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Chapter 14: Q 39 P (page 747)

Question. The following reaction resembles the acid-catalyzed cyclization of squalene oxide. Propose a mechanism for this reaction.

Short Answer

Expert verified

Epoxide ring is highly strained due to which it readily opens up so as to release strain, thus oxygen in epoxide ring abstracts acidic proton from acid, and oxygen acquires positive charge due to which strain on ring further increases, hence it opens up and alkene carbon which is electron rich acts as a nucleophile and is present in same system thus intramolecularly attacks at carbocation generated.

Abstraction of proton and attack of nucleophile at carbocation

Step by step solution

01

Step-1. Abstraction of acidic hydrogen and ring opening of squalene oxide:

Epoxide ring is highly strained due to which it readily opens up so as to release strain, thus oxygen in epoxide ring abstracts acidic proton from acid, and oxygen acquires positive charge due to which strain on ring further increases, hence it opens up and alkene carbon which is electron rich acts as a nucleophile and is present in same system thus intramolecularly attacks at carbocation generated.

Abstraction of proton and attack of nucleophile at carbocation

02

Step-2. Attack of water as a nucleophile at carbocation and formation of product:

Water acts as a nucleophile and attacks at carbocation center, then after neutralization of positive charge on oxygen, we get our required product, that is, 3-hydroxymethyl-1-methyl-cyclopentan-1-ol.

Attack of water as a nucleophile and formation of final product

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Most popular questions from this chapter

Show how you would convert 2-bromocyclohexanol to the following diol. You may use any additional reagents you need.

(a) When ethylene oxide is treated with anhydrous HBr gas, the major product is 1,2-dibromoethane. When ethylene oxide is treated with concentrated aqueous HBr, the major product is ethylene glycol. Use mechanisms to explain these results.

(b) Under base-catalyzed conditions, several molecules of propylene oxide can react to give short polymers. Propose a mechanism for the base-catalyzed formation of the following trimer.

Question. The 2001 Nobel Prize in Chemistry was awarded to three organic chemists who have developed methods for catalytic asymmetric synthesis. An asymmetric (or enantioselective) synthesis is one that converts an achiral starting material into mostly one enantiomer of a chiral product. K. Barry Sharpless (The Scripps Research Institute) developed an asymmetric epoxidation of allylic alcohols that gives excellent chemical yields and greater than 90% enantiomeric excess.

The Sharpless epoxidation uses tert-butyl hydroperoxide, titanium(IV) isopropoxide, and a dialkyl tartarate ester as the reagents. The following epoxidation of geraniol is typical.

  1. Which of these reagents is most likely to be the actual oxidizing agent? That is, which reagent is reduced in the reaction? What is the likely function of the other reagents?
  2. When achiral reagents react to give a chiral product, that product is normally formed as a racemic mixture of enantiomers. How can the Sharpless epoxidation give just one nearly pure enantiomer of the product?
  3. Draw the other enantiomer of the product. What reagents would you use if you wanted to epoxidize geraniol to give this other enantiomer?

(a) Tetramethyloxirane is too hindered to undergo nucleophilic substitution by the hindered alkoxide, potassium tert-butoxide. Instead, the product is the allylic alcohol shown. Propose a mechanism to explain this reaction. What type of mechanism does it follow?

(b) Under mild acid catalysis, 1,1-diphenyloxirane undergoes a smooth conversion to diphenylethanal (diphenylacetaldehyde). Propose a mechanism for this reaction. (Hint: Think Pinacol.)

Show how you would use a protecting group to convert 4-bromobutan-1-ol to hept-5-yn-1-ol.
See all solutions

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