ABSTRACT
Several groups have developed “safety catch” linkers whereby the linking unit is stable to a variety of acidic and basic conditions. The term “safety catch” arises from the fact that the resins normally must be activated prior to release of the product (so the linker is “safe” until intentionally modified to provide an activated ester equivalent). Kenner and Ellman have reported sulfonamide-based linkers that are stable under most reaction conditions [135-137]. Upon chemical activation, the linker serves as an activating group, and the product can be released by reaction with amines or alcohols to produce amides or esters (Fig. 36). Scialdone and coworkers have published several applications of phosgenated oxime (phoxime) resin (5) [138]. The phoxime resin also serves as a reactive equivalent, requiring thermal rather than chemical activation. This polymer-bound reagent traps amines as support-bound urethanes that are thermally labile and can be cleaved by thermolysis to provide isocyanates. The released isocyanate reacts with amines present in the cleavage solution producing highly substituted ureas (Scheme 6) (see also the previous section on resin-to-resin transfer).
