The Development of an aza-Piancatelli Rearrangement.

摘要

Nitrogen containing compounds are ubiquitous in nature, and this element plays a critical role in the biological activity of many useful compounds. The essential role of nitrogen has dictated the need for new methods for constructing organic compounds containing carbon-nitrogen bonds. The present work describes our efforts to develop new reactions that form new carbon-nitrogen bonds through the development of an aza-Piancatelli rearrangement, which is a new method for the construction of a variety of trans-4-amino cyclopentenones from readily available 2-furylcarbinols. Activation of furylcarbinols under acidic conditions initiates a cascade sequence that results in nucleophilic attack by nitrogen onto furan and ultimately terminates by way of a 4pi conrotatory electrocyclization. The electrocyclization step is stereospecific giving products with defined trans stereochemistry on the newly produced cyclopentenone ring. The Piancatelli rearrangement can be considered complimentary to the Nazarov reaction for the formation of substituted cyclopentenones, however with the former being largely underdeveloped with respect to the nucleophilic partner that is incorporated into the final compound. We have utilized lanthanide triflate Lewis acids, which are part of a class of Lewis acidic salts of rare earth metals, to enable the usage of amine nucleophiles with the Piancatelli rearrangement. Challenges were encountered during further developments using the aza-Piancatelli rearrangement due to inherent limitations of the original furylcarbinol system. These limitations were overcome by the utilization donor-acceptor cyclopropanes which act as an entirely new activation platform for the rearrangement. This new activation platform acts as a masked carbocation and does not suffer common side reactions such as elimination or Friedel-Crafts alkylation reactions that can be problematic using traditional furylcarbinol starting materials.