A Remarkable Selectivity Observed in Hetero-Diels–Alder Reactions of Levoglucosenone (LGO) with Thiochalcones: An Experimental and Computational Study †

Levoglucosenone (LGO) smoothly undergoes microwave-assisted hetero-Diels–Alder reactions with thiochalcones in THF solution at 60 °C. The studied reactions are completed after 10 min, and the expected tricyclic 2,3-dihydro-4 H -thiopyran derivatives are formed in a highly regio- and moderately stereoselective manner via competitive exo - and endo -attacks of the 1-thiadiene moiety onto the activated C=C bond of dienophile LGO. Although eight isomers are possible, only the formation of exo , exo - (major) and exo , endo - (minor) cycloadducts was observed. In most cases, isomeric products were separated by preparative layer chromatography and identified by means of spectroscopic methods. Some of the cycloadducts were obtained as single crystalline solids, and X-ray analyses enabled unambiguous confirmation of their structures. In order to explain the observed selectivity of the studied hetero-Diels–Alder reactions, DFT studies were carried out to determine the thermodynamic and kinetic properties of all regio- and stereoisomers. The results of these calculations predict the preferred formation of the two experimentally observed isomers. In addition, remarkable details on the electronic structure of E -1,3-diphenylprop-2-en-1-thione and on involved and hypothetical transition states could be elucidated.