2//-Thiopyran-3,5 -dione

A general preparation of 2-acetonyl-2,4,6-triaryl-2//-thiopyrans 58 was discovered in the reaction of corresponding 2,4,6-triarylthiopyrylium perchlorates with ethanolic acetone catalyzed with various dialkylammonium salts (86GEP235455, 86JPR573). This preparative procedure was successfully extended to cyclohexane- and cyclopentane- 1,2-diones as the carbonyl components (89JPR853 90GEP280324). The action of bases on thiopyrylium salts may caused their dimerization to thiopyranyl derivatives under suitable conditions (89KGS479). 

Analogous cyclizations with 1,5-diketones show that ring closure readily occurs in protic solvents such as acetic acid92"94 or alcohols.94"96 Typical examples include the preparations of 4//-thiopyran derivatives 43, 45, and 47a from the corresponding l,5-diphenyl-l,5-diones 44 and 46, respectively.92,93... 

Thiochroman-3,4-diones react with various diamines to create the thiopyrano[c]pyrazine ring system, and reaction with />-anisaldehyde and NH4OAC gives a thiopyrano[c]imidazole. The base-catalyzed condensation with dibenzyl ketone yields a red [Amax = 495nm, = 8.4 X 103moldm 3cm 1] cyclopenta[c][l]-benzothiopyran-2-one 386 which gives a dibenzo[A/]thiopyran on reaction with DMAD (Scheme 101) <1994T7865>. 

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>. 

Treatment of 4-dimethylaminomethylene-2//-thiopyran-3,5(4//,6//)-dione (575) with hydroxyl-amine hydrochloride in refluxing MeOH gives compound (576) in good yield (Equation (56)) (84JHC1437). 

Both dihydrothiopyran-4-ones 60 and tetrahydrothiopyrano[2,3-h]thiopyran-4,5-diones 61 can be obtained from dialkenoylketene dithioacetals 62. Cleavage of the dithioacetal ring with NaH in DMF is followed by an intramolecular thia-Michael reaction to give 60, which readily... 

Reduced Thiopyrans.—The tetrahydrothiopyran-3,5-dione (73) has been synthesized by treatment of the keto-diester (72) with butoxide, and it is cyclized by barium hydroxide to the furothiopyrone (74), which is a potential synthon for natural furans. Partial reduction of the dione (73) and dehydration gave the dihydrothiopyrone (75). 

Successive treatment of bis-3,3-dimethylallenyl sulphone (357) with butyl-lithium in THF at 0 °C gave a 60% yield of the dithia-adamantane (358), whose structure was confirmed by X-ray crystallography. Photolysis of thietans in the presence of a new triplet sensitizer, l,3-dimethyl-2-thioxoimidazolidine-4,5-dione, has produced interesting results for example, the spiro-xanthone (359) gave the acetate (360), but in u.v. light, and without the sensitizer, the thiopyran (361) was obtained in addition to (360). Among several flavonoids identified in a Chinese plant. Euphorbia lunulata Bge., which is used in the treatment of... 

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