Page 332 - Cascade_Biocatalysis_Integrating_Stereoselective_and_Environmentally_Friendly_Reactions

308 14 Enzymatic Stereoselective Synthesis of -Amino Acids

Table 14.3 Biotransformation of malononitriles.

R Time (h) Yield of amido-nitrile (%)

Furanyl 3 82
Ph 0.75 22
p-OH Ph 0.75 25

3
Most research has been focused on the preparation of β -amino acids, but
the nitrile hydratase–amidase cascade system has also been investigated for the
2
preparation of β -amino acids. Malononitriles contain two nitrile groups separated
by a single carbon, and the regioselective hydrolysis of one of the two nitriles to
the corresponding acid, with subsequent reduction of the carbon–carbon double
2
bond and of the other nitrile to an amino group would yield β -amino acids. This
approach has been described using a Rhodococcus sp. acting on α,α-disubstituted
saturated malononitriles [68], however multiple acid and amide products were
formed.
◦
More recently, three malononitriles were reacted at 30 C with cell pastes of R.
rhodochrous ATCC BAA-870 and the reactions resulted in successful conversion
to the corresponding amido-nitrile products (Scheme 14.5 and Table 14.3). The
configuration of the furanyl amido-nitriles was determined to be Z, by comparison
of our NMR data with that previously described for the E product [69].

CN O NH 2 O NH 2
ATCC BAA-870 C C
R R + R NH 2
CN 30 °C CN C
O
R = furanyl, Ph, p-OH Ph

Scheme 14.5 Conversion of malononitrile derivatives.
Longer reaction times led to the diamide product only, with no acid being
formed. Further conversion of 2-cyano-3-furan-2-yl-but-2-enamide was attempted
using both whole cells of R. rhodochrous ATCC BAA-870 as well as a commercial
amidase enzyme preparation (Pseudomonas aeruginosa, Sigma–Aldrich), but neither
showed conversion of the substrate (unpublished results).

14.4
Conclusion

Nitriles may be converted to amino acids by nitrilases, which catalyze a two-step
3
nitrile hydrolysis reaction, and can provide β -amino acids in high enantiopurity.
The same reaction can be catalyzed by a combination of nitrile hydratase and