Preparation and reactions of silyl nitronates derived from 2,2,2-trifluoronitroethane. Diastereoselective synthesis of trifluoromethyl-substituted aminoethanols and -propanols

Novel transformations of gamma-silyl nitro compounds

Introduction of a gamma-silyl group into nitro compounds of dihydrobenzofuran, dihydrobenzo[b]thiophene, and dihydrofuran allowed new transformations to take place in the presence of a Lewis acid to give the corresponding alpha,beta-unsaturated oximes or multisubstituted dihydrofurans, respectively, in good to excellent yields.[reaction: see text]

Tandem Cycloaddition Chemistry of Nitroalkenes: Preparative and Theoretical Studies on the Stereochemical Course of [3 + 2] Cycloaddition of Cyclic Nitronates

Intermolecular [3 + 2] cycloadditions between two cyclic nitronates and a series of dipolarophiles are examined. High facial selectivity is observed in all cases and is analyzed with the aid of ab initio transition structure calculations. Monosubstituted dipolarophiles reacted with exclusive regiocontrol. Disubstituted dipolarophiles reacted with varying degrees of regiocontrol, which was dependent on the substituent. A theoretical approach for predicting regioselectivity is discussed. Exo selectivity was generally favored due to steric effects, and was especially high with cis-disubstituted dipolarophiles.

(R)-Trifluoro- and Difluoropyruvaldehyde N,S-Ketals: Chiral Synthetic Equivalents of beta-Trifluoro and beta-Difluoro alpha-Amino Aldehydes

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).