A mechanism to account for the given reaction, which scrambles the isotopic labeling, is to be drawn. Concept introduction: The Diels–Alder reaction joins a conjugated diene and a dienophile (either an alkene or an alkyne ) via the formation of two new σ bonds. The product is a six-membered ring of carbon atoms. The Diels–Alder reaction is concerted that is all bonds that are formed and broken do so simultaneously. It requires the cyclic movement of electrons, so it is classified as a pericyclic reaction. Under normal conditions, Diels–Alder reactions tend to be irreversible and proceed under kinetic control. At high temperatures, though, some can undergo the reverse reaction — This is called as a retro Diels–Alder reaction or [ 4 + 2 ] cyclo elimination. The mechanism for this reaction is simply the reverse of a Diels–Alder mechanism, so the entire reaction consists of a single (concerted) step in which six electrons move in a cyclic fashion.

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Chapter 24, Problem 24.53P

Interpretation Introduction

Interpretation:

A mechanism to account for the given reaction, which scrambles the isotopic labeling, is to be drawn.

Concept introduction:

The Diels–Alder reaction joins a conjugated diene and a dienophile (either an alkene or an alkyne) via the formation of two new σ bonds. The product is a six-membered ring of carbon atoms. The Diels–Alder reaction is concerted that is all bonds that are formed and broken do so simultaneously. It requires the cyclic movement of electrons, so it is classified as a pericyclic reaction. Under normal conditions, Diels–Alder reactions tend to be irreversible and proceed under kinetic control. At high temperatures, though, some can undergo the reverse reaction — This is called as a retro Diels–Alder reaction or [4 + 2] cyclo elimination. The mechanism for this reaction is simply the reverse of a Diels–Alder mechanism, so the entire reaction consists of a single (concerted) step in which six electrons move in a cyclic fashion.

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