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Microwave-assisted copper-catalyzed ring expansions of three-membered heterocycles with α-diazo-β-dicarbonyl compounds were investigated. Thiiranes generated 3-acyl-5,6-dihydro-1,4-oxathiines in the presence of copper sulfate and trans -3-acyl-5,6-dihydro-1,4-oxathiines as stereospecific products for 1,2-disubstituted cis -thiiranes through an intramolecular S N 2 process. Oxiranes gave rise to 2-acyl-5,6-dihydro-1,4-dioxines under the catalysis of copper hexafluoroacetylacetonate and cis -3-acyl-5,6-dihydro-1,4-dioxines as stereospecific products for 1,2-disubstituted cis -oxiranes via an intimate ion-pair mechanism. The current method provides a direct and simple strategy in efficient preparation of 3-acyl-5,6-dihydro-1,4-oxathiines and 2-acyl-5,6-dihydro-1,4-dioxines, important agents in medicinal and agricultural chemistry, from readily available thiiranes and oxiranes, respectively. [ABSTRACT FROM AUTHOR]

Synthesis of 3-acyl-5,6-dihydro-1,4-oxathiines has been achieved via reactions of thiiranes and α-diazo-β-1,3-dicarbonyl compounds under microwave and copper sulfate-assisted conditions. The current method provides a direct and simple strategy in efficient preparation of 3-acyl-1,4-oxathiines from readily available thiiranes and trans -3-acyl-5,6-dihydro-1,4-oxathiines as stereospecific products for 1,2-disubstituted cis -thiiranes. The reaction mechanism was also proposed. [ABSTRACT FROM AUTHOR]