title: Preparation of (R)-Fluoropyruvaldehyde N,S-Ketals by Highly Stereospecific Tandem Pummerer Rearrangement/1,2-p-Tolylthio Group Migration of (R)-alpha-(Fluoroalkyl)-beta-sulfinylenamines.
abstract: Fluoropyruvaldehyde N,S-ketals (R)-2 have been prepared in good yields (up to 88%) and ee (up to 79%) from alpha-(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 alpha-carbon and then to the beta-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.
authors.affiliation: Dipartimento di Chimica, Ingegneria Chimica e Materiali, Università di L'Aquila, Via Vetoio, I-67010 Coppito, Italy.
authors.forename: Alessandro
authors.initials: A
authors.lastname: Volonterio
authors.forename: Matteo
authors.initials: M
authors.lastname: Zanda
authors.forename: Pierfrancesco
authors.initials: P
authors.lastname: Bravo
authors.forename: Giovanni
authors.initials: G
authors.lastname: Fronza
authors.forename: Giancarlo
authors.initials: G
authors.lastname: Cavicchio
authors.forename: Marcello
authors.initials: M
authors.lastname: Crucianelli