Glycidic ester (epoxide) synthesis
Darzens glycidic ester synthesis
How the Darzens glycidic ester synthesis works, in 3 steps. Each step shows the structures and the curved arrows that move the electrons.
In short. A 3-step mechanism. Typical reagents: α-halo ester, ketone or aldehyde, NaOEt (or t-BuOK). Conditions: EtOH or THF, 0–25 °C. Bonds formed: C–C; C–O (epoxide ring); bonds broken: α C–H; C=O π; C–Cl. Stereochemistry: Mixture of cis/trans epoxides; set at aldol addition. Rate and selectivity: Aldol addition is reversible; ring closure (SN2) is rate-limiting.
At a glance
Step 1: Base makes the α-halo enolate
Base deprotonates an α-halo ester, forming an enolate that's simultaneously a decent nucleophile and still carries a good leaving group on the same carbon.
Step 2: Aldol-type addition
This enolate attacks a ketone or aldehyde's carbonyl carbon in an ordinary aldol-type addition, forming a β-alkoxide still bearing the halogen.
Step 3: Intramolecular SN2 closes the epoxide
The alkoxide performs an intramolecular backside SN2 attack on the adjacent halogen-bearing carbon, closing an epoxide ring directly — combining an aldol addition and an intramolecular Williamson-type ring closure into one operation, giving a glycidic (epoxy) ester.
Abbreviations and terms
- THF
- Tetrahydrofuran. Cyclic ether solvent; its oxygen lone pairs solvate Li⁺/Mg²⁺ so organometallics stay dissolved and reactive.
- EtOH
- Ethanol. Polar protic solvent.
- KOtBu
- Potassium tert-butoxide. Strong, bulky alkoxide base – favours elimination over substitution.
- NaOEt
- Sodium ethoxide. Alkoxide base matched to ethyl esters (avoids transesterification).
- SN2
- Bimolecular nucleophilic substitution. One concerted step with backside attack and inversion; rate = k[RX][Nu].
How it compares with related reactions
| Darzens glycidic ester synthesis | Ester synthesis from an acid chloride and alcohol | Aza-Wittig reaction | |
|---|---|---|---|
| Reagents | α-halo ester, ketone or aldehyde, NaOEt (or t-BuOK) | Acid chloride + alcohol; pyridine or Et₃N (optional) | R–N=PPh₃ (from R–N₃ + PPh₃, Staudinger), aldehyde or ketone |
| Conditions | EtOH or THF, 0–25 °C | CH₂Cl₂ or Et₂O, 0–25 °C | THF or toluene, 25–110 °C, anhydrous |
| Stereochemistry | Mixture of cis/trans epoxides; set at aldol addition | Alcohol stereocentre retained (C–O bond not broken) | Imine E/Z set at ring collapse; no stereocentre needed |
| Rate / selectivity | Aldol addition is reversible; ring closure (SN2) is rate-limiting | Very fast and irreversible; HCl scavenged by base | Cycloreversion is fast; driven by strong P=O bond (≈ 130 kcal/mol) |
Test yourself
Try to answer each question before opening it.
Which reagents and conditions are typical for the Darzens glycidic ester synthesis?
α-halo ester, ketone or aldehyde, NaOEt (or t-BuOK). Conditions: EtOH or THF, 0–25 °C.
Which bonds are formed and which are broken?
Formed: C–C; C–O (epoxide ring). Broken: α C–H; C=O π; C–Cl.
What is the stereochemical outcome?
Mixture of cis/trans epoxides; set at aldol addition
What controls the rate or the selectivity?
Aldol addition is reversible; ring closure (SN2) is rate-limiting
Study guides that help with this reaction
- Carbonyl chemistry: addition, acyl substitution and enolates
- SN1, SN2, E1 or E2? How to decide which mechanism wins
- Nucleophiles, electrophiles and leaving groups: what makes a reaction go