On heating in toluene to 180” and on treatment with BF,. EtzO in CHzCl, 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-l, 3-diene (isoprene, Sb). In contrast, the formation of one regioisomer is strongly preferred in the BF, ...
