Β-Vetivone is isolated from vetiver, a perennial grass that...

$\beta$-Vetivone is isolated from vetiver, a perennial grass that yields a variety of compounds used in traditional eastern medicine, pest control, and fragrance. In one synthesis, ketone $\textbf{A}$ is converted to $\beta$-vetivone by a two-step process: Michael reaction, followed by intramolecular aldol reaction. (a) What Michael acceptor is needed for the conjugate addition? (See Problem 23.63 for another method to form the bicyclic ring system of $\beta$-vetivone.) (b) Draw a stepwise mechanism for the aldol reaction, which forms the six-membered ring.

$\beta$-Vetivone is isolated from vetiver, a perennial grass that yields a variety of compounds used in traditional eastern medicine, pest control, and fragrance. In one synthesis, ketone $\textbf{A}$ is converted to $\beta$-vetivone by a two-step process: Michael reaction, followed by intramolecular aldol reaction. (a) What Michael acceptor is needed for the conjugate addition? (See Problem 23.63
for another method to form the bicyclic ring system of $\beta$-vetivone.) (b) Draw a stepwise mechanism for the aldol reaction, which forms the six-membered ring.

Key Concepts

Enolate Chemistry

Enolate chemistry focuses on the generation and reactivity of enolate ions, which are formed by deprotonating the alpha position of carbonyl compounds. The resulting enolate is a resonance-stabilized anion that serves as a good nucleophile in carbon–carbon bond forming reactions, such as the aldol reaction. Understanding enolate formation and its behavior under various conditions is crucial for controlling both the regiochemistry and stereochemistry during synthetic transformations.

Intramolecular Aldol Reaction

An intramolecular aldol reaction involves the formation of a new ring system by creating a bond between a nucleophilic enolate and an electrophilic carbonyl group within the same molecule. This process usually begins with the formation of an enolate ion (via deprotonation adjacent to a carbonyl), which then attacks another carbonyl group present in the same molecule. The resulting aldol adduct can undergo dehydration, leading to the formation of a conjugated system and the closure of a ring, which is a highly effective strategy for synthesizing cyclic compounds.

Michael Reaction Mechanism

The Michael reaction is a type of conjugate addition where a nucleophile adds to the beta carbon of an ?,?-unsaturated carbonyl compound. This reaction is pivotal in carbon–carbon bond formation and often involves the formation of a stabilized enolate or other nucleophilic species attacking an electrophilic, conjugated alkene system. By attacking at the ?-position, the overall mechanism allows the construction of complex molecular architectures in a controlled manner.

Michael Acceptor

A Michael acceptor is an electron-deficient alkene, typically conjugated to a carbonyl or another electron-withdrawing group which activates the double bond towards nucleophilic attack. This activation makes it a suitable partner in conjugate addition reactions, allowing nucleophiles to add selectively at the ? position, which is crucial for forming new carbon–carbon bonds in synthetic organic chemistry.

Transcript

00:03 This is the answer to chapter 24, problem number 45 from the smith organic chemistry textbook.

00:11 And this problem talks about beta -vetivone.

00:17 And so, okay, so we're asked what michael acceptor is needed for the conjugate addition.

00:24 And then we're also asked to draw a stepwise mechanism for the aldol reaction that is the second step in this synthesis.

00:32 Okay, and so this is molecule a, i suppose i should label it.

00:39 And so we're asked in part a of this problem, so let me label that as well.

00:44 We're asking part a what michael exceptor is needed to make the starting material for the aldol reaction.

00:54 And so that michael acceptor is going to look like this.

01:07 And so remember a michael acceptor.

01:10 Is going to be an alpha -beta unsaturated carbonyl compound.

01:15 And so that will be like that.

01:20 And when we do the microreaction between this molecule that i just drew and molecule a, we do get this starting material that we need for the aldol step of this synthesis.

01:34 So next we're asked to draw a stepwise mechanism for that aldol step.

01:40 And so that is going to look like this.

01:42 So the first thing that's going to happen is we have some hydroxide ion here.

01:49 And i just drew a couple structures ahead of time just to try to cut down on the time here.

01:54 So that hydroxide ion is going to grab an alpha proton from the starting material.

02:03 And so that is going to get us to this enolate.

02:09 And then the enolate is going to attack the other carbonyl in this molecule and do the aldol.

02:21 Okay.

02:21 And so that second step there is going to be the one that forms our new six -membered ring.

02:28 So bear with me while i draw this.

02:31 That is terrible looking.

02:33 Let's try that again.

02:35 Better.

02:41 Okay.

02:41 So nothing over here on the left side has changed.

02:43 And now on the right, what we have is this newly formed six -membered ring.

02:55 And so our methyl group will be here.

02:59 Let me move this down...

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