Chapter 26.SE, Problem 27MP

Interpretation Introduction

a)

Interpretation:

The first step of the reaction shown involves the nucleophilic substitution of the S atom on the methionine side chain with BrCN to give cyanosulfonium ion [R₂SCN]⁺. The structure of the product formed is to be given and a mechanism for its formation is to be proposed.

Concept introduction:

S atom has two unshared pair of electrons. The nucleophilic attack of an unshared pair of electron present in S on the C of BrCN will displace the bromide ion to yield cyanosulfonium ion. It is a nucleophilic substitution reaction.

To give:

The structure of the product formed in the first step of the reaction shown which involves the nucleophilic substitution of the S atom on the methionine side chain with BrCN to give cyanosulfonium ion [R₂SCN]⁺ along with the mechanism of its formation.

Interpretation Introduction

b)

Interpretation:

The second step of the reaction shown is an internal S_N² reaction, in which the carbonyl oxygen of methionine residue displaces the positively charged sulfur group to form a five membered ring product. The structure of the product formed is to be given and a mechanism for its formation is to be proposed.

Concept introduction:

The internal nucleophilc attack of the unshared pair of electrons on the sulfonium ion can lead to the formation of a five membered ring product.

To give:

The structure of the product formed in the second step of the reaction shown which is an internal S_N² reaction, in which the carbonyl oxygen of methionine residue displaces the positively charged sulfur group to form a five membered ring product and a mechanism for its formation.

Interpretation Introduction

c)

Interpretation:

The third step of the reaction given is a hydrolysis reaction to split the peptide chain to yield a lactone. The structure of the lactone ring formed is to be given and a mechanism for its formation is to be proposed.

Concept introduction:

The imine derivative (CH=NH) when hydrolyzed with aqueous acids breaks to give a carbonyl compound and an amino derivative as products.

To give:

The structure of the lactone ring formed during the hydrolysis of the peptide chain and a mechanism for its formation.

Interpretation Introduction

d)

Interpretation:

The final step of the reaction given is a hydrolysis of the lactone to give the product. A mechanism for this hydrolysis reaction is to be proposed.

Concept introduction:

Lactones upon hydrolysis give hydroxy acids as products.

To propose:

A mechanism for the hydrolysis of the lactone to give the product.