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The irradiation of 2,6-bis(alkylthio)-3,5-diphenyl-4H-thiopyran-4-ones (2) with a medium-pressure mercury arc lamp through a Pyrex filter produced 3,4-bis(alkylthio)-2,5-diphenylcyclopentadienones. The quantum efficiency for formation of cyclopentadienone derivatives was very low (Φ= 0·0002). Sensitization and quenching studies indicate that the reaction involves the triplet state of compounds (2). Cycloadditions of the photoproduct with some dienophiles were studied. The mechanism of the photochemical rearrangement of compounds (2) is discussed.

4H-Thiopyran-4-thiones are ring-opened by sulphide or hydroxide in dimethyl sulphoxide or NN-dimethylformamide, forming anions which undergo intramolecular oxidative coupling to give 6a-thiathiophthens or 3-acylmethylene-3H-1,2-dithioles, respectively. The procedure provides a useful synthesis of 6a-thiathiophthen.

Methinecyanines have been prepared from 1,2- and 1,3-dithiolium salts. The violet compound, previously thought to be a C-betaine derived from bis-[3-(benzo-1,2-dithiole)]methinecyanine, is shown to be 2-(benzo-1,2-dithiol-3-ylidene)-2,3-dihydrobenzo[b]thiophen-3-thione. Two by-products, obtained during an unambiguous synthesis of the latter compound, are shown to be derivatives of 6H-dibenzo[b,f]thieno[3,2-b]thiopyran.

1 H N.m.r. data obtained from the direct observation of benzo- and dibenzo-thiopyran SS-dioxide anions and of some methyl derivatives (I)–(IV) are consistent with a substantially trigonal configuration of the carbanion carbon and with extensive delocalisation of the negative charge within the rings. Evidence is also presented for the occurrence of 1H n.m.r. time scale chemical exchange at the methylene group of 1-benzothiopyran SS-dioxide derivatives in methanol and in the presence of sodium methoxide.

Methylene blue-sensitized photo-oxidation of the 4H-pyran-4-thiones (Ia–c) and 4H-thiopyran-4-thiones (Id, e) gives the corresponding ketones (II).

Methylene Blue-sensitized photo-oxygenation of the 4H-pyran-4-thiones and 4H-thiopyran-4-thiones gives the corresponding ketones. The relative reactivity (β=k1/k2) of these thioketones to dye-sensitized oxygenation has been determined. The reactivity of the thioketones is the same or higher than that of tetramethylethylene. The quenching of the oxygenation by β-carotene suggests that the reactive species is singlet oxygen for the dyesensitized photo-oxygenation.

Irradiation of 2,6-bis(alkylthio)-3,5-diphenyl-4H-thiopyran-4-ones with a medium-pressure mercury lamp with a Pyrex filter produced 3,4-bis(alkylthio)-2,5-diphenylcyclopentadienones.

Ionisation constants of 3-phenyl- and 6-methyl-3-phenyl-2H-thiopyran 1,1-dioxides have been determined spectrophotometrically in water and in methanol. The data are consistent with the corresponding anions being particularly stable. The results are compared with those for other sulphonyl-substituted carbon acids.

Acid-catalysed disproportionation of 1,2,3,4-tetrahydro-8-methoxy-6H-benzo[b,d]thiopyran gives two reduction products, both of which are derived from the intermediate bridgehead thiiranium ion.

Tetrahydro-2-hydroxythiophen, tetrahydro-2-hydroxy(thiopyran), and 2-hydroxythiepan, the first examples of simple cyclic hemithioacetals with sulphur in the ring, have been synthesised. Tautomeric change to the open-chain mercapto-aldehyde occurs much more readily with the seven-than with the five- and the six-membered-ring compounds, but 2-hydroxythiepan, unlike the two lower homologues, does not undergo self-condensation to form a dithioacetal. A modified structure, based on the nuclear magnetic resonance spectrum, is proposed for the dimer of 2,3-dihydrothiophen.

The thiopyran group in the title compounds (7)–(10), which have been synthesized, may represent a reasonable model for the thiopyran SS-dioxide anion. Dipolar forms contribute to the ground state of these compounds as shown by 1H n.m.r. correlations and from reactions with electrophiles. I.r. stretching vibrations of the sulphonyl group of these molecules show considerable bathochromic displacements relative to the standard values, indicating that the sulphonyl group encounters the most favourable conditions for case I conjugation.

Hydrolysis of trans-3,4-dihalogenthiochromans, trans-1,2-dibromo-2,3-dihydro-1H-naphtho[2,1-b]thiopyran, and trans-3,4-dibromo-3,4-dihydro-2H-naphtho[1,2-,b]thiopyran yielded 2-hydroxymethylbenzo[b]thiophen, 2-hydroxymethylnaphtho[2,1-b]thiophen, and 2-hydroxymethylnaphtho[1,2-b]thiophen, respectively. Borohydride reduction of 3-halogenothiochroman-4-ones and related bromodihydronaphthothiopyranones gave cis-3-halogenothiochroman-4-ols and the corresponding cis-bromodihydronaphthopyranols. The 1H n.m.r. spectra of these compounds show that the hetero-ring adopts the ‘sofa’ conformation, with both substituents axial in the trans-isomers, and that the most stable conformation of the cis-isomer is that in which the substituent at the benzylic position is pseudo-axial.

The reaction of prop-2-ene-1-sulphonyl chloride with enamines afforded 3-amino-2-vinylthietan 1,1-dioxides and/or allyl aminovinyl sulphones: no 1,4-cycloadducts were detected. Both the products were converted in the presence of base into 3-amino-3,4-dihydro-2H-thiopyran 1,1-dioxides, whose hydrochlorides underwent deamination to the corresponding thiopyran 1,1-dioxides. The intermediacy of the sulphones in the rearrangement of the vinylthietans to the thiopyrans is suggested; also it is suggested that the cyclisation of the sulphones proceeds through an allylsulphonyl carbanion.