Bis Heteroannulation. 4. Facile Syntheses of Methylene Acids, Methylbutenolides, α-Methyl-γ-lactones, and Related Materials. Total Syntheses of (+/-)-Ligularone and (+/-)-Petasalbine

Article abstract of DOI:10.1021/ja00331a031

Acetylenic oxazoles of proper design undergo and intramolecular Diels-Alder reaction leading directly to fused ring furan derivatives ("bis heteroannulation").With 5-ethoxyoxazoles the corresponding 2-ethoxyfurans are obtained, and these latter materials are excellent precursors for methylene esters, methylene acids, methylbutenolides, α-methyl-γ-lactones, and β-methylfurans.In similar fashion, acetylenic oxazoles unsubstituted in the 2-position have been utilized for highly efficient syntheses of (+/-)-ligularone and (+/-)-petasalbine.