Unit 5: Reactions of Synthetic Importance

Semester 4

BP401T

Introduction to Reactions of Synthetic Importance

Named reactions form the toolbox of the synthetic medicinal chemist. This unit covers highly specific advanced chemical reactions used to construct complex drug molecules. It includes methods for reducing carbonyl groups, dissolving metal reductions for aromatic rings, specific oxidations, and skeletal rearrangements that move carbon atoms to form new C-N or C-C layouts.

Syllabus & Topics

1Metal hydride reduction (NaBH4 and LiAlH4).
2Clemmensen reduction and Wolff-Kishner reduction.
3Birch reduction and its regioselectivity.
4Oppenauer oxidation.
5Dakin reaction.
6Beckmanns rearrangement and Schmidt rearrangement.
7Claisen-Schmidt condensation.

Learning Objectives

Compare the reagents, mechanisms, and specificities of major reduction and oxidation techniques.

Predict the products of Birch reduction based on existing ring substituents.

Draw the detailed step-by-step mechanism of the Beckmann and Schmidt rearrangements.

Identify which synthetic reaction is appropriate for a given transformation during drug synthesis.

Frequently Asked Questions (FAQs)

Q1. What is the Difference Between Clemmensen and Wolff–Kishner Reduction?

Both reactions reduce aldehydes and ketones to alkanes (–C=O to –CH₂–). Clemmensen reduction uses zinc amalgam (Zn(Hg)) in concentrated HCl under strongly acidic conditions and is unsuitable for acid-sensitive compounds. Wolff–Kishner reduction uses hydrazine (NH₂NH₂) with a strong base such as KOH under heat, proceeding under basic conditions and is unsuitable for base-sensitive compounds.

Q2. What is the Birch Reduction?

Birch reduction converts aromatic rings such as Benzene into 1,4-cyclohexadienes. It employs an alkali metal (sodium or lithium) in liquid ammonia along with an alcohol. In substituted aromatic rings, electron-donating groups lead to double bonds at the substituted carbons, whereas electron-withdrawing groups lead to double bonds adjacent to the substituent.

Q3. Explain the Beckmann Rearrangement.

The Beckmann rearrangement is an acid-catalyzed transformation of an oxime into an amide. It is stereospecific, and the group that migrates is the one positioned anti (trans) to the leaving hydroxyl group on the nitrogen atom. This reaction is an important step in the industrial synthesis of Caprolactam for nylon production.

Q4. What is the Oppenauer Oxidation?

Oppenauer oxidation is a mild and selective method for converting secondary alcohols into ketones using aluminum isopropoxide in excess acetone. It is the reverse reaction of the Meerwein–Ponndorf–Verley (MPV) reduction.