Protecting-Group-Free Synthesis of 3-tert-Prenylated Oxindoles: Contiguous All-Carbon Quaternary Centers via Tertiary Neopentyl Substitution

Ruthenium-catalyzed tert-prenylation of isatin 1 occurs efficiently in the absence of N-protecting groups under the conditions of C-C bond-forming transfer hydrogenation employing 1,1-dimethylallene as the prenyl donor. The prenylated adduct, 3-hydroxy-3-tert-prenyl-oxindole 2, is converted to the tertiary neopentyl chloride 3, which participates in nucleophilic substitution by way of an aza-o-xylylene intermediate to furnish adducts 4a-4l. Through tertiary neopentyl substitution, two contiguous all-carbon quaternary centers are established.