Triphenylphosphonium salts bearing an l-alanyl substituent: short synthesis and enantiomeric analysis by NMR

A short, practical stereospecific synthesis of triphenylphosphonium salts bearing an l-(N-benzoyl)-alanyl substituent from l-serine is described. The key step is the ring opening of an oxazoline salt derived from serine with trimethylsilyl halide, giving β-bromo or β-iodo alanine, which were used for the quaternization of triphenylphosphine. The phosphonium salts were obtained in 80% overall yield from serine, and their enantiomeric purity was easily determined by 31P NMR in the presence of a cinchona alcaloid.