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Chapter 27: Q 44. (page 1102)

Question: What type of sigmatropic rearrangement is illustrated in each reaction?

Short Answer

Expert verified

Answer

a. 1,5sigmatropic shift

b.2,3sigmatropic shift

Step by step solution

01

Sigmatropic shift

  • A sigmatropic rearrangement is an intramolecular pericyclic reaction.
  • The reactant’s σ bond is broken, and theπbonds rearrange to form a new bond in the product.
  • In a sigmatropic rearrangement, the number of π bonds is constant in the reactant and product
02

 [1,5] and [2,3] sigmatropic shifts

The 1,5sigmatropic shift is associated with the rearrangement of the group at the first and fifth positions.

The 2,3sigmatropic shift is associated with the rearrangement of the group at the second and third positions.

03

Explanation

a. In the given conversion, the groups at the first position rearrange themselves at the fifth position. Therefore, it is a 1,5sigmatropic shift.

b. In the given conversion, the groups at the second position rearrange themselves at the third position. Therefore, it is a 2,3 sigmatropic shift.

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Most popular questions from this chapter

Question: For each molecular orbital in Figure 27.2, count the number of bonding interactions (interactions between adjacent orbitals of similar phase) and the number of nodes. (a) How do these two values compare for a bonding molecular orbital? (b) How do these two values compare for an antibonding molecular orbital?

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: The endiandric acids comprise a group of unsaturated carboxylic acids isolated from a tree that grows in the rain forests of eastern Australia. The methyl esters of endiandric acids D and E have been prepared from polyene Y by a series of two successive electrocyclic reactions: thermal ring closure of the conjugated tetraene followed by ring closure of the resulting conjugated triene. (a) Draw the structures (including stereochemistry) of the methyl esters of endiandric acids D and E. (b) The methyl ester of endiandric acid E undergoes an intramolecular [4 + 2] cycloaddition to form the methyl ester of endiandric acid A. Propose a possible structure for endiandric acid A.

Question: Draw the product formed when diene M undergoes disrotatory cyclization. Indicate the stereochemistry at new hybridized carbons. Will the reaction occur under thermal or photochemical conditions?

Question: Classify each reaction as an electrocyclic reaction, a cycloaddition, or a sigmatropic rearrangement. Label the σ bonds that are broken or formed in each reaction.
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