Asymmetric [3+3] Cycloaddition of N‐Vinyl Oxindole Nitrones with 2‐Indolylmethanols to Prepare Spirooxindole[1,2]oxazines

We describe a chiral phosphoric acid (CPA) catalyzed asymmetric [3+3] cycloaddition of N ‐viny oxindole nitrones with 2‐indolylmethanols to prepare various spirooxindole[1,2]oxazines in 43–93% yields and 88:12–97:3 enantiomeric ratios. Experimental results revealed that the addition of hexafluoroisopropanol (HFIP) played important roles to help CPA control the reaction reactivity and enantioselectivity. The present method features the construction of a tetrasubstituted chiral carbon stereocenter and the example of N ‐vinyl nitrones in asymmetric [3+3] cycloaddition.