Lanthanide-catalyzed asymmetric 1,3-dipolar cycloaddition of nitrones to alkenes using 3,3′-bis(2-oxazolyl)-1,1′-bi-2-naphthol (BINOL-Box) ligands

New BINOL-derived ligands, 3,3′-bis(2-oxazolyl)-1,1′-bi-2-naphthols (BINOL-Box), bearing chiral bis-oxazoline at the 3,3′-carbons, were synthesized from commercially available 1,1′-bi-2-naphthol (BINOL). With the new ligands obtained, we found that asymmetric 1,3-dipolar cycloaddition reaction of N-benzylidenebenzylamine N-oxide (2) to 3-((E)-2-butenoyl)-1,3-oxazolidin-2-one (1) was catalyzed by BINOL-Box–scandium complexes to give isoxazolidine 3 in high yield with high diastereo- and enantioselectivity. For example, the reaction of 1 with 2 catalyzed by a 6 mol% (S,R)-7d and 5 mol% Sc(OTf)3 complex proceeded to give the endo-3 as the major diastereomer with an endo:exo ratio of 97:3 and 87% ee of the endo-product in the presence of 4 Å molecular sieves. Interestingly, the absolute configuration of the major product was changed according to the kind of additive used.