Competition between hetero-Diels-Alder and cheletropic additions of sulfur dioxide to 2-substituted buta-1,3-dienes. Synthesis of 2-(1-naphthyl)and 2-(2-naphthyl)buta-1,3-diene

Chloroprene (= 2-chlorobuta-1,3-diene; 4b) and electron-rich dienes such as 2-methoxy-(4c), 2-acetoxy(dd), and 2-(phenylseleno)buta-1,3-diene (4e) refused to equilibrate with the corresponding sultines 5 or 6 between -80 and -10degrees in the presence of excess SO2 and an acidic promoter. Isoprene (4a) and 2-(triethylsilyl)(4f), 2-phenyl-(4g), and 2-(2-naphthyl)buta-l,3-diene (4i) underwent the hetero-Diets-Alder additions with SO2 at low temperature. In contrast. 2-(1-naphthyl)buta-1,2-diene (4h) did not. With dienes 4a, 4g, and 4i, the hetero-Diels-Alder additions with SO2 gave the corresponding 4-substituted sultine 5 with high regioselectivity. In the case of 4g + SO2 reversible arrow 5g, the energy barrier for isomerization of 5g to 5-phenylsultine (6g) was similar to that of the cheletropic addition of 4g to give 3-phenylsulfolene (7g). The hetero-Diets-Alder addition of 4f gave a 1:4 mixture of the 4-(triethylsilyl)sultine (5f) and 5-(triethylsilyl)sultine (6f). The preparation of the two new dienes 4h and 4i is reported.