Identify the major product formed in the following reaction sequence:

Step-by-step solution

The given reactant is an enol lactone, specifically a 4-oxa-steroid derivative with a double bond between C5 and C6. The Wittig reagent, methylenetriphenylphosphorane ( H₂C=P(Ph)₃ ), attacks the carbonyl carbon (C3) of the enol lactone. Instead of undergoing the standard elimination to form an enol ether, the tetrahedral intermediate undergoes ring opening. The enolate leaving group is relatively stable, leading to the formation of a ring-opened intermediate containing a ketone at C5 and a phosphonium salt at C3: -