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

Williams and coworlcers have examinai electrophilic additions to chiral
oxazinones 1.34. Generation of the enolate and subsequent alkylation gave the tram
adduct 1.35 in excellent diastereoselectivity (>99%) and reasonable yields (50-90%)
(Scheme 1.16 top)? Only disilazanes are successful at deprotonating the oxazinone 1.34
and the temperature, mode of addition and many other parameters have to be carefully
controlled. Aldol condensations of the boron enolate of 1.34 with aldehydes produce
anti-J3-hydroxy adducts 1.36 in 3857% yield with diastereoselectivities of approximately
5: 1 observed at the carbinol center (Scheme 1.16 bottom)."
I
Scheme 1.16
R'=BOC. Cbz FI2= Me. nPr. Bn. Allyl. CH2C02Et
R~=M~, Pr
The camphor sultam 1.37 introduced by Oppolzer has k en used to direct the
denvatization of glycine enolate equivalent~.~ Deprotonation of the imino sultam
spthon 1.37 followed by alkylation in the presence of HMPA gives the amino acid
derivatives 1.38 with high diastereoselectivities (90-98% de) and excellent yields
(Scheme 1.17). hine hydrolysis under acidic conditions and removal of the auxiliary
with LiOH fumishes a-amino acid 1.6.
15