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Chapter 7: Problem 23

Propose a synthesis of each compound starting from acetylene and any necessary organic and inorganic reagents. (a) 4-Octyne (b) 4-Octanone (c) cis-4-0ctene (d) trans-4-Octene (e) 4-Octanol (f) meso-4,5-0ctanediol

Short Answer

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Question: Propose a step-by-step synthesis of each of the following compounds starting from acetylene and any necessary organic and inorganic reagents: (a) 4-Octyne, (b) 4-Octanone, (c) cis-4-Octene, (d) trans-4-Octene, (e) 4-Octanol, and (f) meso-4,5-Octanediol. Provide the reactions and reagents used in each step for the synthesis. Answer: (a) 4-Octyne: 1. Hydroboration-oxidation of acetylene to yield ethanal. 2. Formation of a Grignard reagent with 6-bromo-1-hexene and magnesium in anhydrous ether. 3. Nucleophilic addition of the Grignard reagent to ethanal. (b) 4-Octanone: 1. Synthesize 4-Octyne as per steps in (a). 2. Ozonolysis of 4-octyne using O3 and DMSO. (c) cis-4-Octene: 1. Hydroboration-oxidation of acetylene to yield ethanal. 2. Prepare Wittig reagent from n-butyllithium, triphenylphosphine, and ethylbromide and react with ethanal. (d) trans-4-Octene: 1. Hydroboration-oxidation of acetylene to yield ethanal. 2. Prepare a stabilized ylide from (n-butyloxy)triphenylphosphonium bromide with a strong base and react with ethanal. (e) 4-Octanol: 1. Synthesize 4-Octyne as per steps in (a). 2. Hydroboration-oxidation of 4-octyne (two successive hydroborations followed by oxidation). (f) meso-4,5-Octanediol: 1. Synthesize 4-Octanol as per steps in (e). 2. Perform Swern oxidation on 4-octanol using oxalyl chloride and DMSO followed by the addition of a base to yield 4-octanal. 3. Form a Grignard reagent from 1,2-dibromoethane and magnesium in anhydrous ether. 4. Add the Grignard reagent to 4-octanal to produce 4,5-octanedione. 5. Reduce 4,5-octanedione using NaBH4 to yield meso-4,5-octanediol.

Step by step solution

01

(a) Synthesis of 4-Octyne

1. Hydroboration-Oxidation: Perform a hydroboration-oxidation reaction on acetylene to yield ethanal (acetaldehyde). 2. Grignard Formation: Prepare a Grignard reagent with 6-bromo-1-hexene and magnesium in the presence of anhydrous ether. 3. Nucleophilic Addition: Add the Grignard reagent to ethanal to produce 4-octyne, which will result from nucleophilic addition followed by the intramolecular elimination of magnesium halide.
02

(b) Synthesis of 4-Octanone

1. Synthesize 4-Octyne: Follow the steps given in (a) to synthesize 4-octyne. 2. Ozonolysis: Perform ozonolysis of 4-octyne using ozone (O3) and dimethylsulfoxide (DMSO) to generate the corresponding 4-octanone.
03

(c) Synthesis of cis-4-Octene

1. Hydroboration-Oxidation: Start by performing a hydroboration-oxidation reaction on acetylene to yield ethanal. 2. Wittig Reaction: Create the necessary Wittig reagent by reacting n-butyllithium and triphenyl-phosphine, followed by addition of ethylbromide. React the Wittig reagent with ethanal in a one-to-one ratio to yield cis-4-octene.
04

(d) Synthesis of trans-4-Octene

1. Hydroboration-Oxidation: Perform a hydroboration-oxidation reaction on acetylene to yield ethanal. 2. Wittig Reaction: Create a stabilized ylide by reacting (n-butyloxy)triphenylphosphonium bromide with a strong base. React the stabilized ylide with ethanal in a one-to-one ratio to yield trans-4-Octene.
05

(e) Synthesis of 4-Octanol

1. Synthesize 4-Octyne: Follow the steps given in (a) to synthesize 4-octyne. 2. Hydroboration-Oxidation: Perform a hydroboration-oxidation reaction on 4-octyne, which may require two successive hydroborations followed by oxidation to obtain 4-octanol.
06

(f) Synthesis of meso-4,5-Octanediol

1. Synthesize 4-Octanol: Follow the steps given in (e) to synthesize 4-octanol. 2. Swern Oxidation: Perform a Swern oxidation on 4-octanol using oxalyl chloride and DMSO followed by the addition of a base to yield 4-octanal. 3. Grignard Reaction: React 1,2-dibromoethane with magnesium in anhydrous ether to form the corresponding Grignard reagent. 4. Nucleophilic Addition: Add the Grignard reagent to 4-octanal to produce 4,5-octanedione. 5. Reduction: Reduce 4,5-octanedione using sodium borohydride (NaBH4) to yield the meso-4,5-octanediol.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Hydroboration-Oxidation
The hydroboration-oxidation reaction is a two-step synthesis method used to convert alkynes to aldehydes or ketones. Initially, a hydroboration occurs when borane (BH3) is added across the triple bond of an alkyne, resulting in an organoborane intermediate. This addition occurs with remarkable regioselectivity, where the boron atom attaches to the less substituted carbon, following Markovnikov's rule in reverse.

In the second step, the organoborane intermediate undergoes oxidation, typically with hydrogen peroxide (H2O2) in aqueous sodium hydroxide (NaOH), to form an alcohol. This alcohol can then easily be converted into an aldehyde or ketone. This reaction is particularly valuable for the synthesis of 4-octanol, where hydroboration-oxidation turns 4-octyne into the desired alcohol.
Grignard Reaction
Named after the French chemist Victor Grignard, the Grignard reaction involves the creation of a Grignard reagent, which is then used to form carbon-carbon bonds in the presence of a carbonyl group. A Grignard reagent is a type of organomagnesium compound formed when a haloalkane reacts with magnesium in anhydrous ether.

Once generated, this highly nucleophilic reagent attacks the electrophilic carbon in the carbonyl group of an aldehyde or ketone. This nucleophilic addition adds an alkyl group to the carbonyl carbon, ultimately leading to the formation of a secondary or tertiary alcohol after protonation. In the context of synthesizing 4-octyne, the Grignard reagent derived from 6-bromo-1-hexene reacts with ethanal to form an extended carbon skeleton.
Ozonolysis
Ozonolysis is a powerful organic reaction where a carbon-carbon double or triple bond is cleaved by ozone (O3), leading to two carbonyl compounds. When applied to alkynes, this process can produce carboxylic acids, ketones, or aldehydes depending on the work-up conditions.

The reaction typically proceeds through the formation of an ozonide intermediate which is then reductively or oxidatively cleaved, commonly using a reducing agent like dimethyl sulfide (DMS) or an oxidative agent like hydrogen peroxide. In synthesizing 4-octanone, the ozonolysis of 4-octyne results in the breaking of the triple bond and the formation of the desired ketone.
Wittig Reaction
The Wittig reaction is a popular method for the synthesis of alkenes. It involves the reaction of a phosphonium ylide (generated from a phosphonium salt and a strong base) with a carbonyl compound, such as an aldehyde or a ketone. The phosphorus ylide acts as a nucleophile, attacking the carbonyl carbon and eventually forming a new carbon-carbon double bond.

An attractive feature of the Wittig reaction is its ability to control the geometry of the resulting alkene. Using different reaction conditions or ylides can favor the formation of either the cis or trans isomer. To synthesize cis-4-octene and trans-4-octene from ethanal, the Wittig reaction is employed with specifically designed ylides to achieve the desired stereochemistry.
Swern Oxidation
Swern oxidation is a technique used to convert primary or secondary alcohols to aldehydes or ketones, respectively, with a high level of selectivity and under mild conditions. The process employs a combination of oxalyl chloride and dimethyl sulfoxide (DMSO), followed by the treatment with a base such as triethylamine.

The transformation begins with the activation of DMSO by oxalyl chloride, leading to the formation of an activated sulfoxide species. This intermediate then oxidizes the alcohol to form the corresponding aldehyde or ketone. Swern oxidation is notable for its clean conversion and minimal by-products, proving essential in the synthesis of 4-octanal as a precursor to meso-4,5-octanediol.
Nucleophilic Addition
Nucleophilic addition is a fundamental type of reaction in organic chemistry where a nucleophile donates a pair of electrons to an electrophile to form a new covalent bond. Carbonyl compounds are especially susceptible to nucleophilic addition due to the partial positive charge on the carbon atom.

In this process, the nucleophile adds across the carbon-oxygen double bond, which, after protonation, leads to the formation of an alcohol. For example, in the synthesis of 4-octyne, a Grignard reagent acts as the nucleophile, adding to the carbonyl carbon of ethanal. This step is pivotal for chain elongation in the preparation of organic compounds.
Reduction Reaction
Reduction reactions are characterized by the gain of electrons or the addition of hydrogen to a molecule. In organic synthesis, reduction reactions are commonly used to transform carbonyl groups into alcohols. A widely used reducing agent is sodium borohydride (NaBH4), which can selectively reduce aldehydes and ketones to primary and secondary alcohols, respectively.

In our context, the reduction of 4,5-octanedione with NaBH4 is a critical step in forming meso-4,5-octanediol. This reagent delivers the necessary hydride ions to the carbonyl carbon, completing the conversion with relative ease and excellent selectivity.

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Most popular questions from this chapter

Draw structural formulas for the major product(s) formed by reaction of 3-hexyne with each of these reagents. (Where you predict no reaction, write NR.) (a) \(\mathrm{H}_{2}\) (excess) \(/ \mathrm{Pt}\) (b) \(\mathrm{H}_{2} /\) Lindlar catalyst (c) \(\mathrm{Na}\) in \(\mathrm{NH}_{3}(l)\) (d) \(\mathrm{BH}_{3}\) followed by \(\mathrm{H}_{2} \mathrm{O}_{2} / \mathrm{NaOH}\) (e) \(\mathrm{BH}_{3}\) followed by \(\mathrm{CH}_{3} \mathrm{COOH}\) (f) \(\mathrm{BH}_{3}\) followed by \(\mathrm{CH}_{3} \mathrm{COOD}\) (g) \(\mathrm{Cl}_{2}(1 \mathrm{~mol})\) (h) \(\mathrm{NaNH}_{2}\) in \(\mathrm{NH}_{3}(l)\) (i) \(\mathrm{HBr}(1 \mathrm{~mol})\) (j) \(\mathrm{HBr}\) (2 mol) (k) \(\mathrm{H}_{2} \mathrm{O}\) in \(\mathrm{H}_{2} \mathrm{SO}_{4} / \mathrm{HgSO}_{4}\)

Hydration of 2-pentyne gives a mixture of two ketones, each with the molecular formula \(\mathrm{C}_{5} \mathrm{H}_{10} \mathrm{O}\). Propose structural formulas for these two ketones and for the enol from which each is derived.

Draw a structural formula for an alkene and dichloroalkane with the given molecular formula that yields the indicated alkyne by each reaction sequence.

Alkyne anions react with the carbonyl groups of aldehydes and ketones to form alkynyl alcohols, as illustrated by the following sequence.

Rimantadine was among the first antiviral drugs to be licensed in the United States for use against the influenza \(A\) virus and in treating established illnesses. It is synthesized from adamantane by the following sequence (we discuss the chemistry of Step 1 in Chapter 8 and the chemistry of Step 5 in Section \(16.8 \mathrm{~A}\) ). Rimantidine is thought to exert its antiviral effect by blocking a late stage in the assembly of the virus. (a) Propose a mechanism for Step 2. Hint: As we shall see in Section 21.1A, reaction of a bromoalkane such as 1-bromoadamantane with aluminum bromide (a Lewis acid, Section 4.7) results in the formation of a carbocation and \(\mathrm{AlBr}_{4}{ }^{-}\). Assume that adamantyl cation is formed in Step 2 and proceed from there to describe a mechanism. (b) Account for the regioselectivity of carbon-carbon bond formation in Step 2 . (c) Describe experimental conditions to bring about Step \(S\). (d) Describe experimental conditions to bring about Step \(4 .\)
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