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

The 3-bromo analog of Williams and CO-worker's oxazinone 1.34 has ben
extensively studied and has been substitutad efficiently by alkyl thiolates, malonates,
alkyl Zn-halides and cuprates in both good yield and high enantio~electivity.~ Coupling
of al1 y lsilanes and si1 y 1 en01 ethers? FriedelCrafts alkylations7' and
trialkylstannylacetylidesU could also be effected under Lewis-acid catalyzed conditions.
The relative stereochemistry of the product remains anti and is postulated to proceed
through iminium species 1.47 after zinc coordination to the bromide 1.46 (Scheme 1.2 1).
The steric factors controlling addition of the electrophile to 1.34 in Scheme 1.16 exert the
sarne influence in the case of the incoming nucleophile in scheme 1.21, resulting in
retention of stereochemistry at the a-carbon.
Belokon et al. have investigated the reactivity of the brorninated Ni-complex 1.40
which was generated as a 2: 1 diastereomeric mixture from 1.39. However, nucleophilic
addition gave the anti adducts with excellent stereocontrol (90-98% ee (S)) and good
yields (6&9û%)."
Scheme 1.21
1.34 * 0 -
CCI*
1.3.4 Synthesàs fiom a-Amino Acids
Br R
a-Amino acids, with their inherent chirality and functionality, provide an
excellent source of starting materials for the synthesis of more complex derivatives.