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Fluorenethione S-oxide undergoes (2 + 4) cycloadditions with azoalkenes to yield 2H-1,2,3-thiadiazine 1-oxides together with a small amount of the regioisomeric 6H-1,3,4-thiadiazine 1-oxides. Fluorenethione, with azoalkenes, undergoes (2 + 4) regiospecific cycloaddition reactions leading to the formation of 6H-1,3,4-thiadiazine derivatives. Diarylsulphines and diarylthiones fail to react with azoalkenes.

Silyl thioketones are easily converted into the corresponding trimers (1,3,5-trithianes) under acidic conditions. Both the α-(cis,trans)- and the β-(cis,cis)-isomers were isolated. The β-diastereoisomers predominate over the α-form. The major isomer from the trimerisation of trimethylsilylphenyl thioketone, upon X-ray analysis, shows a chair-like conformation with the silyl groups all equatorial. Desilylation of the α- and β-trimers of the trimethylsilylphenyl thioketone with fluoride ion affords, stereospecifically, the two isomeric 2,4,6-triphenyl-1,3,5-trithianes.

Diarylsulphines (diaryl thioketone S-oxides) undergo cycloadditions with benzonitrile oxide to yield 1,4,2-oxathiazole 4-oxides, with the exception of thiofluorenone S-oxide which gives a 1,5,2-oxathiazole 5-oxide as the dominant product. From the nature of the products obtained from geometrically isomeric sulphines, the cycloaddition was shown to be stereospecific.

Sulphur monoxide generated in situ by smooth thermolysis of trans- 2,3-diphenylthiiran 1-oxide reacts with triphenylphosphonium-, diphenylsulphonium-, and pyridinium-tetraphenylcyclopentadienylide, leading in all cases to tetraphenylcyclopentadienethione S-oxide. Similar reactions with other representative ylides have been investigated. The reactions are very sensitive to the stability of the ylide and to the electronic nature of the groups directly attached to the ylidic carbanion.

Sulphines (thione S-oxides) undergo cycloaddition reactions with diphenylnitrilimine to yield in a regiospecific manner 1,3,4-thiadiazoline 1-oxides, with the exception of thiofluorene S-oxide, which gave the other regioisomer (a 1,2,3-thiadiazoline 1-oxide) as a by-product. The products obtained from cyclisation with geometrically isomeric sulphines indicate that steric integrity is lost during the reaction. This stereomutation does not come about by sulphine isomerization prior to reaction but is caused by product equilibration afterwards. It is shown that the steric integrity is lost by ring opening–ring closure of the cycloaddition product. A ring expansion of the addition product similar to the penam–cepham conversion is described.

Unambiguous Configurational assignment of syn and anti-isomers of methyl-substituted aromatic Sulphines has been achieved by means of: (a) intermolecular hydrogen bonding with weak acids; (b) intramolecular association in 2-hydroxy-(2′-methyl)thiobenzophenone S-oxide; (c) aromatic solvent induced shift measurements; and (d) carbon-13 n.m.r. spectra. The thermodynamic and kinetic parameters for the thermal interconversion of the isomers have also been determined. These values are discussed in terms of possible isomerisation mechanisms.

The cycloaddition reactions of aromatic sulphines, such as thiobenzophenone S-oxide and thiofluorenone S-oxide, with 2-diazopropane lead to 1,3,4-thiadiazoline 1-oxides in high yields. Diazomethane reacts more sluggishly with sulphines: only with thiofluorenone S-oxide was a cycloadduct obtained. The structure of the cycloadducts has been proven by photochemical extrustion of sulphur monoxide from 2,2-dimethyl-5,5-di-(p-tolyl)-1,3,4-thiadiazoline 1-oxide. In solution the cycloadducts readily undergo a retro-cycloaddition. The adduct from thioxanthen-9-thione SSS′-trioxide and 2-diazopropane not only reverts to the starting sulphine, but also shows a reversed retro-cycloaddition reaction to give diazothioxanthen SS-dioxide and thioacetone S-oxide.