Lewis‐Acid‐Catalyzed (3+2)‐Cycloadditions of Donor‐Acceptor Cyclopropanes with Thioketenes

The reactivity of donor‐acceptor (D‐A) cyclopropanes towards thioketenes was investigated. In a (3+2)‐cycloaddition using Sc(OTf)3 as a Lewis acidic catalyst, the corresponding exocyclic thioenol ethers (2‐methylidene tetrahydrothiophenes) were formed in moderate to good yields. Unsymmetrical thioketenes provided E/Z mixtures at the double bond, with the Z isomer being preferred. A series of dialkyl 2‐methylidene tetrahydrothiophene‐3,3‐dicarboxylates was prepared from (tert‐butyl)iso‐propyl thioketene via Lewis acid catalyzed (3+2)‐cycloaddition with D‐A cyclopropanes. In all studied cases the (Z)‐diastereoisomer was found as the main product.