Organocatalytic Enantioselective Synthesis of Trifluoromethyl-Containing Tetralin Derivatives by Sequential (Hetero)Michael Reaction-Intramolecular Nitrone Cycloaddition.

The enantioselective synthesis of tetralin derivatives bearing a trifluoromethylated all-carbon quaternary stereocenter has been accomplished through a synthetic sequence comprising an organocatalytic β-functionalization of ortho-1-trifluoromethylvinyl-(hetero)aromatic conjugated aldehydes followed by the intramolecular nitrone 1,3-cycloaddition reaction (INCR). Both nitromethane and N-Cbz-hydroxylamine were employed as nucleophiles in the initial organocatalytic conjugate addition step, which provided the chiral information required for the subsequent diastereoselective INCR. Although diastereoselectivity was moderate, good yields and enantioselectivities were, in general, obtained. Interestingly, an inversion of the selectivity in the intramolecular cycloaddition step was observed when either nitromethane or N-Cbz-hydroxylamine was employed. This outcome was studied by means of theoretical calculations, which were in agreement with the experimental results. In addition, the ring opening of the isoxazolidine moiety furnished the corresponding fluorinated diamino alcohols in a very efficient manner. [ABSTRACT FROM AUTHOR]