Synthesis and reactivity of cyclopropanes and cyclopropenes

Activated cyclopropanes have been extensively used in synthetic chemistry as precursors for cycloaddition reactions. The rationale behind this is their ability to undergo ring-opening when activated by a Lewis acid, this can be enhanced further by the presence of a carbocation stabilising group like electron-rich aromatics. The stabilised dipole formed after ring opening can be trapped with suitable electrophiles such as imines and aldehydes via a [3+2] cycloaddition reaction. This results in the synthesis of pyrrolidines and tetrahydrofurans in excellent yields but moderate diastereoselectivity. Similarly, 6-membered heterocycles can be formed via a [3+3] cycloaddition reaction of activated cyclopropanes with nitrones. Now to extend the scope of the methodology, a [3+3] dipolar cycloaddition has been developed using activated 2,3 disubstituted cyclopropane diesters to access a range of highly functionalised oxazines in moderate to good yields (50-75%) and with reasonable diastereoselectivity. The use of activated symmetrical disubstituted cyclopropanes afforded the desired oxazines in a regio- and diastereocontrolled manner, while the use of unsymmetrical cyclopropanes significantly reduced the diastereoselectivity of the reaction. The stereochemistry outcome of the reaction developed was determined by nOe analyses and X-ray diffraction structures could be recorded in some examples. A new methodology has also been developed to gain access to novel N-heterocyclic- and phenol- substituted cyclopropanes in one step from the corresponding cyclopropene via a conjugated addition.