The formation of N-tosyl-2-phenyl-1,3,2-oxazaborolidin-5-one 1 as a result of a condensation reaction between N-tosyl valine 2 and phenyl boronic acid was studied as a model of the formation of chiral oxazaborolidines used as Lewis acidic catalysts for various enantioselective syntheses. Intermediates of the formation of 1 along with those arising from further reactions (of 1) were investigated by electrospray ionization mass spectrometry (ESI-MS). Results of the study indicate that one phenyl boronic acid may react with one or two molecules of 2 and/or with one or two molecules of phenyl boronic acid. In addition, side-products implying dephenylation of 1 and self-condensation of phenyl boronic acid (formation of triphenylboroxine) were found.
Nevalainen, V., Mansikka, T., Kostiainen, R., Simpura, I., & Kokkonen, J. (1999). Mass spektrometric studies on the formation of chiral N-sulphonylated oxazaborolidinones. Tetrahedron: Asymmetry, 10(1), 1-5. https://doi.org/10.1016/S0957-4166(98)00479-0