Solution and Solid Phase Synthesis of Unusual a-Amino Acids From

the desired &substituenf5 onto Evans' chiral oxazolidinone 1 3 or by diastereoselective
cuprate addition to a$-unsaturated-N-acyloxazoiidinones 1.34) followed by a-
brornination. The chiraiity of the B-carbon appears to have little effect on the
stereoselectivity of a-bromination."
Several other systems have been adapted to enantioselectively aminate the a-
carbon and are based on the use of chiral templates developed for the enantioselective
addition to glycine enolates and wili be discussed in more detail in Section 1.5.1.1 (vide
infra).
1.3.2.2 Electrophilic Amination of Enolates
Scheme 1.13
Perhaps the most direct route to a-amino acids is by amination of enolates with
electrophilic reagents. This has only recently ken developed due to the scarcity of
electrophilic sources of nitrogen that participate in C-N bond forming reactions. One of
the first reagents used in this application was di-t-butyl azodicarboxylate, which converts
Li-enolates to a-hydrazido acids.' However, this method suffers from difficulties
associated with the harsh conditions required for the cleavage of the N-N bond in the
newly fomed a-hydrazido acids.
Evans and coworkers have optimized the direct azidation of potassium enolates of
N-acyloxazolidinones 1.30 with trisyl azide, which occws in both good yield and high
12