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Chapter 19: Problem 16

Propose two syntheses of 4-phenyl-2-pentanone, each involving conjugate addition of a lithium diorganocopper reagent.

Short Answer

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Question: Provide two different syntheses of 4-phenyl-2-pentanone using conjugate addition of a lithium diorganocopper reagent. Answer: 1) In the first synthesis, the conjugate addition reaction between 4-phenyl-3-penten-2-one and 1-lithium ethyl copper (EtCuLi) can be performed. This reaction results in the formation of 4-phenyl-2-pentanone. 2) In the second synthesis, 4-phenyl-3-penten-2-one with a different protecting group can be used as the starting material. The conjugate addition reaction between this modified starting material and 1-lithium propyl copper (PrCuLi) can be performed. After the reaction is complete and the protecting group is removed, the desired product, 4-phenyl-2-pentanone, can be obtained again.

Step by step solution

01

Synthesis 1: Identifying the starting materials and lithium diorganocopper reagent

In this first synthesis, we will use the conjugate addition of a lithium diorganocopper reagent, which will result in the addition of a new carbon-carbon bond to the starting material. We need to identify the starting materials which will give us the desired product when reacted with the lithium diorganocopper reagent. For the starting material, we could use an α,β-unsaturated ketone, like 4-phenyl-3-penten-2-one. For the lithium diorganocopper reagent, we could use 1-lithium ethyl copper (EtCuLi) which will be added to the β-position during conjugate addition.
02

Synthesis 1: Conjugate addition reaction

To complete the synthesis, we perform the conjugate addition reaction between the starting material (4-phenyl-3-penten-2-one) and the lithium diorganocopper reagent (EtCuLi). The reagent adds in a 1,4-fashion to the starting material, resulting in the formation of a new carbon-carbon bond. On completing the reaction, we obtain the desired product, 4-phenyl-2-pentanone.
03

Synthesis 2: Identifying the starting materials and lithium diorganocopper reagent

In this second synthesis, we will use a different lithium diorganocopper reagent for the conjugate addition reaction. For the starting material, we could still use an α,β-unsaturated ketone, like 4-phenyl-3-penten-2-one, as we did in synthesis 1. For the lithium diorganocopper reagent, we could use 1-lithium propyl copper (PrCuLi), and use a different protecting group on the carbonyl group of 4-phenyl-3-penten-2-one to differentiate the two synthesis.
04

Synthesis 2: Conjugate addition reaction

Perform the conjugate addition reaction between the starting material (4-phenyl-3-penten-2-one with a different protecting group) and the lithium diorganocopper reagent (PrCuLi). The reagent adds in a 1,4-fashion to the starting material, resulting in the formation of a new carbon-carbon bond. After the reaction is complete, we can remove the carbonyl protecting group to obtain the desired product, 4-phenyl-2-pentanone once again. In both syntheses, 4-phenyl-2-pentanone is produced by conjugate addition of different lithium diorganocopper reagents with the same starting material, 4-phenyl-3-penten-2-one. The key difference between these two syntheses is the choice of lithium diorganocopper reagent used for the conjugate addition reaction.

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