3824
of the reaction (see Table 3). All the N-acetyl amino acids tested racemised, expect for N-acetyl aspartic
acid. In all cases a control reaction was conducted with just the N-acetyl amino acid in acetonitrile and
in each case no significant racemisation was observed (ee >95%).
In conclusion, we have demonstrated for the first time that N-acyl amino acids can be efficiently
racemised using rhodium complexes under comparably mild condition. These results are not only of
significance to kinetic resolution processes, but also of major importance to asymmetric hydrogenations
of N-acylamido acrylic and cinnamic acids. It is possible that at low hydrogen pressure and at high
conversion, small amounts of racemisation take place, thus decreasing the enantiomeric excess of the
reaction.
Acknowledgements
We would like to thank the state of Mecklenburg-Vorpommern for providing financial support for
M.J.H. We also thank Dr. T. Riermeier, Dr. H. Trauthwein and Dr. B. Bosch (all Aventis R&T) for
helpful discussions.
References
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8. General procedure for racemisation experiments: A solution of PCy3 was made up in absolute THF under argon and the
appropriate amount was pipetted into an ACE pressure tube and the THF removed using a stream of argon. The residue was
then dissolved in dry acetonitrile (10 mL) and the N-acyl α-amino acid (0.48 mmol) was added followed by the transition
metal complex as either a solid or a solution in MeCN. The mixture was then heated for between 24 and 48 h at 40–60°C.
A portion of the solution was then taken and derivatised using (trimethylsilyl)diazomethane (2 M in hexane) and methanol
and the enantiomeric excess determined by GC or HPLC analysis.