Draw the neutral organic starting material



Draw the neutral organic starting material.

Concepts and reason
The product of a certain reaction is given and the reactant that is the starting material needs to be given. The reagent used is bromine in dichloromethane. Therefore, the starting material can be alkene which undergoes bromination followed by certain transformation to give the product.

Retrosynthesis: It is a method that is used to obtain the starting material from the target molecule.
Thus, it involves moving backward from the products to the reagents.
It is represented by the arrow image
Retrosynthesis involves disconnection of bonds to obtain the charged species (synthons). The synthons are unstable and hence, their synthetic equivalents are used to perform the function of a synthon.
Target molecule: it is the one whose synthesis needs to be carried out.

Disconnect the bond between the carbon and the oxygen as shown below:
In the above disconnection, the functional groups or the reactive sites are located in the target molecule. The functional group is ether and thus, the site adjacent to the oxygen is disconnected. The disconnection takes place from the side shown because it results in the formation of a more stable synthon where the carbocation formed is secondary.

Further, disconnect the bond between the carbon and bromine in the synthon formed as shown below:
In the above disconnection, further another reactive site present is the alkyl halide bond present between the carbon and the bromine. Hence, the alkyl halide bond is disconnected to get the starting material A. Therefore, the starting material is pent-4-enol.

Explanation for the reaction is as follows:
The synthesis of the target molecule from the starting material takes place as follows:

Firstly, the bromination of double bond takes place to form a bromonium ion intermediate. Then, the oxygen of the alcoholic group attacks from the back side resulting in ring closure. Finally, the removal of proton from the alcoholic oxygen gives the target molecule.


The neutral starting material is as follows: