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- ...
