Polymer bound Burgess reagent

Scheme 7.94 Synthesis of 1,3,4-oxadiazoles utilizing a polymer-bound Burgess reagent.

A polymer-bound Burgess reagent has also been developed. Aside from the mUd, neutral cyclization conditions, this reagent also offers the advantage of a clean reaction with little epimerization and an easy work-up. Examples are listed in Table 8.9. 

A constant interest in the development of new rapid methodologies for the preparation of oxazole hbraries is motivated by their presence in numerous biologically active natural products. Janda and coworkers were hrst to show that oxazoles can be obtained by microwave-assisted treatment of polymer-bound a-acylamino-/f-ketoesters with Burgess reagent [68]. Hydroxybutyl-functionalized /anda/el resin was used for this investigation, with key steps being monitored by on-bead FT-IR. First, a resin-bound acetoacetate was pre-... 

Wipf has also developed a polyethyleneglycol-linked Burgess reagent (35). The major advantages for this polymer bound reagent are improved ease of handling and greater yields in the synthesis of labile oxazolines (36) and thiazolines (37)."... 

In a different approach, microwave-mediated oxazole synthesis utilizing ji-ketoesters bound to a novel polymeric resin has been described [57]. The desired polymer support was prepared by transesterification reactions between tert-butyl j5-ketoesters and hydroxybutyl functionalized Janda/el resin, and subsequent standard diazo transfer. The resulting a-diazo j5-ketoesters have been used for synthesis of an array of oxazoles (Scheme 16.34). Because of the thermal sensitivity of the Burgess reagent used, the temperature was kept rather low, but irradiation for 15 min at 100 °C furnished satisfactory results [57]. Cleavage from the solid support was achieved by diversity-introducing amidation this led to the corresponding oxazole amides in reasonable yields. 

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