title: (R)-Trifluoro- and Difluoropyruvaldehyde N,S-Ketals: Chiral Synthetic Equivalents of beta-Trifluoro and beta-Difluoro alpha-Amino Aldehydes.
abstract: A new, efficient, and stereoselective two-step approach to stereochemically defined chiral nonracemic gamma-tri- and gamma-difluoro beta-amino alcohols (70% to >95% ee) is described, using tri- and difluoropyruvaldehyde N,S-ketals (R)-1a,b as starting materials. Addition of Grignard reagents to (R)-1 occurs with moderate to excellent anti-stereocontrol, depending on the nature of the organomagnesium halides, providing the beta-p-tolylthio beta-benzyloxycarbonylamino secondary carbinols 5. The stereochemical outcome of these reactions can be rationalized by means of a chelated Cram's cyclic model, where the NCbz group is the chelating ligand and the p-tolylthio residue acts as the stereocontrolling "large" group. Reductive displacement of the 2-p-tolylthio substituent of 5 efficiently takes places by means of the NaBH(4)/pyridine system, probably via the corresponding intermediate transient imines 13, providing sulfur-free gamma-tri- and gamma-difluorinated beta-amino alcohols 7 with high levels of anti-stereoselectivity. A considerable shift toward syn-stereoselectivity was obtained performing the reaction on the corresponding phenylacetates 8. Cleavage and reduction of the NHCbz moiety of 7 provided tri- and difluoro analogues of, respectively, norephedrine (11) and ephedrine (12).
authors.affiliation: C.N.R.-Centro di Studio per le Sostanze Organiche Naturali, Via Mancinelli 7, I-20131 Milano, Italy.
authors.forename: Alessandro
authors.initials: A
authors.lastname: Volonterio
authors.forename: Barbara
authors.initials: B
authors.lastname: Vergani
authors.forename: Marcello
authors.initials: M
authors.lastname: Crucianelli
authors.forename: Matteo
authors.initials: M
authors.lastname: Zanda
authors.forename: Pierfrancesco
authors.initials: P
authors.lastname: Bravo