Formal High-Order Cycloadditions of Donor-Acceptor Cyclopropanes with Cycloheptatrienes

Abstract

The design of various cycloaddition/annulation processes is one of the most intriguing challenges in the development of donor-acceptor (D–A) cyclopropane chemistry. In this work, a new class of formal high-order [6+n]-cycloaddition and annulation processes of D–A cyclopropanes with cycloheptatriene systems has been designed and reported, to fill a significant gap in the chemistry of D–A cyclopropanes. The reactivity of methylated cycloheptatrienes from Me1 to Me5 as well as unsubstituted cycloheptatriene was studied in detail under GaCl3 activation conditions, which makes it possible to efficiently generate gallium 1,2-zwitterionic complexes or 1,3-zwitterionic intermediates starting from D–A cyclopropanes, while other Lewis acids are ineffective and non-selective. New examples of formal [6+2]-, [6+3]-, [6+4]-, [6+1]-, and [4+2]-cycloaddition and annulation reactions with cycloheptatrienes along with more complex processes were discovered. Cycloheptatriene itself can also successfully act as a hydride anion donor, which allows the ionic hydrogenation of D–A cyclopropanes to be performed under mild conditions. As a result, a number of efficient and highly diastereoselective protocols for the synthesis of seven-membered carbocycles has been developed.