Following is a retrosynthetic analysis for an intermediate in the industrial synthesis of vitamin \(\mathrm{A}\). (a) Addition of one mole of HCl to isoprene gives 4-chloro-2-methyl-2-butene as the major product. Propose a mechanism for this addition and account its regioselectivity. (b) Propose a synthesis of the vitamin A precursor from this allylic chloride and ethyl acetoacetate.

1. Protonation of the double bond: When HCl reacts with isoprene, the double bond acts as a nucleophile and attacks the proton (H+) from the HCl, forming a carbocation and leaving a negatively charged chloride ion (Cl-). The most stable carbocation is formed at the tertiary carbon, which is the 2-methyl-2-butene structure due to hyperconjugation and induction. \[ \text{Isoprene} + \mathrm{HCl} \rightarrow \text{Carbocation} + \mathrm{Cl}^{\mathrm{-}} \] 2. Attacking by chloride ion: The negatively charged chloride ion (Cl-) then attacks the carbocation at the tertiary carbon, forming the final product 4-chloro-2-methyl-2-butene. \[ \text{Carbocation} + \mathrm{Cl}^{\mathrm{-}} \rightarrow \text{4-chloro-2-methyl-2-butene} \]

The regioselectivity in this reaction can be explained by considering the stability of the carbocations. In general, carbocations that have more substituted carbons (higher degree) are more stable due to the effects of hyperconjugation and the electron-donating inductive effect of the surrounding alkyl groups. In this case, the carbocation forms at the tertiary carbon which leads to a more stable 2-methyl-2-butene structure.

1. Claisen condensation: Perform a Claisen condensation between ethyl acetoacetate and the allylic chloride (4-chloro-2-methyl-2-butene). This would involve the deprotonation of ethyl acetoacetate, followed by the nucleophilic attack of its enolate ion on the allylic chloride (the leaving group will be chloride ion). This results in the formation of a β-ketoester. \[ \text{Ethyl acetoacetate} + \text{4-chloro-2-methyl-2-butene} \rightarrow \text{β-ketoester} + \mathrm{Cl}^{\mathrm{-}} \] 2. Formation of enol: Tautomerize the resulting β-ketoester to its enol form. This involves the proton transfer from the keto group's α-carbon to the carbonyl oxygen. The product will be an enol. 3. Cyclization: The enol product will undergo an intramolecular aldol reaction, which allows the formation of the ring structure that is found in the vitamin A precursor. 4. Dehydration: Remove the hydroxyl group, resulting in the formation of a double bond that leads to the final vitamin A precursor compound. By following these steps, you can successfully synthesize the vitamin A precursor from the allylic chloride and ethyl acetoacetate.