<i>o</i>‑Boronato-
and <i>o</i>‑Trifluoroborato–Phosphonium Salts
Supported by l‑α-Amino Acid Side Chain
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Abstract
The
synthesis of <i>o</i>-boronato- and <i>o</i>-trifluoroborato–phosphonium
salts supported by the l-amino acid side chain is described.
The synthesis of these new class
of amino acid derivatives was achieved by stereoselective quaternization
of <i>o</i>-(pinacolato)boronatophenylphosphine with
β- or γ-iodo amino acid derivatives which are prepared
from l-serine or l-aspartic acid, respectively.
The quaternization of the phosphine was performed using either iodo
amino ester or carboxylic acid derivatives. In addition, free carboxylic
acid and amine derivatives were obtained by saponification or HCl
acidolysis of <i>o</i>-boronato–phosphonium amino
esters, respectively. The usefulness of these compounds in peptide
coupling was demonstrated by coupling an <i>o</i>-boronato–phosphonium
amino ester with an aspartic acid moiety. When the <i>o</i>-boronato–phosphonium amino acid or dipeptide derivatives
were mixed with fluoride, the corresponding <i>o</i>-trifluoroborated
products were cleanly and rapidly obtained in high isolated yields.
The hydrolysis of these compounds at room temperature using a phosphate
buffer pH 7/CD<sub>3</sub>CN mixture has shown only traces of free
fluoride F<sup>–</sup> after several days. Finally, a preliminary
radiolabeling essay has proven the facile [<sup>18</sup>F]-fluoride
incorporation and high stability of the radiolabeled product in aqueous
conditions. Indeed, this new class of boron–phosphonium amino
acid derivatives shows promising properties for their applications
in synthesis and labeling of peptides