Rearrangements of Furan-, Thiophene- and N-Boc-Pyrrole-Derived Donor-Acceptor Cyclopropanes: Scope and Limitations

Rearrangements of furan-, thiophene- and N-Boc-pyrrole-derived donor-acceptor cyclopropanes initiated by BrOnsted acids were investigated. Throughout the study, ketones or aldehydes were utilized as accepting moieties and oxygen, sulfur and Boc-protected nitrogen were used as the donor atoms. Whether a ring-enlargement and thus an annelation of a five-membered ring, or a ring-opening of the three-membered ring followed by rearomatization to the parent heterocycles takes place strongly depends on the substitution pattern of the cyclopropane. Commonly, more substituted substrates led to annelation whereas less substituted substrates tend to undergo rearomatization. In general, good yields were obtained for starting materials with oxygen and nitrogen donors, whereas the reactions with sulfur as donor gave only poor yields.