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

Figure 5.4: A) Chdation controlled mode1 of (E)-lithium enolate of 5.38. B) Nonchelation
del dth (2)-lithium enolate of 538.
Unexpectedly, the 13-addition to glutamates described in chapter four occurs in
higher selectivity than in the case of 12-addition to aspartate derivatives. Of
significance, the extra methylene unit in glutamate 4.69 allows rotation of the (2)-enolate
described in figure 4.10 into a position in which it may chelate with the N-Li bond. As
such, both the (E)- and (%nolate may adopt the same transition state to give high
selectivity in which the geometry of the enolate is imlevant.