Mechanism investigations of the activation process of S-2-[(propan-2-yl)sulfinyl]benzyl (SPSB) glycosides

In our recently developed interrupted Pummerer reaction mediated (IPRm) glycosylation with S-2-[(propan-2-yl)sulfinyl] benzyl (SPSB) glycosides as glycosyl donors, the anomeric leaving groups were recovered in the forms of thiosulfinate, disulfide and thiosulfonate. These products were presumed to be obtained by the hydrolysis and disproportionation of a cyclic thio-sulfonium intermediate. In this study, mass spectrometry and 1H NMR studies were carried out which confirmed that the cyclic thio-sulfonium intermediate exists. Further 18O isotopic labeling reactions revealed that the cyclic thio-sulfonium ion was hydrolyzed to thiosulfinate which further disproportionated to disulfide and thiosulfonate. This study clarifies the activation process of the SPSB glycosides involved glycosylation reactions.