Thiopyrans from thiopyrylium salts

Two principle approaches from thiopyrylium salts to thiopyrans accompanied by C-substitution have been found, e.g., the one-step additions of C-nucleophiles or the two-step procedures involving primary conversions of the salts to nucleophilic intermediates followed by attacks with appropriate electrophiles. 

Tetrasubstituted thiopyrylium salts also provide a variety of products. Thus symmetrically substituted 2,3,5,6-tetraphenylthiopyrylium ion with UA1H4 or the condensed tricyclic salt with NaBH4 gave exclusively 4H-thiopyrans 226a40 or 49 and 228.269,270 Dithieno-4//-thiopyran 16 was obtained similarly from the appropriate thiopyrylium pentachlorostannate.130 On the other hand, asymmetrically substituted substrates 229 with LiAlH4 afforded 2//-thiopyran 230 in yields of 60 to 73% accompanied by 13 to 14% of isomers 231 for the 4-phenyl derivatives.271... 

The cyclization of 2-(l,3-diaryl-3-oxopropyl)cyclohexan-l,3-diones to cyclohexcno[A]thiopyrylium salts by H2S generated in situ from ZnS and acid is only successful with electron-rich aryl substituents. It is suggested that the reaction proceeds through the cyclohexeno[2 ]-4//-thiopyran which is produced in the absence of donor substituents and which yields the thiopyrylium salt on treatment with HCIO4 (Scheme 234) <2000CHE1032>. 

H-Thiopyrans may occasionally be isolated from the reaction of 1,5-diketones, hydrogen sulphide, and acids, although they usually disproportionate under these conditions. Dienones give 2 T-thiopyrans with the same reagents. " [4 + 2] Cycloaddition of 3-dialkylaminovinyl aryl thiones with acrylic acid derivatives and related compounds afforded the 2fT-thiopyrans (10 R = H, OEt R = CN, CONHg, COMe, etc.). The intermediate 4-dialkylamino-A -dihydrothiopyrans were sometimes isolated. Factors affecting the addition were studied, and some reactions of (10) and the derived thiopyrylium salts were described. The preparation of the azido-thiopyrans (11 R = H or Ph) by the action of sodium azide on the thiopyrylium salts has been reported. Their thermolysis afforded substituted pyridines, thiophens, and sulphur, which were the decomposition products of the expected thiazepine intermediates. 

Many further examples have been given of the preparation of mixtures of thiopyrylium salts and the corresponding tetrahydrothiopyrans, by the action of hydrogen sulphide and various acids on 1,5-diketones. Selenopyrylium salts have also been prepared by this method. 2-Amino-6-methylthio-thiopyrylium salts, obtained by methylation of the related 2H-thiopyran-2-thiones, react with aromatic amines to give 2-arylimines (42). 2-Amino-6-arylthiopyrylium salts have also been prepared, from 3-mercaptovinylacrylonitriles, by an extension of a reaction previously used for the synthesis of 2-aminopyrylium and pyridinium salts. Several theoretical treatments have appeared. Non-empirical calculations suggest that the 3d orbitals are used only to a trivial extent," but, in another study, the conclusion is reached that involvement of the d-orbitals may be important, and that contributions of forms of type (43) to the hybrid... 

Thiopyran, derived from ethyl vinyl sulfide, is converted into thiopyrylium fluoroborate by reaction with triphenylcarbenium fluoroborate in 54% overall yield (Scheme 235) <2001EJ02477>. 3-Benzoylisothiochromene is oxidized to the 2-benzothiopyrylium salt in a similar manner <1994J(P1)3129>. 

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