Question

In order for the following transformation to occur thermally, what pericyclic reactions must occur?

A.1. Sigmatropic rearrangement, 2. Electrocyclic ring- closing
B.1. Electrocyclic ring- closing, 2. Diels- Alder
C.1. Sigmatropic rearrangement, 2. Diels- Alder
D.1. Electrocyclic ring- opening, 2. Electrocyclic ring- closing

Answer 1

The reaction in which the net result is the conversion of one pi bond in the sigma bond and vice versa is known as the Electrocyclic reaction and it is a pericyclic type of rearrangement. Two types of such reactions are ring opening and ring closing reactions.

Complete step by step answer:
Ring opening reactions are those reactions which can proceed by both the SN1 and SN2 reaction mechanism based on the situations or the reaction conditions. These reactions are catalysed with the help of acids as well. The ring closing reaction is the reaction in which one or more rings are introduced into a molecule.
The Diels- Alder reaction is the chemical reaction between the conjugated diene and a substituted alkene and is regarded as dienophile. The conjugation is the phenomenon in which the compound exists in alternative single and double bonds. The mechanism of the reaction is concerted in nature.
In the reaction mechanism of the above reaction the Electrocyclic ring opening takes place first in the presence of heat followed by the Diels Alder reaction(4+2 cycloaddition reaction) leading to the formation of the diene formed in the first step of ring opening in the reaction. The mechanism of the given reaction is shown below:

Thus the option B. 1. Electrocyclic ring- closing, 2. Diels- Alder is the correct answer.

Note:
The reaction in which one sigma bond is changed to another sigma bond without the presence of the catalyst is known as the Sigmatropic reaction, it is a type of pericyclic reaction. These reactions are very useful for the conversions within the products.