15.07.04 · orgchem / carbonyl

Carbonyl condensation reactions: Knoevenagel, Michael addition, and Robinson annulation

stub3 tiersLean: nonepending prereqs

Anchor (Master): Carey & Sundberg — Advanced Organic Chemistry, Part B, Ch. 1–2

Intuition Beginner

Carbonyl compounds with a carbon-carbon double bond next to the C=O group are called alpha,beta-unsaturated carbonyls (enones or enals). These conjugated systems have two electrophilic sites: the carbonyl carbon (position 2) and the beta carbon (position 4). A nucleophile can attack either site. Attack at the carbonyl carbon is called 1,2-addition; attack at the beta carbon is called conjugate addition or 1,4-addition.

In conjugate addition, the nucleophile bonds to the beta carbon, the pi electrons shift onto the carbonyl oxygen, and the C=O reforms after protonation. Hard, small nucleophiles (Grignard reagents, hydrides) tend to give 1,2-addition. Soft, stabilised nucleophiles (enolates, cyanide, thiolates) tend to give 1,4-addition because they prefer the more diffuse electrophilicity at the beta position.

The Michael reaction is a conjugate addition in which an enolate (the nucleophile) attacks the beta carbon of an alpha,beta-unsaturated carbonyl (the Michael acceptor). The result is a 1,5-dicarbonyl compound: a new C-C bond has formed between the alpha carbon of the donor and the beta carbon of the acceptor. This is one of the most reliable carbon-carbon bond-forming reactions in organic synthesis.

The Knoevenagel condensation is closely related to the aldol condensation but uses an active methylene compound (a molecule with a $CH_{2}$ group flanked by two electron-withdrawing groups) as the nucleophile. A weak base and a primary or secondary amine catalyst promote the reaction. The product is an alpha,beta-unsaturated carbonyl after dehydration.

The Robinson annulation combines a Michael addition with an intramolecular aldol condensation in a single operation. An enolate adds to an enone (Michael step), and the resulting 1,5-dicarbonyl cyclises and dehydrates to form a new six-membered ring containing an enone. This powerful cascade builds complexity rapidly from simple starting materials.

Visual Beginner

Picture methyl vinyl ketone ( $CH_{2} =CHCOCH_{3}$ , an enone) reacting with the enolate of acetone ( $CH_{2} =C(O^{-}) CH_{3}$ ). In the Michael addition, the enolate carbon attacks the beta carbon of the enone (the terminal $CH_{2}$ of the vinyl group). The double bond electrons shift toward the carbonyl oxygen, giving an enolate intermediate. Protonation yields 2,6-heptanedione ( $CH_{3} COCH_{2} CH_{2} CH_{2} COCH_{3}$ ), a 1,5-diketone.

For the Knoevenagel condensation, picture diethyl malonate ( $CH_{2} (COOEt)_{2}$ ) reacting with benzaldehyde ( $PhCHO$ ). The amine catalyst (piperidine) deprotonates the active methylene to form a stabilised enolate. This attacks the aldehyde carbonyl, and the intermediate dehydrates to give benzylidenemalonate ( $PhCH=C(COOEt)_{2}$ ), an alkene flanked by the phenyl group and two ester groups.

For the Robinson annulation, picture the 1,5-diketone from the Michael addition cyclising. The enolate of one ketone attacks the carbonyl of the other ketone within the same molecule, closing a six-membered ring. Dehydration gives 2-methylcyclohex-2-enone — a new ring formed in a single pot from two simple starting materials.

Worked example Beginner

Predict the product of the Robinson annulation of 2-methylcyclohexan-1,3-dione with methyl vinyl ketone ( $CH_{2} =CHCOCH_{3}$ ).

Step 1 (Michael addition). The enolate of 2-methylcyclohexane-1,3-dione forms at one of the alpha positions flanked by two carbonyls. This stabilised enolate attacks the beta carbon of methyl vinyl ketone in a 1,4-conjugate addition. A new C-C bond forms between the alpha carbon of the dione and the beta carbon of the enone. The product is a 1,5-diketone in which the dione ring is connected through a three-carbon chain to a new ketone.

Step 2 (intramolecular aldol). One of the carbonyls in the chain forms an enolate, which attacks the other ketone carbonyl within the same molecule. The ring closes through a new C-C bond, forming a bicyclic beta-hydroxy ketone.

Step 3 (dehydration). The beta-hydroxy ketone loses water to form a conjugated enone within the new six-membered ring. The product is Wieland-Miescher ketone (or a methyl-substituted analogue), a key building block in steroid synthesis.

The Robinson annulation has converted a simple cyclic dione and an enone into a bicyclic enone with a new six-membered ring, all in one pot.

Check your understanding Beginner

Formal definition Intermediate+

Conjugate addition to alpha,beta-unsaturated carbonyls is governed by the interplay of orbital interactions and thermodynamic stability. The LUMO of an enone has larger coefficients at the beta carbon than at the carbonyl carbon, making the beta carbon the preferred site for attack by soft nucleophiles.

1,2- vs 1,4-addition selectivity

The selectivity between 1,2- (carbonyl) and 1,4- (conjugate) addition depends on the nature of the nucleophile and the electrophile:

Nucleophile type	Preferred pathway	Example
Hard (Grignard, RLi, $H^{-}$ )	1,2-addition	$PhMgBr$ + $PhCH=CHCHO$ gives allylic alcohol
Soft (enolate, malonate, $CN^{-}$ , $RS^{-}$ )	1,4-addition	Malonate + enone gives 1,4-adduct
Borderline ( $RMgX$ with $CuI$ )	1,4-addition	Gilman cuprates give conjugate addition

Hard nucleophiles have a high charge density and react at the most electrophilic site (the carbonyl carbon) under kinetic control. Soft nucleophiles have a more diffuse charge and interact better with the larger LUMO coefficient at the beta carbon. Organocuprates ( $R_{2} CuLi$ , Gilman reagents) are particularly reliable for conjugate addition of alkyl groups to enones, converting Grignard-type nucleophilicity from 1,2 to 1,4.

Thermodynamic control also favours 1,4-addition because the product (a saturated carbonyl) is more stable than the 1,2-product (an alkoxide that reverts to starting material or gives an allylic alcohol). At higher temperatures or with longer reaction times, even hard nucleophiles may give the 1,4-product under thermodynamic control.

Knoevenagel condensation mechanism

The Knoevenagel condensation proceeds through amine-catalysed iminium formation and carbanion addition:

EWG-CH_{2} -EWG’ amine EWG-CH^{-} -EWG’ RCHO EWG-CH(CR(OH))-EWG’ - H_{2} O EWG-C=C(R)-EWG’

The amine catalyst acts as a mild base to deprotonate the active methylene compound, and can also form an iminium intermediate with the aldehyde, increasing its electrophilicity. The dehydration step is strongly favoured because the product alkene is conjugated with one or more electron-withdrawing groups.

Active methylene compounds used in Knoevenagel condensations include malonic esters ( $CH_{2} (COOR)_{2}$ , pK $_{a}$ ~13), malononitrile ( $CH_{2} (CN)_{2}$ , pK $_{a}$ ~11), cyanoacetates ( $NCCH_{2} COOR$ , pK $_{a}$ ~9), and acetylacetone ( $CH_{3} COCH_{2} COCH_{3}$ , pK $_{a}$ ~9). The acidity of the methylene protons determines the base strength required.

Stork enamine Michael addition

Enamines serve as mild, neutral enolate equivalents in Michael additions. A secondary amine (pyrrolidine, morpholine) condenses with a ketone to form the enamine. The enamine beta carbon attacks the Michael acceptor in a conjugate addition. Hydrolysis of the resulting iminium regenerates the ketone with a new alkyl chain at the alpha position. This avoids the need for strong bases and prevents polyalkylation.

Hetero-Michael reactions

Conjugate addition is not limited to carbon nucleophiles. Thiols ( $RSH$ ) add to enones under mild conditions (often without any catalyst) to give thioethers. Amines add to electron-poor Michael acceptors (acrylates, acrylonitrile) in aza-Michael reactions. These hetero-Michael processes are widely used in polymer chemistry and bioconjugation.

Baylis-Hillman reaction

The Baylis-Hillman reaction couples an activated alkene (methyl acrylate, acrylonitrile) with an aldehyde using a nucleophilic catalyst (DABCO, a tertiary amine). The amine adds to the alkene in a conjugate addition, generating an enolate that attacks the aldehyde. Elimination of the amine gives an alpha-methylene-beta-hydroxy product. The reaction is atom-economical but typically slow (hours to days).

Counterexamples to common slips

"Conjugate addition always gives the 1,4-product." With very reactive nucleophiles (Grignard reagents, organolithiums), 1,2-addition to the carbonyl is kinetically favoured. The 1,4-product is thermodynamically favoured but may require equilibration. Cuprate reagents ( $R_{2} CuLi$ ) are specifically designed to bypass 1,2-addition and give clean 1,4-products.
"The Knoevenagel condensation and the aldol condensation are the same reaction." They are mechanistically related (both involve enolate addition to a carbonyl followed by dehydration), but the Knoevenagel uses active methylene nucleophiles with two electron-withdrawing groups and amine catalysis, whereas the aldol uses simple carbonyl enolates with stronger bases. The Knoevenagel product is almost always the dehydrated alkene because of the extra conjugation stabilisation from the second EWG.
"The Robinson annulation always gives a six-membered ring." The classical Robinson annulation produces a six-membered ring because the Michael addition generates a 1,5-dicarbonyl, and the intramolecular aldol closes a six-membered ring. Modified Robinson annulations with tethered substrates can produce five- or seven-membered rings, but these are less favourable and require careful substrate design.

Key mechanism Intermediate+

Robinson annulation: synthesis of 2-methylcyclohex-2-enone from methyl vinyl ketone and cyclohexanone.

The Robinson annulation is a two-stage cascade that builds a bicyclic enone from a cyclic ketone and an enone. The mechanism involves a Michael addition followed by an intramolecular aldol condensation.

Substrates. Cyclohexanone (the Michael donor) and methyl vinyl ketone (MVK, the Michael acceptor).

Stage 1: Michael addition.

Step 1a (enolate formation). A base (NaOEt or KOH) deprotonates the alpha carbon of cyclohexanone, generating the enolate.

Step 1b (conjugate addition). The enolate attacks the beta carbon of MVK in a 1,4-fashion. The C=C pi bond electrons shift onto the carbonyl oxygen, generating a new enolate at the MVK carbonyl.

Step 1c (protonation). The enolate is protonated to give 2-(3-oxobutyl)cyclohexanone — a 1,5-diketone in which the cyclohexanone ring bears a four-carbon side chain terminating in a ketone.

Stage 2: Intramolecular aldol condensation.

Step 2a (enolate formation at the side-chain ketone). The base deprotonates the alpha carbon of the side-chain ketone (the methyl group of the original MVK unit).

Step 2b (ring closure). The enolate attacks the carbonyl carbon of the cyclohexanone ring within the same molecule. A new C-C bond forms, closing a six-membered ring and generating a bicyclic beta-hydroxy ketone.

Step 2c (dehydration). The beta-hydroxy ketone loses water to form the conjugated enone 2-methylbicyclo[4.4.0]dec-1(6)-en-3-one (or simply 2-methyl-2-cyclohexenone fused to a cyclohexane ring).

The overall transformation converts two simple monocyclic starting materials into a bicyclic enone. The Robinson annulation is the cornerstone of steroid synthesis because it builds the six-membered rings (A, B, and C rings of steroids) in a convergent, predictable fashion.

Bridge. The Robinson annulation connects to retrosynthetic analysis 15.10.01 as a classic tandem reaction: a Michael addition disconnection followed by an aldol disconnection on the resulting 1,5-dicarbonyl. In forward synthesis, the annulation builds molecular complexity rapidly, making it indispensable for natural product synthesis. The Wieland-Miescher ketone (the Robinson annulation product of 2-methylcyclohexane-1,3-dione and MVK) is the starting material for over 100 steroid and terpenoid syntheses.

Exercises Intermediate+

Exercise 2 (easy, short-answer).

Explain why a soft nucleophile like the enolate of diethyl malonate gives 1,4-addition to methyl vinyl ketone, whereas a hard nucleophile like methylmagnesium bromide gives predominantly 1,2-addition.

Hint

Consider the orbital coefficients of the LUMO of the enone and how hard and soft nucleophiles interact differently with them.

Answer

The LUMO of an alpha,beta-unsaturated carbonyl has its largest coefficient at the beta carbon (C4), but the carbonyl carbon (C2) is more electrophilic in terms of charge. Hard nucleophiles (Grignard reagents, with high charge density and localised electron pairs) react at the most electrophilic site (C2, the carbonyl carbon) under kinetic control — this is 1,2-addition. Soft nucleophiles (stabilised enolates, with diffuse electron density and polarisable orbitals) interact better with the larger orbital coefficient at C4 (the beta carbon) — this is 1,4-conjugate addition. The malonate enolate, stabilised by two ester groups, is a classic soft nucleophile.

Exercise 3 (medium, short-answer).

Predict the product of a Stork enamine Michael addition: cyclohexanone is converted to its pyrrolidine enamine, which reacts with methyl acrylate ( $CH_{2} =CHCOOMe$ ), followed by acidic hydrolysis.

Hint

The enamine is the nucleophile and adds to the beta carbon of the acrylate. Hydrolysis regenerates the ketone with the new substituent.

Answer

Product: 2-(2-methoxycarbonylethyl)cyclohexanone. The enamine of cyclohexanone attacks the beta carbon of methyl acrylate in a conjugate addition. The iminium intermediate is hydrolysed in aqueous acid to regenerate the cyclohexanone carbonyl and release pyrrolidine. The net result is alpha-alkylation of cyclohexanone with a 2-carbon chain bearing an ester group. The Stork enamine approach avoids the polyalkylation and self-condensation problems of direct enolate alkylation.

Exercise 4 (medium, short-answer).

Draw the Robinson annulation product when 2-methylcyclopentanone reacts with methyl vinyl ketone under basic conditions. Identify the ring junction stereochemistry.

Hint

The Michael addition creates a 1,5-diketone, and the intramolecular aldol closes a six-membered ring. The ring junction is between a five-membered and a six-membered ring.

Answer

The Michael addition of the 2-methylcyclopentanone enolate to MVK installs a four-carbon side chain with a terminal ketone. The intramolecular aldol closes a six-membered ring by enolate attack on the cyclopentanone carbonyl. Dehydration gives a fused bicyclic enone: a five-membered ring fused to a six-membered ring with an enone in the six-membered ring. The ring junction (where the two rings share two carbons) is cis, because the aldol cyclisation proceeds through a chair-like transition state that places the connecting chain in an equatorial orientation.

Exercise 5 (medium, short-answer).

Predict the product of the Baylis-Hillman reaction between methyl acrylate ( $CH_{2} =CHCOOMe$ ) and benzaldehyde ( $PhCHO$ ) catalysed by DABCO. Identify the new stereocentre.

Hint

DABCO adds to methyl acrylate (conjugate addition), generating an enolate. The enolate attacks benzaldehyde. DABCO is then eliminated.

Answer

Product: methyl 2-(hydroxymethyl)-3-phenylacrylate ( $PhCH(OH)C(=CH_{2})COOMe$ ). DABCO adds to methyl acrylate to form a zwitterionic enolate. The enolate carbon attacks the carbonyl of benzaldehyde. Proton transfer and elimination of DABCO gives the alpha-methylene-beta-hydroxy ester. The new stereocentre is the carbon bearing the OH and the phenyl group. The product has an allylic alcohol and an alpha-methylene ester in the same molecule.

Exercise 6 (hard, short-answer).

Design a synthesis of the Wieland-Miescher ketone (8a-methyl-3,4,8,8a-tetrahydronaphthalen-1(2H)-one) from 2-methylcyclohexane-1,3-dione and methyl vinyl ketone. Explain the regiochemistry of the Michael addition step.

Hint

The dione has two enolisable positions. Consider which enolate is more stable and which Michael addition gives the correct connectivity for subsequent ring closure.

Answer

Step 1 (Michael addition). The base deprotonates one of the alpha positions of 2-methylcyclohexane-1,3-dione. The resulting doubly-stabilised enolate (flanked by two carbonyls) attacks the beta carbon of MVK. The regiochemistry is dictated by the thermodynamic enolate: deprotonation occurs at the less substituted alpha position (away from the methyl group) to give the enolate with maximum conjugation. The 1,4-addition installs a four-carbon chain with a terminal ketone.

Step 2 (aldol cyclisation). The enolate of the side-chain ketone attacks the remaining carbonyl of the dione ring, closing a six-membered ring. The product is a fused bicyclic beta-hydroxy ketone.

Step 3 (dehydration). Elimination of water gives the Wieland-Miescher ketone, a conjugated enone in a fused bicyclic framework. This compound is a key intermediate in the synthesis of steroids and terpenes because its ring system matches the A/B ring junction of many natural products. The methyl group at the ring junction controls the stereochemistry of subsequent ring-forming reactions.

Asymmetric Michael addition, organocatalysis, and cascade annulations Master

Modern carbonyl condensation chemistry extends the classical Michael, Knoevenagel, and Robinson reactions into the domains of asymmetric catalysis, organocatalysis, and tandem cascade processes that build multiple bonds and stereocentres in a single operation.

Asymmetric Michael addition

The catalytic asymmetric Michael addition is one of the most studied reactions in organocatalysis. Two principal approaches dominate: organocatalysis using chiral amines (enamine or iminium activation) and phase-transfer catalysis using chiral quaternary ammonium salts.

Enamine catalysis (List-MacMillan). A chiral secondary amine (typically a proline derivative or a diarylprolinol silyl ether) condenses with a ketone donor to form a chiral enamine. The enamine attacks the Michael acceptor from one face, controlled by the steric and electronic environment of the chiral catalyst. The iminium intermediate is hydrolysed to release the product and regenerate the catalyst. Enantioselectivities of >95% ee are routine for aldehyde donors with nitroalkene acceptors.

Iminium catalysis (MacMillan). For reactions where the Michael acceptor is the activated species, a chiral amine forms an iminium ion with an alpha,beta-unsaturated aldehyde. The iminium ion is more electrophilic than the parent enal, and the chiral amine shields one face of the pi system, directing the nucleophile to the opposite face. This approach is effective for the addition of carbon nucleophiles (malonates, nitroalkanes, indoles) to enals.

Phase-transfer catalysis. Chiral quaternary ammonium salts (derived from cinchona alkaloids or binaphthyl amines) catalyse the asymmetric Michael addition of stabilised carbanions (glycine Schiff bases, nitroalkanes) to Michael acceptors in biphasic conditions. The chiral ammonium cation forms an ion pair with the enolate at the interface, controlling the facial selectivity of the addition. The Corey-Deprenyl synthesis of amino acids uses this approach.

Hajos-Parrish reaction

The Hajos-Parrish reaction, independently reported by Hajos and Parrish (Hoffmann-La Roche, 1974) and Eder, Sauer, and Wiechert (Schering, 1971), is the organocatalytic Robinson annulation. Proline catalyses the intramolecular aldol cyclisation of a triketone substrate, giving the Wieland-Miescher ketone in high enantiomeric excess (up to 93% ee with (S)-proline).

The mechanism involves proline forming an enamine with the ketone donor, followed by an intramolecular aldol addition through a chair-like transition state that is sterically controlled by the proline pyrrolidine ring. The reaction is remarkable because it achieves asymmetric induction using a simple, naturally occurring amino acid as the sole chiral source, without any metals. The Hajos-Parrish reaction is considered the founding experiment of asymmetric organocatalysis, predating the formal articulation of the field by List and MacMillan by three decades.

Wieland-Miescher ketone and steroid synthesis

The Wieland-Miescher ketone (WMK), synthesised by Robinson annulation of 2-methylcyclohexane-1,3-dione with methyl vinyl ketone, is the single most important building block in steroid total synthesis. Its fused bicyclic enone framework matches the A/B ring junction of the steroid nucleus.

The enantiomerically pure WMK, available via the Hajos-Parrish reaction, serves as the starting material for syntheses of cortisone, testosterone, cholesterol, and numerous other steroids. The synthetic strategy involves iterative Robinson annulations or conjugate additions to append the C and D rings, followed by functional group manipulation to install the specific oxidation pattern of the target steroid. Notable applications include the Woodward synthesis of cholesterol and the Danishefsky synthesis of cortisone.

Tandem and cascade Michael-aldol processes

The Robinson annulation is the prototypical tandem Michael-aldol process, but modern variants extend the cascade to three or more bond-forming events:

Triple cascade (Enders). A proline-derived catalyst mediates a three-component cascade: an enamine activation (Michael donor), an iminium activation (Michael acceptor), and an intramolecular aldol closure. Three new C-C bonds and up to four stereocentres are set in a single operation with high enantioselectivity.
Conjugate addition-elimination cascades. Nitroalkenes serve as bifunctional Michael acceptors that undergo addition and subsequent elimination, enabling the construction of quaternary stereocentres.
Vinylogous Michael addition. Extended enolates (diene enolates) attack Michael acceptors at the gamma carbon rather than the alpha carbon, forming a new C-C bond that is two atoms further from the original carbonyl. This reactivity is useful for building remote stereocentres.

Knoevenagel in materials science

The Knoevenagel condensation has found applications beyond traditional synthesis. In materials chemistry, the condensation of malononitrile with aromatic aldehydes produces push-pull chromophores for nonlinear optics. In polymer chemistry, Knoevenagel polycondensation produces conjugated polymers for organic electronics. The reaction's tolerance of diverse functional groups and mild conditions makes it suitable for post-synthetic modification of metal-organic frameworks (MOFs) and covalent organic frameworks (COFs).

Connections Master

Enolate chemistry 15.07.03 pending. The Michael addition uses enolates as nucleophiles in conjugate addition, directly extending the enolate reactivity established in the aldol and Claisen condensations. The Robinson annulation is literally a Michael addition followed by an aldol condensation — both reactions from 15.07.03 are combined in one cascade.
Carbonyl nucleophilic addition 15.07.01. Conjugate (1,4-) addition is an alternative reaction pathway for nucleophilic addition to carbonyls. The choice between 1,2- and 1,4-addition depends on the same electronic and steric factors that govern direct carbonyl addition.
Acids and bases 15.03.01. The hard-soft acid-base (HSAB) framework explains the selectivity between 1,2- and 1,4-addition. Hard nucleophiles (high charge density, low polarisability) prefer the hard electrophilic site (carbonyl carbon), while soft nucleophiles (low charge density, high polarisability) prefer the soft site (beta carbon with larger LUMO coefficient).
Retrosynthetic analysis 15.10.01. The Michael disconnection (break a bond between the alpha and beta carbons of a 1,5-dicarbonyl) and the Robinson annulation disconnection (break a six-membered ring enone into a cyclic ketone and an enone) are key transforms in retrosynthetic planning. The Robinson annulation is particularly important for steroid and terpenoid retrosynthesis.
Diels-Alder cycloaddition 15.05.03 pending. The Robinson annulation and the Diels-Alder reaction both build six-membered rings. In synthesis planning, the choice between them depends on the substitution pattern: the Robinson annulation installs an enone, while the Diels-Alder installs an alkene. They are often used sequentially in natural product synthesis.
Enzyme mechanism 15.14.01. Type I and type II polyketide synthases perform iterative Michael-like extensions, building polyene and polyketide chains through enzyme-bound enolate additions to enoyl thioesters. The biological Michael addition is the enzymatic equivalent of the laboratory Robinson annulation cascade.

Historical notes Master

The Michael reaction was first reported by Arthur Michael at Tufts University in 1887. Michael observed that the sodium salt of diethyl malonate reacted with ethyl cinnamate to give a conjugate addition product. Michael initially misassigned the structure, believing the addition to be 1,2- (at the carbonyl) rather than 1,4- (at the beta carbon). The correct 1,4-mechanism was established by later workers, but the reaction retains his name.

The Knoevenagel condensation was reported by Emil Knoevenagel in 1894. Knoevenagel was working in Victor Meyer's laboratory at Heidelberg and discovered that aldehydes reacted with active methylene compounds (malonic acid, malonic esters) in the presence of amines to give alpha,beta-unsaturated products. The role of the amine as both base and iminium-forming catalyst was not fully understood until the mid-twentieth century.

Sir Robert Robinson reported the annulation reaction bearing his name in 1935 while at the University of Oxford. Robinson was studying the biosynthesis of anthocyanins and other natural products and recognised that the tandem Michael-aldol sequence could build the six-membered rings found in steroids and terpenoids. The Robinson annulation became the central reaction in Robinson's landmark synthesis of cholesterol and related steroids, for which he received the Nobel Prize in Chemistry in 1947.

The Hajos-Parrish reaction, reported in 1971-1974, demonstrated that (S)-proline could catalyse an asymmetric intramolecular aldol with high enantioselectivity. This result was underappreciated for decades. When List and MacMillan independently developed asymmetric organocatalysis in 2000, the Hajos-Parrish reaction was recognised as a direct precursor. The 2021 Nobel Prize in Chemistry to List and MacMillan cited the broader field of asymmetric organocatalysis, with the Hajos-Parrish reaction as its earliest example.

The Wieland-Miescher ketone was first prepared by Wieland and Miescher in 1939 and subsequently used in steroid syntheses by Woodward, Stork, Danishefsky, and others. The asymmetric variant via the Hajos-Parrish reaction provides enantiomerically pure WMK, which has been used in the total synthesis of over 50 natural products.

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