Improved Erlenmeyer Synthesis with 5‐Thiazolone and Catalytic Mn(II) Acetate. Crystal Structure Confirmation of the Reaction Stereochemistry

2‐Phenylthiazolin‐5‐one ( 5 , a thioazlactone) condenses with various aldehydes in the presence of the mild base Mn(II) acetate as catalyst in CH₂Cl₂ solution. This leads to the corresponding Erlenmeyer reaction products ( 6 ) in excellent yields in the case of aromatic aldehydes and moderate yields in others. The mildness of the reaction conditions is apparently enabled by the aromaticity of the (putative) intermediate thiazolone anion. The structure and stereochemistry (Z) of the product derived from i‐BuCHO was confirmed by single crystal X‐ray diffraction. This study overcomes key limitations of the classical Erlenmeyer synthesis and also introduces the relatively nontoxic Mn(II) acetate as a reagent in heterocyclic chemistry.