Approaching a Synthesis Problem - Exercise 112

Exercise 12.12 - Approaching a Synthesis Problem

Introduction

This video tackles Exercise 12.12, which involves performing a multi-step synthesis to transform a given starting molecule into a specified product. The focus is on identifying the structural changes required and applying appropriate reactions to achieve the desired transformation.

Main Points

• Identifying Structural Changes:

- Compare the starting material and the product to identify common structures and changes.

- Note the increase in the number of carbons from five in the starting material to six in the product.

- The presence of an aldehyde in the product suggests ozonolysis.

• Increasing Carbon Chain Length:

- To add carbons, use a terminal alkyne treated with sodium amide to create a nucleophile.

- Decide which molecule will be the terminal alkyne and which will be the alkyl halide based on the location of the reactive part in the product.

• Addition and Elimination Reactions:

- Add a halogen (bromine) using anti-Markovnikov addition of HBr in the presence of peroxides.

- Convert the molecule into an alkyl halide with the halogen on the desired carbon.

• Performing the SN2 Reaction:

- Create a nucleophile from a terminal alkyne using sodium amide.

- Conduct an SN2 reaction between the nucleophile and the alkyl halide to form the new carbon-carbon bond.

• Reducing the Number of Carbons:

- The product has one less carbon than the intermediate, indicating the need for ozonolysis.

- Convert the alkyne to an alkene using either hydrogen gas with Lindlar’s catalyst or dissolving metal reduction (sodium in liquid ammonia).

- Perform ozonolysis on the alkene to cleave the carbon-carbon bond and form the aldehyde.

Examples/Evidence

• Mechanistic Steps:

- Detailed steps for each reaction, including the addition of HBr, formation of the nucleophile, SN2 reaction, reduction of the alkyne to an alkene, and ozonolysis.

- Emphasis on controlling the addition and elimination steps to ensure the correct placement of functional groups.

Conclusion

This synthesis problem demonstrates the importance of planning and executing multi-step transformations in organic chemistry. Key steps include identifying structural changes, using appropriate reagents for addition and elimination reactions, and performing ozonolysis to achieve the desired product.