Question

Chrysamehemic acid occurs as a mixture of esters in flowers of the chryxanthemum (pyrethrum) family. Reduction of charysanthemaic acid to its alcohol (Section 17.6A) followed by conversion of the alcohol to its tosylate gives charysaathemyl tosylate. Solvolysis (Section 9.2) of the tosylate gives a mixture of artemesia and yomogi alcobok.

Short answer

Answer: The conversion process involves three steps: 1) Reduction of chrysanthemum acid to its alcohol using a reducing agent like LiAlH4 or NaBH4. 2) Conversion of the alcohol to its tosylate by reacting it with tosyl chloride (TsCl) in the presence of a base like pyridine. 3) Solvolysis of the tosylate, which involves cleavage of the C-O bond, yielding a carbocation and a leaving group, and finally giving a mixture of artemesia and yomogi alcobok.

Step-by-step solution

Reduction of charysanthemaic acid to its alcohol

In order to convert charysanthemaic acid to its corresponding alcohol, we will perform a reduction reaction. This can be done using various reducing agents like lithium aluminum hydride (LiAlH4) or sodium borohydride (NaBH4). These reducing agents will reduce the carboxylic acid group (-COOH) to an alcohol group (-OH).

Conversion of the alcohol to its tosylate

After obtaining the alcohol, the next step is to convert it into its tosylate. This will involve a reaction with tosyl chloride (TsCl) in the presence of a base, such as pyridine. The -OH group of the alcohol will react with the tosyl chloride to form the corresponding tosylate, with the toluene group coming from the tosyl chloride.

Solvolysis of the tosylate

In the final step, we will carry out the solvolysis of the charysaathemyl tosylate. Solvolysis will involve the cleavage of the C-O bond to give a carbocation and a leaving group (the tosyl group). This can be carried out under strong acidic or basic conditions, depending on the desired products. In this case, the solvolysis gives a mixture of artemesia and yomogi alcobok.

Key concepts

Reduction of carboxylic acids

Understanding the reduction of carboxylic acids is fundamental in organic chemistry as it allows the conversion of a carboxylic acid into its corresponding alcohol. This reduction involves the gain of electrons, hence decreasing the oxidation state of carbon in the carboxyl group (-COOH), resulting in the formation of an alcohol group (-OH).

Common reducing agents used for this purpose include lithium aluminum hydride (LiAlH₄), which is a powerful reducing agent, and sodium borohydride (NaBH₄), which is milder and less reactive. The choice of reducing agent depends on the sensitivity of other functional groups present in the molecule. For our charysanthemaic acid example, the reduction might typically proceed with LiAlH₄ due to its effectiveness in reducing carboxylic acids.

During the reduction, the carboxylic acid is protonated and then hydride ions from the reducing agent attack the carbonyl carbon, eventually leading to the breakage of the C=O bond and formation of an alcohol.

Conversion to tosylates

The creation of tosylates is a strategic step in organic transformations. Converting an alcohol to a tosylate enhances its ability to leave a molecule, making it a very good leaving group for further reactions.

To form a tosylate, an alcohol reacts with p-toluenesulfonyl chloride (tosyl chloride, TsCl), in an SN2 reaction where the hydroxyl (-OH) group is substituted by a tosyl group (-OTs). A base like pyridine is often used to absorb the generated hydrochloric acid and to promote the reaction. The tosyl group, which is a sulfonyl functional group derived from toluene, significantly improves the alcohol's capacity as a leaving group, allowing for numerous subsequent reactions, such as nucleophilic substitutions.

The conversion is typically clean and efficient, producing stable tosylates that can be stored and manipulated under a variety of conditions, a characteristic that makes tosylates very useful intermediates in synthesis.

Solvolysis reactions

Solvolysis is a specific type of nucleophilic substitution reaction where the solvent acts as the nucleophile. This process is crucial when we want to induce the cleavage of bonds within reactant molecules. During solvolysis, a compound (usually an alkyl halide or similar substrate like a tosylate) reacts with a solvent, leading to the departure of the leaving group and the formation of a product where the solvent has taken its place.

In the context of charysaathemyl tosylate, solvolysis typically occurs in aqueous or alcohol solutions, causing the tosylate to leave and forming a carbocation intermediate. Subsequent reaction with the solvent gives rise to the final products. The conditions of solvolysis (acidic or basic) can influence the resulting product distribution by stabilizing or destabilizing the intermediates and transition states involved in the mechanism.

Carbocation intermediates

Carbocation intermediates play a pivotal role in many organic reactions, including solvolysis. A carbocation is a positively charged carbon species that is highly reactive due to the carbon atom having only six valence electrons and consequently seeking to complete its octet.

These intermediates are often formed when a leaving group, such as a tosylate, departs from a molecule during a reaction like solvolysis. The stability of carbocations is influenced by several factors, including the degree of alkyl substitution and the presence of electron-releasing or electron-withdrawing groups. Tertiary carbocations are generally more stable than secondary or primary ones due to the effect of hyperconjugation and alkyl group donation of electron density.

In the conversion of charysaathemyl tosylate, once the tosyl group leaves, a carbocation intermediate is formed. The stability of this intermediate and the conditions of the reaction mixture dictate how the carbocation reacts next, leading to different products such as artemesia and yomogi alcobok in our example.