Dedicated to Professor Zhengming Li
Cycloaddition is one of the most powerful tools for the construction of carbo-and heterocyclic skeletons, due to its high efficiency and atom economy. [1] Methylenecyclopropanes (MCPs) are useful building blocks in organic synthesis because they are readily available [2] and display good reactivity. [3] Cycloadditions of MCPs have shown much utility in a variety of synthetic applications. [3] There are three typical modes of cycloaddition of MCPs (Scheme 1): a) the exocyclic mode (mode A) [4] in which the exocyclic carbon-carbon double bond formally serves as a dienophile, b) proximal ring opening of a cyclopropane bond (mode B), [5] and c) ring-opening at the CรC bond that is distal to the alkene (mode C), [6] wherein a trimethylenemethane (TMM) [1a, 7] or a metallacyclobutane [8] transition state may be involved. Most of these cycloadditions proceed under thermal conditions or by catalysis using transition metals. Lewis acids, such as lanthanide triflates, [9] are usually effective in promoting the carbocation-related reactions under mild conditions. Recently, Lewis acid-promoted proximal [10] and distal [11] cycloA C H T U N G T R E N N U N G additions of MCPs have been studied and have attracted considerable attention.
Nitrone is one of the most useful dipoles in 1,3-dipolar cycloadditions. [1a] The groups of Brandi and de Meijere have studied the 1,3-dipolar cycloaddition of MCPs with nitrones for many years. [3] However, all of these cycloadditions are of the exocyclic [3+2] variety [Scheme 2, Eq. ( 2)]. In our investigation of cycloadditions of functionalized MCPs, we have [a] Dr.