1. Chapter 23: Ester EnolatesChapter 23: Ester Enolates
and theand the
Claisen CondensationClaisen CondensationRecall theRecall the
aldolaldol
addition:addition:
Can we doCan we do
this withthis with
esterester
enolates?enolates?
2. The Claisen CondensationThe Claisen Condensation
Driven byDriven by deprotonationdeprotonation of 3-oxoalkanoate (“of 3-oxoalkanoate (“ββ–ketoester”).–ketoester”).
Step 1:Step 1: Ester Enolate FormationEster Enolate Formation
Endothermic as writtenEndothermic as written
Mechanism: Addition-EliminationMechanism: Addition-Elimination
Has to be the same or transesterification will occur
AcidicAcidic
4. Step 4:Step 4: Deprotonation of 3-OxoesterDeprotonation of 3-Oxoester
Step 5:Step 5: Protonation on Aqueous Work-upProtonation on Aqueous Work-up
ClaisenClaisen
This step drives the reaction to the product as the anion.This step drives the reaction to the product as the anion.
5. Products of Claisen condensation areProducts of Claisen condensation are alkyl 3-alkyl 3-
oxoalkanoatesoxoalkanoates (IUPAC), common name(IUPAC), common name ββ-keto--keto-
esters and, generally,esters and, generally, ββ-dicarbonyl-dicarbonyl compoundscompounds
The hydrogens between the two carbonyls areThe hydrogens between the two carbonyls are
unusuallyunusually acidicacidic: Due to inductive effect of: Due to inductive effect of
two carbonyls and resonance in the anion.two carbonyls and resonance in the anion.
6. Low pLow pKKaas mean that alkoxide or hydroxide can make the enolatess mean that alkoxide or hydroxide can make the enolates stoichiometricallystoichiometrically!!
7. 1,3-Dicarbonyl Compounds Form1,3-Dicarbonyl Compounds Form
EnolsEnols ReadilyReadily
O O
H
O O
H
65%35%
100°C
H Bonding in enol is symmetrical:H Bonding in enol is symmetrical:
oror ??
J. Am. Chem. Soc.
2006, 128, 854.
8. Acidity of Claisen product is essential to driveAcidity of Claisen product is essential to drive
the reaction. Without the acidic H, reactionthe reaction. Without the acidic H, reaction
goes in the reverse!goes in the reverse!
No goNo go, starting on the left., starting on the left. Goes from right to leftGoes from right to left, when starting with the, when starting with the
product (made by a different route, as will be seen later).product (made by a different route, as will be seen later).
MechanismMechanism of reverse Claisen condensation:of reverse Claisen condensation:
9. But:But: If one partner hasIf one partner has nono αα--hydrogenshydrogens (not(not
enolizable):enolizable):
Crossed Claisen CondensationCrossed Claisen Condensation
GivesGives mixturesmixtures of products (as in the caseof products (as in the case
of the crossed aldol addition).of the crossed aldol addition).
H O O
O O
+
1. Na+ -OCH2CH3,
CH3CH2OH
2. H+, H2O
H O
O O
80%
Ethyl 3-oxopropanoate
10. Mixed Ketone Claisen CondensationMixed Ketone Claisen Condensation
KetonesKetones areare more acidicmore acidic than esters, functionthan esters, function
asas enolateenolate partners. Their competitive aldolpartners. Their competitive aldol
additions are reversible, but Claisen is not.additions are reversible, but Claisen is not.
pKa = 19pKa = 25
This is general forThis is general for
R
O O
11. Intramolecular Claisen CondensationIntramolecular Claisen Condensation
((Dieckmann CondensationDieckmann Condensation))
Ethyl 2-oxocyclohexanecarboxylate
Mixed Claisen-Dieckmann CondensationMixed Claisen-Dieckmann Condensation
O
O
80%
O
O
O O
O O
1. Na+ -OCH2CH3,
CH3CH2OH
2. H+, H2O
O O
OO
O O
12. Use ofUse of ββ--Dicarbonyl CompoundsDicarbonyl Compounds
O O
acidicacidic
Derived enolates are relatively non-basic,Derived enolates are relatively non-basic,
but good nucleophiles:but good nucleophiles:
O O O O
1. Base
2. RX
RH
Particularly useful forParticularly useful for oxoestersoxoesters derived fromderived from
Claisen condensations, because hydrolysis leadsClaisen condensations, because hydrolysis leads
toto oxoacidsoxoacids whichwhich decarboxylatedecarboxylate..
16. Alkylation of ethyl 3-oxobutanoate (ethyl
acetoacetate), followed by ester
hydrolysis, and finally decarboxylation
gives 3-substituted or 3,3-disubstituted
methyl ketones. This is called the
acetoacetic ester synthesis.
Advantage over direct ketone enolate alkylation:Advantage over direct ketone enolate alkylation:
MuchMuch less basic enolateless basic enolate; obviates E2 for R; obviates E2 for Rsecsec..
18. Starting from diethyl propanedioate (malonic
ester) the same sequence (alkylation,
hydrolysis, decarboxylation) gives 2-alkylated
and 2,2-dialkylated acetic acids, a method
called the malonic ester synthesis.
19. Michael AdditionMichael Addition
Reaction of 3-oxo ester enolates withReaction of 3-oxo ester enolates with αα,,ββ––
unsaturated carbonyls (1,4-addition)unsaturated carbonyls (1,4-addition)
1,4-Addition is the thermodynamic process,1,4-Addition is the thermodynamic process,
1,2-addition does occur, but is reversible.1,2-addition does occur, but is reversible.
20. Robinson AnnulationRobinson Annulation
70%70%
Recall:Recall: Robinson annulationRobinson annulation is a sequence ofis a sequence of MichaelMichael
additionaddition, followed by, followed by intramolecular aldol condensationintramolecular aldol condensation
Goes via:Goes via:
Michael adductMichael adduct
21. αα-Hydroxycarbonyls-Hydroxycarbonyls
Important function in nature and in drugs. HowImportant function in nature and in drugs. How
do we make it? Ideal retrosynthesis is:do we make it? Ideal retrosynthesis is:
Alkanoyl anionAlkanoyl anion
Problem: We cannot make alkanoyl anions from aldehydes,Problem: We cannot make alkanoyl anions from aldehydes,
because bases will either deprotonate alpha to makebecause bases will either deprotonate alpha to make
enolate or add to carbonyl function as nucleophiles.enolate or add to carbonyl function as nucleophiles.
Therefore, we need to useTherefore, we need to use masked alkanoyl anions.masked alkanoyl anions.
R
OHO
R
O O
:
22. 1.1. StoichiometricStoichiometric reagents:reagents: 1,3-Dithia-1,3-Dithia-
cyclohexanecyclohexane (1,3-dithiane) anions(1,3-dithiane) anions
Dithiane is acidic because of theDithiane is acidic because of the polarizabilitypolarizability
of sulfur, stabilizes adjacent charge.of sulfur, stabilizes adjacent charge.
24. Other alkylating agents: AnotherOther alkylating agents: Another
aldehyde/ketone synthesisaldehyde/ketone synthesis
HR
O HS SH, ZnCl2
S S
R
1.
2.
Li
1. R'X
2. HgCl2
R'R
O
1.
2. HgCl2
O
R
O
OH
:
25. 2.2. CatalyticCatalytic version: Alkanal couplingversion: Alkanal coupling
usingusing thiazolium ion catalystthiazolium ion catalyst
Crucial for catalysis is:Crucial for catalysis is:
Six electrons:
Carbene
27. Drawback of the catalytic version: OnlyDrawback of the catalytic version: Only symmetricalsymmetrical coupling.coupling.
In nature: Thiamine (vitamin B1)In nature: Thiamine (vitamin B1)
A = H, ThiamineA = H, Thiamine
A = , thiamine pyrophosphateA = , thiamine pyrophosphate
Editor's Notes
Beri-beri
"A certain very troublesome affliction, which attacks men, is called by the inhabitants Beri-beri (which means sheep). I believe those, whom this same disease attacks, with their knees shaking and legs raised up, walk like sheep. It is a kind of paralysis, or rather Tremor: for it penetrates the motion and sensation of the hands and feet indeed sometimes the whole body..." Jacobus Bonitus, Java, 1630
Thiamine deficiency usually causes weight loss, cardiac abnormalities, and neuromuscular disorders. The classic thiamine deficiency syndrome in humans is beri-beri (sometimes called Kakke). Thiamine is abundant in whole grains, usually in the scutellum (the thin covering of the starchy interior endosperm), but is scarce in the endosperm. Unfortunately beri-beri is still common in parts of southeast Asia where polished rice is a staple and thiamine enrichment programs are not fully in place. Beri-beri is characterized by anorexia (loss of appetite) with subsequent weight loss, enlargement of the heart, and neuromuscular symptoms such as paresthesia (spontaneous sensations, such as itching, burning, etc.), muscle weakness, lassitude (weariness, general weakness), and foot and wrist droop. There are three main types of beri-beri: (1) dry (also neuritic, paraplegic, and pernicious) beri-beri; (2) wet (also edematous or cardiac) beri-beri; (3) and infantile (also acute) beri-beri. Dry beri-beri usually inflicts older adults and affects mainly the peripheral nerves with little cardiac involvement. It is characterized by atrophy (wasting away) and peripheral neuritis (inflammation of nerves) of the legs and paraplegia (paralysis of the lower extremities). In contrast wet beri-beri displays substantial cardiac involvement especially tachycardia (rapid heart beat) in addition to peripheral neuropathy. Edema progresses from the feet upwards to the heart causing congestive heart failure in severe cases. Infantile beri-beri is usually seen in breast-feeding infants whose mothers are thiamine deficient (but not necessarily showing signs of beri-beri). These infants are usually anoretic and often have trouble keeping the milk down. Once the disease begins it moves rapidly causing heart failure in a matter of hours.
Wernicke-Korsakoff Syndrome
Wernicke-Korsakoff Syndrome or Wernicke's encephalopathy is the thiamine deficient disease seen most often in the Western hemisphere. It mainly affects alcoholics due to three reasons: (1) the diets of alcoholics are usually poor; (2) diets rich in carbohydrates (e.g., alcohol or rice) increase the metabolic demands of thiamine; and (3) alcohol inhibits intestinal ATPase which is involved in the uptake of thiamine. Two observations suggest a genetic invovlement with Wernicke-Korsakoff Syndrome: (1) it is much higher in among Europeans than non-Europeans; and (2) transketolase (see above under " Thiamine in the Pentose Phosphate Pathway") from Wernicke-Korsakoff Syndrome patients binds TPP 10 time less strongly than normal transketolase. The symptoms of Wernicke-Korsakoff syndrome include confusion, sixth nerve damage resulting in ophthalmoplegia (paralysis of an eye motor nerve) and nystagmus (rhythmical oscillation of the eyes), psychosis, confabulation, and impaired retentive memory and cognitive function. In severe cases the patient may slip into a coma. A congenital defect in transketolase which causes a low binding affinity for TPP increases the chances of acquiring Wernicke-Korsakoff syndrome.