Polymer-supported Burgess reagent

Cyclodehydrations with PEG-bound Burgess reagents proceeded in high yields (76-98%), with little ( 2%) epimerisation and with fewer side-products. Furthermore, 55 is easily prepared on a large scale and shows an increased stability compared to the standard reagent 54. 

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In the preparation of novel 1,3,4-oxadiazoles 56 from 1,2-diacylhydrazines 55, Brain and coworkers [29] used a highly efficient cydodehydration assisted by use of micro-waves, in THF as solvent, using polymer-supported Burgess reagent (Scheme 8.21). 

Synthesis of 1,3,4-oxadiazoles using polymer-supported Burgess reagent... 

Brain, C.T., Paul, J.M., Loong, Y. and Oakley, P.J., Novel procedure for the synthesis of 1,3,4-oxadiazoles from 1,2-diacylhydrazines using polymer-supported Burgess reagent under microwave conditions, Tetrahedron Lett., 1999, 40, 3275. 

Preparation of the reagent [70] A solution of PEG monomethyl ether 89 (MW = 750 5.88 g, 7.8 mmol) in benzene (20 mL) was dried azeotropically for 24 h in an apparatus fitted with a Dean-Stark trap and subsequently added dropwise to a solution of chlorosulfonyl isocyanate (88) (1.10 g, 7.8 mmol) in dry benzene (20 mL). The mixture was stirred at room temperature for 1 h, then concentrated to dryness. A solution of this residue in benzene (35 mL) was added dropwise to a solution of triethylamine (2.5 mL, 17.3 mmol) in benzene (15 mL). The mixture was stirred for 30 min at room temperature, then filtered, and the solid was dried to yield polymer-supported Burgess reagent 91 (6.2 g, 82%). 

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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