Triphenylphosphonium salts bearing an L-alanyl substituents: 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 -bromo or -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.