1. Chemical Synthesis Methods

(1) Strecker Reaction + Chiral Resolution

• Raw materials: pivalaldehyde, NaCN, ammonia
• Process:
  1. Strecker reaction to form racemic DL‑tert‑leucine
  2. chiral resolution using chiral acids (tartaric acid, dibenzoyl tartaric acid)
  3. purification to obtain L‑tert‑leucine
• Disadvantages: low yield (<50%), highly toxic cyanide, heavy waste, high cost

(2) Asymmetric Strecker / Silylcyanation

• Uses chiral auxiliaries (L-phenylglycinol) and chiral catalysts
• Can directly obtain chiral products without resolution
• Disadvantages: expensive catalysts, complex systems, high environmental pressure

(3) Asymmetric Hydrogenation of Keto Acids

• Substrate: trimethylpyruvic acid
• Catalysts: chiral Rh/Ru complexes (BINAP, DuPhos)
• Disadvantages: high pressure, heavy metal residues, expensive ligands

2. Enzymatic Catalysis (Industrial Mainstream)

(1) Reductive Amination by Leucine Dehydrogenase (LeuDH)

• Substrate: trimethylpyruvic acid + ammonia
• Enzyme: L-leucine dehydrogenase (LeuDH)
• Coenzyme regeneration: coupled with formate dehydrogenase (FDH) or glucose dehydrogenase (GDH)
• Advantages:
  • ee >99%, conversion rate 95–98%
  • mild conditions, atmospheric pressure, no heavy metals
  • low cost with whole-cell biocatalysis
• This is the dominant industrial route worldwide.

(2) Transaminase Method

• Substrate: trimethylpyruvic acid with amino donors (L-alanine, L-glutamate)
• No coenzyme needed, single-enzyme system
• Limitations: substrate inhibition, moderate conversion

(3) Multi-enzyme Coupling System

• Combined use of hydroxy acid dehydrogenase and LeuDH
• High substrate concentration, low inhibition, self-sufficient coenzyme cycle

3. Fermentation Method (Under Development)

• Uses engineered strains: Escherichia coli, Corynebacterium glutamicum, etc.
• Constructs biosynthetic pathways for direct fermentation from glucose
• Current status: low titer, long cycle, difficult purification
• Future trend: promising for ultra-low-cost production via synthetic biology

4. Comparison of Production Methods

Method	Advantages	Disadvantages	Optical Purity	Industrial Status
Chemical Resolution	Low raw material cost, mature	Low yield, toxic, polluting	~98%	Gradually phased out
Asymmetric Chemistry	Good stereoselectivity	Expensive catalysts, heavy metals	95–98%	Small-scale only
LeuDH Enzymatic Method	High ee, green, scalable	Enzyme stability required	>99%	Mainstream
Transaminase Method	No coenzyme needed	Substrate inhibition	98–99%	Pilot scale
Fermentation	Ultra-low raw material cost	Low titer, long cycle	>99%	R&D stage

5. Typical Industrial Process (Enzymatic Route)

1. Preparation of trimethylpyruvic acid solution
2. Whole-cell biocatalysis (engineered LeuDH + FDH strain)
3. Cell removal and filtration
4. Decolorization, concentration and pH adjustment
5. Crystallization, centrifugation and drying
6. Final product: L‑tert‑leucine (ee >99%)