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Chapter 27: Q 56. (page 1104)

Question: Use curved arrows to show how E is converted to F by a two-step reaction sequence consisting of a [1,5] sigmatropic rearrangement followed by a [4 + 2] cycloaddition.

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01

[1,5] sigmatropic rearrangement

It is the shift of a group, mainly hydride, alkyl, or aryl, from C1to C5 carbon in a π-system. This shift can take place in a cyclic as well as in an open-chain structure, hence making this rearrangement more useful.

02

[4 + 2] cycloaddition

It is a pericyclic reaction in which one of the reactants contributes 4 π-electrons to the bonding. At the same time, another reactant contributes 2π-electrons to the bonding giving a six-membered ring.

03

Mechanism of the given reaction

The mechanism of the given reaction is shown hereunder.

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Most popular questions from this chapter

Question: Show how the following starting material is converted to the given product by a series of two pericyclic reactions. Account for the observed stereochemistry.

Question: What product is formed from the [5,5] sigmatropic rearrangement of the following unsaturated ether?

Question: Classify each pericyclic reaction as an electrocyclic reaction, cycloaddition, or sigmatropic rearrangement. Indicate whether the stereochemistry is conrotatory, disrotatory, suprafacial, or antarafacial.

Question: a) What is the structure of C, which is formed by oxy-Cope rearrangement of B with NaOEt?

(b) Draw a stepwise mechanism for the conversion of C to the bicyclic alcohol D.

Question: [4 + 2] Cycloadditions with o-quinones such as B are often complex because a variety of products are possible.a. Draw arrows to illustrate how each product is formed when B reacts with styrene, and label the “diene” and “dienophile” components.


b. Draw arrows to illustrate how each product is formed when B reacts with buta-1,3-diene, and label the “diene” and “dienophile” components.


c. o-Quinone C reacts with diene D to form heterocycle E by a process that involves a cycloaddition followed by a [3,3] sigmatropic rearrangement. Use curved arrows to illustrate how this two-step sequence occurs. E is not formed directly from C by a Diels– Alder reaction.
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