Hetero‐Diels‐Alder‐Reaktion mit 1,3‐Thiazol‐5(4H)‐thionen

Hetro‐Diels‐Alder Reaction with 1,3‐Thiazol‐5(4H)‐thiones On heating in toluene to 180° and on treatment with BF3·Et2O in CH2Cl2 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.