Cyclization of Cyclopropyl Carbonyls and the Homo-Nazarov Reaction

Ring strain confers exceptional reactivity to the cyclopropane ring. Nevertheless, activation of the cyclopropane ring is usually needed to allow ring-opening reactions under mild conditions. The introduction of one or several carbonyl functionalities on cyclopropanes is often used to enhance their electrophilic reactivity. When a donor group is also present at the vicinal position to the carbonyl, more reactive donor-acceptor systems are obtained, which are ideally suited for homo-conjugate addition or cycloaddition reactions. This short review focuses on the special case of intramolecular cyclization reactions. Particular emphasis is devoted to the cyclization of vinyl- and aryl-cyclopropyl ketones for the formation of six-membered rings. This process corresponds to a formal homo-Nazarov cyclization and has been only used rarely in the past. Recently, the scope of the reaction was greatly expanded by the use of a silyl group as cation-stabilizing group and by the first catalytic method for the synthesis of vinyl-cyclopropyl ketones. With these new developments, the basis is now set for the application of this special class of cationic cyclizations to the synthesis of more complex synthetic and natural products.