Steric-Hindrance-Induced Diastereoselective Radical Nitration of 3-Alkylidene-2-oxindoles Followed by Tosylhydrazine-Mediated Sulfonation.

Metal-free radical nitration of the β C-H bond of 3-alkylidene-2-oxindoles with tert -butyl nitrite (TBN) has been explored. Interestingly, ( E )-3-(2-(aryl)-2-oxoethylidene)oxindole and ( E )-3-ylidene oxindole give different diastereomers on nitration. The mechanistic investigation revealed that the diastereoselectivity was controlled by the size of the functional group. Another transformation of 3-(nitroalkylidene) oxindole into 3-(tosylalkylidene) oxindole was performed through metal and oxidant-free tosylhydrazine-mediated sulfonation. Both methods have the advantages of readily available starting materials and operational simplicity.