Substrate Transport Limitation as tool to enhance enantioselectivity in the enzyme synthesis of chiral cyanohydrins
		Pieter Jan Gerrits*, Johannes Brussee and Arne van der Gen
		Leiden Institute of Chemistry, Gorlaeus Laboratories, Leiden University, P.O. Box 9502, NL-2300 RA Leiden, The Netherlands E-mail: [email protected]

Received: 15 August 1999 / Uploaded: 24 August 1999

Objective

Rational design of process characterized by:

• two-phase (fed) batch system
• the use of purified oxynitrilase
• enantiomerically pure products (ee > 98%)
• high yield and through-put (mol.l^-1.h^-1)
• down stream processing with respect to recycling of enzyme and solvents

 

Justification

• the almond enzyme is readily available at low cost
• the reaction products, chiral cyanohydrins

 

Experimental conditions

Figure 1. On the left the reaction scheme showing the general reaction. On the right the double-walled reaction vessel used in the here described synthesis (Copyright J.Marcus 1999)

 

Results

Figure 2. Effect of co-solvent on the stability of the cyanohydrin in hexane and proposed mechanism for the interaction of alcohols with cyanohydrins¹

Figure 3. Proposed mechanism for the decomposition of cyanohydrins under influence of alcohols.

Figure 4. Varying the Phase Volume Ratio (PVR) and monitoring the reaction in time

Figure 5. Varying the enzyme concentration (constant substrates concentration and PVR) and monitoring the reaction rate in time below reaction rate limited enzyme concentration

Figure 6. Influence of enzyme concentration on ee and reaction rate, reaching a steady state, below reaction rate limited enzyme concentration

 

Conclusions

• Do not use alcohols as co-solvent: alcohols influence the stability of cyanohydrins, the kind of alcohol employed has an important effect on the decom-position rate.
• optimization of enzyme conditions can give good results even if the non-enzyme catalyzed reaction is favorite
• There exists a constant ratio between both reactions expressed in an enantioselectivity coefficient
• high PVR reduces reaction rates and enhances the enantioselectivity
• Substrate Transport Limitation is a powerful tool to enhance enantioselectivity in a two phase system

 

Reference

[1]Marcus, J.; Vandermeulen, G.W.M.; Brussee, J.; van der Gen, A. Tetrahedron: Asymmetry 1999, 10, 1617-1622.

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