Thermal azole based Claisen rearrangements.

by Nannapaneni, Dharma; Dormi, Silvana; Rivero, Juliette; McIntosh, Matt

In 2007 Waetzig and Tunge reported the Pd-catalyzed conversion of azole-substituted allylic esters to provide 2-butenyl benzothiazoles. The reaction appeared to proceed via Claisen rearrangement of N-allyl-N,S-ketene acetals. These intruiging results prompted us to explore an uncatalyzed, thermal version of the rearrangement, which had not been previously reported. Direct alkylation of benzothiazole with allylic bromides to give the benzothiazolium salts was followed by their conversion to N,S-ketene acetals using tetramethylguanidine (TMG). Thermal rearrangement at ca. 190 °C provided the 2-butenyl benzothiazoles in good yield. The weak base used to form the ketene acetal afforded broad functional group tolerance. Details of the chem. and scope and limitations of the reactions will be discussed.