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‐( 4d ), and 2‐(phenylseleno)buta‐1,3‐diene ( 4e ) refused to equilibrate with the corresponding sultines 5 or 6 between −80 and −10° in the presence of excess SO₂ and an acidic promoter. Isoprene ( 4a ) and 2‐(triethylsilyl)‐( 4f ), 2‐phenyl‐( 4g ), and 2‐(2‐naphthyl)buta‐1,3‐diene ( 4i ) underwent the hetero‐Diels‐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⇄ 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‐Diels‐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.