text: A total synthesis of progesterone was reported in 1971 by W.S. Johnson. The synthesis begins with reacting the phosphonium salt  with phenyl lithium to produce the phosphonium ylide . The ylide  is reacted with an aldehyde to produce the alkene . The ketal protecting groups of  are hydrolyzed to produce the diketone , which in turn is cyclized to form the cyclopentenone . The ketone of  is reacted with methyl lithium to yield the tertiary alcohol , which in turn is treated with acid to produce the tertiary cation . The key step of the synthesis is the π-cation cyclization of  in which the B-, C-, and D-rings of the steroid are simultaneously formed to produce . This step resembles the cationic cyclization reaction used in the biosynthesis of steroids and hence is referred to as biomimetic. In the next step the enol orthoester is hydrolyzed to produce the ketone . The cyclopentene A-ring is then opened by oxidizing with ozone to produce . Finally, the diketone  undergoes an intramolecular aldol condensation by treating with aqueous potassium hydroxide to produce progesterone.