4-Aryl-5-tosyloxyhexanoates are converted to mixtures of...

4-Aryl-5-tosyloxyhexanoates are converted to mixtures of lactones when exposed to silica or heated with $p$-toluenesulfonic acid in various solvents. The aryl ring must have an EWG for the reaction to proceed. A similar reaction occurs with 4-aryl-5-tosyloxypentanoates, but in this case only $\gamma$-lactones are formed. Suggest a mechanism that accounts for both the observed regioselectivity and stereoselectivity and the requirement for an ERG on the aryl ring.

Key Concepts

Lactone Ring Formation

Lactones are cyclic esters formed by the condensation (intramolecular esterification) of hydroxy acids or related intermediates. The formation of lactones in these reactions involves the formation of a new ester linkage through a nucleophilic attack on a protonated ester carbon (or equivalent electrophilic center) facilitated by acid catalysis. The process can involve a cascade of steps including activation of the leaving group, formation of a cationic intermediate, and subsequent ring closure, all influenced by chain length and electron withdrawing effects of substituents.

Regioselectivity and Stereoselectivity

Regioselectivity refers to the preferential formation of one constitutional isomer over others in a chemical reaction, while stereoselectivity concerns the formation of a specific stereoisomer. In the given lactonization reactions, the nature of the chain length and the substituents, including the EWG on the aryl ring, impact which carbon atoms interact during the cyclization process and the orientation in which bonds form. This influences whether ?- or ?-lactones form, as well as the specific stereochemistry of the product, through differences in transition state stability and steric factors.

Electron-Withdrawing Group (EWG) Effects

An electron-withdrawing group is a substituent that pulls electron density away from other parts of a molecule. In these reactions, the presence of an EWG on the aryl ring is necessary because it modulates the electron density and stability of intermediates formed during the reaction. The EWG stabilizes the developing positive charge in the transition state, thereby facilitating the cyclization reaction and influencing both the regioselectivity and stereoselectivity of the final lactone products.

Intramolecular Cyclization

This concept involves the reaction of a molecule with itself, where a nucleophilic and an electrophilic center within the same molecule interact to form a cyclic structure. In the context of lactone formation, an internal nucleophile (such as a carboxylate oxygen or a carbonyl group) attacks an electrophilic carbon center, leading to ring closure. The intramolecular nature of the reaction often results in higher reaction rates and selectivities compared to intermolecular processes.

Acid Catalysis

Acid catalysis refers to the acceleration of reaction rates through the use of acidic conditions. In the mechanism proposed for lactone formation, an acid such as p-toluenesulfonic acid protonates functional groups to activate them toward nucleophilic attack, facilitate the departure of leaving groups like a tosylate, and lower the energy barrier for cyclization. This step is crucial for generating intermediates that lead to ring closure.

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