4.3. Synthesis of α-Amino Acids
The twenty common amino acids are generated on industrial scale using microorganisms. However, many other alpha-amino acids are synthesized using more traditional methods for use in research or pharmaceutical production. Note that the examples in this text (Sections 4.3.1-4.3.3) are only some of the more traditional and straightforward methods; there are many other reactions or sequences of reactions that may be used to generate α-amino acids.
4.3.1. Reaction: Alkylation of Amidomalonate
It is possible to synthesize an α-amino acid by alkylation and decarboxylation of a 1,3-dicarbonyl compound (Scheme 4.9). This is simply a specialized form of alpha-alkylation and decarboxylation of a 1,3-dicarbonyl (see Sections 2.10.6 to 2.10.6.2.1). After alpha-alkylation hydrolysis and decarboxylation occur in a single step. The overall reaction is normally shown as two steps to indicate this
Scheme 4.9 – Generalized Reaction Equation for Synthesis of an Alpha-Amino Acid Using Alpha-Alkylation and Decarboxylation of Diethyl 2-Acetamidomalonate.
The classical method is to use an amidomalonate, most commonly diethyl 2-acetamidomalonate. This compound is preferred only because it is easily synthesized on industrial scale; there is no special reactivity or reason for this choice beyond convenience. The use of a diester with a pre-existing electron-withdrawing group (the amide) increases the acidity at the alpha-position and allows weaker bases to be used for the alkylation step. The alkyl halide is typically an alkyl bromide or iodide. Some alkyl chlorides work, but many undergo competitive side-reactions. This approach will not work with alkyl fluorides.
In reality this method only works well with some alkyl halides, with many others affording poor-to-acceptable results. However, it is not always intuitive whether a given electrophile will work well or poorly for this reaction. Assume any given alkyl halide will work for this reaction unless otherwise indicated.
The mechanism for this transformation has several stages (Scheme 4.10).
The alpha-alkylation follows the standard base-catalyzed mechanism. First, the base catalyzes the formation of the enol. Second, the nucleophile (enol) gets activated by the “catalyst”. This greatly increases its nucleophilicity. Then the enolate (nucleophile) attacks the carbon of the alkyl halide (electrophile). This creates a new C-C bond and a salt (NaBr). The base is not regenerated and is only catalytic for formation of the enol.
Then, converting the esters to carboxylic acids and the amide to an amine follows the standard acid-catalyzed hydrolysis mechanism. The strong acid reacts with water/alcohol to form hydronium (not shown). This is the active catalyst. First, the electrophile (carbonyl) gets activated by the catalyst. This greatly increases its electrophilicity. Second, the water/alcohol (nucleophile) attacks the activated carbonyl (electrophile). This forms a new bond and adds a lone pair to the oxygen. The solvent then removes a proton from one oxygen (the former nucleophile) and places it on the desired leaving group. This greatly increases its leaving group ability. These steps may be viewed as regeneration of the catalyst and another acid-catalyzed activation. Then the lone pair comes back and reforms the π bond with the carbon, ejecting the leaving group (alcohol/water). Finally, water/alcohol removes a proton, generating the final product and regenerating the catalyst. This occurs for each of the carbonyl-containing functional groups (not shown).
Finally, with the energy provided by additional heating decarboxylation occurs. The mechanism for decarboxylation is again a rearrangement, followed by keto-enol tautomerism (no mechanism shown).
Scheme 4.10 – Reaction Mechanism for Synthesis of Alanine Using Alpha-Alkylation of Diethyl 2-Acetamidomalonate with Bromomethane Followed by Hydrolysis and Decarboxylation.
The stereocentre is formed in the last step, keto-enol tautomerism after decarboxylation. As a result, all stereoselectivity considerations for this reaction are identical to those for alpha-halogenation (see Section 2.7.3.). Assuming there are no pre-existing stereocentres in the intermediate (the group that was added does not have a stereocentre(s)), keto-enol tautomerism will not be stereoselective; if a new stereocentre is formed during this step, it will be formed as an equal mixture of both (R) and (S). If another stereocentre (or multiple stereocentres) exists then the resulting products will be diastereomers; the product is a non-one-to-one mixture of diastereomers. Predicting which diastereomer should be favoured is not required.
4.3.2. Reaction: Reductive Amination of Ketones Near Carboxylic Acids
It is possible to synthesize an α-amino acid by reductive amination of a ketone adjacent to a carboxylic acid (Scheme 4.11). This is fundamentally a specific example of reductive amination (see Section 3.5.1). The ketone and ammonia combine to form an iminium, typically through acid catalysis. Afterwards, the iminium is reduced into an amine using a reducing compound such as sodium cyanoborohydride (NaBH3CN) or sodium borohydride (NaBH4). These require quenching to generate the final product. The reaction is often depicted as three steps to indicate this.
Scheme 4.11 – Generalized Reaction Equation for Synthesis of an Alpha-Amino Acid Using Reductive Amination of a Ketone Adjacent to a Carboxylic Acid.
All considerations for this reaction are identical to standard reductive amination. As before, it is possible to use other strong acids but much more common to use hydrochloric acid (HCl). As before, there are several variations of reductive amination with different reaction conditions. Some variants form an imine rather than an iminium. This requires modified conditions for each step of the reaction sequence. Examples like this will not feature in this text but may be encountered in other sources.
The specific method for the quenching varies but is typically standard acid or base quench conditions. Depending on the exact conditions used a salt may be obtained instead of the amino acid directly. The carboxylate (deprotonated carboxylic acid anion) is obtained if a strong base is used for quenching. The ammonium salt (protonated amine cation) is obtained if a strong acid is used for quenching.
The mechanism for this reaction is functionally identical to the standard reductive amination mechanism (see Section 3.5.1.1; Scheme 4.12).
First, acid-catalyzed iminium formation occurs. The strong acid reacts with the solvent to form an oxonium (cationic oxygen; not shown). This is the active catalyst. The electrophile (carbonyl of the ketone) gets activated by the catalyst. This greatly increases its electrophilicity. Then, the ammonia (nucleophile) attacks activated carbonyl (electrophile). This forms a new bond and adds a lone pair to the oxygen. The solvent then removes a proton from the nitrogen (the former nucleophile) and places it on the oxygen. This greatly increases its leaving group ability. These steps may be viewed as regeneration of the catalyst and another acid-catalyzed activation. Then the lone pair of the nitrogen forms a new π bond with the carbon, ejecting the leaving group (water). Although the solvent does remove a proton, forming the imine and regenerating the catalyst, this is in equilibrium. The iminium is favoured.
Second, hydride reduction occurs. This follows the standard mechanism for reduction but first forms a O-B bond between the oxygen of the carboxylic acid and the boron of the reductant. The base (hydride) removes a proton from the acid (carboxylic acid). The conjugate base/nucleophile (carboxylate) attacks the electrophile (boron). This forms a new O-B bond. The next step is now intramolecular. The nucleophile (hydride) attacks the electrophile (carbon of the iminium). This forms a new bond and adds a lone pair to the nitrogen. Then the lone pair forms a new bond with the boron.
Third, after the reaction is complete water and an acid or base is added to quench, forming the final product (no mechanism shown). The neutral product is shown but the ammonium salt or carboxylate salt may be obtained depending on the specific conditions used.
Scheme 4.12 – Reaction Mechanism for Synthesis of Alanine Using Reductive Amination of 2-Oxopropanoic Acid (Pyruvic Acid) with Ammonia.
All stereoselectivity considerations for this reaction are identical to those for standard reductive amination (see Section 3.5.1.2.). Assuming there are no pre-existing stereocentres in the starting materials, the nucleophilic attack (hydride attacks iminium) will not be stereoselective as attacking from above the iminium is sterically and electronically equivalent to attacking from below the iminium; if a new stereocentre is formed during this step, it will be formed as an equal mixture of both (R) and (S). If another stereocentre (or multiple stereocentres) exists then the resulting products will be diastereomers; the product is a non-one-to-one mixture of diastereomers. Predicting which diastereomer should be favoured is not required.
4.3.3. Reaction: Hydrogenation of Enamides
As an introduction to the idea of enantioselective reactions an additional method of synthesizing α-amino acids is included. This discussion omits many of the specific details (how the reactant is made, how it is saponified, the exact transition state, etc.) to focus on the general ideas of the approach and how enantioselectivity is achieved.
It is possible to synthesize an α-amino acid by hydrogenation of an enamide adjacent to a carboxylic acid (Scheme 4.13). The hydrogenation process can be done several different ways, including enantioselectively.
Scheme 4.13 – Generalized Reaction Equation for Synthesis of an Alpha-Amino Acid Using Hydrogenation of an Enamide Adjacent to a Carboxylic Acid.
The enamide can be synthesized several different ways. The specific details of the possible ways are not relevant. In principle many different amide groups may be used. The overwhelming majority use an amide made from an amine and ethanoic acid (acetic acid). These may be referred to as “acetamides”. After the reaction the amide is typically hydrolysed to generate the amino acid (not shown). This can be done several different ways. The specific details of the possible ways are not relevant.
Several transition metal catalysts may be used for the hydrogenation. Two examples are considered to highlight the difference in outcomes.
4.3.3.1. “Mechanism” and Stereoselectivity
Recall that transition metal catalysts are complex, and most institutions have one or more entire courses dedicated to their study. Because of their complexity an in-depth discussion of the mechanisms for these reactions is beyond the scope of this text. However, a visual approximation is helpful. At an introductory level a detailed understanding of the mechanism is not required.
It is possible to hydrogenate the alkene using a simple transition metal catalyst such as palladium on carbon (Pd/C; Pd(C); Scheme 4.14). This is not commonly performed; the product will be racemic and the other methods of generating racemic α-amino acids are more efficient.
Scheme 4.14 – Hydrogenation of an Enamide Using a Generalized Transition Metal Catalyst Showing Stereochemical Outcomes, Mechanistic Details Omitted.
The products, intermediates, and the transition states that form them are enantiomers. Recall that enantiomers are physically and chemically identical. This means that they will have the same properties (e.g. melting point, boiling point, etc.) but also means they have the same energies. Because the transition states leading to the two products are enantiomers they have identical energies and the speed of formation for the two products is identical (Figure 4.19). This leads to the formation of a racemate.
Figure 4.19 – Generalized Reaction Coordinate for Enantioselectivity-Determining Step of Hydrogenation of an Enamide Using a Generalized Transition Metal Catalyst.
It is possible to hydrogenate the alkene using a chiral transition metal catalyst. There are many such catalysts. An example would be the tetrafluoroborate salt of (R,R)-1,2-bis(o-anisylphenylphosphine)ethane (Figure 4.20). At an introductory level memorizing the exact structure is not important, only the idea is: a transition metal catalyst with ligands that have pre-existing stereocentres.
Figure 4.20 – (R,R)-1,2-Bis(o-anisylphenylphosphine)ethane Tetrafluoroborate.
The specific mechanism for hydrogenation using this catalyst is more complex than that of palladium on carbon. Approximating it as a bulk metal surface (as Figure 4.19) is not as accurate. However, this is significantly easier to understand than more precise depictions. The approximation of the mechanism is not accurate, but the concept is.
The products are enantiomers. However, the catalyst has pre-existing stereocentres. As a result, the intermediates and (more importantly) the transition states that form them are diastereomers. Recall that diastereomers are physically and chemically different. This means that they will have different properties (e.g. melting point, boiling point, etc.) and also means they have different energies. Because the transition states leading to the two products are diastereomers they have different energies and the speed of formation for the two products is different (Figure 4.21). This leads to the formation of a scalemate; one of the two enantiomers is preferentially formed by being formed (much) faster. In this case the enantiomer with absolute configuration (S) is favoured. Unfortunately, understanding why this enantiomer is selected for requires significantly more background in transition states, non-covalent interactions, and much more precise depictions of the mechanism.
Figure 4.21 – Generalized Reaction Coordinate for Enantioselectivity-Determining Step of Hydrogenation of an Enamide Using a Generalized Transition Metal Catalyst with Pre-Existing Stereocentres.
