Scheme 19 Thermal retro-ene reaction of an ethyl alkynyl ether as convenient ketene source for
lactone formation in the total synthesis of (+)-acutiphycin, 84
Scheme 20 Macrolactonization via intramolecular concerted addition of oxygen nucleophiles to
acylketenes
Synthesis of 12- to 16-Membered-Ring Lactones
387