Synthesis of Functionalized 1-Azaspirocyclic Cyclopentanones Using Bronsted Acid or N-Bromosuccinimide Promoted Ring Expansions

Azaspirocyclic ring systems are present in a variety of alkaloids. Functionalized 1-azaspirocyclopentanones (6, 7, 11, 12) can be efficiently constructed through semipinacol ring expansion reactions of 2-(1-hydroxycyclobutyl)-p-toluenesulfonylenamides (4) promoted by either a Bronsted acid ((S)-(+)-10-camphorsulfonic acid or HCl) or N-bromosuccinimide, an electrophilic bromine source. Reactions promoted by N-bromosuccinimide tend to proceed in higher yields (80−95%) and with greater diastereoselectivity (3:1−1:0) compared to those reactions promoted by a Bronsted acid. In addition, N-bromosuccinimide promoted reactions can produce a complementary stereochemical outcome compared to the reactions using Bronsted acid.