Diastereoselective Synthesis of Polysubstituted 2‐Amidino Thiolanes via Cycloaddition of <i>N</i> ‐Thioacyl Amidines With Donor–Acceptor Cyclopropanes

A MgBr 2 ‐catalyzed (3 + 2) cycloaddition between N ‐thioacyl amidines and donor–acceptor cyclopropanes (DACs) has been developed, enabling the highly diastereoselective synthesis of polysubstituted 2‐amidino‐thiolanes in up to 92% yield and with dr values exceeding 99:1. The reaction proceeds under mild conditions and tolerates a broad range of DACs. The structures and relative configurations of the major diastereomers were established by single‐crystal X‐ray diffraction analysis of two representative derivatives. Further derivatization, including sulfur oxidation and an unexpected decarboxylative rearrangement to dihydrothiophenes, highlights the synthetic versatility of the products. DFT calculations suggest that the high diastereoselectivity arises from a lower‐energy cyclization transition state leading to the cis isomer. This work provides efficient access to sulfur‐containing heterocycles bearing the valuable amidine functionality.