Definition
The Schöllkopf method is a stereoselective synthetic protocol for the preparation of α‑amino acids. It employs a chiral oxazolidinone auxiliary derived from glycine to generate a diastereomerically biased enolate, which is subsequently alkylated and hydrolyzed to afford the target amino acid with defined configuration.
Overview
Developed in the 1970s by Klaus Schöllkopf at the Ludwig Maximilian University of Munich, the method provides a general route to both natural and non‑proteinogenic α‑amino acids, especially those bearing β‑substituents. The key steps are:
- Installation of the chiral auxiliary – Glycine is converted into a chiral oxazolidinone (often called “Schöllkopf’s oxazolidinone”) by condensation with a suitable aldehyde or ketone and subsequent cyclisation.
- Generation of the enolate – Deprotonation of the oxazolidinone nitrogen‑protected glycine derivative with a strong base (e.g., lithium diisopropylamide, LDA) yields a lithium enolate that is locked in a defined geometry by the auxiliary.
- Alkylation – The enolate undergoes nucleophilic substitution with an electrophile (alkyl halide, allyl, benzyl, etc.) to install the desired side chain at the α‑carbon. The reaction proceeds with high diastereoselectivity, delivering a single stereoisomer of the alkylated intermediate.
- Cleavage of the auxiliary – Hydrolytic or reductive removal of the oxazolidinone liberates the free α‑amino acid, preserving the configuration introduced in the alkylation step.
The overall sequence converts readily available glycine into a versatile chiral building block, allowing the synthesis of a wide variety of amino acids that are otherwise difficult to access.
Etymology/Origin
The method is named after its inventor, Klaus Schöllkopf, a German organic chemist known for contributions to asymmetric synthesis. “Schöllkopf” is a German surname; the term “method” designates the procedural nature of the synthetic approach.
Characteristics
| Feature | Description |
|---|---|
| Chiral Auxiliary | A bicyclic oxazolidinone derived from glycine; provides steric and electronic control over enolate geometry. |
| Key Reactants | Strong bases (LDA, LiHMDS) for enolate formation; diverse electrophiles for α‑alkylation. |
| Stereocontrol | High diastereoselectivity (typically >90 % de) due to the rigid chiral environment of the auxiliary. |
| Scope | Effective for the synthesis of α‑alkyl, α‑aryl, α‑heteroatom‑substituted amino acids; adaptable to both primary and secondary electrophiles. |
| Limitations | Requires additional steps to install and later remove the auxiliary, which may affect overall atom economy; sensitive to moisture and strong bases. |
| Typical Conditions | Low temperature (−78 °C to 0 °C) for enolate generation; inert atmosphere; polar aprotic solvents such as THF or diethyl ether. |
Related Topics
- Chiral Auxiliaries in Asymmetric Synthesis – Other auxiliaries (e.g., Evans’ oxazolidinones, Oppolzer’s sultam) that function similarly to induce stereochemistry.
- Ellman’s Sulfinamide Method – An alternative protocol for asymmetric synthesis of amines and amino acids using a chiral sulfinyl auxiliary.
- Strecker Synthesis – Classical route to α‑amino acids, providing a contrast in terms of stereochemical control.
- Asymmetric Alkylation of Enolates – General strategies for introducing chiral centers via enolate chemistry.
- Non‑Proteinogenic Amino Acids – Amino acids not encoded by the genetic code, many of which are accessed through the Schöllkopf method.