218. Hetero-Diels-Alder Reaction with 1,3-thiazol-5(4H)-Thiones

Article abstract of DOI:10.1002/hlca.19880710825

On heating in toluene to 180 deg and on treatment with BF3*Et2O in CH2Cl2 at room temperature, 1,3-dienes react with the C=S group of 1,3-thiazol-5(4H)-thiones 1 in a reversible Diels-Alder reaction to give spiro<4.5>heterocycles of type 6.A 1:1 mixture of two regioisomeric cycloadducts is formed in the thermal reaction with 2-methylbuta-1,3-diene (isoprene, 5b).In contrast, the formation of one regioisomer is strongly preferred in the BF3-catalyzed reaction.Frontier-orbital control as well as steric factors seem to be responsible for the observed regioselectivity.BF3-Catalyzed, cyclic 1,3-dienes and 1 also undergo a smooth Diels-Alder reaction.Whereas cyclohexa-1,3-diene (5c) reacts with 1a and 1b to give a single isomer (presumably the 'exo'-adduct), cyclopenta-1,3-diene (5d) leads to a ca. 3:1 mixture of 'exo'- and 'endo'-isomer.