Benzothiopyrylium salts, polar 4 + 2-cycloaddition

Benzothiopyrylium salts undergo a polar [4+2+] cycloaddition to give l,10a-dihydro-4/f-4a-thioniaphenanthrene salts, which afford 2-substituted 2ff-l-benzothiopyrans on treatment with nucleophiles <96JCS(P1)2227>. 

Thiopyrylium ions have been shown to behave as effective dienophiles. Shimizu and co-workers demonstrated that 2-benzothiopyrylium salts undergo [4+2+]-type cationic polar cycloadditions with various 1,3-dienes yield benzo-fused bicyclic sulfonium salts <1994J(P1)3129> (see Section 8.32.2.1.2 of CHEC-II(1996)). However, in this study the authors observed that cycloaddition of 2-benzothiopyrylium salt 40 with 2,3-dimethylbuta-l,3-diene 41a with prolonged reaction times caused a decrease in yield of the cycloadduct 22a (see entries 1 and 2 in Table 5 and Equation 10). In fact, stirring of the reaction mixture for 30 h did not afford the cycloadduct at all, see entry 3, but gave an undetermined complex mixture. 

Table 5 Polar cycloadditions of 2-benzothiopyrylium salt 40 with dimethylbuta-1,3-diene 41a (Equation 10)...

Benzothiopyrylium salts have also been shown to undergo [4+2+]-polar cycloadditions with 1,3-dienes in dry 1,2-dichloroethane at room temperature <1996J(P1)2227>. Treatment of 43 with 2,3-dimethylbuta-l,3-diene 41a afforded benzo-fused bicyclic sulfonium salts 22, as shown in Equation (12) and Table 6. The 4-cyano-substituted benzothiopyrylium salt 43d proved to be the most reactive of these substrates (see entry 4). 

Table 7 Polar cycloadditions of 1 -benzothiopyrylium salts 43 with isoprene 41 b (Equation 13)...

Benzothiopyrylium and dibenzothiopyrylium salts also take part in polar cycloaddition reactions with 1,3-dienes to form fused bicyclic sulfonium salts 338 and 339, respectively <1996J(P1)2227, 1997T4611>. Alcohols cleave an S-C bond to give mainly a but-2-enyl derivative 340 with some of the terminal alkenyl product 341 (Schemes 73 and 74). 

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