The construction of the BCD-ring system of C19-diterpene alkaloids was initiated by development of a ring expansion reaction of cyclic enamino-esters with propiolic acid esters, leading from the 5-membered ring ketone 3 to the 7-membered ring ketone 10. Epoxidation and stereospecific reductive epoxide ring opening to give hydroxy ester 18 were subsequent key-steps which eventually furnished cyclic β-keto ester 22. This versatile intermediate ...