Ch. 14 - Ethers, Epoxides, and Thioethers

Wade9th EditionOrganic ChemistryISBN: 9780135213728Not the one you use?Change textbook

Wade 9th Edition

Ch. 14 - Ethers, Epoxides, and Thioethers

Problem 53

Chapter 14, Problem 53

Under the right conditions, the following acid-catalyzed double cyclization proceeds in remarkably good yields. Propose a mechanism. Does this reaction resemble a biological process you have seen?

Verified step by step guidance

Step 1: Protonation of the carbonyl group - The acid catalyst (H⁺) protonates the oxygen atom of the ketone group, increasing the electrophilicity of the carbonyl carbon. This makes the carbonyl carbon more susceptible to nucleophilic attack.

Step 2: Formation of the first ring - The double bond in the molecule acts as a nucleophile and attacks the protonated carbonyl carbon, leading to the formation of a new C-C bond and the first cyclization. This step generates a carbocation intermediate.

Step 3: Rearrangement and second cyclization - The carbocation formed in the previous step undergoes a rearrangement, and another double bond in the molecule acts as a nucleophile to attack the carbocation, forming the second ring. This results in a bicyclic intermediate.

Step 4: Deprotonation and stabilization - The intermediate formed undergoes deprotonation to stabilize the structure, leading to the formation of the final bicyclic compound with an alcohol group.

Step 5: Biological resemblance - This reaction resembles biological processes such as terpene biosynthesis, where carbocation intermediates and cyclizations are key steps in forming complex cyclic structures.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Acid-Catalyzed Reactions

Acid-catalyzed reactions involve the use of an acid to increase the rate of a chemical reaction. In these reactions, the acid donates a proton (H+) to a reactant, enhancing its electrophilicity and facilitating bond formation. This mechanism is crucial in organic chemistry, particularly in reactions like esterification and cyclization, where the formation of cyclic structures is promoted by protonation of functional groups.

Cyclization Mechanisms

Cyclization mechanisms refer to the processes by which linear or branched molecules form cyclic structures. This can occur through various pathways, including nucleophilic attack, electrophilic addition, or radical processes. Understanding the specific steps involved in cyclization, such as the formation of new bonds and the rearrangement of electrons, is essential for predicting the products and yields of these reactions.

Biological Analogies in Organic Reactions

Many organic reactions can be likened to biological processes, particularly those catalyzed by enzymes. Enzymes often facilitate cyclization and other transformations in a highly efficient and selective manner, mimicking acid-catalyzed reactions in the lab. Recognizing these parallels can provide insights into reaction mechanisms and help in understanding how nature achieves complex transformations under mild conditions.

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Related Practice

Textbook Question

Propylene oxide is a chiral molecule. Hydrolysis of propylene oxide gives propylene glycol, another chiral molecule.

(a) Draw the enantiomers of propylene oxide.

(b) Propose a mechanism for the acid-catalyzed hydrolysis of pure (R)-propylene oxide.

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Textbook Question

One of the crowning achievements of natural products synthesis was Bryostatin 1, published by Professor Gary Keck (University of Utah; Journal of the American Chemical Society, 2011, 133, 744–747). The Bryostatins are a family of compounds isolated from aquatic invertebrates known as Bryozoans. The compounds are of interest for a variety of biological effects, including anti-cancer activity and reversing brain damage in rodents.

(d) How many chiral centers are in this molecule?

(e) Using the number of chiral centers you reported in part (d), calculate the number of stereoisomers possible at these chiral centers. (Ignore stereoisomers at double bonds.)

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Textbook Question

An acid-catalyzed reaction was carried out using methyl cellosolve (2-methoxyethanol) as the solvent. When the 2-methoxyethanol was redistilled, a higher-boiling fraction (bp 162°C) was also recovered. The mass spectrum of this fraction showed the molecular weight to be 134. The IR and NMR spectra are shown here. Determine the structure of this compound, and propose a mechanism for its formation.

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Textbook Question

A compound of molecular formula C₈H₈O gives the IR and NMR spectra shown here. Propose a structure, and show how it is consistent with the observed absorptions.

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Textbook Question

In 2012, a group led by Professor Masayuki Satake of the University of Tokyo reported the isolation and structure determination of a toxin from a marine algal bloom that decimated the fish population off the New Zealand coast in 1998. Extensive mass spectrometry and NMR experiments ultimately led to the structure shown below, named Brevisulcenal-F. (See Journal of the American Chemical Society, 2012, 134, 4963–4968.) This structure holds the record for the largest number of fused rings, at 17.

(a) How many ether groups are present?

(b) How many alcohol groups are present? Classify the alcohols as 1° or 2° or 3°.