Question

Question: Identify the lettered compounds in the following reaction schemes. Each reaction sequence was used in the synthesis of a natural product.

a.

b.

Short answer

Answer

a.

b.

Step-by-step solution

Step-by-Step solutionStep 1: Reaction with a phosphorous halide (Forming A)

Alcohol reacts with phosphorous halides to give the corresponding halogen alkanes by replacement reaction.

The alcohol reacts with phosphorous tribromide to give the corresponding alkyl bromide.

Reaction with phosphorous halide

Reaction with methyllithium (Forming B)

The alkyl alkyne ether would undergo deprotonation to form an acetylide anion in the presence of the strong base methyllithium $\left({{\mathrm{CH}}_3}^{-}{\mathrm{Li}}^{+}\right)$. The acetylide anion abstracts the cation to form lithium substituted alkyl alkyne ether.

Reaction with methyl lithium

Dehalogenation (Forming C)

The carbon-bromine bond breaks in the presence of the strong base alkyl lithium. The acetylide anion would attack electrophilic carbon to give the required natural product.

Formation of the natural product

Nucleophilic substitution reaction (Involving D)

The acetylide anion would open the epoxide by an ${\mathrm{S}}_{\mathrm{N}}2$reaction forming a new carbon-carbon bond that undergoes hydration to form the corresponding alcohol.

Ring-opening reaction of an epoxide

Tosylation reaction (Involving E)

The alkyl alcohol reacts with tosyl chloride to give the corresponding alkyl tosylate through a nucleophilic substitution reaction.

Tosylation reaction

Alkylation reaction (Involving F)

The alcohol is deprotonated by the strong base sodium hydride (NaH) that undergo alkylation in the presence of ethyl iodide to form the corresponding ether.

Alkylation reaction

Methylation reaction (involving G)

The sodium hydride would deprotonate the ether to form the acetylide anion that would undergo alkylation by methyl iodide to give the required natural product.

Formation of the natural product