SINGLE AND DOUBLE ASYMMETRIC INDUCTION IN THE CYCLOADDITION OF CHIRAL NITRONES TO ACHIRAL AND CHIRAL VINYLPHOSPHINE OXIDES

The asymmetric 1,3-dipolar cycloadditions of the chiral alpha,beta-dialkoxynitrones (4S)-(Z)-N-(2,2-dimethyl-1,3-dioxolan-4-yl)methylenebenzylamineN-oxide 3 and (4S,5S)-(Z)-N-(2,2,5-trimethyl-1,3-dioxolan-4-yl) methylenebenzylamine N-oxide 4a to diphenylvinylphosphine oxide 2 give, besides complex mixtures of all the possible regio- and diastereoisomers, one major isomer, 5a and 8a respectively, consisting of 65% of the total isomeric amount. This compounds have been assigned the erythro C(3)-C(4') configurations deriving from the most reactive conformation A or B through an endo transition state. The cycloaddition of the same nitrones with racemic and enantiomerically pure (-)-S-methylphenylvinylphosphine oxide (1) allowed the study of the double asymmetric induction. The selectivity towards the erythro compounds increases remarkably to ca. 40:1 for the endo approach, indicating that (2S)-nitrones 3 or 4a and (S)-phosphine oxide 1 constitute a matched pair of reactants. The synthesized phosphinylisoxazolidines can be conveniently transformed into selectively protected C-phosphinyl-aminotriols. An illustrative example of the synthesis of the novel fully and selectively protected 1-phosphinyl-seco-daunosamine 22 is reported.