Question: Muscalure, the sex pheromone of the common housefly, can be prepared by a reaction sequence that uses two nucleophilic substitutions. Identify compounds A–D in the following synthesis of muscalure.

A nucleophile can be prepared by the abstraction of an acidic base. The nucleophile tends to form a bond with an electrophile by the donation of a lone pair of electrons.

The terminal alkyne has the tendency to act as a nucleophile because it has acidic hydrogen that can be abstracted.

Therefore, a carbanion is generated by the abstraction of hydrogen by a base.

The alkyl halide has electrophilic carbon at the C-X bond; therefore, it acts as an electrophile. The nucleophile attacks at the alkyl halide.

The ethyne is a terminal alkyne and has two acidic hydrogen atoms. The NaH is a strong base; therefore, it abstracts the acidic hydrogen atom of ethyne. This abstraction forms compound A, which is a carbanion.

Now, carbanion A attacks at the alkyl halide (an electrophile) and forms the product B.

The compound B has remained a terminal alkyne which is abstracted by the base NaH. This gives the compound C.

The compound C has a triple bond which is reduced by the hydrogenation and forms the product D.

Mechanism