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 tetrasubstituted chiral carbon stereocenter and the example of N‐vinyl nitrones in asymmetric [3+3] cycloaddition.