Propose a mechanism for each of the following reactions:

a.

b.

Reactions in which a new ring structure is introduced into the reactant are called cyclization reactions.

Cyclization reactions generally occur at elevated temperatures.

A cyclization reaction is favored only if the ring structure formed in the reaction is stable. Therefore, reactions involving the formation of a five-membered or six-membered ring are most favored. If the reaction produces a three-membered ring, the backward reaction will be favored at equilibrium.

Reactions in which a single molecule undergoes reactions within itself to change the connectivity of functional groups are called rearrangement reactions. In these reactions, there is no change in the molecular formula of the compound, but the structural formula changes.

A rearrangement reaction occurs based on the stability of a compound. Compounds undergo rearrangement to attain the most stable or the lowest energy structure. The electronegativities of functional groups also influence the rearrangement in compounds.

a. The unsaturated alpha carbon of the cyclic ether attacks the electrophilic halogen-containing carbon removing the halide. This makes the alpha carbon electrophilic, and the hydroxyl group attacks this carbon forming a heterocyclic five-carbon ring.

Deprotonation by a base gives the final product. The mechanism of the reaction can be given as:

Mechanism for reaction (a)

b. The proton abstraction by the base makes the oxygen negatively charged and attacks the alpha carbon of the sulfonyl group. This produces a five-membered heterocyclic ring.

Rearrangement within the molecule gives the final stable product. The mechanism can be given as follows:

Mechanism for reaction (b)