(a) (i) In the presence of sodium ethoxide, the following transformation occurs. Explain : (ii) Propose a suitable mechanism for…

(a)

(i) Transformation of 2-methyl-2-carbethoxycyclopentanone The reaction proceeds via a retro-Dieckmann/Dieckmann sequence rather than a standard Claisen condensation. The starting material possesses a quaternary α-carbon bearing the ester group, which lacks an acidic proton. Consequently, the base (NaOEt) cannot form an enolate at this position. Instead, the ethoxide ion attacks the ketone carbonyl, forming a tetrahedral intermediate. This intermediate undergoes a retro-Dieckmann ring-opening, cleaving the C–C bond to yield an open-chain diethyl 2-methylhexanedioate. The newly revealed methylene group (CH₂) adjacent to the ester is acidic; deprotonation by ethoxide generates an enolate. This enolate then undergoes an intramolecular Dieckmann cyclization by attacking the other ester carbonyl. The resulting β-keto ester product possesses an acidic α-hydrogen. The formation of the stabilized enolate of this product drives the equilibrium toward the cyclized form. Finally, the addition of HCl reprotonates the enolate to yield the neutral 2-methyl-5-carbethoxycyclopentanone (ethyl 3-methyl-2-oxocyclopentanecarboxylate).

(ii) Mechanism for the cyclisation of the epoxy-diene The transformation is a cationic cyclization initiated by acid. The epoxide oxygen is protonated by H₂SO₄, increasing the electrophilicity of the ring. The epoxide ring opens at the more substituted carbon to generate a stable tertiary carbocation. The pendant alkene then attacks this cation intramolecularly in a Markovnikov fashion, forming the first six-membered ring and a new secondary carbocation. A second intramolecular cyclization occurs as the remaining alkene attacks this new cation, closing the second ring to form a bicyclic cation. Water acts as a nucleophile to trap the final carbocation, yielding the alcohol. The reaction proceeds through chair-like transition states, which favor the formation of a trans-fused decalin system due to steric minimization.

(b)

(i) Hydration of 3,3-dimethyl-1-butene (1) The reaction does not yield the product shown because the initially formed carbocation is unstable. Protonation of the alkene at C1 generates a secondary carbocation at C2. This cation is adjacent to a quaternary carbon (C3). A 1,2-methyl shift (Wagner–Meerwein rearrangement) occurs, moving a methyl group from C3 to C2. This rearrangement converts the secondary cation into a more stable tertiary cation at C3. Water then attacks this tertiary cation, and subsequent deprotonation yields 2,3-dimethylbutan-2-ol as the major product. (2) To synthesize 3,3-dimethyl-2-butanol (the unrearranged product), one must use oxymercuration–demercuration. The reagents are 1) Hg(OAc)₂, H₂O and 2) NaBH₄. In this mechanism, the alkene reacts with the mercuric ion to form a bridged mercurinium ion. Water attacks the more substituted carbon of the bridge. Because the intermediate is a bridged ion rather than a free carbocation, 1,2-shifts (rearrangements) cannot occur. The final reduction with NaBH₄ replaces the mercury group with hydrogen, yielding the unrearranged alcohol.

(ii) Styrene + BrCCl₃, hν The reaction is a radical chain addition known as the Kharasch reaction.

1. Initiation: Light (hν) causes homolysis of BrCCl₃ to form ·CCl₃ and Br· radicals.
2. Propagation: The ·CCl₃ radical adds to the terminal CH₂ of the styrene. This regioselectivity is driven by the formation of a benzylic radical intermediate, which is stabilized by resonance with the aromatic ring.
3. Propagation (second step): The benzylic radical abstracts a bromine atom from another molecule of BrCCl₃ to form the product and regenerate the ·CCl₃ radical.

The major product is PhCH(Br)CH₂CCl₃.

(c)

(i) 2-(3-oxopentyl)cyclohexanone + KOH, then HA This is an intramolecular aldol condensation. The base (KOH) deprotonates the α-carbon of the ethyl ketone side chain. The resulting enolate attacks the ketone carbonyl of the cyclohexanone ring. This forms a six-membered ring (the new ring) fused to the original ring. The initial aldol product (a β-hydroxy ketone) undergoes dehydration under the reaction conditions (or upon heating with acid) to form the conjugated enone. Product: 1-methyl-4,4a,5,6,7,8-hexahydronaphthalen-2(3H)-one.

(ii) Quinuclidine derivative + H⁺ This is a pinacol (or semipinacol) rearrangement. Protonation of the hydroxyl group on the diphenyl-substituted carbon leads to the loss of water, generating a carbocation at that position. This cation is highly stabilized by the two phenyl rings. To relieve steric strain and form a stable carbonyl, a C–C bond of the bicyclic amine framework migrates to the cationic center. This migration expands the ring size and converts the adjacent hydroxyl group into a ketone. Product: A ring-expanded bicyclic ketone: the C–C bond of the bicyclic amine framework migrates to the diphenylcarbinyl cation, one ring grows by one carbon atom and the carbinol-bearing ring carbon becomes a ketone; the two phenyl groups remain attached at the migration terminus.

(iii) Benzosuberone + NH₂OH, then Al₂O₃ This is a Beckmann rearrangement. First, the ketone reacts with hydroxylamine (NH₂OH) to form an oxime. The oxime is then heated over alumina (Al₂O₃). The group anti to the oxime hydroxyl group migrates to the nitrogen atom, accompanied by the loss of water. In this cyclic ketone, the migration of the ring carbon expands the seven-membered ring to an eight-membered ring. Product: 3,4,5,6-tetrahydro-1-benzazocin-2(1H)-one (a benzo-fused eight-membered lactam formed by aryl migration).

(iv) Tetraester + EtO⁻ This is a Dieckmann condensation. The substrate contains multiple ester groups. The central carbon is fully substituted (quaternary) and has no acidic protons. However, the terminal methylene group (CH₂) adjacent to an ester is acidic. Ethoxide deprotonates this CH₂ to form an enolate. This enolate attacks the ester carbonyl on the other end of the chain. The resulting intermediate expels an ethoxide ion to form a cyclic β-keto ester. Product: A cyclopentanone ring bearing a β-keto triester functionality.