Bis heteroannulation. 4. Facile syntheses of methylene acids, methylbutenolides, .alpha.-methyl-.gamma.-lactones, and related materials. Total syntheses of (.+-.)-ligularone and (.+-.)-petasalbine
Bis heteroannulation. 4. Facile syntheses of methylene acids, methylbutenolides, .alpha.-methyl-.gamma.-lactones, and related materials. Total syntheses of (.+-.)-ligularone and (.+-.)-petasalbine