Phospholyl(borane) Amino Acids and Peptides: Stereoselective Synthesis and Fluorescent Properties with Large Stokes Shift

The synthesis of
phospholyl­(borane) amino acids was stereoselectively
achieved by reaction of phospholide anion with iodo α-amino
ester derived from l-aspartic acid or l-serine,
followed by in situ complexation with borane. Phospholyl­(borane)
amino acids are easy to store and can be subjected to direct transformation
into the corresponding free phospholyl, gold complex, oxide or sulfur
derivatives as well as phospholinium salts, thus offering a variety
of side chains. After selective deprotection of carboxylic function
or amine, C- or N- peptide coupling with an alanine moiety proved
the possible incorporation into peptides. Such phospholyl amino acid
and peptide derivatives exhibit fluorescent properties with a large
Stokes shift (160 nm) and fluorescence up to 535 nm, depending on
the phosphole aromaticity and the chemical environment. These phospholyl­(borane)
amino acids constitute a new class of unnatural amino acids useful
for structure–activities relationship studies and appear to
be promising fluorophores for the development of labeled peptides.