Preparation of (R)-Fluoropyruvaldehyde N,S-Ketals by HighlyStereospecific Tandem Pummerer Rearrangement/1,2-p-TolylthioGroup Migration of (R)-alfa-(Fluoroalkyl)-beta-sulfinylenamines

Fluoropyruvaldehyde N,S-ketals (R)-2 have been prepared in good yields (up to 88%) and ee (up to 79%) from R-(fluoroalkyl)-beta-sulfinylenamines (R)-(Z)-1, through a new self-immolative tandem sequential process, consisting of a Pummerer reaction, promoted by trifluoroacetic anhydride, followed by a 1,2-migration of the p-tolylthio group, triggered by addition of silica gel or aqueous base. Each transfer of stereogenic center, from sulfur to the R-carbon and then to the alfa-carbon, occurs with an average degree of enantioselectivity up to 94:6. Cis geometry between the sulfinyl and the amino groups of the starting enamine (R)-1 is necessary for achieving high level of stereocontrol, since neighboring group participation by the N-Cbz amino group prevents the sulfinyl center from racemization promoted by trifluoroacetic anhydride. NMR studies have shown that imines 3 are intermediate products of the Pummerer rearrangement, which are stable in the reaction environment.