid-phase synthesis has two extra variables the resin and linker. Unwanted reactions on either can cause major problems. The reactivity of the resin, normally polystyrene, is typically very low and not a major concern. Linkers, however, are designed to be broken, so they must have some level of reactivity. The cleavage conditions must be strictly avoided during all the synthetic steps. Because photochemical reactions are uncommon in mainstream synthesis, photo-chemically labile linkers are attractive in solid-phase synthesis. 

Most of the mass spectrometry applications for combinatorial chemistry will be described in the following sections of this chapter. Here we will give a short overview of MS techniques utilized for the characterization of resin-bound molecules. The majority of publications in this field describe applications of matrix-assisted laser desorption ionization (MALDI), combined with time-of-flight (TOF) detection. The major difference of MS application for analysis of resin-bound molecules from the above-described NMR and IR applications is that analyte should not be covalently bound to solid support prior to mass measurement. Detachment of compound molecules from resin can be done chemically (for example, by bead exposure to TFA vapors) [30,31] or photochemically, such that cleavage, desorption, and ionization of molecules occur simultaneously upon stimulation by laser radiation [32], Since the... 

Depending on the chemical structure of the anchor and chemistry of its attachment to the resin, the product can be cleaved at the end of the synthesis either with acid, base or nucleophilic cleavage reagents, hydrogenolysis, enzymatic, palladium-catalyzed or photochemical, oxidative and reductive cleavage methods. In addition, the safety-catch anchors can be chemically modified at the end of the synthesis to provide a structure which is subsequently cleavable. 

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