Pd-Catalyzed Cyclization of Propargylamides to Oxazoles

Significance

Studies towards the tandem cyclization of N-propargylic 1-allyl-2-indolecarbox­amides in the presence of an oxidizing agent led, instead of the expected tetracyclic heterocycle, to an unexpected oxazole (R = 1-allylindole) in 62% yield. The result prompted a more detailed study of this reaction varying oxidants and catalysts. Two sets of conditions (A and B) were found to be superior to others, for example, the use of MnO₂ as oxidant completely inhibited the reaction. Thus, a set of aromatic and heteroaromatic propargylic amides were tested and afforded the oxazole ­carbaldehydes in modest yield. Achievement of higher yields was not possible due to a competing decomposition of the amides at high temperatures.