Gk. Hsiao et Dg. Hangauer, A FACILE SYNTHESIS OF TERT-BUTYL 5,5-TETRAMETHYL-1,3,2-DIOXABOROLAN-2-YL)PROPIONATE - AN ORTHOGONALLY PROTECTED BORONIC ACID ANALOG OF ASPARTIC-ACID, Synthesis, (7), 1998, pp. 1043-1046
tert-Butyl -(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)prop an ortho
gonally protected analog of aspartic acid wherein the side chain carbo
xylic acid is replaced with a boronic acid moiety, was conveniently sy
nthesized by alkylation of N-(diphenylmethylene)glycine tertbutyl este
r with pinacol (chloromethyl)boronate as the key step. After selective
cleavage of the tert-butyl ester this unnatural amino acid was couple
d with L-isoleucinamide using a mixed a anhydride method. The two dias
tereomers of the resulting dipeptides were separated by reversed phase
HPLC and their absolute configurations were established by oxidizing
the boronic acids to the corresponding hydroxy groups followed by NMR
and HPLC correlation with authentic Cbz-L-Ser-L-Ile-NH2.